# Propagating Density Spikes in Light-Powered Motility-Ratchets

## TL;DR

This paper demonstrates how a flashing light pattern can induce and control long-distance, propagating density spikes in active colloids, with potential applications in targeted drug delivery and colloid segregation.

## Contribution

It introduces a novel method of using a periodic light-field to generate and manipulate propagating density spikes in phototactic active colloids, combining experiments and simulations.

## Key findings

- Density spikes resemble shock waves and propagate with minimal dispersion.
- The propagation can be controlled by the flashing protocol and colloid speed.
- Potential applications include drug delivery and colloid sorting.

## Abstract

Combining experiments and computer simulations, we use a spatially periodic and flashing light-field to direct the motion of phototactic active colloids. Here, the colloids self-organize into a density spike pattern, which resembles a shock wave and propagates over long distances, almost without dispersing. The underlying mechanism involves a synchronization of the colloids with the light-field, so that particles see the same intensity gradient each time the light-pattern is switched on, but no gradient in between (for example). This creates a pulsating transport whose strength and direction can be controlled via the flashing protocol and the self-propulsion speed of the colloids. Our results might be useful for drug delivery applications and can be used to segregate active colloids by their speed.