Active colloidal propulsion over a crystalline surface

Abstract

We study both experimentally and theoretically the dynamics of chemically self-propelled Janus colloids moving atop a two-dimensional crystalline surface. The surface is a hexagonally close-packed monolayer of colloidal particles of the same size as the mobile one. The dynamics of the self-propelled colloid reflects the competition between hindered diffusion due to the periodic surface and enhanced diffusion due to active motion. Which contribution dominates depends on the propulsion strength, which can be systematically tuned by changing the concentration of a chemical fuel. The mean-square displacements (MSDs) obtained from the experiment exhibit enhanced diffusion at long lag times. Our experimental data are consistent with a Langevin model for the effectively two-dimensional translational motion of an active Brownian particle in a periodic potential, combining the confining effects of gravity and the crystalline surface with the free rotational diffusion of the colloid. Approximate analytical predictions are made for the MSD describing the crossover from free Brownian motion at short times to active diffusion at long times. The results are in semi-quantitative agreement with numerical results of a refined Langevin model that treats translational and rotational degrees of freedom on the same footing.

Optimal slip velocities of micro-swimmers with arbitrary axisymmetric shapes

“… Scanning electron microscope (SEM) image of a single half-coated Janus particle; inset: the dark blue shows the location of the platinum (Pt) cap (Choudhury et al. 2017). The SEM image of a phototactic swimmer, which consists of a haematite particle extruded from a colloidal bead (Aubret & Palacci 2018).…”

Negative differential mobility and trapping in active matter systems

“…Such clustering effects occur for both driven diffusive [3,4,5,6,7,8] and run-and-tumble systems [3,7,8,9]. Studies of active matter systems generally focus on samples with featureless substrates, but recent work has addressed the behavior of active matter interacting with more complex environments [2], such as random [10,11,12] or periodic obstacle arrays [13,14,15], pinning arrays or rough landscape substrates [15,16,17], or funnel arrays [18], as well as mixtures of active and passive particles [19]. In run-and-tumble disk systems, studies of the average flux through an obstacle array in the presence of an additional external drift force [10] show that for low activity or short run times, the active disks have Brownian characteristics and are easily trapped; however, for increasing run persistence length or activity, the trapping is reduced and the flux of disks through the system increases.…”

Engineering the Dynamics of Active Colloids by Targeted Design of Metal–Semiconductor Heterojunctions

“…Colloidal particles suspended in a fluid may become self‐propelling when a catalyzed chemical reaction takes place at the interface between the particle and the fluid through which motion occurs . If this reaction generates a local concentration gradient, a chemiosmotic flow over the particle's surface may arise.…”