Particle separation by standing surface acoustic waves inside a sessile droplet

Biological and chemical diagnostic procedures often require the analyte to be completely separated from many components found in complex physiological fluids, rapid particle concentration and separation for preparation techniques are critical to the success of subsequent analytical procedures. In this work, we established an acoustofluidic platform based on standing surface acoustic waves (SSAWs) and analyzed the influence of the droplet contact angle, which is a crucial step to particle separation and purity improvement. In particular, we show that the effect of radio frequency (RF), RF power and particle diameter on the contact angle of a sessile droplet. Additionally, we explain the causes of the position distribution of the suspended particles in the droplets and the mechanism of particle separation. We use a SSAW to achieve a continuous effect on one or two kinds of suspended particles in the droplet and record in real-time the positions at which the particles are distributed at different contact angles and the final conditions at which the particles are separated. As noted in this SSAW-based report, to achieve complete separation of the two kinds of suspended particles, the contact angle of the droplet should be as small as possible, and a higher frequency should be employed. To quickly reduce the contact angle of the droplets in the separation experiment, high power or high frequency is preferable.