Boundaries can steer active Janus spheres

Abstract

The advent of autonomous self-propulsion has instigated research towards making colloidal machines that can deliver mechanical work in the form of transport, and other functions such as sensing and cleaning. While much progress has been made in the last 10 years on various mechanisms to generate self-propulsion, the ability to steer self-propelled colloidal devices has so far been much more limited. A critical barrier in increasing the impact of such motors is in directing their motion against the Brownian rotation, which randomizes particle orientations. In this context, here we report directed motion of a specific class of catalytic motors when moving in close proximity to solid surfaces. This is achieved through active quenching of their Brownian rotation by constraining it in a rotational well, caused not by equilibrium, but by hydrodynamic effects. We demonstrate how combining these geometric constraints can be utilized to steer these active colloids along arbitrary trajectories.

Brownian dynamics of confined suspensions of active microrollers

“…These studies neglect all Brownian motion because their magnitude is estimated to be small compared with the active forces. Other works that focus on active suspensions have been published but in general they do not include Brownian fluctuations or many-body hydrodynamic interactions [19][20][21].…”

A Pickering Emulsion Route to Swimming Active Janus Colloids

“…We focus specifically on spherical Janus colloids of the type shown in Figure 1 . This type of swimming device has been found to display a wide range of interesting colloidal phenomena, including autonomous guidance effects (gravitaxis,16 chemotaxis,17 and boundary steering),18 and has also been the subject of many currently untested theoretical proposals for high volume fraction collective phenomena 19. An advantage of Janus spheres as a system to explore emergent behavior is that they move without issuing bubbles and so their interactions via chemical “wakes” and hydrodynamics are amenable to being analyzed and experimentally observed, whereas nanotubes produce considerable convective flow due to bubble release 20.…”

Self-diffusiophoresis of chemically active colloids

“…For instance, active particles can exhibit taxis: under certain conditions, they can reliably reorient and migrate in response to cues (such as gravitational fields [15,77], the topography of bounding surfaces [73,74], externally imposed flows [78], or fluid interface-response flows [79]) in their environment. Here we shall consider the possible effects of gravity and external flow.…”

“…Experimental observations of such surface-bounded states for a few types of chemically active particles and various confining geometries have been reported recently [72][73][74]. (Note that in these studies the analysis is more involved due to the fact that the force and the torque due to gravity are relevant and thus have to be included in the corresponding force and torque balance (Eq.…”