(349i) Enhanced Dissolution of Liquid Microdroplets Under Planar Extensional Flow

The dissolution of liquid microdroplets is of great importance in many fields of science and engineering with a wide range of applications in food and pharmaceutical industry. Here, we describe a novel microfluidic method to study the dissolution of freely suspended liquid microdroplets into an immiscible phase under planar extensional flow. Micron-sized (15-60 µm) benzyl benzoate and n-decanol droplets are generated by vigorous agitation in aqueous solution containing a surfactant. The microdroplets are subsequently delivered into a microfluidic device where they are hydrodynamically trapped at the stagnation point of a planar extensional flow generated by two opposing laminar streams at the junction of two perpendicular microchannels.

Due to their low solubility, benzyl benzoate and n-decanol microdroplets are characteristically stable in aqueous solutions with negligible change in size under no flow conditions. However, under planar extensional flow, we observed that benzyl benzoate and n-decanol droplets exhibit enhanced dissolution revealed by a rapid change in droplet diameter where droplets (tens of microns in diameter) are completely dissolved typically within less than an hour. The swift change in droplet diameter can be explained by a flow-induced dissolution mechanism. To describe the observed phenomena, we adapted a numerical solution to the convection-diffusion equation, developed by Zhang-Yang-Mao, taking into account both the diffusive and the convective mass transfer from a liquid drop under an extensional flow. The numerical solution relates the rate of change in diameter (dR/dt) to several parameters including diffusivity and the solubility of the droplet phase in the host phase, relative viscosities of the droplet and host phase, density of the droplet phase, and the Peclet number.

The numerical solution to the convection-diffusion equation successfully explains the observed rate of change in droplet size from the droplet dissolution experiments under various flow conditions. To our knowledge, this is the first experimental demonstration of flow-induced dissolution of microdroplets under extensional flow. This unique method paves the way for a rapid, dynamic determination of solubility and diffusion coefficient for liquids with low miscibility in aqueous solutions, enabling potential new applications in materials science and engineering, and pharmaceutical sciences.