Defying gravity: Drops that climb up a vertical wall of their own accord.

Drops that move uphill on a gradient surface have been introduced in the past. In this paper, however, we present drops that climb a surface that does not have a gradient to begin with. In our study, Octadecylamine in Tetradecane (ODA/TD) drops were placed on either vertical or horizontal mica surfaces, and both the cases show spreading and retraction that initiate the motion of the ODA/TD drops. On horizontal surfaces, initially, the drop spreads in all directions. Then, after some time, which is a function of the solute concentration, the rear edge of the drop jerks in the direction opposite to spreading with a retraction that reminds breaststroke swimming motion: the front sides keep spreading while the back retracts, followed by the sides closing on themselves and pushing all the liquid forward which is the only place that never retracts. The front side of the drop then spreads faster in a way that reminds the circle that the arms make during breaststroke. The back and front sides of the drop continue to shrink and expand, respectively, with a net result of moving forward. The reason this motion can happen, is that the drop self-creates a local interfacial gradient at its surrounding. The direction of this self-induced interfacial gradient is against the gravity for inclined surfaces and is random if the surface is horizontal. Tilting the surface results in a local gradient that is preferentially opposite to the direction of gravity, hence the drop's motion results in climbing up. The drop leaves behind it a surfactant covered, but otherwise dry, surface. To the best of our knowledge such a system has not been explored before.

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