Enhanced gravitational trapping of bottom-heavy Janus particles over parallel microgrooves

We report a systematic study on the barrier-crossing dynamics of bottom-heavy self-propelled particles (SPPs) over a one-dimensional periodic potential landscape U₀(x), which is fabricated on a microgroove-patterned polydimethylsiloxane (PDMS) substrate. From the measured steady-state probability density function (PDF) P(x;F₀) of the SPPs with different self-propulsion forces F₀, we find that the escape dynamics of slow-rotating SPPs over the periodic potential U₀(x) can be well described by an activity-dependent potential Ũ₀(x;F₀) under the fixed angle approximation. A theoretical model is developed to include the effects of the gravitational-torque-induced alignment on the polar angle θ and the hydrodynamic wall alignment on the azimuthal angle φ as well as their influence on the self-propulsion speed v₀. By introducing a proper average of the activity-dependent potential Ũ₀(x;F₀) over all possible particle orientations, our model explains the enhanced trapping effect on the bottom-heavy Janus particles. The obtained theoretical results are in good agreement with both the experimental and active Brownian particle simulation results. This work thus provides a thermodynamics description of the non-equilibrium barrier crossing of the Janus particles with nonuniform angular distributions over periodic potentials.