Academic literature on the topic 'Shear Induced Microscopic Dynamic'

The viscosity of colloidal suspensions varies with shear rate, an important effect encountered in many natural and industrial processes. Although this non-Newtonian behavior is believed to arise from the arrangement of suspended particles and their mutual interactions, microscopic particle dynamics are difficult to measure. By combining fast confocal microscopy with simultaneous force measurements, we systematically investigate a suspension’s structure as it transitions through regimes of different flow signatures. Our measurements of the microscopic single-particle dynamics show that shear thinning results from the decreased relative contribution of entropic forces and that shear thickening arises from particle clustering induced by hydrodynamic lubrication forces. This combination of techniques illustrates an approach that complements current methods for determining the microscopic origins of non-Newtonian flow behavior in complex fluids.

We formulate a particle and force level, activated dynamics-based statistical mechanical theory for the continuous startup nonlinear shear rheology of ultradense glass-forming hard sphere fluids and colloidal suspensions in the context of the elastically collective nonlinear Langevin equation approach and a generalized Maxwell model constitutive equation. Activated structural relaxation is described as a coupled local-nonlocal event involving caging and longer range collective elasticity which controls the characteristic stress relaxation time. Theoretical predictions for the deformation-induced enhancement of mobility, the onset of relaxation acceleration at remarkably low values of stress, strain, or shear rate, apparent power law thinning of the steady-state structural relaxation time and viscosity, a nonvanishing activation barrier in the shear thinning regime, an apparent Herschel–Buckley form of the shear rate dependence of the steady-state shear stress, exponential growth of different measures of a yield or flow stress with packing fraction, and reduced fragility and dynamic heterogeneity under deformation were previously shown to be in good agreement with experiments. The central new question we address here is the defining feature of the transient response—the stress overshoot. In contrast to the steady-state flow regime, understanding the transient response requires an explicit treatment of the coupled nonequilibrium evolution of structure, elastic modulus, and stress relaxation time. We formulate a new quantitative model for this aspect in a physically motivated and computationally tractable manner. Theoretical predictions for the stress overshoot are shown to be in good agreement with experimental observations in the metastable ultradense regime of hard sphere colloidal suspensions as a function of shear rate and packing fraction, and accounting for deformation-assisted activated motion appears to be crucial for both the transient and steady-state responses.

Grain growth under shear annealing is crucial for controlling the properties of polycrystalline materials. However, their microscopic kinetics are not well understood because individual atomic trajectories are difficult to track. Here, we study grain growth with single-particle kinetics in colloidal polycrystals using video microscopy. Rich grain-growth phenomena are revealed in three shear regimes, including the normal grain growth (NGG) in weak shear melting–recrystallization process in strong shear. For intermediate shear, early stage NGG is arrested by built-up stress and eventually gives way to dynamic abnormal grain growth (DAGG). We find that DAGG occurs via a melting–recrystallization process, which naturally explains the puzzling stress drop at the onset of DAGG in metals. Moreover, we visualize that grain boundary (GB) migration is coupled with shear via disconnection gliding. The disconnection-gliding dynamics and the collective motions of ambient particles are resolved. We also observed that grain rotation can violate the conventional relation R×θ=constant (R is the grain radius, and θ is the misorientation angle between two grains) by emission and annihilation of dislocations across the grain, resulting in a step-by-step rotation. Besides grain growth, we discover a result in shear-induced melting: The melting volume fraction varies sinusoidally on the angle mismatch between the triangular lattice orientation of the grain and the shear direction. These discoveries hold potential to inform microstructure engineering of polycrystalline materials.

Steady and oscillatory shear 3-D simulations of electro- and magnetorheology in uniaxial and biaxial fields are presented, and compared to the predictions of the chain model. These large scale simulations are three dimensional, and include the effect of Brownian motion. In the absence of thermal fluctuations, the expected shear thinning viscosity is observed in steady shear, and a striped phase is seen to rapidly form in a uniaxial field, with a shear slip zone in each sheet. However, as the influence of Brownian motion increases, the fluid stress decreases, especially at lower Mason numbers, and the striped phase eventually disappears, even when the fluid stress is still high. In a biaxial field, an opposite trend is seen, where Brownian motion decreases the stress most significantly at higher Mason numbers. To account for the uniaxial steady shear data we propose a microscopic chain model of the role played by thermal fluctuations on the rheology of ER and MR fluids that delineates the regimes where an applied field can impact the fluid viscosity, and gives an analytical prediction for the thermal effect. In oscillatory shear, a striped phase again appears in a uniaxial field, at strain amplitudes greater than ~0.15, and the presence of a shear slip zone creates strong stress nonlinearities at low strain amplitudes. In a biaxial field, a shear slip zone is not created, and so the stress nonlinearities develop only at expected strain amplitudes. The nonlinear dynamics of these systems is shown to be in good agreeement with the Kinetic Chain Model.

