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1
Zhao, Guanjia, Emma J. E. Stuart, and Martin Pumera. "Enhanced diffusion of pollutants by self-propulsion." Physical Chemistry Chemical Physics 13, no. 28 (2011): 12755. http://dx.doi.org/10.1039/c1cp21237k.
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Jurado Romero, Arnau, Carles Calero, and Rossend Rey. "Enhancement of swimmer diffusion through regular kicks: analytic mapping of a scale-independent parameter space." Journal of Statistical Mechanics: Theory and Experiment 2024, no. 6 (June 21, 2024): 063201. http://dx.doi.org/10.1088/1742-5468/ad4024.
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Abstract Depending on their mechanism of self-propulsion, active particles can exhibit time-dependent, often periodic, propulsion velocity. The precise propulsion velocity profile determines their mean square displacement and their effective diffusion coefficient at long times. Here, we demonstrate that any periodic propulsion profile results in a larger diffusion coefficient than the corresponding case with constant propulsion velocity. We investigate, in detail, periodic exponentially decaying velocity pulses, expected in propulsion mechanisms based on sudden absorption of finite amounts of energy. We show, both analytically and with numerical simulations, that in these cases the effective diffusion coefficient can be arbitrarily enhanced with respect to the case with constant velocity equal to the average speed. Our results may help interpret, in a new light observations on the diffusion enhancement of active particles.
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Wang, Xin, Zhongju Ye, Shen Lin, Lin Wei, and Lehui Xiao. "Nanozyme-Triggered Cascade Reactions from Cup-Shaped Nanomotors Promote Active Cellular Targeting." Research 2022 (June 21, 2022): 1–15. http://dx.doi.org/10.34133/2022/9831012.
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Self-propelled nanomotors have shown enormous potential in biomedical applications. Herein, we report on a nanozyme-powered cup-shaped nanomotor for active cellular targeting and synergistic photodynamic/thermal therapy under near-infrared (NIR) laser irradiation. The nanomotor is constructed by the asymmetric decoration of platinum nanoparticles (PtNPs) at the bottom of gold nanocups (GNCs). PtNPs with robust peroxidase- (POD-) like activity are employed not only as propelling elements for nanomotors but also as continuous O2 generators to promote photodynamic therapy via catalyzing endogenous H2O2 decomposition. Owing to the Janus structure, asymmetric propulsion force is generated to trigger the short-ranged directional diffusion, facilitating broader diffusion areas and more efficient cellular searching and uptake. This cascade strategy combines key capabilities, i.e., endogenous substrate-based self-propulsion, active cellular targeting, and enhanced dual-modal therapy, in one multifunctional nanomotor, which is crucial in advancing self-propelled nanomotors towards eventual therapeutic agents.
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Chen, Shuai, Zhi Zhang, Yu Zhang, and Yong Sha. "A three-dimensional multiphase numerical model for the influence of Marangoni convection on Marangoni self-driven object." Physics of Fluids 34, no. 4 (April 2022): 043308. http://dx.doi.org/10.1063/5.0082893.
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By means of coordinate transformation and the volume-of-fluid-level set multiphase flow method, a three-dimensional multiphase numerical model is established to simulate a Marangoni self-driven object. The forces on the Marangoni self-driven object are discussed as the driving force, viscous resistance, and pressure resistance. A typical disk-shaped, Marangoni self-driven object driven by the diffusion of camphor from its tail to water is utilized to perform a numerical study. Its motion evolution and force change in the whole process are represented quantitatively alongside the flow field and camphor concentration distribution in the flow domain. Meanwhile, the influence of Marangoni convection, which is induced by camphor diffusion at the moving gas–liquid interface, on surfer motion is surveyed. The results presented in this work can improve understanding of self-driven Marangoni propulsion since self-driven object motion and fluid movement details are difficult to acquire experimentally.
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Feng, Mudong, and Michael K. Gilson. "A Thermodynamic Limit on the Role of Self-Propulsion in Enhanced Enzyme Diffusion." Biophysical Journal 116, no. 10 (May 2019): 1898–906. http://dx.doi.org/10.1016/j.bpj.2019.04.005.
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Popescu, Mihail N., and Szilveszter Gáspár. "Analyte Sensing with Catalytic Micromotors." Biosensors 13, no. 1 (December 28, 2022): 45. http://dx.doi.org/10.3390/bios13010045.
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Catalytic micromotors can be used to detect molecules of interest in several ways. The straightforward approach is to use such motors as sensors of their “fuel” (i.e., of the species consumed for self-propulsion). Another way is in the detection of species which are not fuel but still modulate the catalytic processes facilitating self-propulsion. Both of these require analysis of the motion of the micromotors because the speed (or the diffusion coefficient) of the micromotors is the analytical signal. Alternatively, catalytic micromotors can be used as the means to enhance mass transport, and thus increase the probability of specific recognition events in the sample. This latter approach is based on “classic” (e.g., electrochemical) analytical signals and does not require an analysis of the motion of the micromotors. Together with a discussion of the current limitations faced by sensing concepts based on the speed (or diffusion coefficient) of catalytic micromotors, we review the findings of the studies devoted to the analytical performances of catalytic micromotor sensors. We conclude that the qualitative (rather than quantitative) analysis of small samples, in resource poor environments, is the most promising niche for the catalytic micromotors in analytical chemistry.
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Tătulea-Codrean, Maria, and Eric Lauga. "Artificial chemotaxis of phoretic swimmers: instantaneous and long-time behaviour." Journal of Fluid Mechanics 856 (October 12, 2018): 921–57. http://dx.doi.org/10.1017/jfm.2018.718.
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Phoretic swimmers are a class of artificial active particles that has received significant attention in recent years. By making use of self-generated gradients (e.g. in temperature, electric potential or some chemical product) phoretic swimmers are capable of self-propulsion without the complications of mobile body parts or a controlled external field. Focusing on diffusiophoresis, we quantify in this paper the mechanisms through which phoretic particles may achieve chemotaxis, both at the individual and the non-interacting population level. We first derive a fully analytical law for the instantaneous propulsion and orientation of a phoretic swimmer with general axisymmetric surface properties, in the limit of zero Péclet number and small Damköhler number. We then apply our results to the case of a Janus sphere, one of the most common designs of phoretic swimmers used in experimental studies. We next put forward a novel application of generalised Taylor dispersion theory in order to characterise the long-time behaviour of a population of non-interacting phoretic swimmers. We compare our theoretical results with numerical simulations for the mean drift and anisotropic diffusion of phoretic swimmers in chemical gradients. Our results will help inform the design of phoretic swimmers in future experimental applications.
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Menzel, Andreas M. "Statistics for an object actively driven by spontaneous symmetry breaking into reversible directions." Journal of Chemical Physics 157, no. 1 (July 7, 2022): 011102. http://dx.doi.org/10.1063/5.0093598.
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Propulsion of otherwise passive objects is achieved by mechanisms of active driving. We concentrate on cases in which the direction of active drive is subject to spontaneous symmetry breaking. In our case, this direction will be maintained until a large enough impulse by an additional stochastic force reverses it. Examples may be provided by self-propelled droplets, gliding bacteria stochastically reversing their propulsion direction, or nonpolar vibrated hoppers. The magnitude of active forcing is regarded as constant, and we include the effect of inertial contributions. Interestingly, this situation can formally be mapped to stochastic motion under (dry, solid) Coulomb friction, however, with a negative friction parameter. Diffusion coefficients are calculated by formal mapping to the situation of a quantum-mechanical harmonic oscillator exposed to an additional repulsive delta-potential. Results comprise a ditched or double-peaked velocity distribution and spatial statistics showing outward propagating maxima when starting from initially concentrated arrangements.
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Zaid, Irwin M., Jörn Dunkel, and Julia M. Yeomans. "Lévy fluctuations and mixing in dilute suspensions of algae and bacteria." Journal of The Royal Society Interface 8, no. 62 (February 23, 2011): 1314–31. http://dx.doi.org/10.1098/rsif.2010.0545.
