Abstract
Liquid-liquid emulsion systems are usually stabilized by additives, known as surfactants, which can be observed in various environments and applications such as oily bilgewater, water-entrained diesel fuel, oil production, food processing, cosmetics, and pharmaceuticals. One important factor that stabilizes emulsions is the lowered interfacial tension (IFT) between the fluid phases due to surfactants, inhibiting the coalescence. Many studies have investigated the surfactant transport behavior that leads to corresponding time-dependent lowering of the IFT. For example, the rate of IFT decay depends on the phase in which the surfactant is added (dispersed vs continuous) due in part to differences in the near-surface depletion depth. Other key factors, such as the viscosity ratio between the dispersed and continuous phases and Marangoni stress, will also have an impact on surfactant transport and therefore the coalescence and emulsion stability. In this feature article, the measurement techniques for dynamic IFT are first reviewed due to their importance in characterizing surfactant transport, with a specific focus on macroscale versus microscale techniques. Next, equilibrium isotherm models as well as dynamic diffusion and kinetic equations are discussed to characterize the surfactant and the time scale of the surfactant transport. Furthermore, recent studies are highlighted showing the different IFT decay rates and its long-time equilibrium value depending on the phase into which the surfactant is added, particularly on the microscale. Finally, recent experiments using a hydrodynamic Stokes trap to investigate the impact of interfacial surfactant transport, or "mobility", and the phase containing the surfactant on film drainage and droplet coalescence will be presented.
| Original language | English (US) |
|---|---|
| Pages (from-to) | 14904-14923 |
| Number of pages | 20 |
| Journal | Langmuir |
| Volume | 36 |
| Issue number | 49 |
| DOIs | |
| State | Published - Dec 15 2020 |
Bibliographical note
Funding Information:The authors thank Prof. Charles Schroeder, Dr. Anish Shenoy, and Dinesh Kumar from the University of Illinois at Urbana–Champaign for assistance in setting up the Stokes trap. The authors also thank Prof. Joseph Zasadzinski from the Department of Chemical Engineering and Materials Science at the University of Minnesota for helpful discussions. This material is based upon work supported by the Humphreys Engineer Center Support Activity under Contracts No. W912HQ18C0024 and W912HQ20C0041, corresponding to DOD Strategic Environmental Research and Development Program (SERDP) projects WP18-1031 and WP19-1407. The DOD SERDP support includes support for Y.C. This work was also partially funded and carried out in collaboration with the Donaldson Company, including support for S.N. The authors acknowledge helpful discussions with colleagues at the Donaldson Company, including Dr. Davis Moravec, Dr. Brad Hauser, and Dr. Andrew Dallas. Portions of this work were conducted at the Minnesota Nano Center, which is supported by the National Science Foundation through the National Nano Coordinated Infrastructure Network (NNCI) under award number ECCS-1542202.
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© 2020 American Chemical Society.