Aiden Huffman | Applied Mathematics, University of Waterloo
From Forces to Functions: Surface Tension in Active Droplets
Active droplets describe a large family of physical systems. For our purposes, the term active refers to a droplet's ability to move within a surrounding fluid or process chemical reactions. The term droplet refers to liquid-liquid phase separations, which one can draw analogies from oil droplets within water. Frequently, the mobility of active droplets results from variations in the surface tension at the interface between the droplet and the fluid that contains it; while the chemical reactivity is a consequence of the droplets separating reactive material from the surrounding fluid.
Generally, the dynamics of an individual droplet are well understood. However, active droplets exhibit surprising emergent behaviour when studied in aggregate. For example, collections of similar active droplets can organize spatially, perform metal extraction (Ban et al. 2014), or deliver cargo to a specific destination (Li et al. 2018). Unlike comparable platforms, such as microcapsules or Janus particles, active droplets have a deformable interface which material can diffuse through. The permeability of active droplets is essential for chemically active droplets to access reagents within the fluid. Moreover, recent work has shown that phase separation is an adaptive process within biological cells. It stresses the importance of understanding the emergent behaviours of active droplets, which could self-assemble and perform tasks within biological systems.
We present three recent experiments involving active droplets. Each experiment relies on surface tension effects but harnesses them and the surrounding environment differently to produce the observed behaviour. We will pose several questions from these experiments and present some conjectures to answer them. For instance, can chemotactic droplets improve substrate conversion by transporting enzymes compared to freely diffusing enzymes? Are changes in surface tension induced by shifts in temperature able to mix material in highly viscous droplets efficiently? How can we organize droplets into prescribed formations or have them collectively produce complex flow fields? Finally, we will describe a diffuse-interface method to describe the dynamics numerically. Diffuse-interface methods have successfully described thermocapillary flows (Antanovskii 1995, Jasnow & Vinals 1996), droplet fusing and break-up (Jacqmin 1996, Lowengrub and Truskinovsky 1998), and droplet evaporation and impact (Lee et al. 2021, Khatavkar et al. 2007)