Academic literature on the topic 'Micro-swimmer'

We propose a computational method to solve optimal swimming problems, based on the boundary integral formulation of the hydrodynamic interaction between swimmer and surrounding fluid and direct constrained minimization of the energy consumed by the swimmer. We apply our method to axisymmetric model examples. We consider a classical model swimmer (the three-sphere swimmer of Golestanian et al.) as well as a novel axisymmetric swimmer inspired by the observation of biological micro-organisms.

A squirmer model achieves propulsion by generating surface squirming velocities. This model has been used to analyze the movement of micro-swimmers, such as microorganisms and Janus particles. Although squirmer motion has been widely investigated, motions of two connected squirmers, i.e., a dumbbell squirmer, remain to be clarified. The stable assembly of multiple micro-swimmers could be a key technology for future micromachine applications. Therefore, in this study, we investigated the swimming behavior and stability of a dumbbell squirmer. We first examined far-field stability through linear stability analysis, and found that stable forward swimming could not be achieved by a dumbbell squirmer in the far field without the addition of external torque. We then investigated the swimming speed of a dumbbell squirmer connected by a short rigid rod using a boundary element method. Finally, we investigated the swimming stability of a dumbbell squirmer connected by a spring. Our results demonstrated that stable side-by-side swimming can be achieved by pullers. When the aft squirmer was a strong pusher, fore and aft swimming were stable and swimming speed increased significantly. The findings of this study will be useful for the future design of assembled micro-swimmers.

The membrane tension of some kinds of ciliates has been suggested to regulate upward and downward swimming velocities under gravity. Despite its biological importance, deformation and membrane tension of a ciliate have not been clarified fully. In this study, we numerically investigated the deformation of a ciliate swimming freely in a fluid otherwise at rest. The cell body was modelled as a capsule with a hyperelastic membrane enclosing a Newtonian fluid. Thrust forces due to the ciliary beat were modelled as torques distributed above the cell body. The effects of membrane elasticity, the aspect ratio of the cell's reference shape, and the density difference between the cell and the surrounding fluid were investigated. The results showed that the cell deformed like a heart shape, when the capillary number was sufficiently large. Under the influence of gravity, the membrane tension at the anterior end decreased in the upward swimming while it increased in the downward swimming. Moreover, gravity-induced deformation caused the cells to move gravitationally downwards or upwards, which resulted in a positive or negative geotaxis-like behaviour with a physical origin. These results are important in understanding the physiology of a ciliate's biological responses to mechanical stimuli.

Recent advances in micro-machining allow very small cargos, such as single red blood cells, to be moved by outfitting them with tails made of micrometre-sized paramagnetic particles yoked together by polymer bridges. When a time-varying magnetic field is applied to such a filament, it bends from side to side and propels itself through the fluid, dragging the load behind it. Here, experimental data and a mathematical model are presented showing the dependence of the swimming speed and direction of the magnetic micro-swimmer upon tunable parameters, such as the field strength and frequency and the filament length. The propulsion of the filament arises from the propagation of bending waves between free and tethered ends: here we show that this gives the micro-swimmer a gait that is intermediate between a eukaryotic cell and a waggled elastic rod. Finally, we extract from the model design principles for constructing the fastest swimming micro-swimmer by tuning experimental parameters.

The hydrodynamics of a flagellated micro-organism is investigated when swimming close to a planar free-slip surface by means of numerical solutions of the Stokes equations obtained via a boundary element method. Depending on the initial conditions, the swimmer can either escape from the free-slip surface or collide with the boundary. Interestingly, the micro-organism does not exhibit a stable orbit. Independently of escape or attraction to the interface, close to a free-slip surface, the swimmer follows a counter-clockwise trajectory, in agreement with experimental findings (Di Leonardo et al., Phys. Rev. Lett., vol. 106 (3), 2011, 038101). The hydrodynamics is indeed modified by the free surface. In fact, when the same swimmer moves close to a no-slip wall, a set of initial conditions exists which result in stable orbits. Moreover, when moving close to a free-slip or a no-slip boundary, the swimmer assumes a different orientation with respect to its trajectory. Taken together, these results contribute to shed light on the hydrodynamical behaviour of micro-organisms close to liquid–air interfaces which are relevant for the formation of interfacial biofilms of aerobic bacteria.

