Paper Number
FE11 

Session
Foams, Emulsions, Surfactants, and Micelles

Title
The impact of viscous stress and Marangoni stress on the micro-scale droplet film drainage time

Presentation Date and Time
October 11, 2021 (Monday) 4:10

Track / Room
Track 3 / Meeting Room A-B

Authors (Click on name to view author profile)

1. Chen, Yun (University of Minnesota, Mechanical Engineering)
2. Dutcher, Cari (University of Minnesota, Mechanical Engineering)

Author and Affiliation Lines (in printed abstract book)
Yun Chen and Cari Dutcher
Mechanical Engineering, University of Minnesota, Minneapolis, MN 55455

Speaker / Presenter 
Chen, Yun

Keywords
experimental methods; emulsions; microfluidics; surfactants

Text of Abstract
Liquid-liquid droplet emulsions are widely observed in various applications such as bilgewater, food processing, and water-entrained diesel fuels. The dispersed droplets are usually stabilized by the surfactant molecules that are adsorbed onto the droplet interfaces. The presence of the surfactant molecules usually causes the reduction of droplet interfacial tension, and therefore, inhibits the droplet coalescence. Despite the impact of interfacial tension on the droplet coalescence, other factors will also affect the droplet stability. When two droplets are moving close to each other, a thin film forms between the two droplets and must drain before they can coalesce. In this scenario, the time scale for the film drainage is significant to quantify the emulsion stability. Studies have shown that this film drainage time can be influenced by the viscous stress and Marangoni stress at the droplet interface. The viscous stress at the interface is determined by the viscosity ratio between the dispersed and continuous phases, while the Marangoni stress is induced by the gradient of the adsorbed surfactant molecules along the interface. For different liquid-liquid systems, both viscous stress and Marangoni stress can affect the film drainage to a certain extent, which will be the focus of this work. In this presentation, systematic droplet coalescence experiments will be presented. In particular, systems with different viscosity ratios between the dispersed and continuous phases are studied to understand the impact of viscous stress. In addition, the film drainage time is also characterized for surfactants with different concentrations to understand the impact of the Marangoni stress on the droplet coalescence.