Ultrasonic surface acoustic wave-assisted separation of microscale droplets with varying acoustic impedance

In droplet-based microfluidic platforms, precise separation of microscale droplets of different chemical compo-sition is increasingly necessary for high-throughput combinatorial chemistry in drug discovery and screening assays. A variety of droplet sorting methods have been proposed, in which droplets of the same kind are translocated. However, there has been relatively less effort in developing techniques to separate the uniform-sized droplets of different chemical composition. Most of the previous droplet sorting or separation techniques either rely on the droplet size for the separation marker or adopt on-demand application of a force field for the droplet sorting or separation. The existing droplet microfluidic separation techniques based on the in-droplet chemical composition are still in infancy because of the technical difficulties. In this study, we propose an acoustofluidic method to simultaneously separate microscale droplets of the same volume and dissimilar acoustic impedance using ultrasonic surface acoustic wave (SAW)-induced acoustic radiation force (ARF). For extensive investigation on the SAW-induced ARF acting on both cylindrical and spherical droplets, we first performed a set of the droplet sorting experiments under varying conditions of acoustic impedance of the dispersed phase fluid, droplet velocity, and wave amplitude. Moreover, for elucidation of the underlying physics, a new dimensionless number ARD was introduced, which was defined as the ratio of the ARF to the drag force acting on the droplets. The experimental results were comparatively analyzed by using a ray acoustics approach and found to be in good agreement with the theoretical estimation. Based on the findings, we successfully demonstrated the simultaneous separation of uniform-sized droplets of the different acoustic impedance under continuous application of the acoustic field in a label-free and detection-free manner. Insomuch as on-chip, precise separation of multiple kinds of droplets is critical in many droplet microfluidic applications, the proposed acoustofluidic approach will provide new prospects for microscale droplet separation.

Automated summary

In droplet-based microfluidic platforms, there is increasing demand for precise separation of microscale droplets of different chemical composition. Most existing techniques focus on separating droplets of the same kind, while there is less effort on separating uniform-sized droplets of different chemical composition. This study proposes an acoustofluidic method using ultrasonic surface acoustic wave (SAW)-induced acoustic radiation force (ARF) to simultaneously separate uniform-sized droplets of different acoustic impedance. The experimental results show that the proposed method provides a label-free and detection-free approach for on-chip, precise separation of multiple kinds of droplets in microfluidic applications.