Abstract This paper aims to investigate the influence of periodicity temperature change on the properties of dry granular materials in macroscopic and microscopic. A series of cyclic thermal consolidation tests have been carried out based on the discrete element method (DEM) that incorporate particles’ volumetric thermal expansion coefficient. The simulation of the direct shear test was carried out on the samples after thermal cycling. Results showed that thermally-induced volumetric strain accumulation of the specimen can be calculated by the DEM model, based on the two-dimensional particle flow code (PFC2D) software. The lateral pressure degraded concomitantly thanks to decreases in particles’ horizontal contact during periodic thermal cycling. In addition, the shear dilatancy level decreases during the shearing process with the number of thermal cycles. Both the size and anisotropy of the normal contact force and contact number and the force chain are affected by the temperature cycle. Finally, the results of this paper have a certain reference for the engineering practice, such as thermal piles or others, when granular materials are subjected to thermal cycling.

The presence and the microscopic origin of normal stress differences in dense suspensions under simple shear flows are investigated by means of inertialess particle dynamics simulations, taking into account hydrodynamic lubrication and frictional contact forces. The synergic action of hydrodynamic and contact forces between the suspended particles is found to be the origin of negative contributions to the first normal stress difference $N_{1}$ , whereas positive values of $N_{1}$ observed at higher volume fractions near jamming are due to effects that cannot be accounted for in the hard-sphere limit. Furthermore, we found that the stress anisotropy induced by the planarity of the simple shear flow vanishes as the volume fraction approaches the jamming point for frictionless particles, while it remains finite for the case of frictional particles.

In this communication, we offer a theoretical explanation for the results of recent experiments that examine the stress response of a dilute suspension of bacteria (wild-type E. coli) subjected to step changes in the shear rate (Lopez et al., Phys. Rev. Lett., vol. 115, 2015, 028301). The observations include a regime of negative apparent shear viscosities. We start from a kinetic equation that describes the evolution of the single-bacterium orientation probability density under the competing effects of an induced anisotropy by the imposed shear, and a return to isotropy on account of stochastic relaxation mechanisms (run-and-tumble dynamics and rotary diffusion). We then obtain analytical predictions for the stress response, at leading order, of a dilute bacterial suspension subject to a weak but arbitrary time-dependent shear rate profile. While the predicted responses for a step-shear compare well with the experiments for typical choices of the microscopic parameters that characterize the swimming motion of a single bacterium, use of actual experimental values leads to significant discrepancies. The incorporation of a distribution of run times leads to a better agreement with observations.

Al-Mg-Si series aluminum alloy is a heat-treatment-strengthened alloy. Research on the impact resistance of Al-Mg-Si series aluminum alloy is of great significance to expand its application in engineering. Taking 6082-T6 aluminum alloy as the concrete research object, using the split Hopkinson pressure bar (SHPB) device, the dynamic mechanical response of the material under different temperatures and average strain rates was studied, and the service performance of the material under extreme conditions was determined. The absolute temperature rise was introduced to optimize the existing constitutive model. The results show that when the environment temperature is 298.15~473.15 K under high-speed impact, the internal thermal softening effect of the material is dominant in the competition with the work hardening, resulting in a decrease in the flow stress of the material. Through the analysis of the real stress–strain curve, it was found that the elastic modulus of the material was negatively correlated with the strain rate, negatively correlated with the temperature, and showed an obvious temperature-softening effect. Yield strength was negatively correlated with temperature and positively correlated with strain rate, which showed an obvious strain rate hardening effect. Based on SEM microscopic analysis, it was found that under given conditions, adiabatic shear bands appeared in some samples, and their internal structures demonstrated obvious change. It was judged that when high-speed impact occurs, cracks are induced at the shear bands, and the cracks will continue to develop along the adiabatic shear bands, resulting in many oblique cracks which will gradually become larger and eventually lead to material failure. Finally, based on the model, the strain rate and temperature softening terms were improved, and a rise in adiabatic temperature rise was introduced. The improved model can better describe the strain rate effect of the material and accurately describe its flow stress. It provides a theoretical basis for the engineering application of materials.

We investigated the effect of mechanical stimuli on mouse embryonic development from the 2-cell to blastocyst stage to evaluate physical factors affecting embryonic development. Shear stress (SS) applied to embryos using two mechanical vibration systems (MVSs) was calculated by observing microscopic images of moving embryos during mechanical vibration (MV). The MVSs did not induce any motion of the medium and the diffusion rate using MVSs was the same as that under static conditions. Three days of culture using MVS did not improve embryonic development. MVS transmitted MV power more efficiently to embryos than other systems and resulted in a significant decrease in development to the morula or blastocyst stage after 2 days. Comparison of the results of embryo culture using dynamic culture systems demonstrated that macroscopic diffusion of secreted materials contributes to improved development of mouse embryos to the blastocyst stage. These results also suggest that the threshold of SS and MV to induce negative effects for mouse embryos at stages earlier than the blastocyst may be lower than that for the blastocyst, and that mouse embryos are more sensitive to physical and chemical stimuli than human or pig embryos because of their thinner zona pellucida.

