Solvent evaporation controls shape of complex droplets

Complex emulsions are attractive for applications like cosmetics, drug delivery, and sensing. Now, David A. Weitz and colleagues at Harvard University have a new way to manipulate these systems: They tuned the structure of double emulsion droplets—droplets inside droplets—through solvent evaporation (ACS Appl. Mater. Interfaces 2018, DOI: 10.1021/acsami.8b13216).

Weitz’s team encased drops of a polyvinyl alcohol (PVA) aqueous solution in a mixture of two polymers dissolved in dichloromethane solvent, then bathed the double-layer droplets in PVA solution identical to their inner fluid. Gradually, solvent in the polymer layer diffuses into the bath and evaporates, and the polymer duo thins and solidifies into a tough shell. If a concentration gradient of solvent exists in the bath, this behavior happens asymmetrically. That causes the inner fluid to flow within the polymer shell, deforming the droplet.

By tailoring both the osmotic pressure and solvent concentration gradient in the bath, researchers can control the shape changes of the complex droplets, which did not rupture in any of the reported experiments. And the method would enable researchers to create a robust shell from the more diluted, low-viscosity fluids typically used to make such emulsions, Weitz says.

Understanding how researchers can make systems that reshape themselves in response to chemical gradients is important to the development of future active and adaptive materials, says Lauren D. Zarzar of The Pennsylvania State University, who studies reconfigurable emulsions and was not involved with the research.

“The ability to dynamically alter the shape of droplets could be useful for applications such as tunable optics, where the curvature of a droplet influences its lens characteristics, or for sensing, where droplet shape could be a reporter of analyte gradients in solution,” she says.

Weitz says his team is working to find more ways to apply the group’s technology to drug delivery, among other applications.