Droplet Formation via Solvent Shifting in a Microfluidic Device

TL;DR

This study investigates droplet formation and radial motion in a microfluidic device using solvent shifting, combining experimental setup with analytical and numerical diffusion models to understand phase diagram conditions and droplet dynamics.

Contribution

It provides detailed analysis of droplet formation via solvent shifting in microfluidics, integrating phase diagrams and diffusion modeling to elucidate droplet behavior and motion.

Findings

Droplets form between binodal and spinodal in the phase diagram.

Radial motion driven by solutal Marangoni effect toward the channel center.

Droplets are rapidly dissolved upon reaching single-phase region.

Abstract

Solvent shifting is a process in which a non-solvent is added to a solvent/solute mixture and extracts the solvent. The solvent and the non-solvent are miscible. Because of solution supersaturation a portion of the solute transforms to droplets. In this paper, based on this process, we present an investigation on droplet formation and their radial motion in a microfluidic device in which a jet is injected in a co-flowing liquid stream. Thanks to the laminar flow, the microfluidic setup enables studying diffusion mass transfer in radial direction and obtaining well-defined concentration distributions. Such profiles together with Ternary Phase Diagram (TPD) give detailed information about the conditions for droplet formation condition as well as their radial migration in the channel. The ternary system is composed of ethanol (solvent), de-ionized water (non-solvent) and divinyle benzene (solute). We employ analytical/numerical solutions of the diffusion equation to obtain concentration profiles of the components. We show that in the system under study droplets are formed in a region of the phase diagram between the binodal and the spinodal, i.e. via a thermally activated process. The droplets are driven to the channel centerline by the solutal Marangoni effect but are not able to significantly penetrate into the single-phase region, where they get rapidly dissolved. Therefore, the radial motion of the binodal surface carries the droplets to the centerline where they get collected.