Translational and rotational dynamics of a self-propelled Janus probe in crowded environments.

We computationally investigate the dynamics of a self-propelled Janus probe in crowded environments. The crowding is caused by the presence of viscoelastic polymers or non-viscoelastic disconnected monomers. Our simulations show that the translational as well as rotational mean square displacements have a distinctive three-step growth for fixed values of self-propulsion force, and steadily increase with self-propulsion, irrespective of the nature of the crowder. On the other hand, in the absence of crowders, the rotational dynamics of the Janus probe is independent of self-propulsion force. On replacing the repulsive polymers with sticky ones, translational and rotational mean square displacements of the Janus probe show a sharp drop. Since different faces of a Janus particle interact differently with the environment, we show that the direction of self-propulsion also affects its dynamics. The ratio of long-time translational and rotational diffusivities of the self-propelled probe with a fixed self-propulsion, when plotted against the area fraction of the crowders, passes through a minimum and at higher area fraction merges to its value in the absence of the crowder. This points towards the decoupling of the translational and rotational dynamics of the self-propelled probe at an intermediate area fraction of the crowders. However, such translational-rotational decoupling is absent for passive probes.

Directing the Diffusion of a Nonmagnetic Nanosized Active Particle with External Magnetic Fields.

With the help of molecular dynamics simulations we show that an arbitrary nonmagnetic active particle with a size below one micrometer, being immersed in a suspension of magnetic nanoparticles, can diffuse faster along the direction of an applied field than perpendicular to the latter. This effect is demonstrated in monodisperse and polydisperse systems of magnetic nanoparticles for magnetic fields of moderate strength. The ability to direct a nonmagnetic active particle along the magnetic field stems from the formation of chains of magnetic nanoparticles aligned with the field direction. Such chains form effective channels through which the active particle can diffuse. We combine the investigations of the diffusion and transport efficiency of the active particle parallel and perpendicular to the field with the structural analysis of the magnetic nanoparticle system and find that the ability to direct an active particle of a given size can be maximized by changing magnetic particle concentration. The optimal transport efficiency is achieved at a concentration of magnetic material that provides a mean width of the effective tunnels that matches the effective size of the active particle.

Tracer diffusion in a sea of polymers with binding zones: mobile vs. frozen traps.

We use molecular dynamics simulations to investigate the tracer diffusion in a sea of polymers with specific binding zones for the tracer. These binding zones act as traps. Our simulations show that the tracer can undergo normal yet non-Gaussian diffusion under certain circumstances, e.g., when the polymers with traps are frozen in space and the volume fraction and the binding strength of the traps are moderate. In this case, as the tracer moves, it experiences a heterogeneous environment and exhibits confined continuous time random walk (CTRW) like motion resulting in a non-Gaussian behavior. Also the long time dynamics becomes subdiffusive as the number or the binding strength of the traps increases. However, if the polymers are mobile then the tracer dynamics is Gaussian but could be normal or subdiffusive depending on the number and the binding strength of the traps. In addition, with increasing binding strength and number of polymer traps, the probability of the tracer being trapped increases. On the other hand, removing the binding zones does not result in trapping, even at comparatively high crowding. Our simulations also show that the trapping probability increases with the increasing size of the tracer and for a bigger tracer with the frozen polymer background the dynamics is only weakly non-Gaussian but highly subdiffusive. Our observations are in the same spirit as found in many recent experiments on tracer diffusion in polymeric materials and question the validity of using Gaussian theory to describe diffusion in a crowded environment in general.