One of the principal mechanisms by which surfaces and interfaces affect microbial life is by perturbing the hydrodynamic flows generated by swimming. By summing a recursive series of image systems, we derive a numerically tractable approximation to the three-dimensional flow fields of a stokeslet (point force) within a viscous film between a parallel no-slip surface and a no-shear interface and, from this Green’s function, we compute the flows produced by a force- and torque-free micro-swimmer. We also extend the exact solution of Liron & Mochon (J. Engng Maths, vol. 10 (4), 1976, pp. 287–303) to the film geometry, which demonstrates that the image series gives a satisfactory approximation to the swimmer flow fields if the film is sufficiently thick compared to the swimmer size, and we derive the swimmer flows in the thin-film limit. Concentrating on the thick-film case, we find that the dipole moment induces a bias towards swimmer accumulation at the no-slip wall rather than the water–air interface, but that higher-order multipole moments can oppose this. Based on the analytic predictions, we propose an experimental method to find the multipole coefficient that induces circular swimming trajectories, allowing one to analytically determine the swimmer’s three-dimensional position under a microscope.

Recent strides in micro- and nanofabrication technology have enabled researchers to design and develop new micro- and nanorobots for biomedicine and environmental monitoring. Due to its non-invasive remote actuation and convenient navigation abilities, magnetic propulsion has been widely used in micro- and nanoscale robotic systems. In this article, a highly efficient Janus microdimer swimmer propelled by a rotating uniform magnetic field was investigated experimentally and numerically. The velocity of the Janus microdimer swimmer can be modulated by adjusting the magnetic field frequency with a maximum speed of 133 μm·s−1 (≈13.3 body length s−1) at the frequency of 32 Hz. Fast and accurate navigation of these Janus microdimer swimmers in complex environments and near obstacles was also demonstrated. This efficient propulsion behavior of the new Janus microdimer swimmer holds considerable promise for diverse future practical applications ranging from nanoscale manipulation and assembly to nanomedicine.

A device constructed from a filament of paramagnetic beads connected to a human red blood cell will swim when subject to an oscillating magnetic field. Bending waves propagate from the tip of the tail toward the red blood cell in a fashion analogous to flagellum beating, making the artificial swimmer a candidate for studying what has been referred to as ‘flexible oar’ micro-swimming. In this study, we demonstrate that under the influence of a rotating field the artificial swimmer will perform ‘corkscrew’-type swimming. We conduct numerical simulations of the swimmer where the paramagnetic tail is represented as a series of rigid spheres connected by flexible but inextensible links. An optimal range of parameters governing the relative strength of viscous, elastic and magnetic forces is identified for swimming speed. A parameterization of the motion is extracted and examined as a function of the driving frequency. With a continuous elastica/resistive force model, we obtain an expression for the swimming speed in the low-frequency limit. Using this expression we explore further the effects of the applied field, the ratio of the transverse field to the constant field, and the ratio of the radius of the sphere to the length of the filament tail on the resulting dynamics.

