Scaling of the formation of cyclodextrin-based droplets in a flow-focusing microchannel

Cyclodextrin (CD)-based droplets have received considerable attention in health and body care applications, and a real-time approach to actively control the size and generation frequency of the droplets is desirable. Here we study the droplet formation experimentally inside a flow-focusing microchannel using aqueous CD solutions as the continuous phase, and different oils including glycerol trioleate (TG), oleic acid (OA) and tetradecane (TD) as the dispersed phase, respectively. We identify three distinct flow regimes: threading, dripping and jamming, whose boundaries change as the oil is varied. We further measure the size and generation frequency of the droplets as a function of the CD concentration and the flow rate ratio of the two liquid phases in the dripping regime. Remarkably the least viscous oil of TD forms droplets with the largest size and lowest generation frequency. Building on our recent findings for the dynamic anchoring patterns of CD microcrystals on droplets, we demonstrate that interfacial elastic behavior plays an important role in the droplet formation, and develop a scaling relation by including the interfacial elasticity to predict the size and generation frequency of the droplets, which is consistent with the experimental results.