Fig. 3. Acoustic microstreaming results of the microrobot in Newtonian and non-Newtonian fluids.
(A) 2D axisymmetric numerical simulations of the acoustic microstreaming fields, including the flow shear rate, velocity, and particle trajectory for Newtonian (DI water), shear-thinning, and viscoelastic fluid (mucus). The oscillation amplitude and resonance frequency of the microbubble membrane were set to 500 nm and 380 kHz, respectively. (B) The experimental trajectories of 2-μm tracer particles in a PBS medium visualizing the microstreaming flow in front of the microrobots, fixed on the substrate, under ultrasound actuation of 380 kHz and 1 Vpp (see movie S2). The streamline pattern observed in the experiments matches well with the particle trajectory simulations for the Newtonian fluid in (A). (C) Time-lapse images of the acoustic microstreaming in mucus 6%; the mucus microstructure was stretched and displaced in front of the microrobot orifice during the oscillation of the robot microbubble (movie S2). The microstreaming pattern in mucus is similar to the combination of shear-thinning and viscoelastic particle trajectory simulation in (A). Scale bars, 10 μm (A) and 25 μm (B and C).