Medium exchange and enrichment of microparticles encapsulated in microscale droplets*

Droplet microfluidic medium exchange and enrichment of microparticles have long been pursued but remained problematic because the simultaneous and precise control over the droplet interface and suspended microparticles is difficult. This work proposes a new method for in-droplet microparticle medium exchange and enrichment using acoustic waves. Under the influence of the acoustic field within a microchannel, the droplets and encapsulated particles experience acoustic radiation force induced by wave scattering at the liquid/liquid and liquid/solid interfaces. Based on the acoustic radiation force, the medium of in-droplet microparticles was exchanged, and the number of the in-droplet microparticles was increased to the desired quantity. The microparticle concentration location within droplets has been also investigated based on the dimensionless parameters: Helmholtz number and acoustic radiation force factor. The proposed acoustic wave-based method is expected to offer new perspectives to microparticle-based assays in droplet-based microfluidic applications.