On-Demand Viscosity Gradients Turn Nanoliter Droplets into Programmable Sources of Enriched Picoliter Droplets.

Droplet microfluidics has revolutionized diagnostics, chemical synthesis, and biotechnology. While droplet merging and mixing are routine operations, the opposite processes─coupled enrichment and pinch-off of reagents, analytes, or products─is nontrivial. If accessed, these operations are highly valuable, enabling improved reaction kinetics, increased assay sensitivity, and purification. In this work, we present coupled enrichment and on-demand emission of picoliter-scale daughter droplets from nanoliter-scale parents. The reported approach leverages electrokinetically driven viscosity gradients within droplets to modulate droplet dynamics. Specifically, localized enrichment of a charged viscosity modifier (the polyelectrolyte, alginate) and a charged target species (here, an anionic fluorescent tracer) is accomplished by in-droplet ion concentration polarization (ICP). The results presented here demonstrate that emission is favored by a steep viscosity gradient and that the volume of daughter droplets is programmable. These findings potentiate unprecedented control over droplet-mediated chemistry and bioassays via a tunable process reminiscent of vesicle packaging and pinch-off in biological cells.