The context In this thesis, we investigated the relation between the mechanical properties of a material and the state of its microscopic constituents, with a particular focus on the yielding transition in soft materials. Yielding represents one of the key modes of mechanical failure and characterizes the mechanical response of a wide class of materials, from metals to polymers, from biological tissues to emulsions. Despite being a complex phenomenon, encompassing a wide range of length- and time-scales [6], [9], [3], yielding is characterized by an associated phenomenology that displays a number of universal features, that are largely material-independent [8], [2]. It is this universality that makes soft materials very convenient model systems for understanding yielding: on one hand, they usually display a structural organization at length scales that can be probed by means of optical techniques; on the other hand, as they are "soft", it is very easy to obtain large deformations or induce yielding even upon the application of moderate forces. Experimentally, the greatest challenge of a multi-scale (from micro to macro) study of yielding in soft materials lies in imposing a macroscopic deformation on a soft sample while monitoring the microscopic degrees of freedom. Characterization of the microscopic states through far field scattering techniques suffers from lack of spatial resolution, as the output of these experiments typically represent a spatial average performed over a large (and possibly heterogeneous) volume. At the same time, imaging techniques usually suffer from poor statistics, since usually a better spatial resolution comes at the expense of the extension of the field of view. The possibility to combine a good spatial resolution and a statistically robust characterization is crucial in the study of a complex phenomenon, that has an intrinsic stochastic nature, a considerable sample to sample variability and a strong sensitivity to spatial heterogeneity. Our methodological contribution Here we have developed an integrated approach (rheology protocol and image analysis) to study the microscopic dynamics of soft materials, both at rest and under oscillatory shear strain. All the techniques and methods that we have refined provide a quantitative characterization of the statics (e.g. spatial arrangement and positional correlation,...) and dynamics (e.g. relaxation of density modes, determination of particle trajectories, dynamical activity,...) of the soft material under scrutiny. Most of the proposed tools pivot around Differential Dynamic Microscopy that combines the experimental setup of optical microscopy with an image processing aimed at extracting scattering-like information about the sample at different wave vectors. Two basic methodological advances obtained in this work, which were succesfully applied to samples at rest and under shear, include Image Windowing - A pre-processing step for DDM analysis aimed at removing artifacts due to the image finite size. This tool revealed to be essential to extract the correct dynamics over a larger wave-vector range, as detailed in the associated peer-reviewed article [5]. DDM-rheology - Microrheology is a family of techniques aimed at characterizing the linear mechanical properties of a material via the measurement of the MSD of embedded micrometric tracers . The most diffuse route to microrheology is particle tracking, that requires to individually identify the tracers and their trajectories. We have shown how DDM can be used to obtain the same information, under a wide range imaging conditions. More details are available also in the published peer-reviewed article [4]. To study the dynamics under shear, we used a large amplitude oscillatory shear (LAOS) approach, applied to simple yield stress fluids. This choice was motivated by the fact that in LAOS experiments, simple yield stress fluids reach a stationary condition even in the non-linear regime. The so-obtained stationary state allows for a statistically significant characterization of the microscopic state. To achieve this goal we have acted along two main lines: we have developed an echo-DDM approach to monitor and quantify the shear-induced dynamics at different wave-vectors: images of the sample are acquired with a frame rate equal to the deformation rate, in order to be sensible only to irreversible plastic rearrangements. we have monitored intermittency and heterogeneity of the shear-induced dynamics by computing activity maps (in direct space) and q-resolved dynamic susceptibility chi 4(q;dt) (in reciprocal space) to go beyond average quantities like the intermediate scattering function or the particle MSD. Results on the physics of yielding LAOS experiments with the simple yield stress fluid (Carbopol 971 P NF) provided interesting results on the physics behind the yielding transition. Through the study of Lissajous plots we have shown that in the yielding regime the mechanical response of the material is non-linear but yet harmonic, that is, accurately described in terms of phase-independent viscoelastic moduli G'' (loss modulus) and G' (storage modulus), respectively. This property allows to treat the sample in the non-linear regime as an "effective viscoelastic material", with distinct mechanical properties from the sample at rest. In other words, it should be conceptually feasible to study the linear response of the shared material to small harmonic perturbations superimposed on the LAOS, obtaining an effective frequency-dependent dynamic modulus. According to basic concepts of linear rheology, whether this effective dynamic modulus presents or not a relaxation time provides a criterion to discriminate a solid-like to a liquid-like behaviour [7]. In this context, with the additional hypothesis that the storage and loss moduli crosses each other once (if ever), we can thus interpret the crossover point where G'' = G' (occurring for c:o: = 66%, and marked by a red circle in Fig. 1.1) as the point at which the relaxation time becomes equal to the shearing period. As a consequence, under the hypothesis that the relaxation time monotonically decreases with the shear amplitude, we expect the yielding point to be located at gamma crossover. As our experimental setup did not allow a direct measurement of the linear response of the sheared material, we probed the state of the shared material via microscopy observations of the spontaneous dynamics of embedded tracer particles. EchoDDM measurements highlighted the presence of plastic dynamics: in the high-q limit we found a diffusive scaling of the relaxation rate with the wavevector, with an effective diffusion coefficient which is strongly dependent on the shear amplitude (Fig. 1.1 inset top right corner). Experiments with tracers of different sizes show that the shear-induced diffusion coefficient is inversely proportional to the particle radius, in analogy with the Stokes-Einstein relation. The validity of this scaling law is intriguing and opens to the possibility of pushing even further the analogy between a yielding material and viscoelastic material at rest. Because of limits imposed by our setup on the duration of the experiments, we are not always able to unambiguously discriminate between free and constrained diffusion, so that the presence of a diffusive dynamics on small spatial length scales should not be necessarily interpreted as a proxy for the "fluidization" of the sample. We could instead consider "fluidized" a sample where the effective diffusion coefficient D is such to allow a particle with the typical size of a microgel to displace by an amount comparable with its own size within the duration of the experiment. This operative criterion defines a threshold DT for the effective diffusion coefficient (marked by the red line in Fig. 1.1 inset top right corner). By applying this criterion we obtain gamma yielding 45%, which is within the nonlinear regime identified in LAOS experiments, but below the crossover point, consistently with the picture described before. Perspectives and open questions on yielding. The approach proposed in this work enables one overcoming some of the key experimental limitations in the study of soft yield-stress materials. For this reason it should be applied to other systems to understand the generality of the observed behaviour and possibly new aspects of the shear-induced dynamics. One of the open issues remains the exact location of the yielding threshold. Our experiments provides an upper boundary, but further investigation and longer measurements are needed to increase the precision in locating the transition point. Also, the comparison with ad hoc mechanical tests aimed at measuring the linear response to small perturbation in the non-linear regime1 will be key to validate the consistency of the proposed scenario and the identified yielding criterion. We suggest that the proposed scenario for the yielding transition in simple YSF under LAOS forcing could provide an ideal starting point also for understanding more complex non-linear and non-stationary phenomena linked to yielding. For this reason the very next steps for the follow-up to this work is the application of this method to other simple YSF. This work includes also a first body of preliminary results on a different sample: a depletion gel. In particular, we showed that we can impose a controlled homogeneous deformation profile. Unfortunately, EchoDDM experiments on this sample didn’t provide fully satisfactory results. In particular, the genuine dynamic signal originated from shear-induced plastic rearrangement in the sample was almost completely buried by the spurious effects (large-scale drifts or flows) due to the unstable confinement of the sample. Beside optimizing the experimental set-up, a possible strategy to improve the quality of the optical signal could be to study gels formed by larger colloidal particles. This should increase the "effective resolution" of the experiment. In fact, we expect the amplitude of shear-induced displacements to scale with the particle size, which is the only relevant length-scale in the system. Methodological perspectives The self-consistent approach developed for the DDM-microrheology is applicable also outside of the context of microrheology: it is a promising tool for a more accurate characterization of the colloids dynamics. Given the close analogy that we have drawn between sheared samples and viscoelastic materials, valid both at the macroscopic (as shown by Lissajous plots), and at microscopic level (as suggested by the validity of the scaling of the diffusion coefficient with the particle size) we believe that the application of microrheological methods to sheared samples could provide interesting information. In particular, we think that DDM-microrheology could be the ideal tool to extract the MSD of tracers embedded in a sheared sample For example the so-called superposition rheology [10]. and to extract via a suitable GSER an effective dynamic modulus. Another interesting development is to apply the DDM analysis to characterize the in-cycle dynamics. This is expected to be particularly interesting for intermediate shear amplitudes, i.e. in the non-linear regime, below the yielding point. Methods to decouple affine and non affine displacements have been recently proposed [1], but, to the best of our knowledge, they have not been applied yet to yielding systems.