It is known that active particles induce emerging patterns as a result of their dynamic interactions, giving rise to amazing collective motions, such as swarming or clustering. Here we present a systematic numerical study of self-propelling particles; our main goal is to characterize the collective behavior of suspensions of active particles as a result of the competition among their propulsion activity and the intensity of an attractive pair potential. Active particles are modeled using the squirmer model. Due to its hydrodynamic nature, we are able to classify the squirmer swimmer activity in terms of the stress it generates (referred to as pullers or pushers). We show that these active stresses play a central role in the emergence of collective motion. We have found that hydrodynamics drive the coherent swimming between swimmers while the swimmer direct interactions, modeled by a Lennard-Jones potential, contributes to the swimmers' cohesion. This competition gives rise to two different regimes where giant density fluctuations (GDF) emerge. These two regimes are differentiated by the suspension alignment; one regime has GDF in aligned suspensions whereas the other regime has GDF of suspensions with an isotropic orientated state. All the simulated squirmer suspensions shown in this study were characterized by a thorough analysis of global properties of the squirmer suspensions as well as a complementary cluster analysis. Active matter refers generically to systems composed of self-driven units, active particles, each capable of converting stored or ambient free energy into systematic movement. Examples of active systems are found at all length scales and could be classified in living and nonliving systems such as microorganisms, tissues and organisms, animal groups, self- propelled colloids and artificial nanoswimmers. Specifically, at the micro and nano scale we find an enormous range of interesting systems both biological and artificial; e.g. spermatozoa that fuse with the ovum during fertilization, the bacteria that inhabit our guts, the protozoa in our ponds, the algae in the ocean; these are but a few examples of a wide biological spectrum. In the artificial world we have self- healing colloidal crystals and membranes as well as self- assembled microswimmers and robots. Experiments in this field are now developing at a very rapid pace and new theoretical ideas are needed to bring unity to the field and identify "universal" behavior in these internally driven systems. One important feature of active matter is that their elements can develop emergent, coordinated behavior; collective motion constitutes one of the most common and spectacular example. Collective motion is ubiquitous and at every scale, from herds of large mammals to amoeba and bacteria colonies, down to the cooperative behavior of molecular motors in the cell. The behavior of large fish schools and the dance of starling flocks at dusk are among the most spectacular examples. From a physical perspective collective motion emerges from a spontaneous symmetry breaking that allows for long-range orientational orden The different mechanisms responsible for such symmetry breaking are still not completely understood. We have performed a systematic numerical study of interactive micro-swimmer suspensions building on the squirmer model, introduced by Lighthill. Since the squirmer identifies systematically the hydrodynamic origin of self-propulsion and stress generation it provides a natural scheme to scrutinize the impact that the different features associated to self-propulsion in a liquid medium have in the collective dynamics of squirmer suspensions. In this abstract we describe the simulation scheme and how squirmers are modeled, then some of the main results are discussed and finally we conclude emphasizing the main implications of the results obtained.
Los sistemas activos se definen como materiales fuera del equilibrio termodinámico compuestos por muchas unidades interactuantes que individualmente consumen energía y colectivamente generan movimiento o estreses mecánicos. Ejemplos se pueden encontrar en un enorme rango de escalas de longitud, desde el mundo biológico hasta artificial, incluyendo organismos unicelulares, tejidos y organismos pluricelulares, grupos de animales, coloides auto-propulsados y nano-nadadores artificiales. Actualmente se están desarrollando experimentos en este campo a un ritmo muy veloz, en consecuencia son necesarias nuevas ideas teóricas para traer unidad al campo de estudio e identificar comportamientos “universales” en estos sistemas propulsados internamente. El objetivo de esta tesis es el estudiar mediante simulaciones numéricas, el comportamiento colectivo de un modelo de micro-nadadores. En particular, el modelo de squirmers, donde el movimiento del fluido es axi-simétrico. Existen estructuras coherentes que emergen de estos sistemas así que, el entender si las estructuras coherentes son generadas por la firma hidrodinámica intrínseca de los squirmers individuales o por un efecto de tamaño finito se vuelve algo de primordial importancia. Nosotros también estudiamos la influencia que tiene la geometría en la aparición de estructuras coherentes, la interacción directa entre las partículas, la concentración, etc.

