Mobile Surface Charge Can Immobilize the Air/Water Interface

Abstract

Recent Surface Force Apparatus measurements on thin film drainage as a bubble approaches a surface are reinterpreted in terms of a new model for the air/water interface. In this model, surface charge at the interface can be convected and can diffuse along the surface as the film drains. This creates surface tension gradients, since surface tension includes a charge-dependent contribution from the double-layer free energy. Although this electrocapillary effect is relatively small, we show here that the gradients are large enough to create Marangoni effects that influence the hydrodynamic flow in real systems. The new model can account for observed changes in hydrodynamic boundary condition from mobile initially to immobile during early stages of film drainage, and back to partially mobile as drainage progresses. Experimental film profiles for a millimeter-size bubble in 1 mM KCl solution driven toward a mica surface at 27 μm/s are reasonably well described with this mobile surface charge model. At longer times, there are still features of the experimental data that remain to be explained, which suggests further modeling is warranted.

Local Meniscus Curvature During Steady-State Evaporation from Micropillar Arrays

“…Yang et al recently demonstrated better agreement with modeling and experiments using a no-slip boundary condition for capillary-driven flow in microchannels . This liquid–gas interface immobilization finding is not unique , but also not uncontested . Marangoni effects, due to surface tension gradients, have been suggested as one possible explanation for how the liquid–air interface could be immobilized .…”

“…21 This argument is made in the context of prior literature, which has demonstrated how surface tension gradients can lead to no-slip behavior and the mechanisms that can generate surface tension gradients including surfactant contamination 23 or mobile surface charge. 22 Because of heating in our tests, there also must be surface tension gradients present because of the temperature dependence of water surface tension. This thermal-driven effect in porous media is well-known and can give rise to Marangoni convective flows.…”

“…This liquid–gas interface immobilization finding is not unique , but also not uncontested . Marangoni effects, due to surface tension gradients, have been suggested as one possible explanation for how the liquid–air interface could be immobilized . Meanwhile, the presence of nontrivial Marangoni effects are expected in thin film evaporation due to thermal gradients .…”

“…24 Marangoni effects, due to surface tension gradients, have been suggested as one possible explanation for how the liquid−air interface could be immobilized. 22 Meanwhile, the presence of nontrivial Marangoni effects are expected in thin film evaporation due to thermal gradients. 25 In this work, we consider both no-slip and slip conditions when numerically determining geometric-specific, curvature-dependent permeabilities.…”

Interfacial Tension Sensor for Low Dosage Surfactant Detection

“…The motion of gas bubbles is known to be affected by even the smallest amount of surface-active impurities, as these molecules adsorb at the gas-liquid interface and change the tangential mobility at the bubble surface [1][2][3]. In high-purity water (or surface-active molecule free) electrolyte solutions, in the low shear stress case (i.e., smaller bubbles or gasliquid interface moving slowly), the gas-liquid interface deviates from being tangentially mobile [4][5][6][7][8]. For the higher shear stress (i.e., larger bubbles or gas-liquid interface moving fast) the gas-liquid interface stays tangentially mobile [9,10].…”