Peripheral vascular grafts are used for the treatment of peripheral arterial disease and arteriovenous grafts for vascular access in end stage renal disease. The development of neo-intimal hyperplasia and thrombosis in the distal anastomosis remains the main reason for occlusion in that region. The local haemodynamics produced by a graft in the host vessel is believed to significantly affect endothelial function. Single spiral flow is a normal feature in medium and large sized vessels and it is induced by the anatomical structure and physiological function of the cardiovascular system. Grafts designed to generate a single spiral flow in the distal anastomosis have been introduced in clinical practice and are known as spiral grafts. In this work, spiral peripheral vascular and arteriovenous grafts were compared with conventional grafts using ultrasound and computational methods to identify their haemodynamic differences. Vascular-graft flow phantoms were developed to house the grafts in different surgical configurations. Mimicking components, with appropriate acoustic properties, were chosen to minimise ultrasound beam refraction and distortion. A dual-beam two-dimensional vector Doppler technique was developed to visualise and quantify vortical structures downstream of each graft outflow in the cross-flow direction. Vorticity mapping and measurements of circulation were acquired based on the vector Doppler data. The flow within the vascular-graft models was simulated with computed tomography based image-guided modelling for further understanding of secondary flow motions and comparison with the experimental results. The computational assessments provided a three-dimensional velocity field in the lumen of the models allowing a range of fluid dynamic parameters to be predicted. Single- or double-spiral flow patterns consisting of a dominant and a smaller vortex were detected in the outflow of the spiral grafts. A double- triple- or tetra-spiral flow pattern was found in the outflow of the conventional graft, depending on model configuration and Reynolds number. These multiple-spiral patterns were associated with increased flow stagnation, separation and instability, which are known to be detrimental for endothelial behaviour. Increased in-plane mixing and wall shear stress, which are considered atheroprotective in normal vessels, were found in the outflow of the spiral devices. The results from the experimental approach were in agreement with those from the computational approach. This study applied ultrasound and computational methods to vascular-graft phantoms in order to characterise the flow field induced by spiral and conventional peripheral vascular and arteriovenous grafts. The results suggest that spiral grafts are associated with advanced local haemodynamics that may protect endothelial function and thereby may prevent their outflow anastomosis from neo-intimal hyperplasia and thrombosis. Consequently this work supports the hypothesis that spiral grafts may decrease outflow stenosis and hence improve patency rates in patients.