Les micro-organismes sont omniprésents sur Terre. Ils ont développé l'autopropulsion pour explorer leur environnement et coloniser de nouvelles niches écologiques. Certains d'entre eux sont pathogènes et déclenchent des inflammations lorsqu'ils entrent en contact avec des cellules épithéliales. Alors que la nature hydrodynamique de leur mouvement est plutôt bien comprise dans les fluides newtoniens, il reste encore beaucoup à comprendre lorsqu'ils interagissent mécaniquement avec leur environnement, soit par la présence d'obstacles géométriques, soit en raison de la nature non-newtonienne de leur milieu de nage. Dans cette thèse, nous examinons la motilité des micro-nageurs bactériens (E. coli) dans deux conditions physiologiques, particulièrement pertinentes dans le contexte biophysique des infections bactériennes à travers le mucus intestinal : premièrement dans le cas d'une géométrie confinée entre deux surfaces parallèles, et deuxièmement dans du mucus intestinal d'origine animale. Tout d'abord, nous réalisons des expériences à l'aide d'un dispositif de suivi, dit "Lagrangien", qui nous permet de capturer les trajectoires des bactéries tout en visualisant leur corps et leurs flagelles pendant plusieurs centaines de secondes. Nous l'utilisons pour comprendre les effets des surfaces lorsqu'elles explorent un environnement confiné. Le confinement ralentit la propagation d'E. coli en les piégeant sur les surfaces et en interrompant les "runs" qu'elles effectuent depuis l'environnement 3D. Les résultats expérimentaux sont rationalisés à l'aide d'un modèle stochastique qui rend compte de la dynamique interne complexe à l'origine des réorientations actives d'E. coli. Le mouvement sur les surfaces est spécifiquement étudié, et la variabilité interindividuelle observée dans les propriétés de nage est questionnée sous le prisme de leurs morphologies, en particulier leur nombre de flagelles. Nous nous attachons ensuite à comprendre le mouvement d'E. coli dans du mucus intestinal, qui est extrait de deux groupes différents de porcelets que nous comparons. Après un processus de purification, les différents échantillons sont caractérisés par une expérience in vitro originale dans laquelle des bactéries ont pénétré une barrière de mucus, d'où émerge une "longueur de pénétration" caractérisant une "qualité de mucus", complétée par des mesures rhéologiques et optiques. La longueur de pénétration varie de 100 à 1000 microns selon l'échantillon et semble dépendre davantage de la taille de la structure que de la macro-rhéologie. Des signatures rhéologiques différentes sont observées, avec ou sans l'influence de l'histoire du cisaillement. Cette étude préliminaire offre de nombreuses perspectives, à la fois physiques (microscopie OCT / diffusion des rayons X / microrhéologie) et médicales (outil de diagnostic pour les patients / utilisation de souches bactériennes sélectionnées). Pour obtenir une vision temporelle du processus de pénétration, l'apprentissage automatique est finalement utilisé pour étendre l'utilisation du dispositif de suivi lagrangien aux fluides optiquement complexes, avec succès pour le mucus. Il est démontré que les bactéries explorent le mucus dix fois plus lentement que l'eau et qu'elles sont bloquées au bout de quelques minutes. Les résultats et les protocoles expérimentaux développés dans cette thèse étendent l'état de l'art sur le sujet des micro-nageurs en termes méthodologiques, tout en fournissant de nouvelles données sur les schémas de nage et la pénétration dans les fluides viscoélastiques
Microorganisms are ubiquitous on Earth. They developed self-propulsion to explore their environment and colonize new ecological niche. Some of them are pathogens and trigger inflammation when in contact with epithelial cells. While the hydrodynamical nature of their motion is rather well understood in Newtonian fluids, there is still much to understand when they interact mechanically with their environment either through the presence of geometric obstacles or stemming from the non-Newtonian nature of their swimming environments. In this thesis, we take a look at the motility of bacterial microswimmers (E. coli) under two physiologically conditions, especially relevant in the biophysical context of bacterial infections through intestinal mucus: firstly in the case of a confined geometry between two parallel surfaces, and secondly in intestinal mucus of animal origin. First, we perform experiments with E. coli using an in-house tracking device that allows us to capture the trajectories of bacteria while visualizing their bodies and flagella for long periods of time. We use it to understand the effects of surfaces as they explore a confined environment. Confinement slows the spread of E. coli by trapping them on surfaces and interrupting the "runs" they take from the bulk. Experimental results are rationalized with a stochastic model that accounts for the complex internal dynamics that result in active reorientations of E. coli. The motion at surfaces is specifically studied, and the interindividual variability observed in the swimming properties is questioned under the prism of their morphologies, especially their number of flagella. We then turn to understanding the movement of E. coli in intestinal mucus, which is extracted from two different groups of piglets that are compared. After a purification process, the different samples are characterized by an original in vitro experiment in which bacteria have penetrated a mucus barrier, from which emerges a "penetration length" characterizing a "mucus quality", complemented by rheological and optical measurements. The penetration length ranges from 100 to 1000 microns depending on the sample and seems to depend more on the structure size than on the macrorheology. Different rheological signatures are observed with and without the influence of the shear history. This preliminary study offers many perspectives, both physical (OCT microscopy / X-ray scattering / microrheology) and medical (diagnostic tool for patients / use of selected bacterial strains). To get a temporal view of the penetration process, machine learning is finally used to extend the use of the Lagrangian tracking device to optically complex fluids, successfully implemented for mucus. Bacteria are shown to explore mucus ten times slower than water, and to get blocked after a few minutes. The results and experimental protocols developed in this thesis extend the state-of-the-art on the subject of microswimmers in methodological terms, while also providing some new data on swimming patterns and penetration into viscoelastic fluids