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Swimming micro-organisms rely on effective mixing strategies to achieve efficient nutrient influx. Recent experiments, probing the mixing capability of unicellular biflagellates, revealed that passive tracer particles exhibit anomalous non-Gaussian diffusion when immersed in a dilute suspension of self-motile Chlamydomonas reinhardtii algae. Qualitatively, this observation can be explained by the fact that the algae induce a fluid flow that may occasionally accelerate the colloidal tracers to relatively large velocities. A satisfactory quantitative theory of enhanced mixing in dilute active suspensions, however, is lacking at present. In particular, it is unclear how non-Gaussian signatures in the tracers' position distribution are linked to the self-propulsion mechanism of a micro-organism. Here, we develop a systematic theoretical description of anomalous tracer diffusion in active suspensions, based on a simplified tracer-swimmer interaction model that captures the typical distance scaling of a microswimmer's flow field. We show that the experimentally observed non-Gaussian tails are generic and arise owing to a combination of truncated Lévy statistics for the velocity field and algebraically decaying time correlations in the fluid. Our analytical considerations are illustrated through extensive simulations, implemented on graphics processing units to achieve the large sample sizes required for analysing the tails of the tracer distributions.
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Chen, Xiao, and Yaner Yan. "Enhanced Diffusion and Non-Gaussian Displacements of Colloids in Quasi-2D Suspensions of Motile Bacteria." Materials 17, no. 20 (October 14, 2024): 5013. http://dx.doi.org/10.3390/ma17205013.
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In the real world, active agents interact with surrounding passive objects, thus introducing additional degrees of complexity. The relative contributions of far-field hydrodynamic and near-field contact interactions to the anomalous diffusion of passive particles in suspensions of active swimmers remain a subject of ongoing debate. We constructed a quasi-two-dimensional microswimmer–colloid mixed system by taking advantage of Serratia marcescens’ tendency to become trapped at the air–water interface to investigate the origins of the enhanced diffusion and non-Gaussianity of the displacement distributions of passive colloidal tracers. Our findings reveal that the diffusion behavior of colloidal particles exhibits a strong dependence on bacterial density. At moderate densities, the collective dynamics of bacteria dominate the diffusion of tracer particles. In dilute bacterial suspensions, although there are multiple dynamic types present, near-field contact interactions such as collisions play a major role in the enhancement of colloidal transport and the emergence of non-Gaussian displacement distributions characterized by heavy exponential tails in short times. Despite the distinct types of microorganisms and their diverse self-propulsion mechanisms, a generality in the diffusion behavior of passive colloids and their underlying dynamics is observed.
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BOSTAN, MIHAI, and JOSE ANTONIO CARRILLO. "ASYMPTOTIC FIXED-SPEED REDUCED DYNAMICS FOR KINETIC EQUATIONS IN SWARMING." Mathematical Models and Methods in Applied Sciences 23, no. 13 (September 16, 2013): 2353–93. http://dx.doi.org/10.1142/s0218202513500346.
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We perform an asymptotic analysis of general particle systems arising in collective behavior in the limit of large self-propulsion and friction forces. These asymptotics impose a fixed speed in the limit, and thus a reduction of the dynamics to a sphere in the velocity variables. The limit models are obtained by averaging with respect to the fast dynamics. We can include all typical effects in the applications: short-range repulsion, long-range attraction, and alignment. For instance, we can rigorously show that the Cucker–Smale model is reduced to a Vicsek-like model without noise in this asymptotic limit. Finally, a formal expansion based on the reduced dynamics allows us to treat the case of diffusion reducing the Cucker–Smale model with diffusion to the non-normalized Vicsek model as in Ref. 29. This technique follows closely the gyroaverage method used when studying the magnetic confinement of charged particles. The main new mathematical difficulty is to deal with measure solutions in this expansion procedure.
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Fritz, J. H., and U. Seifert. "Thermodynamically consistent model of an active Ornstein–Uhlenbeck particle." Journal of Statistical Mechanics: Theory and Experiment 2023, no. 9 (September 1, 2023): 093204. http://dx.doi.org/10.1088/1742-5468/acf70c.
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Abstract Identifying the full entropy production of active particles is a challenging task. We introduce a microscopic, thermodynamically consistent model, which leads to active Ornstein–Uhlenbeck statistics in the continuum limit. Our minimal model consists of a particle with a fluctuating number of active reaction sites that contribute to its active self-propulsion on a lattice. The model also takes ordinary thermal noise into account. This approach allows us to identify the full entropy production stemming from both thermal diffusion and active driving. Extant methods based on the comparison of forward and time-reversed trajectory underestimate the physical entropy production when applied to the Langevin equations obtained from our model. Constructing microscopic Markovian models can thus provide a benchmark for determining the entropy production in non-Markovian active systems.
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Rangaig, Norodin A. "Thermodynamic description of active brownian particle driven by fractional gaussian noise." Physica Scripta 99, no. 2 (January 31, 2024): 025024. http://dx.doi.org/10.1088/1402-4896/ad20be.
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Abstract As a natural extension of the recent results on the thermodynamics of an active Brownian particle (self-propelled), we study the thermodynamics of an active Brownian particle (ABP) driven by fractional Gaussian noise (FGN). To serve as a prelude of the main results, we start from the conventional Markov process but with time dependent diffusion coefficient, where deviation in integral fluctuation relation (IFR) for total entropy production requires a general definition of the temperature, following the same case for a Brownian particle. In other words, the general temperature definition for this case is independent to the statistics of the rotational motion. We then proceed with the main problem of the paper, which is an active Brownian particle driven by fractional Gaussian noise. Under the assumption that self-propulsion is even under time-reversal, temperature is defined as well as the distance on how far the IFR for total entropy production deviates from the standard definition by adopting the standard definition of trajectory-level entropy and the joint probability of ABP. Furthermore, second law-like concept based on the found deviation is derived, as well as a generalized Clausius inequality. Lastly, magnitude of this deviation diminishes in the case of pure white noise.
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Sandoval, Mario, Navaneeth K. Marath, Ganesh Subramanian, and Eric Lauga. "Stochastic dynamics of active swimmers in linear flows." Journal of Fluid Mechanics 742 (February 21, 2014): 50–70. http://dx.doi.org/10.1017/jfm.2013.651.
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AbstractMost classical work on the hydrodynamics of low-Reynolds-number swimming addresses deterministic locomotion in quiescent environments. Thermal fluctuations in fluids are known to lead to a Brownian loss of the swimming direction, resulting in a transition from short-time ballistic dynamics to effective long-time diffusion. As most cells or synthetic swimmers are immersed in external flows, we consider theoretically in this paper the stochastic dynamics of a model active particle (a self-propelled sphere) in a steady general linear flow. The stochasticity arises both from translational diffusion in physical space, and from a combination of rotary diffusion and so-called run-and-tumble dynamics in orientation space. The latter process characterizes the manner in which the orientation of many bacteria decorrelates during their swimming motion. In contrast to rotary diffusion, the decorrelation occurs by means of large and impulsive jumps in orientation (tumbles) governed by a Poisson process. We begin by deriving a general formulation for all components of the long-time mean square displacement tensor for a swimmer with a time-dependent swimming velocity and whose orientation decorrelates due to rotary diffusion alone. This general framework is applied to obtain the convectively enhanced mean-squared displacements of a steadily swimming particle in three canonical linear flows (extension, simple shear and solid-body rotation). We then show how to extend our results to the case where the swimmer orientation also decorrelates on account of run-and-tumble dynamics. Self-propulsion in general leads to the same long-time temporal scalings as for passive particles in linear flows but with increased coefficients. In the particular case of solid-body rotation, the effective long-time diffusion is the same as that in a quiescent fluid, and we clarify the lack of flow dependence by briefly examining the dynamics in elliptic linear flows. By comparing the new active terms with those obtained for passive particles we see that swimming can lead to an enhancement of the mean-square displacements by orders of magnitude, and could be relevant for biological organisms or synthetic swimming devices in fluctuating environmental or biological flows.
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Ouyang, Wu, Feipeng Pan, Lei Wang, and Ruicong Zheng. "Frictional Wear Behavior of Water-Lubrication Resin Matrix Composites under Low Speed and Heavy Load Conditions." Polymers 16, no. 19 (September 29, 2024): 2753. http://dx.doi.org/10.3390/polym16192753.
