Abstract:
Surface tension or buoyancy-driven instabilities are at the origin of shape modifications and motion of drops. They result from the coupling of physico-chemical processes among which evaporation, solubilization, solute transfer, surfactant adsorption/desorption, aggregation and chemical reactions. Desired processes can be selected by the choice of chemicals and experimental conditions [1]. We have observed a remarkable succession of dynamical regimes for dichloromethane (CH2Cl2, 25 μL) drops setting on aqueous solutions of cetyltrimethylammonium bromide (CTAB)[2]. The surfactant concentration, used as a control parameter, induces an amazing range of shapes and motion patterns. At low surfactant concentration, they are related to spreading and dewetting processes giving rise to directional motion or pulsations. At intermediate concentrations, rotational motion becomes predominant. At the highest concentration, drops have a polygonal rim featuring several small tips. Coupled to these shape-forming processes is the emission of smaller droplets. This system is the first example of such a sequence of highly ordered patterns induced by coupled hydrodynamic instabilities. We will mainly focus on the pulsating regimes observed in the presence of 0.5 mM CTAB in the aqueous phase [3]. The detailed analysis including the drop behavior during the induction period (before pulsations) [4], volume effects and container size was performed. The addition of CTAB also in the drop modifies the observed oscillations: frequency is increased but also the resulting patterns. The drop shows a perfectly circular expanding rim but the most impressive changes are obtained during the receding phase of the film. A flower-like pattern of an amazing regularity is obtained together with the emission of smaller droplets in a mode never observed before. The patterns are related to dynamical features which relevance covers most of the behaviors obtained in any application involving liquid/liquid interfacial processes such as spreading, dewetting, Marangoni-driven motion or drop division.
1. V. Pimienta and C. Antoine, Curr. Opin. Colloid Interface Sci., 19, 290, (2014).
2. V. Pimienta, M. Brost, N. Kovalchuk, S. Bresch and O. Steinbock, Angew. Chem. Int. Ed 50, 10728(2011).
3. C. Antoine and V. Pimienta ,Langmuir, 29, 14935 (2013).
4. C. Antoine, J. Irvoas, K. Schwarzenberger, K. Eckert, F. Wodlei, and V. Pimienta, J. Phys. Chem. Lett. 7, 520, (2016).