Ce travail de thèse cherche à mieux caractériser le comportement des matériaux hétérogènes sous cisaillement avec une approche multi-échelles (macro-méso-microscopique).Cela est rendu possible en développant un montage innovant qui couple un rhéomètre à un système d’imagerie de speckle résolue spatialement et temporellement (RheoSpeckle). Nous montrons la validation de notre expérience en l’appliquant sur deux matériaux parfaits : un solide et un liquide. Sur le solide, on mesure le champ de déplacement sur les images de speckle avec une résolution meilleure que 1 µm. Puis on prouve l’élasticité du matériau à l’échelle microscopique. Sur le liquide, la taille des nanoparticules est déterminée avec un excellent accord avec la spécification du fabriquant. Le champ de vitesse dans l’entrefer du Couette est calculé avec une bonne précision sur un temps inférieur à 1 s et avec une résolution spatiale de 100 µm sur 5mm. La dynamique microscopique d’une solution brownienne est étudiée et l’influence du cisaillement sur la décorrélation est déterminée. Nous montrons les capacités de notre expérience à étudier des matériaux hétérogènes en l’appliquant sur une solution concentrée de micelles géantes. La rhéologie linéaire est étudiée en rhéometrie classique mais aussi en utilisant l’imagerie du speckle. La rhéologie non linéaire de ce matériau est déterminé en rhéometrie (macro) mais aussi en calculant le champ de vitesse et l’intensité des images de speckle (méso) ou on caractérise les bandes de cisaillement qui se forment à partir d’un cisaillement critique. En fin la relaxation spatio-temporelle des bandes de cisaillement (micro) est caractérisée. On observe pour la première fois l’existence de deux temps de relaxation après l’arrêt du cisaillement et que la relaxation des bandes est relativement lente
This work tries to better characterize the behavior of homogeneous and heterogeneous materials under shear with a multi-scale approach (macro-meso-micro-scopic). To do that, we have developed an innovative setup by coupling a rheometer to a speckle imaging geometry witch is spatially and temporally resolved (RheoSpeckle). We validate our experience using two perfect materials: a solid and a pure viscous fluid. On a solid sample, we calculate the displacement field on the speckle images with a resolution better than 1 µm. we demonstrate than, the microscopic elasticity of this material. On a pure viscous fluid, we measured the nanoparticle’s size with excellent accuracy. When a constant shear rate is applied, the velocity profile is measured with a time less than 1 s with a spatial resolution of 100 µm over 5 mm. The microscopic dynamic of a Brownian solution under shear is probed and the shear induced on the decorrelation of the intensity correlation function is studied. We show the capabilities of our experience using a concentrated solution of wormlike micelles. The linear rheology is studied using rheometric measurements and our speckle imaging system. Nonlinear rheology is studied using rheometric measurements (macro), but also by calculating the velocity filed and the intensity of speckle images (meso). With mesocopics measurements, the formation of shear banding is proved and characterized. Finally, the spatio-temporal relaxation (micro) of shear bands of this material is studied. We show for the first time the existence of two relaxations times after shear and that the relaxation of bands is relatively slow