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Resin matrix composites are commonly utilized in water-lubricated stern tube bearings for warship propulsion systems. Low-speed and high-load conditions are significant factors influencing the tribological properties of stern tube bearings. The wear characteristics of resin-based laminated composites (RLCs), resin-based winding composites (RWCs), and resin-based homogeneous polymer (RHP) blocks were investigated under simulated environmental conditions using a ring-on-block wear tester. Simulated seawater was prepared by combining sodium chloride with distilled water. The wetting angle, coefficient of friction (COF), and mass loss were measured and compared. Additionally, their surface morphologies were examined. The results indicate a significant increase in the COFs for the three materials with an increased speed or load under dry conditions. The COF of the RLCs is the lowest, indicating that it has superior self-lubricating properties. In wet conditions, the COFs of the three materials decrease with an increasing speed or load, exhibiting a pronounced hydrodynamic effect. The COF and mass loss of RWCs are the highest, while RLCs and RHP exhibit lower COFs and mass loss values. The reticulated texture and flocculent fibers on the surface of RLC enhance the heat diffusion and improve the material wettability and water storage capacity. The surface of RWC is dense, and the friction area under dry conditions is melted and brightened. The surface of RHP is smooth, while the worn material forms an agglomerate and exhibits susceptibility to burning and blackening under dry conditions. The laminated formation method demonstrates superior tribological performance throughout the wear evolution process.
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Wang, Xiaolu, Martin In, Christophe Blanc, Paolo Malgaretti, Maurizio Nobili, and Antonio Stocco. "Wetting and orientation of catalytic Janus colloids at the surface of water." Faraday Discussions 191 (2016): 305–24. http://dx.doi.org/10.1039/c6fd00025h.
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Janus colloidal particles show remarkable properties in terms of surface activity, self-assembly and wetting. Moreover they can perform autonomous motion if they can chemically react with the liquid in which they are immersed. In order to understand the self-propelled motion of catalytic Janus colloids at the air–water interface, wetting and the orientation of the catalytic surface are important properties to be investigated. Wetting plays a central role in active motion since it determines the contact between the fuel and the catalytic surface as well as the efficiency of the transduction of the chemical reaction into motion. Active motion is not expected to occur either when the catalytic face is completely out of the aqueous phase or when the Janus boundaries are parallel to the interfacial plane. The design of a Janus colloid possessing two hydrophilic faces is required to allow the catalytic face to react with the fuel (e.g. H2O2 for platinum) in water and to permit some rotational freedom of the Janus colloid in order to generate propulsion parallel to the interfacial plane. Here, we discuss some theoretical aspects that should be accounted for when studying Janus colloids at the surface of water. The free energy of ideal Janus colloidal particles at the interface is modeled as a function of the immersion depth and the particle orientation. Analytical expressions of the energy profiles are established. Energetic aspects are then discussed in relation to the particle’s ability to rotate at the interface. By introducing contact angle hysteresis we describe how the effects of contact line pinning modifies the scenario described in the ideal case. Experimental observations of the contact angle hysteresis of Janus colloids at the interface reveal the effect of pinning; and orientations of silica particles half covered with a platinum layer at the interface do not comply with the ideal scenarios. Experimental observations suggest that Janus colloids at the fluid interface behave as a kinetically driven system, where the contact line motion over the defects decorating the Janus faces rules the orientation and rotational diffusion of the particle.
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Khodabocus, M. I., M. Sellier, and V. Nock. "Slug Self-Propulsion in a Capillary Tube Mathematical Modeling and Numerical Simulation." Advances in Mathematical Physics 2016 (2016): 1–16. http://dx.doi.org/10.1155/2016/1234642.
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A composite droplet made of two miscible fluids in a narrow tube generally moves under the action of capillarity until complete mixture is attained. This physical situation is analysed here on a combined theoretical and numerical analysis. The mathematical framework consists of the two-phase flow phase-field equation set, an advection-diffusion chemical concentration equation, and closure relationships relating the surface tensions to the chemical concentration. The numerical framework is composed of the COMSOL Laminar two-phase flow phase-field method coupled with an advection-diffusion chemical concentration equation. Through transient studies, we show that the penetrating length of the bidroplet system into the capillary tube is linear at early-time regime and exponential at late-time regime. Through parametric studies, we show that the rate of penetration of the bidroplet system into the capillary tube is proportional to a time-dependent exponential function. We also show that this speed obeys the Poiseuille law at the early-time regime. A series of position, speed-versus-property graphs are included to support the analysis. Finally, the overall results are contrasted with available experimental data, grouped together to settle a general mathematical description of the phenomenon, and explained and concluded on this basis.
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Cheng, Zhiguo, and Bing Wang. "The diffusion behaviour of coupled particle rings driven by self-propelled particles in a two-dimensional reflection channel." Physica Scripta, July 17, 2024. http://dx.doi.org/10.1088/1402-4896/ad648f.
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Abstract Investigated the diffusion behaviour of self-propelled coupled particle rings in a two-dimensional channel considering particle collisions. The channel geometry and noise regulation play crucial roles in directing transport within the system. Observed a significant alteration in the diffusion behaviour of the particle rings at specific stages of the collision process, accompanied by corresponding changes in the diffusion coefficient. As the modulation phase shift increases, the mean square displacement (MSD) of the particle rings displays periodic fluctuations. The binding force between the particle rings partially restricts the growth of the MSD. An increase in white noise intensity enhances the diffusion behaviour. The impact of self-propulsion speed is influenced by the modulation parameters. The sign of the modulation parameter dictates the correlation of the self-propulsion speed. Furthermore, the number of particle rings in the channel introduces a complex effect on the diffusion behaviour.
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Picella, Francesco, and Sébastien Michelin. "Confined self-propulsion of an isotropic active colloid." Journal of Fluid Mechanics 933 (December 23, 2021). http://dx.doi.org/10.1017/jfm.2021.1081.
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To spontaneously break their intrinsic symmetry and self-propel at the micron scale, isotropic active colloidal particles and droplets exploit the nonlinear convective transport of chemical solutes emitted/consumed at their surface by the surface-driven fluid flows generated by these solutes. Significant progress was recently made to understand the onset of self-propulsion and nonlinear dynamics. Yet, most models ignore a fundamental experimental feature, namely the spatial confinement of the colloid, and its effect on propulsion. In this work the self-propulsion of an isotropic colloid inside a capillary tube is investigated numerically. A flexible computational framework is proposed based on a finite-volume approach on adaptative octree grids and embedded boundary methods. This method is able to account for complex geometric confinement, the nonlinear coupling of chemical transport and flow fields, and the precise resolution of the surface boundary conditions, that drive the system's dynamics. Somewhat counterintuitively, spatial confinement promotes the colloid's spontaneous motion by reducing the minimum advection-to-diffusion ratio or Péclet number, ${Pe}$ , required to self-propel; furthermore, self-propulsion velocities are significantly modified as the colloid-to-capillary size ratio $\kappa$ is increased, reaching a maximum at fixed ${Pe}$ for an optimal confinement $0<\kappa <1$ . These properties stem from a fundamental change in the dominant chemical transport mechanism with respect to the unbounded problem: with diffusion now restricted in most directions by the confining walls, the excess solute is predominantly convected away downstream from the colloid, enhancing front-back concentration contrasts. These results are confirmed quantitatively using conservation arguments and lubrication analysis of the tightly confined limit, $\kappa \rightarrow 1$ .
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Ryabov, Artem, and Mykola Tasinkevych. "Enhanced diffusivity in microscopically reversible active matter." Soft Matter, 2022. http://dx.doi.org/10.1039/d2sm00054g.
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Nayak, Shubhadip, Sohom Das, Poulami Bag, Tanwi Debnath, and Pulak K. Ghosh. "Driven transport of active particles through arrays of symmetric obstacles." Journal of Chemical Physics 159, no. 16 (October 25, 2023). http://dx.doi.org/10.1063/5.0176523.
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We numerically examine the driven transport of an overdamped self-propelled particle through a two-dimensional array of circular obstacles. A detailed analysis of transport quantifiers (mobility and diffusivity) has been performed for two types of channels, channel I and channel II, that respectively correspond to the parallel and diagonal drives with respect to the array axis. Our simulation results show that the signatures of pinning actions and depinning processes in the array of obstacles are manifested through excess diffusion peaks or sudden drops in diffusivity, and abrupt jumps in mobility with varying amplitude of the drive. The underlying depinning mechanisms and the associated threshold driving strength largely depend on the persistent length of self-propulsion. For low driving strength, both diffusivity and mobility are noticeably suppressed by the array of obstacles, irrespective of the self-propulsion parameters and direction of the drive. When self-propulsion length is larger than a channel compartment size, transport quantifiers are insensitive to the rotational relaxation time. Transport with diagonal drives features self-propulsion-dependent negative differential mobility. The amplitude of the negative differential mobility of an active particle is much larger than that of a passive one. The present analysis aims at understanding the driven transport of active species like, bacteria, virus, Janus particle etc. in porous medium.
