Acoustic Streaming in Solutions with Acousto-Responsive Particles Using Finite Element Analysis

Microgels have the potential to be used as microscale transporters that release an active agent when excited externally. In recent work, poly(N-isopropylacrylamide) has been reported to be responsive to ultrasound through particle agglomeration, resulting in turbidity as a visual indicator of energy absorption. The evaluation of turbidity evolution for understanding sub-microscale processes is distorted by ultrasound-induced acoustic streaming (AS) as well as nonisothermal flow (NF) caused by the transducer’s heat loss. This work presents a 2D-axisymmetric numerical model of a transducer attached to a container filled with an aqueous solution as a liquid resonator operating at 2.38 MHz. The dominance of AS over the superposing NF is assessed with temperature and bulk viscosity as input quantities. Transducer characteristics are incorporated and validated using impedance matching. A streaming pattern of the microgel being actuated from previous work is reanalyzed as a validation method for the numerical model presented. The bulk viscosity of the solution is adapted according to validation data, and the resulting streaming pattern agrees with the measurements. The positions of the vortex centers show deviations of up to 0.5 mm and 0.3 mm in the x- and y-direction, respectively. Thus, evaluating the dominating effects within the solution depending on varying material properties or geometry is feasible.