The impact of viscous stress and Marangoni stress on the micro-scale droplet coalescence*

Liquid-liquid droplet emulsions are ubiquitous in systems such as bilgewater, food processing, and water-entrained diesel fuels. The dispersed droplets are usually stabilized by the surfactant molecules absorbed onto the interface, which reduces the interfacial tension (IFT) force, and therefore, inhibits the droplet coalescence. In addition to IFT, other factors can also impact droplet stability. When two droplets approach each other, a thin film forms between the droplets and must drain before they can coalesce. Thus, the emulsion stability is determined by the time scale for the film drainage. Studies have shown that the film drainage time can be influenced by the viscous stress and Marangoni stress at the droplet interface. Based on different liquid-liquid systems, both viscous stress and Marangoni stress will inhibit the film drainage to a certain extent. In this work, systematic droplet coalescence experiments will be presented. Systems with different viscosity ratios are investigated to understand the impact of viscous stress. In addition, the film drainage time is also measured for surfactants with different concentrations. Scaling analysis based on the Marangoni number will be performed to understand the effect of the Marangoni stress on the droplet coalescence.