Micro-scale slender swimmers are frequently encountered in nature and recently in micro-robotic applications. The swimming mechanism examined in this article is based on small transverse axi-symmetrical travelling wave deformations of a cylindrical long shell. In very small scale, inertia forces become negligible and viscous forces dominate most propulsion mechanisms being used by micro-organisms and robotic devices. The present paper proposes a compact design principle that provides efficient power to propel and maneuver a micro-scale device. Shown in this paper is a numerical analysis which couples the MEMS structure to the surrounding fluid. Analytical results compare the proposed mechanism to commonly found tail (flagella) driven devices, and a parametric comparison is shown suggesting it has superior performance. Numerical studies are preformed to verify the analytical model. Finally, a macro-scale demonstrator swimming in an environment with similar Reynolds numbers to the ones found in small scale is shown and its behavior in the laboratory is compared to the theory.

Autonomous micro-swimming robots can be utilized to perform specialized procedures such as in vitro or in vivo medical tasks as well as chemical surveillance or micro manipulation. Maneuverability of the robot is one of the requirements that ensure successful completion of its task. In micro fluidic environments, dynamic trajectories of active micro-swimming robots must be predicted reliably and the response of control inputs must be well-understood. In this work, a reduced-order model, which is based on the resistive force theory, is used to predict the transient, coupled rigid body dynamics and hydrodynamic behavior of bio-inspired artificial micro-swimmers. Conceptual design of the micro-swimmer is biologically inspired: it is composed of a body that carries a payload, control and actuation mechanisms, and a long flagellum either such as an inextensible whip like tail-actuator that deforms and propagates sinusoidal planar waves similar to spermatozoa, or of a rotating rigid helix similar to many bacteria, such as E. Coli. In the reduced-order model of the micro-swimmer, fluid’s resistance to the motion of the body and the tail are computed from resistive force theory, which breaks up the resistance coefficients to local normal and tangential components. Using rotational transformations between a fixed world frame, body frame and the local Frenet-Serret coordinates on the helical tail we obtain the full 6 degrees-of-freedom relationship between the resistive forces and torques and the linear and rotational motions of the swimmer. In the model, only the tail’s frequency (angular velocity for helical tail) is used as a control input in the dynamic equations of the micro-swimming robot. The reduced-order model is validated by means of direct observations of natural micro swimmers presented earlier in the literature and against; results show very good agreement. Three-dimensional, transient CFD simulations of a single degree of freedom swimmer is used to predict resistive force coefficients of a micro-swimmer with a spherical body and flexible tail actuator that uses traveling plane wave deformations for propulsion. Modified coefficients show a very good agreement between the predicted and actual time-dependent swimming speeds, as well as forces and torques along all axes.

