Light controllable active matter
Proteorhodopsin (PR) is a light driven proton pump which uses photon energy to pump protons out of the inner membrane of bacteria. The resulting electrochemical gradient can drive the rotation of the flagellar motor so that controllable proteorhodopsin puts a “solar panel” on every cell, allowing to remotely control its swimming speed with light.
These light powered microswimmers can be embedded inside artificial micromachines and employed as efficient and controllable propellers. When freely swimming in a dense suspension, they can instead provide a light controllable active fluid, whose density can be accurately controlled in space and time by projecting dynamical light patterns. We show that an initially homogeneous lawn of these light controlled bacteria can be made to morph between different shapes. We discuss non-local effects arising from memory in light response and propose a feedback control strategy that allows to use bacteria to accurately display a grayscale complex image.