Microfluidic control on nano-plasmonic thin films using Marangoni effect

In this study, we demonstrate the rapid switching of flow direction in a narrow parallel plate channel filled with water by using the thermoplasmonic Marangoni effect. A gold island film prepared in the channel is used as a thermoplasmonic heater, on which a continuous wave (CW) laser is focused to generate a micro bubble. By displacing the laser spot from the bubble center, Marangoni vortex flows are developed adjacent to the bubble. The direction of the observed flow significantly changes depending on the applied laser power. When the laser power is square-wave modulated at 5 Hz, the flow direction instantaneously switches in response to the power, and polystyrene microspheres dispersed in the water are arranged in a discrete pattern. The flow direction switching is observed for laser power modulation frequency of up to 40 Hz, which indicates that the time constant of the flow direction switching is at least of the order of 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. Consequently, the thermoplasmonic Marangoni flows are useful for the dynamic and flexible flow control and microparticle manipulation in a microfluidic channel.