Density Modulations in Active Colloidal Systems through Orthogonal Propulsion Control and Sensory Delays

Recent advancements in active colloidal systems aim to mimic key characteristics of biological microswimmers, particularly their adaptive motility in response to environmental changes. While many approaches rely on externally imposing a variable propulsive force, achieving true autonomous and self-regulating adaptation to the environment remains limited. In this study, we take a step in this direction and develop Janus microswimmers driven by electrohydrodynamic flows that autonomously adjust their propulsion dynamics in response to varying illumination and exposure to chemical agents. Our Janus particles are silica colloids partially coated with titania, which self-propel via induced-charge electrophoresis under uniform AC electric fields. Since titania is photoconductive, it increases its conductivity under UV illumination, which thereby regulates the propulsion velocity independent of and orthogonally to the applied electric field. Crucially, the velocity adaptation happens spontaneously but requires a finite amount of time. This sensory delay leads to enhanced microswimmer localization in response to spatiotemporal light modulations compared with the typical case of instantaneous response considered for synthetic microswimmers. Additionally, the particles spontaneously adapt their response time in the presence of chemicals, here methanol, which affect the lifetime of charge carriers and lead to a concentration-dependent response. By harnessing these dynamics, akin to those of biological microswimmers, we control both local and global particle behavior, presenting exciting opportunities for adaptive active matter systems.