Spatial Patterning of Micromotor Aggregation and Flux

Abstract

Using a spatially varying light pattern with light activated semi‐conductor based magnetic micromotors, we study the difference in micromotor flux between illuminated and non‐illuminated regions in the presence and absence of an applied magnetic field. We find that the magnetic field enhances the flux of the motors which we attribute to a straightening of the micromotor trajectories which decreases the time they spend in the illuminated region. We also demonstrate spatially patterned light‐induced aggregation of the micromotors and study its time evolution at various micromotor concentrations. Although light induced aggregation has been observed previously, spatial patterning of aggregation demonstrates a further means of control which could be relevant to swarm control or self‐assembly applications. Overall, these results draw attention to the effect of trajectory shape on the flux of active colloids as well as the concentration dependence of aggregation and its time dependence within a spatially patterned region, which is not only pertinent to self‐assembly and swarm control, but also provides insight into the behavior of active matter systems with spatially varying activity levels.

Upstream mobility and swarming of light activated micromotors

“…With a magnetic field applied, the micromotors move in a wide range of directions rather than along the direction of the magnetic field. We have observed this previously 24 and attribute it to differences in the magnetic coating on the micromotors or to the formation of aggregates which possess a magnetic moment that is misaligned with their direction of motion.…”

“…A magnetic field acts to make the trajectories of these micromotors more linear due to a suppression of their natural active rotation. 24 Therefore, we applied a magnetic field by placing a permenant magnet near the sample, resulting in less rotation of the micromotors and more persistent motility upstream.…”

“…Swarms of particles have been generated using magnetic fields, 8,9,11 chemical effects, 12,13 electric fields, 14,15 thermal gradients, 16 and acoustic waves. [17][18][19][20] Swarm-like schools of light-active particles have also been formed 21 as well as self-attractive clusters, [22][23][24] although control of such collections of light-active units in flows has not yet been demonstrated.…”