碩士
國立交通大學
土木工程學系
99
River discharge in Taiwan varies a lot during flooding season; this situation often results in unstable river channel. The intensive erosion of bedrock during flood may also endanger the stability of cross-river structures, especially for cases of river bed composed of soft rocks. This study makes use of numerical simulation as “virtual erosion test” to explore the mechanisms of rock erosion. In the simulation, rock material is modeled as a granular assemblage with inter-particle bonding; the erosion process is simulated as particles’ release due to de-bonding. Virtual rock specimen are subjected to boundary loads from either bed shear stress or saltating particle to simulates physical erosion experiments. The purpose of this thesis is to investigate the important factors that may control the erodibility of soft rock subjected to bed shear stress and particle saltation; also, the dissipated energy and bonding failure associated with the occurrence of rock erosion are studied. For erosion due to bed shear stress, simulated results show that the number of de-bonded particles raises as the bed shear stress increases. It is found the shear stress required for the initiation of a de-bonded particle in fresh soft rock material is significantly higher than the typical in-situ bed shear stress that may occur in a flood. It appears clear water current alone may hardly erode a fresh rock material. However, it is common to see the degradation or weathering of soft rock exposed to the periodical variation of water level (i.e., subjected to drying-wetting cycles). These weakening processes are likely to cause the rock material become more erodible. Saltating abrasion can be a consequence of impacts of gravels traveling along with water flow; the impact results in the local failure and causes abrasion of river-bed rock material. The major factors affecting the erodibility and erosion rate are examined through a series of virtual erosion tests. Simulated results show that more de-bonded particles may occur for a condition with higher impact speed, higher impact angle, larger gravel size, higher Young’s modulus, or lower rock strength. Besides, a good correlation relationship between the number of de-bonded particles and the accumulated dissipated energy is notable.

碩士
長庚大學
光電工程研究所
102
In this thesis, we established a real time interferometric differential-phase measurement platform, in order to analyze thermal induced shear wave on the interface of air and soft material. From theory of harmonic shear wave equation deduced by Stress and strain in soft material, the relation between rate of phase change or oscillation frequency regarding of harmonic shear wave and its elasticity coefficient in shear is developed. Hence, this set up is able to measure the oscillation frequency of harmonic shear wave of soft material in real time. This results in a time dependent elasticity of tested soft material which can be measured precisely and non-invasively. In the future, this proposed method can be applied in biomedical applications such as non-invasive cancer cell diagnosis in terms of variation of cell membrane elasticity in hospital.

碩士
輔仁大學
化學系
107
P3HT/PCBM blend films have been used in the photoactive layer of polymer solar cells. In the blend film, bulk-heterojuction (BHJ) structure can enhance contact area between materials and facilitate the exciton dissociation. In addition, controlling molecular aggregation of conjugating polymer has been a critical issue for polymer solar cells. Higher crystalline of P3HT is benificial to absorption spectra and carrier mobility. Thermal annealing has been used to improve crystalline of P3HT in many references. However, thermal annealing results in phase separation due to poor compatibility. Here, the new processes are applied to improve crystalline of polymer prior to coating process and decrease probability of phase separation so that minimum/or no post-treatment .With respect to materials, in high PCE polymer solar cells, low band-gap conjugated copolymer has been widely used to enhance absorption spectra recently, however, they are not often crystallizable because of different monomers used in the polymer backbones. Therefore, this study will apply the new processes to conjugated copolymer (pBCN). Part one, synergistic effect of dynamic cooling/freeze drying process is applied to pBCN/PCBM blend to enhance aggregation of pBCN and decrease agglomeration of PCBM. The dynamic-cooling process allows pBCN molecules to aggregate in solution into a more organized structure during the cooling process; the freeze-drying process prevents severe agglomeration of PCBM during the solvent removing process. To improve stability of blend films, we add additive (bis-PCBM) to decrease agglomeration of PCBM after thermal annealing. Part two, a shear–induced-crystallization (SIC) process is applied to the polymer solution prior to coating process. Experimental results indicate that after applying SIC process to a crystallizable polymer, pBCN, aggregation of pBCN is enhanced than that from spin-coating process. Additionally, film absorption study shows that aggregation of pBCN does not affected by addition of PCBM, which makes the SIC process feasible for the fabrication of polymer solar cells.

In this article, we review the basic physics of viscoelastic phase separation including fracture phase separation. We show that with an increase in the ratio of the deformation rate of phase separation to the slowest mechanical relaxation rate the type of phase separation changes from fluid phase separation, to viscoelastic phase separation, to fracture phase separation. We point out that there is a physical analogy of this to the transition of the mechanical fracture behaviour of materials under shear from liquid-type, to ductile, to brittle fracture. This allows us to discuss phase separation and shear-induced instability of disordered materials including soft matter, on the same physical ground. Finally it should be noted that what we are going to describe in this article has not necessarily been firmly established and there still remain many open problems to be studied in the future.

