Droplet Manipulation Using Acoustic Streaming Induced by a Vibrating Membrane

We
present a simple method for on-demand manipulation of aqueous
droplets in oil. With numerical simulations and experiments, we show
that a vibrating membrane can produce acoustic streaming. By making
use of this vortical flow, we manage to repulse the droplets away
from the membrane edges. Then, by simply aligning the membrane at
45° to the flow, the droplets can be forced to follow the membrane’s
boundaries, thus steering them across streamlines and even between
different oil types. We also characterize the repulsion and steering
effect with various excitation voltages at different water and oil
flow rates. The maximum steering frequency we have achieved is 165
Hz. The system is extremely robust and reliable: the same membrane
can be reused after many days and with different oils and/or surfactants
at the same operating frequency.