Microfluidic control on nanoplasmonic thin films using Marangoni effect

Abstract

The rapid switching of flow direction in a thin microfluidic chamber filled with water is demonstrated using the thermoplasmonic Marangoni effect. A gold island film is used as a thermoplasmonic heater on which a continuous-wave laser is focused to generate a microbubble and develop Marangoni flows around it. The direction of the observed flow significantly changes depending on the laser power. When the laser power is square-wave modulated, the flow direction instantaneously switches in response to the power, creating a discrete pattern of polystyrene microspheres. Flow direction-switching is observed for laser power modulation frequencies of up to 40 Hz, which indicates that the time constant of the flow direction switching is on the order of at least several milliseconds. This rapid flow direction switching is attributed to the fast response of both the thermoplasmonic effect of the gold nanoparticles and the Marangoni effect on the bubble surface.