Drawbacks persist relating to irreversibility of leaflet resection, time-consuming leaflet reconstruction with sliding annuloplasty, monoleaflet function, and systolic anterior motion (SAM) risk. Graded neochordal reconstruction mitigates many of these but has the challenge of precise sizing and possibility of leaving excessive tissue, risking SAM. When this reconstruction is based on stress analysis and shear analysis methods the outcome gives the best results. Short term evaluation has been done with good outcomes.

ABSTRACT During hypervelocity impacts, target rocks are subjected to shock wave compression with high pressures and differential stresses. These differential stresses cause microscopic shear-induced deformation, which can be observed in the form of kinking, twinning, fracturing, and shear faulting in a range of minerals. The orientation of these shear-induced deformation features can be used to constrain the maximum shortening axis. Under the assumption of pure shear deformation, the maximum shortening axis is parallel to the maximum principal axis of stress, σ1, which gives the propagation direction of the shock wave that passed through a rock sample. In this study, shocked granitoids cored from the uppermost peak ring of the Chicxulub crater (International Ocean Discovery Program [IODP]/International Continental Drilling Project [ICDP] Expedition 364) were examined for structures formed by shearing. Orientations of kink planes in biotite and basal planar deformation features (PDFs) in quartz were measured with a U-stage and compared to a previous study of feather feature orientations in quartz from the same samples. In all three cases, the orientations of the shortening axis derived from these measurements were in good agreement with each other, indicating that the shear deformation features all formed in an environment with similar orientations of the maximum principal axis of stress. These structures formed by shearing are useful tools that can aid in understanding the deformational effects of the shock wave, as well as constraining shock wave propagation and postshock deformation during the cratering process.

Abstract In the present work, we experimentally and numerically investigated the dynamics of a millimeter-sized cavitation bubble generated nearby a solid surface. In experiments, bubbles are induced by a focused Nd-YAG laser generating plasma. A specimen of the commercially pure aluminum surface was placed nearby a bubble at varying relative wall distances. Here, the relative wall distance is a ratio of the distance between the bubble center and specimen surface, and the maximum radius of the bubble. In experiments, we captured the bubble’s dynamics by back illumination method using a highspeed camera. Damage obtained was characterized by an optical microscope and profilometer. The surface profiles and damage patterns quantified the damage characteristics. The three-dimensional flow was captured numerically by solving the Navier-Stokes equations in an Euler-Euler approach with barotropic equations of state. The computations were performed assuming both water and vapor as compressible phases. The dynamics of a single bubble obtained in computations were compared with the experiments for shapes and collapsing times. The computed characteristics of flow around a bubble near the solid surface, e.g. impact velocities and pressures were also discussed. Additionally, the dynamics of a microscopic bubble collapse near the surface was also investigated to compute collapse-induced wall shear rate and flow around the collapsing bubble. The results of numerical simulations were compared with the existing experimental data. The comparisons showed, a good qualitative and quantitative agreement. Overall, the numerical method well reflected the dynamics bubble up to three collapses and resolved flow around the bubble. The statistical data of pits obtained are also useful in deriving loads induced by a single bubble collapse. Overall, this work extensively comprises the single cavitation bubble dynamics and induced damage. This article summarizes the investigations of Sagar (2018) and Sagar & el Moctar (2020).

Focal adhesions (FAs) and their associated integrins are thought to act as mechanosensors and transducers of shear stress into intracellular biochemical signals. However, to date there exists no quantification of the magnitude of forces generated at integrin molecules in response to apically-applied fluid shear stress. Thus, we used finite element analysis of fluid dynamics and cellular stresses to compute FA stresses from solid models of focally-adhered endothelial cells. These models were developed from quantitative 3-D microscopy and total internal reflection fluorescence (TIRF) microscopy of calcein-stained endothelial cells. Extrusion coupling variables mapped stresses from the macroscale cell model to individual microscale 3-D models of FAs determined from quantitative TIRF. Included in the microscale FA model were moduli for subcellular matrix (SCM) (e.g. hyaluronan, hyaluronaic acid and other glycocalyx constituents) and extracellular matrix (ECM) (e.g. collagen, fibronectin). Integrin forces were estimated from assumed bonds densities and computed FA stresses. Maximal bond tension obtained from the simulation for a single integrin-extracellular matrix (ECM) bond was .1pN. Thus, it is unlikely that integrin-ECM bonds or chemical activities are appreciably affected by shear stress. The computational model, however, supports an alternative model of activation in and reorganization of FAs in which shear stress-induced forces cause the membrane to bend toward and away from the ECM immediately upstream and downstream of the FA, respectively. The simulation also suggests that the elasticity of the SCM plays an important role in modulating shear-induced FA reorganization. These results support a new model of endothelial cell activation by shear stress in which integrins and FAs participate in the directional biasing of force-induce signaling but do not initiate it.

