The collective motion of thermophoretic-type active particle suspension under spatial–temporal modulation

The mesoscopic collective behavior of a thermophoretic-type active particle suspension under a spatial–temporal modulated excitation field is experimentally studied. By using a digital light processing chip, a sinusoidal-like spatial–temporal varying laser pattern (intensity wave) is projected on the particle suspension to modulate the particle propulsion strength through the thermophoretic effect and to exert spatial confinement through the optical trapping effect. For static and slow (wave speed ≪ particle propulsion speed) modulations, in contrast to the passive particles trapped around the wave crest, the self-propulsion strength splits the particle distribution into two branches adjacent to the edges of confinement similar to the bacteria system, and the profile of particle distribution moves coherently with the modulation wave. Once the wave speed increases to the order of the particle propulsion speed, the splitting of particle distribution disappears. Furthermore, it is remarkable that the forward propagating modulation-induced certain fraction of backward streaming particles are observed. This is a generic behavior of a self-propelling active particle suspension, which is not found in the bacteria system. This finding should be useful for particle manipulation at the mesoscopic scale.