Experimental study of acoustic streaming in microchannels generated by pulsed focused ultrasound

Pulsed focused ultrasounds (p-FUS) are gaining interest across a range of applications, such as regenerative medicine, neurostimulation and targeted drug delivery, offering a non-invasive therapeutic approach. In order to gain insight into the hydrodynamic effects potentially induced by p-FUS in biological tissues, the present acoustofluidic study investigates the ability of focused ultrasound to generate acoustic streaming in a viscous fluid confined in a microchannel. Through micro-particles image velocimetry (µPIV) measurements, it analyses the flow patterns induced by p-FUS in a rectangular cross-section microchannel, corresponding to half of the ultrasonic wavelength. The analysis confirmed good repetability across replicates, despite minor variations introduced by manual assembly. It is shown that the flow patterns reach the equilibrium within seconds and that the average streaming velocity varies quadratically with the duty cycle characterizing p-FUS. Moreover, the results indicate that the streaming velocity magnitude depends on the amount of acoustic energy delivered to the channel. Interestingly, the induced flow exhibits a strongly three-dimensional structure, revealing spatial dynamics that, to our knowledge, have not been previously observed in microscale acoustic streaming studies. These results lay the foundation for understanding the interaction of p-FUS with a confined fluid at the microscale, opening new avenues for investigating more complex networks and porous media representing biological tissues.