Danov Krassimir D, Valkovska Dimitrina S, Kralchevsky Peter A
Laboratory of Chemical Physics and Engineering, Faculty of Chemistry, University of Sofia, 1 James Bourchier Avenue, 1164 Sofia, Bulgaria.
J Colloid Interface Sci. 2003 Nov 1;267(1):243-58. doi: 10.1016/s0021-9797(03)00596-4.
We investigate the effect of surfactant on the hydrodynamic stability of a thin liquid film formed between two emulsion drops or gas bubbles, which are moving along a narrow capillary. A ganglion (deformed drop or bubble in a pore) is covered by an adsorption monolayer of surfactant. Due to the hydrodynamic viscous friction, the surfactant is dragged from the front part of a moving ganglion toward its rear part. Consequently, the front and rear parts are, respectively, depleted and enriched in adsorbed surfactant. When such two ganglia move one after another, surfactant molecules desorb from the rear part of the first ganglion and are transferred by diffusion, across the intermediate liquid film, to the front part of the second ganglion. This leads to the appearance of a diffusion-driven hydrodynamic instability, which may cause coalescence of the two neighboring drops or bubbles. The coalescence occurs through a dimple-like perturbation in the film thickness, which is due to a local lowering in the pressure caused by a faster circulation of the liquid inside the film, which in turn is engendered by the accelerated surfactant diffusion across the thinner parts of the film. The developed theory predicts the critical distance between the two ganglia, which corresponds to the onset of coalescence, and its dependence on the radius of the capillary channel, velocity of motion, surfactant concentration and type of the operative surface forces. The results can be useful for a better understanding and quantitative description of the processes accompanying the flow of emulsions and foams though porous media.
我们研究了表面活性剂对在两个乳液滴或气泡之间形成的薄液膜流体动力学稳定性的影响，这两个液滴或气泡沿着狭窄的毛细管移动。一个神经节（孔隙中的变形液滴或气泡）被表面活性剂的吸附单层覆盖。由于流体动力粘性摩擦，表面活性剂从移动神经节的前部被拖向其后部。因此，前部和后部的吸附表面活性剂分别减少和富集。当这样的两个神经节相继移动时，表面活性剂分子从第一个神经节的后部解吸，并通过扩散穿过中间液膜转移到第二个神经节的前部。这导致出现扩散驱动的流体动力学不稳定性，这可能会导致两个相邻液滴或气泡的聚结。聚结通过膜厚度中的酒窝状扰动发生，这是由于膜内液体更快循环导致的局部压力降低引起的，而这又是由表面活性剂在膜较薄部分的加速扩散所引发的。所发展的理论预测了两个神经节之间的临界距离，该距离对应于聚结的开始，以及它对毛细管通道半径、运动速度、表面活性剂浓度和有效表面力类型的依赖性。这些结果有助于更好地理解和定量描述乳液和泡沫通过多孔介质流动时伴随的过程。
