Configurable thermoacoustic streaming by laser-induced temperature gradients.

mControlling the streaming flow in acoustically actuated microchannels enables the targeted motion of suspended micro-objects. This can offer novel approaches for rare cell studies or cell sorting in medicine or basic biology. In this work, we utilize the temperature dependence of the acoustic body force, which originates from the interaction of an acoustic field with gradients in compressibility or density. A temperature gradient was optically induced inside an acoustofluidic microchannel by the absorption of light and the resulting streaming flow was measured by particle tracking in three dimensions. Inside a microfluidic channel, two different thermal fields were investigated for a fixed sound field, both in experiments and in simulations. The results show that shifting the location of the heat source from the center to the side of the channel leads to a transition from four streaming rolls to two rolls in the plane normal to the laser incidence. By modulating the optical absorbance of the medium, the streaming velocity can be tuned such that higher absorption leads to faster thermoacoustic streaming. Further, for higher absorbance, we observe increasing velocity components in the direction of the laser due to asymmetric heat generation along the beam.