Spontaneous spinning of a dichloromethane drop on an aqueous surfactant solution.

We report a series of experiments carried out with a dichloromethane drop deposited on the surface of an aqueous solution containing a surfactant, cetyltrimethylammonium bromide. After an induction stage during which the drop stays axisymmetric, oscillations occur along the contact line. These oscillations are succeeded by a spectacular spontaneous spinning of the drop. The latter quickly takes the form of a two-tip 'rotor' and the spinning rate stabilizes at a constant value, no longer varying despite the gradual changes of the drop shape and size. The drop eventually disappears due to the continual dissolution and evaporation of dichloromethane. Schlieren visualizations and particle image velocimetry are used to establish a consistent scenario capable of explaining the evolution of the system. The Marangoni effect induced by the dissolution of dichloromethane in the drop vicinity is shown to be responsible for the observed dynamics. Arguments borrowed from dynamical systems theory and from an existing low-order model allow us to explain qualitatively why the system selects the spinning configuration. The geometry of the immersed part of the drop is shown to play a crucial role in this selection process, as well as in the regulation of the spinning rate.

Air humidity effects on water-drop icing.

The canonical problem of the icing of a water drop lying on a cold substrate is revisited to take into account the effects of atmospheric conditions on the icing front kinetics and on the tip formation. Here, we demonstrate both experimentally and theoretically that the air humidity induces liquid-vapor phase change at the icing droplet interface and that the associated heat transfer has a strong influence on both the icing front kinetics and the iced drop shape. The experimental results obtained in this study, as well as results from literature, compare well to a modified Stefan model accounting for the effects of humidity, showing a good agreement with the experimental data of both the front kinetics and tip angle.

Equilibrium thickness of large liquid lenses spreading over another liquid surface.

This article discusses the equilibrium states and more particularly the equilibrium thickness of large lenses of a liquid spread over the surface of a denser liquid. Both liquids are supposed to be nonvolatile and immiscible. Taking into account the effect of intermolecular forces in addition to the sign of the spreading parameters leads to four possible states. The three first are similar to the states of equilibrium of a liquid spread on a solid surface: total wetting where the floating liquid spreads until it reaches an equilibrium thickness on the order of the molecular size, partial wetting where the floating liquid forms a lens of macroscopic thickness in equilibrium with a "dry" bath, and pseudopartial wetting where the floating liquid spreads as a lens of macroscopic thickness in equilibrium with a thin film covering the bath. The last regime, called pseudototal wetting, consists of a macroscopic lens of the floating liquid covered with a thin film of the bath. These four regimes are described through a free-energy minimization, and their equilibrium thicknesses are predicted. A comparison of this model with experimental results available in the literature and dedicated experiments for the pseudototal wetting state are reported.