title: Altering Emulsion Stability with Heterogeneous Surface Wettability.
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.
authors.affiliation: Mechanical Engineering, University of Science and Technology Beijing, Beijing 100083, China.
authors.forename: Qiang
authors.initials: Q
authors.lastname: Meng
authors.affiliation: Soft Matter, Fluidics and Interfaces, University of Twente, Enschede 7500 AE, The Netherlands.
authors.forename: Yali
authors.initials: Y
authors.lastname: Zhang
authors.affiliation: Mechanical Engineering, University of Science and Technology Beijing, Beijing 100083, China.
authors.forename: Jiang
authors.initials: J
authors.lastname: Li
authors.affiliation: Soft Matter, Fluidics and Interfaces, University of Twente, Enschede 7500 AE, The Netherlands.
authors.forename: Rob G H
authors.initials: RG
authors.lastname: Lammertink
authors.affiliation: State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China.
authors.forename: Haosheng
authors.initials: H
authors.lastname: Chen
authors.affiliation: Department of Mechanical Engineering, University of Alberta, Edmonton, Alberta T6G 2G8, Canada.
authors.forename: Peichun Amy
authors.initials: PA
authors.lastname: Tsai