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

## TL;DR

This paper introduces Janus microswimmers that autonomously adjust their movement in response to light and chemicals, mimicking biological behavior.

## Contribution

The study introduces autonomous propulsion control in microswimmers using orthogonal stimuli and sensory delays.

## Key findings

- Janus particles adjust propulsion velocity independently of the applied electric field under UV illumination.
- Sensory delays in response to light improve microswimmer localization in spatiotemporal modulations.
- Chemical exposure alters response time, enabling concentration-dependent adaptive behavior.

## Abstract

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.