We numerically investigated the transient dynamics of a passive colloidal particle in a periodic array of planar counter-rotating convection rolls at high Péclet numbers. We discovered that the distributions of first-exit times out of a single convection roll exhibit distinct regimes: an exponential tail, due to the noise-assisted diffusion of the trapped particle from the center toward the edge of the roll, and a peak structure emerging from a power-law background, associated with the transverse and longitudinal Brownian diffusion of the particle within the rolls’ flow boundary layers. These results are interpreted analytically and related to the earlier literature on the topic.

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.

It is often desirable to know the controlling mechanism of the survival probability of nano- or microscale particles in small cavities such as, e.g., confined submicron particles in fiber beds of high-efficiency filter media or ions/small molecules in confined cellular structures. Here, we address this issue based on a numerical study of the escape kinetics of inertial Brownian colloidal particles from various types of cavities with single and multiple pores. We consider both the situations of strong and weak viscous damping. Our simulation results show that as long as the thermal length is larger than the cavity size, the mean exit time remains insensitive to the medium viscous damping. On further increasing damping strength, a linear relation between the escape rate and damping strength emerges gradually. This result is in sharp contrast to the energy barrier crossing dynamics where the escape rate exhibits a turnover behavior as a function of the damping strength. Moreover, in the ballistic regime, the exit rate is directly proportional to the pore width and the thermal velocity. All these attributes are insensitive to the cavity as well as the pore structures. Further, we show that the effects of pore structure variation on the escape kinetics are conspicuously different in the low damping regimes compared to the overdamped situation. Apart from direct applications in biology and nanotechnology, our simulation results can potentially be used to understand diffusion of living or artificial micro/nano-objects, such as bacteria, virus, Janus particles, etc., where memory effects play dictating roles.

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.

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.