Nanoparticle manipulation within a microscale acoustofluidic droplet

Manipulation of nanoparticles suspended in a droplet can be achieved through the use of low frequency vibration. Upon actuation two types of flows are present in the droplet, a first order flow at the frequency of vibration, and a second order flow in the form of acoustic streaming which is steady state in nature. The former causes collection of nanoparticles, the latter causes swirling of nanoparticles. Through consideration of the two effects, this paper shows that a reduction in fluid height and an associated slight rise in actuation frequency combine to make it possible to manipulate particles in the order of a few hundred nanometers at an excitation frequency of approximately 200 Hz. The amplitude of excitation also plays a role; the increase of amplitude heightens acoustic streaming so this parameter cannot be used as a shortcut to improved ability to collect nanoparticles, hence the need for the analysis of the role of droplet height is presented here.