Abstract
Emulsions–liquid droplets dispersed in another immiscible liquid–are widely used in a broad spectrum of applications, including food, personal care, agrochemical, and pharmaceutical products. Emulsions are also commonly present in natural crude oil, hampering the production and quality of petroleum fuels. The stability of emulsions plays a crucial role in their applications, but controlling the stability without external driving forces has been proven to be difficult. Here we show how heterogeneous surface wettability can alter the stability and dynamics of oil-in-water emulsions, generated by a co-flow microfluidic device. We designed a useful methodology that can modify a micro-capillary of desired heterogeneous wettability (e.g., alternating hydrophilic and hydrophobic regions) without changing the hydraulic diameter. We subsequently investigated the effects of flow rates and heterogeneous wettability on the emulsion morphology and motion. The experimental data revealed a universal critical timescale of advective emulsions, above which the microfluidic emulsions remain stable and intact, whereas below they become adhesive or inverse. A simple theoretical model based on a force balance can be used to explain this critical transition of emulsion dynamics, depending on the droplet size and the Capillary number–the ratio of viscous to surface effects. These results give insight into how to control the stability and dynamics of emulsions in microfluidics with flow velocity and different wettability.
Highlights
Multiphase flow in porous media is a common theme and plays a significant role in soil remediation, enhanced oil recovery, membrane technology, and biological and medical applications
We report the invention of an effective method to modify a micro-capillary with heterogeneous surface wettability of a desired pattern
We found a universal critical convective time scale, Δt*, delineating the unaltered and changing emulsion dynamics
Summary
Multiphase flow in porous media is a common theme and plays a significant role in soil remediation, enhanced oil recovery, membrane technology, and biological and medical applications. We examine the effects of surface wettability on the stability of (oil-in-water) emulsions traveling in a micro-capillary (see Fig. 1), which mimics a simple pore-scale structure in a porous medium. For water-in-oil droplets produced by a microfluidic flow-focusing structure, as Ca increases from 10−4 to 10, four different regimes of lubrication film can be delineated: wetting (h = 0), thin film (h → 0), thick film (h ~ Ca2/3 for Ca > 10−1), and constant thicker film (h/d ~ 0.11)[27]. We elucidate the significant influence of heterogeneous surface wettability on the morphology and dynamics of microfluidic emulsions. The static contact angles were measured as 27 ± 3° and 97 ± 5° with a water droplet of 2 μL on the flat hydrophilic and hydrophobic surfaces, respectively. The corresponding contact angle hysteresis is about 70° (detailed materials and methods are described below)
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