" Light driven motion of active microswimmers "

We report on active colloids which can undergo light-driven self-propelled motion. The mechanism of self-propulsion is based on generation of local hydrodynamic flow at each micro-particle resulting from light-driven diffusioosmosis (LDDO) [1]. At the heart of this process is photosensitive azobenzene containing surfactant which undergoes a reversible trans-cis photo-isomerization with corresponding changes in hydrophobicity of the whole surfactant molecule. Active microparticles consist of porous silica and are negatively charged in water. When porous silica particles are dispersed in the solution of cationic photosensitive surfactant, they absorb partially the surfactant in trans-state, but the more hydrophilic cis-isomers are expelled out of the particles. Under illumination with blue light promoting trans-cis photo-isomerization, a continuous local hydrodynamic flow at a single particle is formed where cis-isomers leave the particle, while trans-isomers flow inside. The particles become active when one side of the porous surface is covered with a metal layer.
[1] D. Feldmann, S. R. Maduar, M. Santer, N. Lomadze, O. I. Vinogradova, S. Santer Scientific Reports 6 (2016) 36443