Phase-Dependent Surfactant Transport on the Microscale: Interfacial Tension and Droplet Coalescence

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.

Development of Thermally Responsive PolyNIPAm Microcarrier for Application of Cell Culturing—Part I: A Feasibility Study

“…The interfacial tensions between water and cyclohexane containing 0.02 g/mL of Span 80 ® rises from 6.30 ± 0.03 mN/m to 6.58 ± 0.13 mN/m upon adding in 10 wt% of CTAB. A similar finding was reported by Chen et al (2020) in a study of the stability of dispersed droplets in light mineral oil containing Span 80 ® [ 42 ]. It was found that the droplets tended to coalesce as the interfacial tension of the immiscible liquids increased, leading to the formation of larger particles [ 42 ].…”

“…A similar finding was reported by Chen et al (2020) in a study of the stability of dispersed droplets in light mineral oil containing Span 80 ® [ 42 ]. It was found that the droplets tended to coalesce as the interfacial tension of the immiscible liquids increased, leading to the formation of larger particles [ 42 ]. Moreover, the particle size of polymer is affected by different surfactant types.…”

Unraveling the Interaction of Water-in-Oil Emulsion Droplets via Molecular Simulations and Surface Force Measurements

“…B2 had the highest F max . Although there have been reports that the dynamic IFT of a droplet can be size-dependent, , the diffusion of surface-active molecules from the bulk to the interface played important roles in causing such differences. By applying a different bulk concentration in the simulations to effectively ″expedite″ the diffusion, we might have reached a similar surface coverage at the water/toluene interface as compared to experiments, which led to the good qualitative comparison.…”

Surface Tension of the Oxide Skin of Gallium-Based Liquid Metals

“…Generally, surface tension measurements of liquids are performed via force-based measurements, i.e., Wilhelmy plate 19 and du Nouÿ ring, 19 pressure-based approaches, i.e., pendant drop, 20 (maximum) bubble pressure, 21 and spinning drop method, 22 and so on. 23 For the measurement of metals, the surface tension is measured via sessile drop, 24 pendant drop, 25 and levitated drop methods 26 in a high-temperature furnace, while oxygen is excluded. A prerequisite for these methods is that an equilibrium is established in the time scale of the experiment.…”