Clustering of chemically propelled nanomotors in chemically active environments.

Synthetic nanomotors powered by chemical reactions have been designed to act as vehicles for active cargo transport, drug delivery, and a variety of other uses. Collections of such motors, acting in consort, can self-assemble to form swarms or clusters, providing opportunities for applications on various length scales. While such collective behavior has been studied when the motors move in a chemically inactive fluid environment, when the medium in which they move is a chemical network that supports complex spatial and temporal patterns, through simulation and theoretical analysis we show that collective behavior changes. Spatial patterns in the environment can guide and control motor collective states, and interactions of the motors with their environment can give rise to distinctive spatiotemporal motor patterns. The results are illustrated by studies of the motor dynamics in systems that support Turing patterns and spiral waves. This work is relevant for potential applications that involve many active nanomotors moving in complex chemical or biological environments.

Inertial effects on rectification and diffusion of active Brownian particles in an asymmetric channel.

Micro- and nano-swimmers, moving in a fluid solvent confined by structures that produce entropic barriers, are often described by overdamped active Brownian particle dynamics, where viscous effects are large and inertia plays no role. However, inertial effects should be considered for confined swimmers moving in media where viscous effects are no longer dominant. Here, we study how inertia affects the rectification and diffusion of self-propelled particles in a two-dimensional, asymmetric channel. We show that most of the particles accumulate at the channel walls as the masses of the particles increase. Furthermore, the average particle velocity has a maximum as a function of the mass, indicating that particles with an optimal mass Mop * can be sorted from a mixture with particles of other masses. In particular, we find that the effective diffusion coefficient exhibits an enhanced diffusion peak as a function of the mass, which is a signature of the accumulation of most of the particles at the channel walls. The dependence of Mop * on the rotational diffusion rate, self-propulsion force, aspect ratio of the channel, and active torque is also determined. The results of this study could stimulate the development of strategies for controlling the diffusion of self-propelled particles in entropic ratchet systems.

Diffusion of chiral active particles in a Poiseuille flow.

We study the diffusive behavior of chiral active (self-propelled) Brownian particles in a two-dimensional microchannel with a Poiseuille flow. Using numerical simulations, we show that the behavior of the transport coefficients of particles, for example, the average velocity v and the effective diffusion coefficient D_{eff}, strongly depends on flow strength u_{0}, translational diffusion constant D_{0}, rotational diffusion rate D_{θ}, and chirality of the active particles Ω. It is demonstrated that the particles can exhibit upstream drift, resulting in a negative v, for the optimal parameter values of u_{0}, D_{θ}, and Ω. Interestingly, the direction of v can be controlled by tuning these parameters. We observe that for some optimal values of u_{0} and Ω, the chiral particles aggregate near a channel wall and the corresponding D_{eff} are enhanced. However, for the nonchiral particles (Ω=0), D_{eff} is suppressed by the presence of Poiseuille flow. It is expected that these findings have a great potential for developing microfluidic and lab-on-a-chip devices for separating the active particles.

Mass separation in an asymmetric channel.

We present a mechanism to sort out particles of different masses in an asymmetric channel, where the entropic barriers arise naturally and control the diffusion of these particles. When particles are subjected to an oscillatory force, with the scaled amplitude a and frequency ω, the mean particle velocity exhibits a bell-shaped behavior as a function of the particle mass, indicating that particles with an optimal mass m_{op} drift faster than other particles. By tuning a and ω, we get an empirical relation to estimate m_{op}∼(aω^{2})^{-0.4}. An additional static bias, applied in the opposite direction of the rectified velocity, would push the particles of lighter mass to move in its direction while the others drift opposite to it. This study is useful to design lab-on-a-chip devices for separating particles of different masses.

Confined diffusion in a random Lorentz gas environment.

We study the diffusive behavior of biased Brownian particles in a two dimensional confined geometry filled with the freezing obstacles. The transport properties of these particles are investigated for various values of the obstacle density η and the scaling parameter f, which is the ratio of work done to the particles to available thermal energy. We show that, when the thermal fluctuations dominate over the external force, i.e., small f regime, particles get trapped in the given environment when the system percolates at the critical obstacle density η_{c}≈1.2. However, as f increases, we observe that particle trapping occurs prior to η_{c}. In particular, we find a relation between η and f which provides an estimate of the minimum η up to a critical scaling parameter f_{c} beyond which the Fick-Jacobs description is invalid. Prominent transport features like nonmonotonic behavior of the nonlinear mobility, anomalous diffusion, and greatly enhanced effective diffusion coefficient are explained for various strengths of f and η. Also, it is interesting to observe that particles exhibit different kinds of diffusive behaviors, i.e., subdiffusion, normal diffusion, and superdiffusion. These findings, which are genuine to the confined and random Lorentz gas environment, can be useful to understand the transport of small particles or molecules in systems such as molecular sieves and porous media, which have a complex heterogeneous environment of the freezing obstacles.

Diffusion of interacting particles in a channel with reflection boundary conditions.

The diffusive transport of biased Brownian particles in a two-dimensional symmetric channel is investigated numerically considering both the no-flow and the reflection boundary conditions at the channel boundaries. Here, the geometrical confinement leads to entropic barriers which effectively control the transport properties of the particles. We show that compared to no-flow boundary conditions, the transport properties exhibit distinct features in a channel with reflection boundary conditions. For example, the nonlinear mobility exhibits a nonmonotonic behavior as a function of the scaling parameter f, which is a ratio of the work done to the particles to available thermal energy. Also, the effective diffusion exhibits a rapidly increasing behavior at higher f. The nature of reflection, i.e., elastic or inelastic, also influences the transport properties firmly. We find that inelastic reflections increase both the mobility and the effective diffusion for smaller f. In addition, by including the short range interaction force between the Brownian particles, the mobility decreases and the effective diffusion increases for various values of f. These findings, which are a signature of the entropic nature of the system, can be useful to understand the transport of small particles or molecules in systems such as microfluidic channels, membrane pores, and molecular sieves.