Opto-thermoelectric microswimmers

Optical actuation: microparticle swimming Dielectric-metal microstructures that are propelled by light can swim and navigate in fluids. Xiaolei Peng and coworkers from the University of Texas, US and Tsinghua University in Beijing, China, fabricated polystyrene microparticles that were half-coated with gold and suspended in a solution of water and 0.2 mM of cetyltrimenthylammonium chloride (CTAC). When illuminated with a defocused laser beam, asymmetric optical absorption and heating leads to a temperature gradient and an opto-thermoelectric field that propels the microparticles in the direction of the temperature gradient. Swimming velocities of up to nearly 20 mu m/s were measured for 2.1 mu m-sized particles illuminated with a 1064nm-wavelength laser beam with a low optical intensity of 0.03 mW/mu m(2). The use of a second focused laser beam makes it possible to rotate the microparticles and thus control the direction of motion. Inspired by the "run-and-tumble" behaviours of Escherichia coli (E. coli) cells, we develop opto-thermoelectric microswimmers. The microswimmers are based on dielectric-Au Janus particles driven by a self-sustained electrical field that arises from the asymmetric optothermal response of the particles. Upon illumination by a defocused laser beam, the Janus particles exhibit an optically generated temperature gradient along the particle surfaces, leading to an opto-thermoelectrical field that propels the particles. We further discover that the swimming direction is determined by the particle orientation. To enable navigation of the swimmers, we propose a new optomechanical approach to drive the in-plane rotation of Janus particles under a temperature-gradient-induced electrical field using a focused laser beam. Timing the rotation laser beam allows us to position the particles at any desired orientation and thus to actively control the swimming direction with high efficiency. By incorporating dark-field optical imaging and a feedback control algorithm, we achieve automated propelling and navigation of the microswimmers. Our opto-thermoelectric microswimmers could find applications in the study of opto-thermoelectrical coupling in dynamic colloidal systems, active matter, biomedical sensing, and targeted drug delivery.