Droplet mixing and liquid property tracking using an electrodynamic plate resonator

Acoustic streaming in a sessile droplet can be achieved, e.g., by surface acoustic wave transducers or piezoelectric actuators. We present a novel method using a plate resonator, which is electrodynamically actuated. The liquid droplet rests on a round plate, which is suspended by spring structures and oscillates in its own plane at a resonance frequency around 1 kHz. The resonator loading depends on mass density and (complex) viscosity of the liquid in the vicinity of the contact surface. Actuating the oscillation at the resonance frequency yields high displacement amplitudes, which consecutively leads to acoustic streaming capable of effective stirring within the droplet. The method is used to quantify the stability of two-phase systems such as emulsions and suspensions. After a defined stirring period, the resonator loading is measured every few seconds. Resonance frequency shift and bandwidth are evaluated to derive the viscosity of the liquid two-phase system and monitor the phase-separation.