Abstract Wall shear stress induced by blood flow affects morphology and physiology of endothelial cells. The specific mechanisms alternating shapes and functions have not been identified in endothelial cells, because of the lack of a detailed description of the flow near the cell surface. To clarify the mechanism, an analysis of the flow on subcellular scale is required. We therefore developed velocimetry using expanded cell model (Fukushima et al. 1997). The method, however, needs some improvements in order to discuss wall shear stress distribution on subcellular scale. In this paper we deal with determination of three-dimensional velocity field and wall shear stress distribution and verify the reliability by comparing experimental results with theoretical solution.

Abstract Wall shear stress induced by blood flow affects morphology and physiology of endothelial cells. The specific mechanisms of mechano-biological interactions alternating shapes and functions have not been identified in the cells, because of the lack of a detailed description of microscopic flow near the cell surface. We therefore developed velocimetry using expanded cell model and demonstrated that the microscopic flow depended on three-dimensional cell shape (Fukushima et al., 1997, 1998). Furthermore, we determined wall shear stress distribution on cultured endothelial cells experimentally.

Dorsal surfaces and upstream regions around ostia of aortic branches are favored sites of atherosclerosis. Both asymmetrical stresses in branch walls and disturbed flow patterns have been suggested as contributing to this localization. In the present study, fluorescence images of the thoracic aortic tree of C57 mice were obtained using quantum dot (Qdot) bioconjugate markers for vascular cell adhesion molecule-1 (VCAM-1) and two-photon excitation laser scanning microscopy. The images show that dorsal surfaces and upstream regions of intercostal ostia have a higher intensity of VCAM-1 than the downstream region. We also investigated blood flow patterns and wall shear stress (WSS) in the descending aorta and proximal intercostal branches of C57 mice using micro-CT imaging and ultrasound velocity measurements, combined with computational fluid dynamics (CFD). The latter investigation showed that dynamical wall deformation caused by pulsatile pressure around the ostia induces blood flow patterns which create lower and oscillating WSS in the upstream region and dorsal surface than in the distal region. Comparisons of the Qdot marker and CFD studies demonstrate that the distribution of greater expression of VCAM-1 corresponds with lower and oscillating WSS around the branch ostia. Thus, local wall deformation may contribute to disturbed flow patterns that are known to be associated with increased VCAM-1 expression.

Implantable blood recirculation devices such as ventricular assist devices (VADs) and more recently the temporary total artificial heart (TAH-t) are promising bridge-to-transplant (BTT) solutions for patients with end-stage cardiovascular disease. However, blood flow in and around certain non-physiological geometries, mostly associated with pathological flow around mechanical heart valves (MHVs) of these devices, enhances shear stress-induced platelet activation, thereby significantly promoting flow induced thrombogenicity and subsequent complications such as stroke, despite a regimen of post-implant antithrombotic agents. Careful characterization of such localized high shear stress trajectories in these devices by numerical techniques and corresponding experimental measurements of their accentuated effects on platelet activation and sensitization, is therefore critical for effective design optimization of these devices (reducing the occurrence of pathological flow patterns formation) for minimizing thrombogenicity [1].

Abstract A plethora of studies have investigated the motion of a liquid droplet in the vicinity of a smooth surface, incurred by shear flow, parabolic flow or gravity. However, there are few studies that consider the roughness of the surface that could affect the droplet motion. In this study, we employ a 3D spectral boundary element method for interfacial dynamics to examine the droplet translation, migration, and deformation in the vicinity of a rough surface due to shear flow. The roughness feature of the surface is comparable to the size of the droplet and is simulated with sinusoidal functions. Topologies of epoxy coating surfaces are also considered in the computations. The roughness and profile of the coating surface is obtained by atomic force microscopy. The computational results show that the surface roughness affects significantly the behavior of a deformable droplet near the surface, including its deformation and migration speed. In return, the dynamics of the droplet also influences the stress distribution on the rough surface. The results of this study could provide theoretical foundation in the prediction of particle induced erosion corrosion of organic coatings.

Blood recirculating devices, which include ventricular assist devices and prosthetic heart valves, are necessary for some patients suffering from end-stage heart failure and valvular diseases. However, disturbed flow patterns in these devices cause shear-induced platelet activation and aggregation. Thromboembolic complications resulting from this platelet behavior necessitates lifelong anticoagulant therapy for patients implanted with such devices. In addition, blood recirculating device manufacturers mostly test and optimize their products for hemolysis, which occurs at shear stresses ten-fold higher than required for platelet activation. The relative paucity of optimization for flow-induced thrombogenicity is further exacerbated by the fact that there are few predictive shear-induced platelet activation models.