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Bag, Poulami, Shubhadip Nayak, and Pulak Kumar Ghosh. "Particle-Wall Alignment Interaction and Active Brownian Diffusion Through Narrow Channels." Soft Matter, 2024. http://dx.doi.org/10.1039/d4sm00848k.
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We numerically examine the impacts of particle-wall alignment interactions on active species diffusion through a structureless narrow two-dimensional channel. We consider particle-wall interaction to depend on the self-propulsion velocity direction...
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Zhu, Guangpu, and Lailai Zhu. "Self-propulsion of an elliptical phoretic disk emitting solute uniformly." Journal of Fluid Mechanics 974 (November 7, 2023). http://dx.doi.org/10.1017/jfm.2023.858.
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Self-propulsion of chemically active droplets and phoretic disks has been studied widely; however, most research overlooks the influence of disk shape on swimming dynamics. Inspired by experimentally observed prolate composite droplets and elliptical camphor disks, we employ simulations to investigate the phoretic dynamics of an elliptical disk that emits solutes uniformly in the creeping flow regime. By varying the disk's eccentricity $e$ and the Péclet number $Pe$ , we distinguish five disk behaviours: stationary, steady, orbiting, periodic and chaotic. We perform a linear stability analysis (LSA) to predict the onset of instability and the most unstable eigenmode when a stationary disk transitions spontaneously to steady self-propulsion. In addition to the LSA, we use an alternative approach to determine the perturbation growth rate, illustrating the competing roles of advection and diffusion. The steady motion features a transition from a puller-type to a neutral-type swimmer as $Pe$ increases, which occurs as a bimodal concentration profile at the disk surface shifts to a polarized solute distribution, driven by convective solute transport. An elliptical disk achieves an orbiting motion through a chiral symmetry-breaking instability, wherein it repeatedly follows a circular path while simultaneously rotating. The periodic swinging motion, emerging from a steady motion via a supercritical Hopf bifurcation, is characterized by a wave-like trajectory. We uncover a transition from normal diffusion to superdiffusion as eccentricity $e$ increases, corresponding to a random-walking circular disk and a ballistically swimming elliptical counterpart, respectively.
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Hu, Han-Xian, Yi-Fan Shen, Chao Wang, and Meng-Bo Luo. "Dynamics of a two-dimensional active polymer chain with a rotation-restricted active head." Soft Matter, 2022. http://dx.doi.org/10.1039/d2sm01139e.
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The rotation of the active Brownian particle (ABP) at the head is reduced by the connected passive polymer. The propulsive diffusion coefficient of the whole polymer originated from the self-propulsion force can be described by a scaling relation.
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Chao, Xichen, Katherine Skipper, C. Patrick Royall, Silke Henkes, and Tanniemola B. Liverpool. "Traveling Strings of Active Dipolar Colloids." Physical Review Letters 134, no. 1 (January 6, 2025). https://doi.org/10.1103/physrevlett.134.018302.
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We study an intriguing new type of self-assembled active colloidal polymer system in 3D. It is obtained from a suspension of Janus particles in an electric field that induces parallel dipoles in the particles as well as self-propulsion in the plane perpendicular to the field. At low volume fractions, in experiment, the particles self-assemble into 3D columns that are self-propelled in 2D. Explicit numerical simulations combining dipolar interactions and active self-propulsion find an activity dependent transition to a string phase by increasing dipole strength. We classify the collective dynamics of strings as a function of rotational and translational diffusion. Using an anisotropic version of the Rouse model of polymers with active driving, we analytically compute the strings’ collective dynamics and center of mass motion, which matches simulations and is consistent with experimental data. We also discover long range correlations of the fluctuations along the string contour that grow with the active persistence time, a purely active effect that disappears in the thermal limit. Published by the American Physical Society 2025
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Yariv, Ehud, and Sébastien Michelin. "Phoretic self-propulsion at large Péclet numbers." Journal of Fluid Mechanics 768 (February 27, 2015). http://dx.doi.org/10.1017/jfm.2015.78.
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We analyse the self-diffusiophoresis of a spherical particle animated by a non-uniform chemical reaction at its boundary. We consider two models of solute absorption, one with a specified distribution of interfacial solute flux and one where this flux is governed by first-order kinetics with a specified distribution of rate constant. We employ a macroscale model where the short-range interaction of the solute with the particle boundary is represented by an effective slip condition. The solute transport is governed by an advection–diffusion equation. We focus upon the singular limit of large Péclet numbers, $\mathit{Pe}\gg 1$. In the fixed-flux model, the excess-solute concentration is confined to a narrow boundary layer. The scaling pertinent to that limit allows the problem governing the solute concentration to be decoupled from the flow field. The resulting nonlinear boundary-layer problem is handled using a transformation to stream-function coordinates and a subsequent application of Fourier transforms, and is thereby reduced to a nonlinear integral equation governing the interfacial concentration. Its solution provides the requisite approximation for the particle velocity, which scales as $\mathit{Pe}^{-1/3}$. In the fixed-rate model, large Péclet numbers may be realized in different limit processes. We consider the case of large swimmers or strong reaction, where the Damköhler number $\mathit{Da}$ is large as well, scaling as $\mathit{Pe}$. In that double limit, where no boundary layer is formed, we obtain a closed-form approximation for the particle velocity, expressed as a nonlinear functional of the rate-constant distribution; this velocity scales as $\mathit{Pe}^{-2}$. Both the fixed-flux and fixed-rate asymptotic predictions agree with the numerical values provided by computational solutions of the nonlinear transport problem.
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Datta, Agniva, Carsten Beta, and Robert Großmann. "Random walks of intermittently self-propelled particles." Physical Review Research 6, no. 4 (December 16, 2024). https://doi.org/10.1103/physrevresearch.6.043281.
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Motivated by various recent experimental findings, we propose a dynamical model of intermittently self-propelled particles: active particles that recurrently switch between two modes of motion, namely an active run state and a turn state, in which self-propulsion is absent. The durations of these motility modes are drawn from arbitrary waiting-time distributions. We derive the expressions for exact forms of transport characteristics like mean-square displacements and diffusion coefficients to describe such processes. Furthermore, the conditions for the emergence of sub- and superdiffusion in the long-time limit are presented. We give examples of some important processes that occur as limiting cases of our system, including run-and-tumble motion of bacteria, Lévy walks, hop-and-trap dynamics, intermittent diffusion and continuous-time random walks. Published by the American Physical Society 2024
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Suzuki, Tamako, and Hideyuki Sawada. "Analysis of convection flow of a self-propelled alcohol droplet in an exoskeleton frame." ROBOMECH Journal 11, no. 1 (June 13, 2024). http://dx.doi.org/10.1186/s40648-024-00278-y.
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AbstractThis study aims to analyze the convection flow of a self-propelled 1-pentanol droplet. The droplets move spontaneously when 1-pentanol droplets are dropped into an aqueous 1-pentanol solution. This self-propulsion is due to the interfacial tension gradient caused by the concentration differences. The shape of the droplet is closely related to its behavior because the shape of the droplet changes the interfacial tension gradient. In this study, an exoskeleton is used to fix the droplet shape. In our preliminary experiments, we observed Marangoni convection in droplets dropped in exoskeleton frames with boomerang and round holes. The results showed that a large difference in surface tension was necessary to control the self-propulsion of the 1-pentanol droplets. Herein, we prepared two exoskeletons with different holes, an elongated symmetrical elliptical shape, and an asymmetrical shape to fix the shape of the droplet. The droplets were then dropped into each exoskeleton, and the droplet behavior, Marangoni convection inside the droplet, and convection in the aqueous phase were analyzed. We found that the direction of the self-propulsion of the droplet was determined by these exoskeletons, particularly in the case of the asymmetrical exoskeleton, and the direction of self-propulsion was fixed in one direction. Marangoni convection was observed in the droplet from the direction of lower surface tension to that of higher surface tension. In the aqueous phase, two convections were generated from the aqueous phase to the droplet because of the diffusion of 1-pentanol. In particular, when an asymmetrical exoskeleton was used, two convections of different sizes and velocities were observed in the aqueous phase. Based on these experimental results, the relationship between droplet behavior and convection is discussed.
