Complex shapes and dynamics of dissolving drops of dichloromethane.

There is a growing interest in synthetic, chemical systems capable of undergoing autonomous shape changes and/or self-motion. Important examples include solid objects such as catalytic Au/Pt nanorods, mechanically responsive gels driven by oscillating reactions, and liquid systems in which self-motion is induced by surface-tension gradients. The latter class of systems includes iodine/iodide-containing oil droplets on glass surfaces under aqueous solutions of stearyltrimethylammonium chloride as well as drop motion on an alkylsilane-treated silicon surface with spatial “wettability” changes. Droplet motion on air–water interfaces is usually driven by aMarangoni effect involving temperature or concentration gradients. A typical example are pentanol droplets on water, which depending on the drop volume, perform erratic or unidirectional motion and also show very disorganized forms of droplet fission. This fission can extend from the millimeter-scale down to nanoscopic micelles. Herein, we investigate the dynamics of water-saturated dichloromethane (CH2Cl2, 25 mL) droplets on aqueous solutions of cetyltrimethylammonium bromide (CTAB). Figure 1 is a qualitative phase diagram describing the macroscopic dynamics in the CH2Cl2/CTAB system in terms of the elapsed reaction time and the surfactant concentration. The data are representative for the fourth and fifth drops added. The diagram shows a variety of complex drop shapes and dynamics that we identified to be the most characteristic ones. For each concentration we have studied the dynamics of five successive drops. In general these dichloromethane-accumulating experiments reveal no marked differences; however, the fourth and fifth drops deviate from their predecessors during the late stages of dissolution. These altered dynamics typically match the behavior of drops at a slightly higher CTAB concentration. The life time of the dichloromethane drops varies systematically between approximately 20 and 90 s. This large range is mainly caused by changes in the initial induction period during which drops are stationary and have a circular rim. Complex phenomena are found only for surfactant concentrations above a critical value [CTAB]crit 0.25 mmolL . In the absence of surfactant or below [CTAB]crit, the drops spread out over a large area and solubilize rapidly (< 15 s) without noteworthy macroscopic features. For surfactant concentration close to [CTAB]crit (left column in Figure 1), the initial dichloromethane drops are relatively flat. We also observe that the water surface around the drop supports a disk-shaped film. The macroscopic dynamics involve several successive stages: During the first stage, the edge of the film breaks into small droplets that are continuously ejected and quickly disappear. Then the drop abruptly moves away from the center of the film to maneuver back and forth along a nearly stationary line. During these lateral oscillations, each directional change causes the expulsion of a line of small droplets. After a while, the drop starts to move steadily along a circular orbit larger than its own diameter. Finally all motion ceases, the drop becomes circular again, shrinks and vanishes. At a surfactant concentration of 0.5 mmolL 1 (second column in Figure 1), the dichloromethane drops undergo a similar sequence of motion patterns. However, the dynamics Figure 1. Qualitative description of the drop evolution in the CH2Cl2/ CTAB system at five different concentrations of the surfactant CTAB. The typical life time of the dissolving droplets ranges between 20 and 90 s. The time axis is not to scale as the diagram emphasizes distinct, successive states in the drop evolution. Single arrows indicate rotation of the drop around its geometrical center. The double arrow indicates that the drop moves back and forth along a fixed line. The field of view of all frames corresponds to 13 13 mm.