Paper Number
RI7 

Session
Rheology and Mobility at Interfaces

Title
Interfacial tensions and film drainage times with surfactant stabilized emulsions: Towards improved liquid-liquid separation

Presentation Date and Time
October 13, 2021 (Wednesday) 1:55

Track / Room
Track 5 / Ballroom 6

Authors (Click on name to view author profile)

  1. Bachnak, Rana B. (University of Minnesota Twin Cities, Mechanical Engineering)
  2. Moravec, Davis B. (Donaldson company)
  3. Hauser, Brad G. (Donaldson company)
  4. Andrew, Dallas J. (Donaldson company)
  5. Dutcher, Cari (University of Minnesota, Mechanical Engineering)

Author and Affiliation Lines (in printed abstract book)
Rana B. Bachnak1, Davis B. Moravec2, Brad G. Hauser2, Dallas J. Andrew2 and Cari Dutcher1
1Mechanical Engineering, University of Minnesota Twin Cities, Minneapolis, MN 55414; 2Donaldson company, Bloomington, MN 55431

Speaker / Presenter 
Bachnak, Rana B.

Keywords
experimental methods; theoretical methods; emulsions; interfacial mobility; micelles; microfluidics; surfactants

Text of Abstract
The interfacial properties of surfactant stabilized liquid-liquid systems inform liquid separations and treatment strategies, such as coalescence. Liquid-liquid separation becomes harder in the presence of surfactants that make droplets more stable. Therefore, the effective removal of the dispersed phase droplets requires understanding of the effect of surfactant presence in these emulsions on their dynamics and stability. In this presentation, we highlight this effect using measurements performed across a range of droplet size, surfactant concentration, and viscosity ratios. The first factor studied here is the interfacial tension (IFT), as well as the characteristic timescale required for the IFT to reach equilibrium. Dynamic IFT measurements are performed here at two length scales: a millimeter scale using pendant drop experiments and a microscale using microfluidic tensiometry, with systems of light and heavy mineral oil containing varied concentrations of SPAN80 surfactant. It is found that the IFT decays faster in the case of micro-scale droplets due to a shift from diffusion-limit to adsorption-limited surfactant transport mechanism. In addition, as expected, the equilibrium IFT decreases with increasing SPAN80 concentration, and the decay rate of the dynamic IFT is higher at greater surfactant concentrations. It was also found that the equilibrium IFT increases with increasing viscosity ratio of the outer to the inner phase, and that the rate of IFT decay is lower at a greater viscosity ratio. The surfactant diffusivity and interfacial adsorption and desorption rates are extracted by fitting a surfactant diffusion and equation of state equations to the dynamic IFT measurements. The second factor studied here is the time for the thin film to drain between two coalescing droplets. Similar to dynamic IFT, the film drainage time is closely tied to the stability of the droplet, with faster film drainage events indicative of less stable emulsions. This relation is shown in this work by presenting the film d