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Khatri, Narender, and Raymond Kapral. "Inertial effects on rectification and diffusion of active Brownian particles in an asymmetric channel." Journal of Chemical Physics, March 7, 2023. http://dx.doi.org/10.1063/5.0141696.
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Micro- and nano-swimmers moving in a fluid solvent confined by structures that produce entropic barriers are often described by overdamped active Brownian particle dynamics, where viscous effects are large and inertia plays no role. However, inertial effects should be considered for confined swimmers moving in media where viscous effects are no longer dominant. Here, we study how inertia affects the rectification and diffusion of self-propelled particles in a two-dimensional asymmetric channel. We show that most of the particles accumulate at the channel walls as the masses of the particles increase. Furthermore, the average particle velocity has a maximum as a function of the mass, indicating that particles with an optimal mass $M^{*}_{\rm op}$ can be sorted from a mixture with particles of other masses. In particular, we find that the effective diffusion coefficient exhibits an enhanced diffusion peak as a function of the mass, which is a signature of the accumulation of most of the particles at the channel walls. The dependence of $M^{*}_{\rm op}$ on the rotational diffusion rate, self-propulsion force, aspect ratio of the channel, and active torque is also determined. The results of this study could stimulate the development of strategies for controlling the diffusion of self-propelled particles in entropic ratchet systems.
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Schmidt, Falko, Hana Šípová-Jungová, Mikael Käll, Alois Würger, and Giovanni Volpe. "Non-equilibrium properties of an active nanoparticle in a harmonic potential." Nature Communications 12, no. 1 (March 26, 2021). http://dx.doi.org/10.1038/s41467-021-22187-z.
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AbstractActive particles break out of thermodynamic equilibrium thanks to their directed motion, which leads to complex and interesting behaviors in the presence of confining potentials. When dealing with active nanoparticles, however, the overwhelming presence of rotational diffusion hinders directed motion, leading to an increase of their effective temperature, but otherwise masking the effects of self-propulsion. Here, we demonstrate an experimental system where an active nanoparticle immersed in a critical solution and held in an optical harmonic potential features far-from-equilibrium behavior beyond an increase of its effective temperature. When increasing the laser power, we observe a cross-over from a Boltzmann distribution to a non-equilibrium state, where the particle performs fast orbital rotations about the beam axis. These findings are rationalized by solving the Fokker-Planck equation for the particle’s position and orientation in terms of a moment expansion. The proposed self-propulsion mechanism results from the particle’s non-sphericity and the lower critical point of the solution.
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Reichert, Julian, Leon F. Granz, and Thomas Voigtmann. "Transport coefficients in dense active Brownian particle systems: mode-coupling theory and simulation results." European Physical Journal E 44, no. 3 (March 2021). http://dx.doi.org/10.1140/epje/s10189-021-00039-4.
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Abstract We discuss recent advances in developing a mode-coupling theory of the glass transition (MCT) of two-dimensional systems of active Brownian particles (ABPs). The theory describes the structural relaxation close to the active glass in terms of transient dynamical density correlation functions. We summarize the equations of motion that have been derived for the collective density-fluctuation dynamics and those for the tagged-particle motion. The latter allow to study the dynamics of both passive and active tracers in both passive and active host systems. In the limit of small wave numbers, they give rise to equations of motion describing the mean-squared displacements (MSDs) of these tracers and hence the long-time diffusion coefficients as a transport coefficient quantifying long-range tracer motion. We specifically discuss the case of a single ABP tracer in a glass-forming passive host suspension, a case that has recently been studied in experiments on colloidal Janus particles. We employ event-driven Brownian dynamics (ED-BD) computer simulations to test the ABP-MCT and find good agreement between the two for the MSD, provided that known errors in MCT already for the passive system (i.e., an overestimation of the glassiness of the system) are accounted for by an empirical mapping of packing fractions and host-system self-propulsion forces. The ED-BD simulation results also compare well to experimental data, although a peculiar non-monotonic mapping of self-propulsion velocities is required. The ABP-MCT predicts a specific self-propulsion dependence of the Stokes–Einstein relation between the long-time diffusion coefficient and the host-system viscosity that matches well the results from simulation. An application of ABP-MCT within the integration-through transients framework to calculate the density-renormalized effective swim velocity of the interacting ABP agrees qualitatively with the ED-BD simulation data at densities close to the glass transition and quantitatively for the full density range only after the mapping of packing fractions employed for the passive system. Graphic abstract
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Adersh, F., M. Muhsin, and MAMATA SAHOO. "Transition from random self-propulsion to rotational motion in a non-Markovian microswimmer." Communications in Theoretical Physics, December 5, 2024. https://doi.org/10.1088/1572-9494/ad9a8b.
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Abstract We study the motion of an inertial microswimmer in a non-Newtonian environment with a finite memory and present the theoretical realization of an unexpected transition from its random self-propulsion to rotational (circular or elliptical) motion. Further, the rotational motion of the swimmer is followed by spontaneous local direction reversals yet with a steady state angular diffusion. Moreover, the advent of this behaviour is observed in the oscillatory regime of the inertia-memory parameter space of the dynamics. We quantify this unconventional rotational motion of microswimmer by measuring the time evolution of direction of its instantaneous velocity or orientation. By solving the generalized Langevin model of non-Markovian dynamics of an inertial active Ornstein-Uhlenbeck particle, we show that the emergence of the rotational (circular or elliptical) trajectory is due to the presence of both inertial motion and memory in the environment.
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Guo Rui-Xue and Ai Bao-Quan. "Directed Transport of Deformable Self-propulsion Particles in an Asymmetric Periodic Channel." Acta Physica Sinica, 2023, 0. http://dx.doi.org/10.7498/aps.72.20230825.
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Research on molecular motors, which effectively convert chemical energy into mechanical energy in living organisms, is currently at the forefront of study in the biology and physics realms. The dynamic process of their guided movement, along with the crucial role they play in intra-cellular material transport, has significantly piqued the interest of multiple research institutions, both domestically and internationally. Theoretical and experimental perspectives have allowed detailed examinations of the motion attributes of these molecular motors. Of notable importance is the Brownian ratchet model. It provides an illustration of a non-equilibrium system that transforms thermal fluctuation into guided transport by utilizing temporal or spatial asymmetry. The workings of this mechanism have been extensively explored and studied across fields including physics, biology and nanotechnology. Investigations into a variety of ratchets and identification of optimum conditions contribute to a deeper understanding of guided Brownian particle transport.<br/>Preceding studies on ratchet systems have largely concentrated on the rectification motion of diverse types of particles-active, polar and chiral-in asymmetric structures. However, the transport of deformable particles in asymmetric channels has been relatively unexamined. Particles in soft material systems such as cell monolayers, tissues, foams, and emulsions are frequently deformable. The shape deformation of these soft particles significantly impacts the system's dynamic behavior. Thus, understanding the guided transport of these deformable particles within confined structures is crucial.<br/>For greater clarity on the subject, we conducted numerical simulations on the guided transportation of active, deformable particles within a two-dimensional, periodic, asymmetric channel. We aimed to identify the factors that influence the transport of these particles within confined structures. The main feature of the deformable particle model is that the particle's shape is characterized by multiple degrees of freedom. For active deformable particles, self-propulsion speed disrupts thermodynamic equilibrium, leading to guided transport in spatially asymmetric conditions. Our findings demonstrate that a particle's direction of movement is entirely defined by the channel's asymmetric parameter, and it tends to gravitate towards increased stability. Augmenting particle self-propulsion speed and particle softening can facilitate ratchet transport. When v0 is large, the particle's tensile effect becomes more apparent, and particle softening significantly enhances directed transport. In contrast, an increase in density and rotational diffusion can slow particle rectification. Increased density can obstruct particles, making channel passage more difficult. Elevated rotational diffusion reduces persistence length, challenging particle transition through channels. With constant density, greater numbers of particles will also encourage rectification. These research findings offer valuable insights into the transportation behaviors of deformable particles in confined structures. They also deliver crucial theoretical support for applicable experiments within the field of soft matter.
