Spontaneous spinning of a dichloromethane drop upon a CTAB solution

An intriguing phenomenon arises when a dichloromethane drop is laid upon a 10 mM solution of CTAB, a surfactant. The circular drop contour starts to oscillate, leading potentially to the triangular mode 3 of deformation. The system keeps evolving and turns into a self-rotating system with a characteristic helix-like shape. The spinning lasts until the dichloromethane drop entirely dissolves and evaporates, which takes approximately 12 seconds for a 20 uL initial drop. To study the phenomenon, and elucidate the origin of this spontaneous symmetry breaking, we record the drop spinning with a diascopic optical setup. This device allows us to determine the drop shape and measure the spinning velocity. Surprisingly, this velocity is found to be very stable during the spinning phase and does not depend on the depth of water. In a complementary approach, we determine the horizontal fluid velocity field around the drop at several distances from the surface by using two-dimensional PIV. Owing to the three surface tensions at the contact line, a radial Marangoni flow moves the water surface outward. This primary diverging flow combines with flows resulting from the high density (1.33) and volatility of the dichloromethane, resulting in a complex and composite pattern around the drop.