Active Brownian Motion with Orientation-Dependent Motility: Theory and Experiments

Abstract

Combining experiments on active colloids, whose propulsion velocity can be controlled via a feedback loop, and theory of active Brownian motion, we explore the dynamics of an overdamped active particle with a motility that depends explicitly on the particle orientation. In this case, the active particle moves faster when oriented along one direction and slower when oriented along another, leading to an anisotropic translational dynamics which is coupled to the particle's rotational diffusion. We propose a basic model of active Brownian motion for orientation-dependent motility. Based on this model, we obtain analytic results for the mean trajectories, averaged over the Brownian noise for various initial configurations, and for the mean-square displacements including their anisotropic non-Gaussian behavior. The theoretical results are found to be in good agreement with the experimental data. Our findings establish a methodology to engineer complex anisotropic motilities of active Brownian particles, with potential impact in the study of the swimming behavior of microorganisms subjected to anisotropic driving fields.

Hydrodynamics can determine the optimal route for microswimmer navigation

“…Interestingly, however, while for such macroagents or dry active particles in constant external fields the shortest path is optimal 7,50 , for microswimmers excursions can pay off, as we will see in the following. Note that the considered model can be tested with programmed active colloids [51][52][53] but should also be relevant for biological microswimmers which often swim at an almost constant speed and are able to control their self-propulsion direction on demand. These swimmers are often in contact with (remote) walls or interfaces which drastically influence their overall swimming speed and direction of motion.…”

Dynamics of active particles with space-dependent swim velocity

“…Moreover, the effect of inertia should be considered for the case where the swim velocity depends on space. 75,76,101 Finally, we mention a recent experimental and theoretical study by Sprenger et al, who proposed an experimental method to modulate the particle rotational diffusivity 102,103 in systems of active magnetic dumbbells. The extension of our result to that case, i.e.…”

Reconfigurable artificial microswimmers with internal feedback

“…Realizing artificial microswimmers with similar adaptation capabilities and autonomous behavior might substantially impact technologies ranging from optimal transport to sensing and microrobotics 3 . Focusing on adaptation, existing approaches at the colloidal scale mostly rely on external feedback, either to regulate motility via the spatiotemporal modulation of the propulsion velocity and direction [4][5][6][7][8] or to induce shape changes via the same magnetic or electric fields [9][10][11] , which are also driving the particles. On the contrary, endowing artificial microswimmers with an internal feedback mechanism, which regulates motility in response to stimuli that are decoupled from the source of propulsion, remains an elusive task.…”