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Ramesh, Prashanth, Babak Vajdi Hokmabad, Dmitri O. Pushkin, Arnold J. T. M. Mathijssen, and Corinna C. Maass. "Interfacial activity dynamics of confined active droplets." Journal of Fluid Mechanics 966 (July 4, 2023). http://dx.doi.org/10.1017/jfm.2023.411.
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Active emulsions can spontaneously form self-propelled droplets or phoretic micropumps. However, it remains unclear how these active systems interact with their self-generated chemical fields, which can lead to emergent chemodynamic phenomena and multistable interfacial flows. Here, we simultaneously measure the flow and chemical concentration fields using dual-channel fluorescence microscopy for active micropumps, i.e. immobilised oil droplets that dynamically solubilise in a supramicellar aqueous surfactant solution. With increasing droplet radius, we observe (i) a migration of vortices from the posterior to the anterior, analogous to a transition from pusher- to puller-type swimmers, (ii) a bistability between dipolar and quadrupolar flows and, eventually, (iii) a transition to multipolar modes. We also investigate the long-time dynamics. Together, our observations suggest that a local build-up of chemical products leads to a saturation of the surface, which controls the propulsion mechanism. These multistable dynamics can be explained by the competing time scales of slow micellar diffusion governing the chemical buildup and faster molecular diffusion powering the underlying transport mechanism. Our results are directly relevant to phoretic micropumps, but also shed light on the interfacial activity dynamics of self-propelled droplets and other active emulsion systems.
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Han, Hyeong-Tark, Sungmin Joo, Takahiro Sakaue, and Jae-Hyung Jeon. "Nonequilibrium diffusion of active particles bound to a semiflexible polymer network: Simulations and fractional Langevin equation." Journal of Chemical Physics 159, no. 2 (July 10, 2023). http://dx.doi.org/10.1063/5.0150224.
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In a viscoelastic environment, the diffusion of a particle becomes non-Markovian due to the memory effect. An open question concerns quantitatively explaining how self-propulsion particles with directional memory diffuse in such a medium. Based on simulations and analytic theory, we address this issue with active viscoelastic systems where an active particle is connected with multiple semiflexible filaments. Our Langevin dynamics simulations show that the active cross-linker displays superdiffusive and subdiffusive athermal motion with a time-dependent anomalous exponent α. In such viscoelastic feedback, the active particle always exhibits superdiffusion with α = 3/2 at times shorter than the self-propulsion time (τA). At times greater than τA, the subdiffusive motion emerges with α bounded between 1/2 and 3/4. Remarkably, active subdiffusion is reinforced as the active propulsion (Pe) is more vigorous. In the high Pe limit, athermal fluctuation in the stiff filament eventually leads to α = 1/2, which can be misinterpreted with the thermal Rouse motion in a flexible chain. We demonstrate that the motion of active particles cross-linking a network of semiflexible filaments can be governed by a fractional Langevin equation combined with fractional Gaussian noise and an Ornstein–Uhlenbeck noise. We analytically derive the velocity autocorrelation function and mean-squared displacement of the model, explaining their scaling relations as well as the prefactors. We find that there exist the threshold Pe (Pe∗) and crossover times (τ∗ and τ†) above which active viscoelastic dynamics emerge on timescales of τ∗≲ t ≲ τ†. Our study may provide theoretical insight into various nonequilibrium active dynamics in intracellular viscoelastic environments.
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Ryabov, Artem, and Mykola Tasinkevych. "Diffusion coefficient and power spectrum of active particles with a microscopically reversible mechanism of self-propelling." Journal of Chemical Physics, August 9, 2022. http://dx.doi.org/10.1063/5.0101520.
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Catalytically active macromolecules are envisioned as key building blocks in development of artificial nanomotors. However, theory and experiments report conflicting findings regarding their dynamics. The lack of consensus is mostly caused by a limited understanding of specifics of self-propulsion mechanisms at the nanoscale. Here, we study a generic model of a self-propelled nanoparticle that does not rely on a particular mechanism. Instead, its main assumption is the fundamental symmetry of microscopic dynamics of chemical reactions: the principle of microscopic reversibility. Significant consequences of this assumption arise if we subject the particle to an action of an external time-periodic force. The particle diffusion coefficient is then enhanced compared to the unbiased dynamics. The enhancement can be controlled by the force amplitude and frequency. We also derive the power spectrum of particle trajectories. Among new effects stemming from the microscopic reversibility are the enhancement of the spectrum at all frequencies and sigmoid-shaped transitions and a peak at characteristic frequencies of rotational diffusion and external forcing. The microscopic reversibility is a generic property of a broad class of chemical reactions, therefore we expect that the presented results will motivate new experimental studies aimed at testing of our predictions. This could provide new insights into dynamics of catalytic macromolecules.
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Shi Zi-Xuan, Jin Yan, Jin Yi-Yang, Tian Wen-De, Zhang Tian-Hui, and Chen Kang. "Gel transition of active triblock copolymers." Acta Physica Sinica, 2024, 0. http://dx.doi.org/10.7498/aps.73.20240796.
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The self-propulsion of active matter leads to many non-equilibrium self-organization phenomena, and the conformational freedom of polymer chains can produce unique equilibrium self-assembly behaviors, which stimulates cross-disciplinary research between active matter and polymer physics. In this work, we use molecular dynamics simulations to investigate the modulation of self-propulsion activity on the gel transition of ABA triblock copolymers. The research results indicate that under different active forces and attractive strengths, the gel states formed by ABA copolymers can be divided into three types: Stable Polymer Gels with stable percolation paths and uniform spatial distribution, Dynamic Polymer Gels with constantly changing percolation paths and strands conformation, and Collapsed Polymer Gels aggregating into large percolating clusters. The spatial uniformity of active gels is related not only to the concentration fluctuation during the formation of the network, but also to the inconsistent movement of the network chains caused by the activity, which is manifested in the rotation of crosslinking points in the flexible system and the directional movement of the bundles along their contour directions in the semi-flexible and rigid systems. In terms of topological conformation of polymer networks, when the attractive strength between A blocks is strong, the proportion of loop increases with the active force. When attractive strength is weak, inter and intra chain binding is unstable, and the conformation is easily changed by the activity drive, noise and other chain collisions, so the proportion of loop decreases with the active force. The branching number of crosslinking points varies with the active force, which is not only affected by the attraction strength, but also related to the rigidity of the network chain. Generally, the branch number of crosslinking points in semi-flexible networks is larger than that in flexible and rigid networks. In addition，the directional motion of active polymers induces anomalous diffusion in Stable Polymer Gels. This study enhances understanding of the collective behavior of active polymers and provides a reference for the design and application of active polymeric materials.
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Ender, Hendrik, and Jan Kierfeld. "From diffusive mass transfer in Stokes flow to low Reynolds number Marangoni boats." European Physical Journal E 44, no. 1 (January 2021). http://dx.doi.org/10.1140/epje/s10189-021-00034-9.
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Abstract We present a theory for the self-propulsion of symmetric, half-spherical Marangoni boats (soap or camphor boats) at low Reynolds numbers. Propulsion is generated by release (diffusive emission or dissolution) of water-soluble surfactant molecules, which modulate the air–water interfacial tension. Propulsion either requires asymmetric release or spontaneous symmetry breaking by coupling to advection for a perfectly symmetrical swimmer. We study the diffusion–advection problem for a sphere in Stokes flow analytically and numerically both for constant concentration and constant flux boundary conditions. We derive novel results for concentration profiles under constant flux boundary conditions and for the Nusselt number (the dimensionless ratio of total emitted flux and diffusive flux). Based on these results, we analyze the Marangoni boat for small Marangoni propulsion (low Peclet number) and show that two swimming regimes exist, a diffusive regime at low velocities and an advection-dominated regime at high swimmer velocities. We describe both the limit of large Marangoni propulsion (high Peclet number) and the effects from evaporation by approximative analytical theories. The swimming velocity is determined by force balance, and we obtain a general expression for the Marangoni forces, which comprises both direct Marangoni forces from the surface tension gradient along the air–water–swimmer contact line and Marangoni flow forces. We unravel whether the Marangoni flow contribution is exerting a forward or backward force during propulsion. Our main result is the relation between Peclet number and swimming velocity. Spontaneous symmetry breaking and, thus, swimming occur for a perfectly symmetrical swimmer above a critical Peclet number, which becomes small for large system sizes. We find a supercritical swimming bifurcation for a symmetric swimmer and an avoided bifurcation in the presence of an asymmetry. Graphic abstract
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Huang, Chuqi, Natalie P. Pinchin, Chia‐Heng Lin, Irving Hafed Tejedor, Matthew Gene Scarfo, Hamed Shahsavan, and Abdon Pena‐Francesch. "Self‐Propelled Morphing Matter for Small‐Scale Swimming Soft Robots." Advanced Functional Materials, October 2024. http://dx.doi.org/10.1002/adfm.202413129.
