Driven transport of active particles through arrays of symmetric obstacles.

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.

Directed Autonomous Motion and Chiral Separation of Self-Propelled Janus Particles in Convection Roll Arrays.

Self-propelled Janus particles exhibit autonomous motion thanks to engines of their own. However, due to the randomly changing direction of such motion they are of little use for emerging nanotechnological and biomedical applications. Here, we numerically show that the motion of chiral active Janus particles can be directed, subjecting them to a linear array of convection rolls. The rectification power of self-propulsion motion here can be made to be more than 60%, which is much larger than earlier reports. We show that rectification of a chiral Janus particle's motion leads to conspicuous segregation of dextrogyre and levogyre active particles from a racemic binary mixture. Further, we demonstrate how efficiently the rectification effect can be exploited to separate dextrogyre and levogyre particles when their intrinsic torques are distributed with Gaussian statistics.

Escape kinetics of self-propelled particles from a circular cavity.

We numerically investigate the mean exit time of an inertial active Brownian particle from a circular cavity with single or multiple exit windows. Our simulation results witness distinct escape mechanisms depending on the relative amplitudes of the thermal length and self-propulsion length compared to the cavity and pore sizes. For exceedingly large self-propulsion lengths, overdamped active particles diffuse on the cavity surface, and rotational dynamics solely governs the exit process. On the other hand, the escape kinetics of a very weakly damped active particle is largely dictated by bouncing effects on the cavity walls irrespective of the amplitude of self-propulsion persistence lengths. We show that the exit rate can be maximized for an optimal self-propulsion persistence length, which depends on the damping strength, self-propulsion velocity, and cavity size. However, the optimal persistence length is insensitive to the opening windows' size, number, and arrangement. Numerical results have been interpreted analytically based on qualitative arguments. The present analysis aims at understanding the transport controlling mechanism of active matter in confined structures.

Advection-enhanced diffusion in biased convection arrays.

We numerically investigated the transport of a passive colloidal particle in a one-dimensional periodic array of planar counter-rotating convection rolls at high Péclet numbers. We show that advection-enhanced diffusion is drastically suppressed by an external transverse bias but strongly reinforced by a longitudinal drive of appropriate intensity. Both effects are magnified by imposing free-slip flows at the array's edges. The dependence of the diffusion constant on an external forcing is interpreted as a measure of the fluid-mechanical robustness of the flow boundary layer mechanism governing diffusion in convection rolls.

Active diffusion limited reactions.

We investigate the one- and two-dimensional diffusion limited reactions A + A → 0 and A + B → 0 with A active Janus particles and B passive particles in thermal equilibrium. We show that by increasing the self-propulsion time of the A particles, the reactant densities decay faster, at least for time transients of potential interest for chemical applications, e.g., to develop smart drug delivery protocols. Asymptotic and transient density decays obey power laws with exponents that depend on the actual annihilation reaction and its dimensionality.

Active microswimmers in a finite two dimensional trap: The role of hydrodynamic interaction.

We investigate the dynamics of two identical artificial active particles suspended in a free-standing fluid film with a trap of finite radius in an acoustic tweezer. In the two dimensional Oseen approximation, their hydrodynamic coupling is long ranged, which naturally raises the question as under what conditions they can simultaneously reside in the trap. We determine a critical value of the hydrodynamic coupling below which that happens and study the ensuing active pair dynamics inside the trap. For larger couplings, only one particle sits in the trap, while the other diffuses freely until it eventually replaces the particle in the trap. Such a mechanism repeats itself with a characteristic noise-dependent mean residence-retrapping time.

Activated barrier crossing dynamics of a Janus particle carrying cargo.

We numerically study the escape kinetics of a self-propelled Janus particle, carrying a cargo, from a meta-stable state. We assume that the cargo is attached to the Janus particle by a flexible harmonic spring. We take into account the effect of the velocity field created in the fluid due to movements of the dimer's components, by considering a space-dependent diffusion tensor (Oseen tensor). Our simulation results show that the synchronization between barrier crossing events and the rotational relaxation process can enhance the escape rate to a large extent. Also, the load carrying capability of a Janus particle is largely controlled by its rotational dynamics and self-propulsion velocity. Moreover, the hydrodynamic interaction, conspicuously, enhances the escape rate of the Janus-cargo dimer. The most important features in escape kinetics have been justified based on analytic arguments.

Hydrodynamic interaction of trapped active Janus particles in two dimensions.

The dynamics of a pair of identical artificial microswimmers bound inside two harmonic traps, in a thin sheared fluid film, is numerically investigated. In a two-dimensional Oseen approximation, the hydrodynamic pair coupling is long-ranged and proportional to the particle radius to film thickness ratio. On increasing such ratio above a certain threshold, a transition occurs between a free regime, where each swimmer orbits in its own trap with random phase, and a strong synchronization regime, where the two swimmers strongly repel each other to an average distance larger than both the trap distance and their free orbit diameter. Moreover, the swimmers tend to synchronize their positions opposite the center of the system.

Diffusion of active dimers in a Couette flow.

We study the 3D dynamics of an elastic dimer consisting of an active swimmer bound to a passive cargo, both suspended in a Couette flow. Using numerical simulations, we determine the diffusivity of such an active dimer in the presence of long-range hydrodynamic interactions for different values of its self-propulsion speed and the Couette flow. We observe that the effect of hydrodynamic interactions is greatly enhanced under the condition that self-propulsion is strong enough to contrast the shear flow. The magnitude of the effect grows with the size of the dimer's constituents relative to their distance, which makes it appreciable under experimental conditions.

Two-dimensional dynamics of a trapped active Brownian particle in a shear flow.

We model the two-dimensional dynamics of a pointlike artificial microswimmer diffusing in a harmonic trap subject to the shear flow of a highly viscous medium. The particle is driven simultaneously by the linear restoring force of the trap, the drag force exerted by the flow, and the torque due to the shear gradient. For a Couette flow, elliptical orbits in the noiseless regime, and the correlation functions between the particle's displacements parallel and orthogonal to the flow are computed analytically. The effects of thermal fluctuations (translational) and self-propulsion fluctuations (angular) are treated separately. Finally, we discuss how to extend our approach to the diffusion of a microswimmer in a Poiseuille flow. These results provide an accurate reference solution to investigate, both numerically and experimentally, hydrodynamics corrections to the diffusion of active matter in confined geometries.