Basheva Elka S, Kralchevsky Peter A, Danov Krassimir D, Stanimirova Rumyana D, Shaw Neil, Petkov Jordan T
Department of Chemical and Pharmaceutical Engineering, Faculty of Chemistry and Pharmacy, Sofia University, Sofia 1164, Bulgaria.
Department of Chemical and Pharmaceutical Engineering, Faculty of Chemistry and Pharmacy, Sofia University, Sofia 1164, Bulgaria.
J Colloid Interface Sci. 2020 Sep 15;576:345-355. doi: 10.1016/j.jcis.2020.05.048. Epub 2020 May 19.
New dynamic phenomena can be observed in evaporating free liquid films from colloidal solutions with bimodal particle size distribution. Such distributions are formed in a natural way in mixed (slightly turbid) solutions of cationic and anionic surfactants, where nanosized micelles coexist with micronsized precipitated particles.
Without evaporation of water, the films thin down to thickness < 100 nm. Upon water evaporation from the film, one observes spontaneous film thickening (above 300 nm) and appearance of a dynamic vortex with a spot of thinner film in the center. The vortex wall has a stepwise profile with step-height equal to the effective micelle diameter (ca. 8 nm) and up to 20-30 stratification steps.
For thicknesses greater than 100 nm, stratification in foam films from micellar solutions has never been observed so far. It evidences for the formation of a thick colloidal crystal of micelles in the evaporating film. The role of the bigger, micronsized particles is to form a filtration cake in the Plateau border, which supports the thick film. The developed quantitative mechanical model shows that the stepwise vortex profile is stabilized by the balance of hydrodynamic and surface tension forces. Vortex is observed not only in films from catanionic surfactant solutions, but also in films from silica and latex particle suspensions, which contain smaller surfactant micelles.
在具有双峰粒径分布的胶体溶液蒸发形成的自由液膜中，可以观察到新的动态现象。这种分布以自然方式形成于阳离子和阴离子表面活性剂的混合（略浑浊）溶液中，其中纳米尺寸的胶束与微米尺寸的沉淀颗粒共存。
在不蒸发水的情况下，液膜变薄至厚度小于100纳米。当液膜中的水蒸发时，可以观察到液膜自发增厚（超过300纳米），并出现一个动态涡旋，其中心有一层较薄的液膜。涡旋壁具有阶梯状轮廓，阶梯高度等于有效胶束直径（约8纳米），且有多达20 - 30个分层阶梯。
到目前为止，对于厚度大于100纳米的情况，在胶束溶液的泡沫液膜中从未观察到分层现象。这证明在蒸发的液膜中形成了胶束的厚胶体晶体。较大的微米尺寸颗粒的作用是在 Plateau 边界形成滤饼，支撑厚液膜。所建立的定量力学模型表明，阶梯状涡旋轮廓通过流体动力学力和表面张力的平衡得以稳定。不仅在阴阳离子表面活性剂溶液的液膜中观察到涡旋，在含有较小表面活性剂胶束的二氧化硅和乳胶颗粒悬浮液的液膜中也观察到了涡旋。
