37 research outputs found
Boundaries can steer active Janus spheres
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
Cross-stream migration of active particles
For natural microswimmers, the interplay of swimming activity and external
flow can promote robust motion, e.g. propulsion against ("upstream rheotaxis")
or perpendicular to the direction of flow. These effects are generally
attributed to their complex body shapes and flagellar beat patterns. Here,
using catalytic Janus particles as a model experimental system, we report on a
strong directional response that occurs for spherical active particles in a
channel flow. The particles align their propulsion axes to be nearly
perpendicular to both the direction of flow and the normal vector of a nearby
bounding surface. We develop a deterministic theoretical model of spherical
microswimmers near a planar wall that captures the experimental observations.
We show how the directional response emerges from the interplay of shear flow
and near-surface swimming activity. Finally, adding the effect of thermal
noise, we obtain probability distributions for the swimmer orientation that
semi-quantitatively agree with the experimental distributions
Topographical pathways guide chemical microswimmers
Achieving control over the directionality of active colloids is essential for
their use in practical applications such as cargo carriers in microfluidic
devices. So far, guidance of spherical Janus colloids was mainly realized using
specially engineered magnetic multilayer coatings combined with external
magnetic fields. Here, we demonstrate that step-like sub-micron topographical
features can be used as reliable docking and guiding devices for chemically
active spherical Janus colloids. For various topographic features (stripes,
squares or circular posts) docking of the colloid at the feature edge is robust
and reliable. Furthermore, the colloids move along the edges for significantly
long times, which systematically increase with fuel concentration. The observed
phenomenology is qualitatively captured by a simple continuum model of
self-diffusiophoresis near confining boundaries, indicating that the chemical
activity and associated hydrodynamic interactions with the nearby topography
are the main physical ingredients behind the observed behaviour.Comment: 18 pages, 12 figure