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AbstractAquatic insects have developed versatile locomotion mechanisms that have served as a source of inspiration for decades in the development of small‐scale swimming robots. However, despite recent advances in the field, efficient, untethered, and integrated powering, actuation, and control of small‐scale robots remains a challenge due to the out‐of‐equilibrium and dissipative nature of the driving physical and chemical phenomena. Here, we have designed small‐scale, bioinspired aquatic locomotors with programmable deterministic trajectories that integrate self‐propelled chemical motors and photoresponsive shape‐morphing structures. A Marangoni motor system is developed integrating structural protein networks that self‐regulate the release of chemical fuel with photochemical liquid crystal network (LCN) actuators that change their shape and deform in and out of the surface of water. While the diffusion of fuel from the motor system regulates the propulsion, the dissipative photochemical deformation of LCNs provides locomotors with control over the directionality of motion at the air‐water interface. This approach gives access to five different but interchangeable modes of locomotion within a single swimming robot via morphing of the soft structure. The proposed design, which mimics the mechanisms of surface gliding and posture change of semiaquatic insects such as water treaders, offers solutions for autonomous swimming soft robots via untethered and orthogonal power and control.
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Shapira, Dekel, and Doron Cohen. "Emergence of Sinai Physics in the stochastic motion of passive and active particles." New Journal of Physics, June 6, 2022. http://dx.doi.org/10.1088/1367-2630/ac7609.
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Abstract A particle that is immersed in a uniform temperature bath performs Brownian diffusion, as discussed by Einstein. But Sinai has realized that in a "random environment" the diffusion is suppressed. Follow-up works have pointed out that in the presence of bias $f$ there are delocalization and sliding transitions, with threshold value $f_c$ that depends on the disorder strength. We discuss in a critical way the emergence of Sinai physics for both passive and active Brownian particles. Tight-binding and Fokker-Planck versions of the model are addressed on equal footing. We assume that the transition rates between sites are enhanced either due to a driving mechanism or due to self-propulsion mechanism that are induced by an irradiation source. Consequently, counter intuitively, the dynamics becomes sub-diffusive and the relaxation modes become over-damped. For a finite system, spontaneous delocalization may arise, due to residual bias that is induced by the irradiation.
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Shapira, Dekel, and Doron Cohen. "Emergence of Sinai Physics in the stochastic motion of passive and active particles." New Journal of Physics, June 6, 2022. http://dx.doi.org/10.1088/1367-2630/ac7609.
Full textAbstract:
Abstract A particle that is immersed in a uniform temperature bath performs Brownian diffusion, as discussed by Einstein. But Sinai has realized that in a "random environment" the diffusion is suppressed. Follow-up works have pointed out that in the presence of bias $f$ there are delocalization and sliding transitions, with threshold value $f_c$ that depends on the disorder strength. We discuss in a critical way the emergence of Sinai physics for both passive and active Brownian particles. Tight-binding and Fokker-Planck versions of the model are addressed on equal footing. We assume that the transition rates between sites are enhanced either due to a driving mechanism or due to self-propulsion mechanism that are induced by an irradiation source. Consequently, counter intuitively, the dynamics becomes sub-diffusive and the relaxation modes become over-damped. For a finite system, spontaneous delocalization may arise, due to residual bias that is induced by the irradiation.
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Liu, Rong‐Kun, Yanling Guo, Jia Jia, Qian Sun, Hong Zhao, and Jie‐Xin Wang. "Asymmetric Assembly in Microdroplets: Efficient Construction of MOF Micromotors for Anti‐Gravity Diffusion." Small, June 5, 2024. http://dx.doi.org/10.1002/smll.202402819.
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AbstractJanus‐micromotors, as efficient self‐propelled materials, have garnered considerable attention for their potential applications in non‐agitated liquids. However, the design of micromotors is still challenging and with limited approaches, especially concerning speed and mobility in complex environments. Herein, a two‐step spray‐drying approach encompassing symmetrical assembly and asymmetrical assembly is introduced to fabricate the metal‐organic framework (MOF) Janus‐micromotors with hierarchical pores. Using a spray‐dryer, a symmetrical assembly is first employed to prepare macro‐meso‐microporous UiO‐66 with intrinsic micropores (<0.5 nm) alongside mesopores (≈24 nm) and macropores (≈400 nm). Subsequent asymmetrical assembly yielded the UiO‐66‐Janus loaded with the reducible nanoparticles, which underwent oxidation by KMnO4 to form MnO2 micromotors. The micromotors efficiently generated O2 for self‐propulsion in H2O2, exhibiting ultrahigh speeds (1135 µm s−1, in a 5% H2O2 solution) and unique anti‐gravity diffusion effects. In a specially designed simulated sand‐water system, the micromotors traversed from the lower water to the upper water through the sand layer. In particular, the as‐prepared micromotors demonstrated optimal efficiency in pollutant removal, with an adsorption kinetic coefficient exceeding five times that of the micromotors only possessing micropores and mesopores. This novel strategy fabricating Janus‐micromotors shows great potential for efficient treatment in complex environments.
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Yariv, Ehud. "Two-dimensional phoretic swimmers: the singular weak-advection limits." Journal of Fluid Mechanics 816 (March 10, 2017). http://dx.doi.org/10.1017/jfm.2017.126.
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Because of the associated far-field logarithmic divergence, the transport problem governing two-dimensional phoretic self-propulsion lacks a steady solution when the Péclet number $\mathit{Pe}$ vanishes. This indeterminacy, which has no counterpart in three dimensions, is remedied by introducing a non-zero value of $\mathit{Pe}$, however small. We consider that problem employing a first-order kinetic model of solute absorption, where the ratio of the characteristic magnitudes of reaction and diffusion is quantified by the Damköhler number $\mathit{Da}$. As $\mathit{Pe}\rightarrow 0$ the dominance of diffusion breaks down at distances that scale inversely with $\mathit{Pe}$; at these distances, the leading-order transport represents a two-dimensional point source in a uniform stream. Asymptotic matching between the latter region and the diffusion-dominated near-particle region provides the leading-order particle velocity as an implicit function of $\log \mathit{Pe}$. Another scenario involving weak advection takes place under strong reactions, where $\mathit{Pe}$ and $\mathit{Da}$ are large and comparable. In that limit, the breakdown of diffusive dominance occurs at distances that scale as $\mathit{Da}^{2}/\mathit{Pe}$.
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Guan, Mingyang, Weiquan Jiang, Luoyi Tao, Guoqian Chen, and Joseph H. W. Lee. "Migration of confined micro-swimmers subject to anisotropic diffusion." Journal of Fluid Mechanics 985 (April 25, 2024). http://dx.doi.org/10.1017/jfm.2024.349.
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Shear-induced migration of elongated micro-swimmers exhibiting anisotropic Brownian diffusion at a population scale is investigated analytically in this work. We analyse the steady motion of confined ellipsoidal micro-swimmers subject to coupled diffusion in a general setting within a continuum homogenisation framework, as an extension of existing studies on macro-transport processes, by allowing for the direct coupling of convection and diffusion in local and global spaces. The analytical solutions are validated successfully by comparison with numerical results from Monte Carlo simulations. Subsequently, we demonstrate from the probability perspective that symmetric actuation does not yield net vertical polarisation in a horizontal flow, unless non-spherical shapes, external fields or direct coupling effects are harnessed to generate steady locomotion. Coupled diffusivities modify remarkably the drift velocity and vertical migration of motile micro-swimmers exposed to fluid shear. The interplay between stochastic swimming and preferential alignment could explain the diverse concentration and orientation distributions, including rheological formations of depletion layers, centreline focusing and surface accumulation. Results of the analytical study shed light on unravelling peculiar self-propulsion strategies and dispersion dynamics in active-matter systems, with implications for various transport problems arising from the fluctuating shape, size and other external or inter-particle interactions of swimmers in confined environments.
