Abstract

Understanding the interactions between active colloids and their complex surroundings is essential for both fundamental research and engineering applications. Recently, synthetic self-propelled particles (SPPs) have been developed as a simplified model system for studying the generic physical properties behind active colloids. Despite considerable progress in theoretical and numerical studies over the past decade, there is an urgent need for experimental investigations. In this thesis, I report four experiments studying the non-equilibrium behaviors of synthetic SPPs interacting with complex external potentials.
In the first experiment, we use a thin polydimethylsiloxane (PDMS) substrate with parallel microgroove patterns to construct a periodic gravitational potential U0(x) for the SPPs moving above. From the particle trajectories, we measure the non-equilibrium steady-state probability distribution function (PDF) P(x; v0) and the dwell time td of the SPPs across the microgrooves. We find the activity of SPPs can assist particles in barrier crossing, reducing the effective barrier height. An effective equilibrium approach based on fixed angle approximation is proposed to describe the non-equilibrium barrier crossing of slow-rotating SPPs in a strong trapping potential.

The second experiment studies the ratchet effect of SPPs in an asymmetric potential generated by the ratchet microgrooves. We measure the number of events that SPPs escape the asymmetric potential in the forward and backward directions. A net particle transport towards one direction is observed. We provide a thermodynamics description to understand the non-equilibrium particle transport for slow-rotating SPPs.

In the third experiment, we study the dynamics of SPPs with non-uniform angular distributions in a periodic gravitational potential U0(x). From the measured PDF P(x; v0) of the particles, we find the activity of the bottom-heavy SPPs enhances the gravitational trapping. A theoretical model is developed for the slow-rotating, bottom-heavy Janus particles across a periodic potential, which shows a good agreement with the observed enhanced trapping effect in the experiment.

In the fourth experiment, we study the dynamic clustering of two SPPs interacting via hard-sphere repulsion. We find the activity of SPPs in a single file can introduce an effective attraction. The measured effective pair potential of two SPPs is well described by a lattice-based model for exclusive random walks.