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

Area of Science:

• Biotechnology
• Chemical Synthesis
• Diagnostics
• Microfluidics

Background:

• Droplet microfluidics enables advancements in diagnostics, chemical synthesis, and biotechnology.
• While droplet merging and mixing are established, coupled enrichment and pinch-off remain challenging operations.

Purpose of the Study:

• To present a novel method for coupled enrichment and on-demand emission of picoliter-scale droplets from nanoliter-scale parent droplets.
• To demonstrate precise control over droplet dynamics for improved reaction kinetics, assay sensitivity, and purification.

Main Methods:

• Utilized electrokinetically driven viscosity gradients within droplets to modulate droplet dynamics.
• Employed in-droplet ion concentration polarization (ICP) for localized enrichment of a charged viscosity modifier (alginate) and a charged target species (fluorescent tracer).

Main Results:

• Demonstrated that droplet emission is favored by a steep viscosity gradient.
• Showcased that the volume of the emitted daughter droplets is programmable.
• Achieved coupled enrichment and on-demand emission of picoliter droplets.

Conclusions:

• The developed approach offers unprecedented control over droplet-mediated chemistry and bioassays.
• The tunable process mimics vesicle packaging and pinch-off in biological cells.
• This method has significant potential for advancing microfluidic applications.