Homogenization of Solute Concentration in Vertically Coalescent Droplets Using Acoustic Wave

Precise control over the solute concentration in microdroplet arrays is essential for biochemical analyses. We propose using acoustic waves to homogenize the solute concentration within a fused microdroplet comprising a solute-containing lower droplet that is brought into contact with an upper droplet of pure water. Applying audible-range acoustic waves to the fused droplet promotes solute diffusion, which can be combined with a repetitive contact-fusion-separation process to control the solute concentration within each droplet. We applied acoustic-wave mixing to a droplet array with a concentration gradient and achieved a dispensing accuracy of 0.14 ng.