10 research outputs found
Transport of a dilute active suspension in pressure-driven channel flow
Confined suspensions of active particles show peculiar dynamics characterized
by wall accumulation, as well as upstream swimming, centerline depletion and
shear-trapping when a pressure-driven flow is imposed. We use theory and
numerical simulations to investigate the effects of confinement and non-uniform
shear on the dynamics of a dilute suspension of Brownian active swimmers by
incorporating a detailed treatment of boundary conditions within a simple
kinetic model where the configuration of the suspension is described using a
conservation equation for the probability distribution function of particle
positions and orientations, and where particle-particle and particle-wall
hydrodynamic interactions are neglected. Based on this model, we first
investigate the effects of confinement in the absence of flow, in which case
the dynamics is governed by a swimming Peclet number, or ratio of the
persistence length of particle trajectories over the channel width, and a
second swimmer-specific parameter whose inverse measures the strength of
propulsion. In the limit of weak and strong propulsion, asymptotic expressions
for the full distribution function are derived. For finite propulsion,
analytical expressions for the concentration and polarization profiles are also
obtained using a truncated moment expansion of the distribution function. In
agreement with experimental observations, the existence of a
concentration/polarization boundary layer in wide channels is reported and
characterized, suggesting that wall accumulation in active suspensions is
primarily a kinematic effect which does not require hydrodynamic interactions.
Next, we show that application of a pressure-driven Poiseuille flow leads to
net upstream swimming of the particles relative to the flow, and an analytical
expression for the mean upstream velocity is derived in the weak flow limit. In
stronger imposed flows .....
On the distribution and swim pressure of run-and-tumble particles in confinement
The spatial and orientational distribution in a dilute active suspension of
non-Brownian run-and-tumble spherical swimmers confined between two planar hard
walls is calculated theoretically. Using a kinetic model based on coupled
bulk/surface probability density functions, we demonstrate the existence of a
concentration wall boundary layer with thickness scaling with the run length,
the absence of polarization throughout the channel, and the presence of sharp
discontinuities in the bulk orientation distribution in the neighborhood of
orientations parallel to the wall in the near-wall region. Our model is also
applied to calculate the swim pressure in the system, which approaches the
previously proposed ideal-gas behavior in wide channels but is found to
decrease in narrow channels as a result of confinement. Monte-Carlo simulations
are also performed for validation and show excellent quantitative agreement
with our theoretical predictions
Vapor-Driven Propulsion of Catalytic Micromotors
Chemically-powered micromotors offer exciting opportunities in diverse fields, including therapeutic delivery, environmental remediation, and nanoscale manufacturing. However, these nanovehicles require direct addition of high concentration of chemical fuel to the motor solution for their propulsion. We report the efficient vapor-powered propulsion of catalytic micromotors without direct addition of fuel to the micromotor solution. Diffusion of hydrazine vapor from the surrounding atmosphere into the sample solution is instead used to trigger rapid movement of iridium-gold Janus microsphere motors. Such operation creates a new type of remotely-triggered and powered catalytic micro/nanomotors that are responsive to their surrounding environment. This new propulsion mechanism is accompanied by unique phenomena, such as the distinct off-on response to the presence of fuel in the surrounding atmosphere, and spatio-temporal dependence of the motor speed borne out of the concentration gradient evolution within the motor solution. The relationship between the motor speed and the variables affecting the fuel concentration distribution is examined using a theoretical model for hydrazine transport, which is in turn used to explain the observed phenomena. The vapor-powered catalytic micro/nanomotors offer new opportunities in gas sensing, threat detection, and environmental monitoring, and open the door for a new class of environmentally-triggered micromotors
Motion-based threat detection using microrods: experiments and numerical simulations
Motion-based chemical sensing using microscale particles has attracted considerable recent attention. In this paper, we report on new experiments and Brownian dynamics simulations that cast light on the dynamics of both passive and active microrods (gold wires and gold–platinum micromotors) in a silver ion gradient. We demonstrate that such microrods can be used for threat detection in the form of a silver ion source, allowing for the determination of both the location of the source and concentration of silver. This threat detection strategy relies on the diffusiophoretic motion of both passive and active microrods in the ionic gradient and on the speed acceleration of the Au–Pt micromotors in the presence of silver ions. A Langevin model describing the microrod dynamics and accounting for all of these effects is presented, and key model parameters are extracted from the experimental data, thereby providing a reliable estimate for the full spatiotemporal distribution of the silver ions in the vicinity of the source
Vapor-Driven Propulsion of Catalytic Micromotors
Chemically-powered micromotors offer exciting opportunities in diverse fields, including therapeutic delivery, environmental remediation, and nanoscale manufacturing. However, these nanovehicles require direct addition of high concentration of chemical fuel to the motor solution for their propulsion. We report the efficient vapor-powered propulsion of catalytic micromotors without direct addition of fuel to the micromotor solution. Diffusion of hydrazine vapor from the surrounding atmosphere into the sample solution is instead used to trigger rapid movement of iridium-gold Janus microsphere motors. Such operation creates a new type of remotely-triggered and powered catalytic micro/nanomotors that are responsive to their surrounding environment. This new propulsion mechanism is accompanied by unique phenomena, such as the distinct off-on response to the presence of fuel in the surrounding atmosphere, and spatio-temporal dependence of the motor speed borne out of the concentration gradient evolution within the motor solution. The relationship between the motor speed and the variables affecting the fuel concentration distribution is examined using a theoretical model for hydrazine transport, which is in turn used to explain the observed phenomena. The vapor-powered catalytic micro/nanomotors offer new opportunities in gas sensing, threat detection, and environmental monitoring, and open the door for a new class of environmentally-triggered micromotors
Three-dimensional kinetic simulations of active suspensions: effect of chemotaxis and steric interaction
In this work, the effects of chemotaxis and steric interactions in active suspensions are analyzed by extending the kinetic model proposed by Saintillan and Shelley [1, 2]. In this model, a conservation equation for the active particle configuration is coupled to the Stokes equation for the flow arising from the force dipole exerted by the particles on the fluid. The fluid flow equations are solved spectrally and the conservation equation is solved by second-order finite differencing in space and second-order Adams-Bashforth time marching.