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Kelidou, Maria, Mohammad Fazelzadeh, Baptiste Parage, Marinde van Dijk, Twan Hooijschuur, and Sara Jabbari-Farouji. "Active string fluids and gels formed by dipolar active Brownian particles in 3D." Journal of Chemical Physics 161, no. 10 (September 13, 2024). http://dx.doi.org/10.1063/5.0215545.
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Self-propelled particles possessing permanent magnetic dipole moments occur naturally in magnetotactic bacteria and can be built into man-made systems such as active colloids or micro-robots. Yet, the interplay between self-propulsion and anisotropic dipole–dipole interactions on dynamic self-assembly in three dimensions (3D) remains poorly understood. We conduct Brownian dynamics simulations of active dipolar particles in 3D, focusing on the low-density regime, where dipolar hard spheres tend to form chain-like aggregates and percolated networks with increasing dipolar coupling strength. We find that strong active forces override dipolar attractions, effectively inhibiting chain-like aggregation and network formation. Conversely, activating particles with low to moderate forces results in a fluid composed of active chains and rings. At strong dipolar coupling strengths, this active fluid transitions into an active gel, consisting of a percolated network of active chains. Although the overall structure of the active gel remains interconnected, the network experiences more frequent configurational rearrangements due to the reduced bond lifetime of active dipolar particles. Consequently, particles exhibit enhanced translational and rotational diffusion within the active fluid of strings and active gels compared to their passive counterparts. We quantify the influence of activity on aggregate topology as they transition from branched structures to unconnected chains and rings. Our findings are summarized in a state diagram, delineating the impact of dipolar coupling strength and active force magnitude on the system.
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Urso, Mario, Martina Ussia, Filip Novotný, and Martin Pumera. "Trapping and detecting nanoplastics by MXene-derived oxide microrobots." Nature Communications 13, no. 1 (June 22, 2022). http://dx.doi.org/10.1038/s41467-022-31161-2.
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AbstractNanoplastic pollution, the final product of plastic waste fragmentation in the environment, represents an increasing concern for the scientific community due to the easier diffusion and higher hazard associated with their small sizes. Therefore, there is a pressing demand for effective strategies to quantify and remove nanoplastics in wastewater. This work presents the “on-the-fly” capture of nanoplastics in the three-dimensional (3D) space by multifunctional MXene-derived oxide microrobots and their further detection. A thermal annealing process is used to convert Ti3C2Tx MXene into photocatalytic multi-layered TiO2, followed by the deposition of a Pt layer and the decoration with magnetic γ-Fe2O3 nanoparticles. The MXene-derived γ-Fe2O3/Pt/TiO2 microrobots show negative photogravitaxis, resulting in a powerful fuel-free motion with six degrees of freedom under light irradiation. Owing to the unique combination of self-propulsion and programmable Zeta potential, the microrobots can quickly attract and trap nanoplastics on their surface, including the slits between multi-layer stacks, allowing their magnetic collection. Utilized as self-motile preconcentration platforms, they enable nanoplastics’ electrochemical detection using low-cost and portable electrodes. This proof-of-concept study paves the way toward the “on-site” screening of nanoplastics in water and its successive remediation.
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Jayaram, Ashreya, and Thomas Speck. "Effective dynamics and fluctuations of a trapped probe moving in a fluid of active hard discs." Europhysics Letters, June 16, 2023. http://dx.doi.org/10.1209/0295-5075/acdf1a.
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Abstract We study the dynamics of a single trapped probe surrounded by self-propelled active particles in two dimensions. In the limit of large size separation, we perform an adiabatic elimination of the small active particles to obtain an effective Markovian dynamics of the large probe, yielding explicit expressions for the mobility and diffusion coefficient. To calculate these expressions, we perform computer simulations employing active Brownian discs and consider two scenarios: non-interacting bath particles and purely repulsive interactions modeling volume exclusion. We keep the probe-to-bath size ratio fixed and vary the propulsion speed of the bath particles. The positional fluctuations of a trapped probe are accessible in experiments, for which we test the prediction from the adiabatic elimination. Although the approximations cause a discrepancy at equilibrium, the overall agreement between predicted and measured probe fluctuations is very good at larger speeds.
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Martín-Roca, José, Emanuele Locatelli, Valentino Bianco, Paolo Malgaretti, and Chantal Valeriani. "Tangentially active polymers in cylindrical channels." SciPost Physics 17, no. 4 (October 8, 2024). http://dx.doi.org/10.21468/scipostphys.17.4.107.
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We present an analytical and computational study characterizing the structural and dynamical properties of an active filament confined in cylindrical channels. We first outline the effects of the interplay between confinement and polar self-propulsion on the conformation of the chains. We observe that the scaling of the polymer size in the channel, quantified by the end-to-end distance, shows different anomalous behaviours under different confinement and activity conditions. In particular, we report scaling exponents that are markedly different from their passive counterparts. Interestingly, we show that the universal relation, describing the ratio between the end-to-end distance of passive polymer chains in cylindrical channels and in bulk is broken by activity. Finally, we show that the long-time diffusion coefficient under confinement can be rationalised by an analytical model, that takes into account the presence of the channel and the elongated nature of the polymer.
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Peng, Xia, Mario Urso, and Martin Pumera. "Metal oxide single-component light-powered micromotors for photocatalytic degradation of nitroaromatic pollutants." npj Clean Water 6, no. 1 (March 14, 2023). http://dx.doi.org/10.1038/s41545-023-00235-z.
Full textAbstract:
AbstractMass transfer is a key parameter in heterogeneous reactions. Micro/nanomachines, a promising technology for environmental applications, significantly enhance the performance of conventional purification treatments because of the active motion ability and thus enhanced diffusion (superdiffusion) of these photocatalysts, which in turn leads to dramatically improved mass transfer and higher degradation capability compared to stationary microparticles. However, the design of micromotors generally involves noble metals, for instance, Au and Pt, to achieve an effective autonomous motion. Considering the expensive fabrication cost and complicated steps, we present Pt-free single-component light-powered WO3 micromotors capable of enhanced diffusion and effective degradation of nitroaromatic compounds in water. These microswimmers, synthesized by a hydrothermal method, which is highly scalable at low cost, followed by calcination, exhibit fuel-free light-driven motion due to asymmetric light irradiation. Picric acid (PA) and 4-nitrophenol (4-NP) were selected as representative nitroaromatic contaminants and photocatalytically decomposed by WO3 micromotors thanks to the close contact with the micromotors promoted by their self-propulsion. This work provides a low-cost, sustainable, scalable method for enhancing mass transfer by creating moving catalysts with broad application potential for water cleanup.
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Ji, Yuxing, Yanan Pan, Xuemei Ma, Yan Ma, Zhongxiang Zhao, and Qiang He. "pH‐Sensitive Glucose‐Powered Nanomotors for Enhanced Intracellular Drug Delivery and Ferroptosis Efficiency." Chemistry – An Asian Journal, November 6, 2023. http://dx.doi.org/10.1002/asia.202300879.
Full textAbstract:
We propose a glucose‐powered Janus nanomotor where two faces are functionalized with glucose oxidase (GOx) and polydopamine‐Fe3+ chelates (PDF), respectively. In the glucose fuel solution, the GOx on the one side of these Janus nanomotors catalytically decomposes glucose fuels into gluconic acid and hydrogen peroxide (H2O2) to drive them at a speed of 2.67 μm/s. The underlying propulsion mechanism is the glucose‐based self‐diffusiophoresis owing to the generated local glucose concentration gradient by the enzymatic reaction. Based on the enhanced diffusion motion, such nanomotors with catalytic activity increase the contact probability towards cells and subsequently exhibit excellent capabilities for Fe3+ ions delivery and H2O2 generation for enhancing ferroptosis efficiency for inducing cancer cell death. In particular, the Fe3+ ions are released from nanomotors in a slightly acidic environment, and subsequently generate toxic hydroxyl radicals via Fenton reactions, which accumulation reactive oxygen species (ROS) level (~300%) and further lipid peroxidation (LPO) strengthened ferroptosis therapy for cancer treatment. Such a pH‐sensitive nanomotor with efficient diffusion can exhibit Fe ions overloading and sufficient H2O2 to promote ferroptosis efficiency, which provides great potential for future cancer precise therapy.