Passive and active tracer dynamics in crowded environment and in magnetic fluid:

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Using molecular dynamics simulations we have investigated 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 showed 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. More recently, our group has worked on the tracer diffusion in polymer gel and in particular how size, shape of the tracer and the flexibility of the gel affect the dynamics and in particular on the emergence of non-Gaussian heavy tailed dynamics of the probe. In addition, we are currently investigating the dynamics of active Janus probe in crowded environment and the decoupling of its translational-rotational dynamics as observed at moderate crowding. Another related topic of research is controlling the diffusion of nanosized non-magnetic active particle in a magnetic fluid by external magnetic field.