First, the dynamics in suspensions of oxytactic run-and-tumble bacteria confined in thin liquid films surrounded by air is investigated. These bacteria modify their tumbling behavior by making temporal comparisons of the oxygen concentration, and, on average, swim towards high concentrations of oxygen. The kinetic model proposed by Saintillan and Shelley [1, 2] is modified to include run-and-tumble effects and oxygentaxis. The spatio-temporal dynamics of the oxygen and bacterial concentration are analyzed. For small film thicknesses, there is a weak migration of bacteria to the boundaries, and the oxygen concentration is high inside the film as a result of diffusion; both bacterial and oxygen concentrations quickly reach steady states. Above a critical film thickness (approximately 200 micron), a transition to chaotic dynamics is observed and is characterized by turbulent-like 3D motion, the formation of bacterial plumes, enhanced oxygen mixing and transport into the film, and hydrodynamic velocities of magnitudes up to 7 times the single bacterial swimming speed. The simulations demonstrate that the combined effects of hydrodynamic interactions and oxygentaxis create collective three-dimensional instabilities which enhances oxygen availability for the bacteria. Our simulation results are consistent with the experimental findings of Sokolov et al. [3], who also observed a similar transition with increasing film thickness.
Next, the dynamics in concentrated suspensions of active self-propelled particles in a 3D periodic domain are analyzed. We modify the kinetic model of Saintillan and Shelley [1, 2] by including an additional nematic alignment torque proportional to the local concentration in the equation for the rotational velocity of the particles, causing them to align locally with their neighbors (Doi and Edwards [4]). Large-scale three- dimensional simulations show that, in the presence of such a torque both pusher and puller suspensions are unstable to random fluctuations and are characterized by highly nematic structures. Detailed measures are defined to quantify the degree and direction of alignment, and the effects of steric interactions on pattern formation will be presented. Our analysis shows that steric interactions have a destabilizing effect in active suspensions
Fluid Dynamics of Active Suspensions: The Effects of Interparticle Interactions, External Fields and Confinement
This thesis is devoted to understanding the effect of inter-particle interactions, external fields and confinement in active suspensions. Active suspensions, such as a bath of swimming micro-organisms, have microstructural elements which are motile and exert active stresses on the suspending fluid. The internally generated stresses in active suspensions lead to an intrinsic coupling between the swimmer configurations and the immersing fluid.The first theme of the thesis focuses on hydrodynamically driven self-organization in active suspensions. We first study the dynamics of concentrated active suspensions in a 3D periodic domain using a coupled Smoluchowski-Stokes kinetic model and discover novel instabilities for both rear-actuated (pusher) and front-actuated (puller) swimmers, characterized by giant number density fluctuations, due to the coupled effects of hydrodynamic and steric interactions. Next, we incorporate chemotactic run-and-tumble effects in the kinetic model to study the dynamics in thin films of aerotactic bacteria. A transition to chaotic dynamics beyond a critical film thickness is reported, in agreement with experiments, and shown to be a consequence of the coupling between aerotactic response of bacteria and hydrodynamic disturbance flows. The second theme focuses on the sole interplay between motility and confinement in dilute suspensions, ignoring the effect of inter-particle interactions. First, we investigate the dynamics of a confined suspension of Brownian swimmers using a simple kinetic model by prescribing a no-flux condition on the probability distribution function of particle configurations and explain several peculiar dynamics reported in experiments, viz., wall accumulation, as well as upstream swimming, centerline depletion and shear-trapping when a pressure-driven flow is imposed. Next, we calculate the swim pressure of non-Brownian run-and-tumble spherical swimmers using a kinetic model based on coupled bulk/surface probability density functions.The third theme focuses on the effect of confinement on active self-organization. We discover a symmetry-breaking phase-transition to a spontaneous flowing state with net fluid pumping beyond a critical concentration in a strongly confined channel. The framework for studying confined active suspensions is also extended to explore geometric control of active self-organization in circular and other complex domains