Separation of two-different-sized microparticles inside moving droplets with travelling surface acoustic waves

Abstract

Since microfluidic systems can contain more than two kinds of microscale objects, such as cells and microparticles, separation of a sample is one of the important prerequisites for analysis. A control of the positions of microscale objects is crucial for practical applications in not only continuous-flow-based but also droplet-based systems. To date, there have been a few studies on the position control of particles in a moving droplet, but they are limited to practical applications since they are either separation of microparticles with one size or separation relying on particles' specific properties (i.e., magnetism and electric polarity). In this study, we achieved an on-demand, label free, in-droplet separation based on the size utilizing an acoustic method. We demonstrated a lateral position control, which is related to a direction of channel width, of polystyrene particles of 5 and 10 μm in diameter inside the moving water-in-oil droplet using travelling surface acoustic waves. The location of the in-droplet particles in the direction of channel width was manipulated by acoustic radiation force of travelling surface acoustic waves, thereby being moved laterally in the direction of the wave propagation. With two frequencies of travelling surface acoustic waves, two-different-sized particles were translocated at the different lateral positions in the droplet. After droplet splitting at a Y-junction, in-droplet separation was then realized.