We present, for the first time, a micro scale 1D swimmer powered by cardiamyocytes. It consists of a PDMS “flagella” with a head. Cardiomyocytes are plated near the head. Synchronized contraction of a small number of cells generates an elastic deformation wave in the flagella, which interacts with the fluid. Solid-fluid interaction results in a net propulsive force on the swimmer, which drives it through the fluid overcoming the drag.

Micro swimming robots that mimic the motion of micro organisms can carry out a variety of medical tasks including drug delivery, micro surgery and minimally invasive diagnostic tasks. Micro organisms such as spermatozoa and bacteria use their flagella to propel themselves. The artificial micro swimmer presented in this study is composed of a body that carries a medical payload, and one wave propagating tail attached to it. In this study, forces and torques exerted on the tail structure by the surrounding fluid are computed with the help of corresponding force coefficients. Rigid body dynamics computations are carried out by four-dimensional quaternion configuration to eliminate numerical error accumulation during matrix integrations, and, hence, instantaneous rotation matrix for rigid body rotation is extracted from the quaternion. Propulsive force obtained by waving tail is balanced by the drag force on the micro swimmers’ total wet surface and dynamic behavior of the micro swimmer is obtained as a rigid body motion. The effect of swimmer and waving geometry is parameterized to study the swimming behavior. Simulations carried out to explore the effect of wave length, wave amplitude, driving frequency. Translational and rotational velocities and hydrodynamic power requirements are presented for each individual set of design parameters. Validity of the model is tested by comparing the numerical results and finite element simulation results. Lastly, the model is modified to utilize the mobility matrix coefficients obtained from inertia eliminated finite element simulations governed by time dependent Navier-Stokes equations.

Micro swimming robots offer many advantages in biomedical applications, such as delivering potent drugs to specific locations in targeted tissues and organs with limited side effects, conducting surgical operations with minimal damage to healthy tissues, treatment of clogged arteries, and collecting biological samples for diagnostic purposes. Reliable navigation techniques for micro swimmers need to be developed to improve the localization of robots inside the human body in future biomedical applications. In order to estimate the dynamic trajectory of magnetically propelled micro swimmers in channels, that mimic blood vessels and other conduits, fluid-micro robot interaction and the effect of the channel wall must be understood well. In this study, swimming of one-link robots with helical tails is modeled with Stokes equations and solved numerically with the finite element method. Forces acting on the robot are set to zero to enforce the force-free swimming and obtain forward, lateral and angular velocities that satisfy the constraints. Effects of the number of helical waves, wave amplitude, relative size of the cylindrical head of micro swimmer and the radial position on angular and linear velocity vectors of micro swimmer are presented.

This paper describes a unique non-vibratory traveling wave generator, which performs as a propulsion mechanism or pump in low Reynolds number (Re) environments, i.e. in highly viscous fluids or in micro scales. In low Re numbers the dynamics of a moving body is governed mainly by fluid drag effects, therefore mobility must rely on non-inertial, non-time reversible trajectories, e.g., traveling waves. The proposed device generates axisymmetric transverse traveling waves along an elastic cylindrical shell. The waving surface induces a vorticity and pressure fields in the adjacent fluid, which result in propagation of the swimmer opposite to the wave direction, or alternatively, in pumping fluid in the wave propagation direction. The dynamics of the mechanical multi-cam device ensures, assuming negligible friction, zero actuation shaft torque, while for non-negligible friction it is demonstrated that torque oscillations are dramatically decreased, yielding a small non-zero mean torque. These properties are achieved by choosing a specific angular phase between successive cams, so that the ratio of the number of wavelengths times a harmonic index, over the number of cams is non-integer. The effects of wave discretization on the tempo-spatial frequency content of the cylindrical envelope are also studied. The analytical analysis is accompanied by numerical examples, and demonstrated with an experimental working prototype.