RESUMEN
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.
RESUMEN
A liquid drop may spread faster on surfaces when surfactants are added. Here we show that after some time the spreading in such systems can, under certain conditions, spontaneously reverse to retraction and the droplet pulls itself back, receding from areas it has just recently wetted, elevating its center of mass in a jerklike motion. The duration from drop placement to the onset of retraction ranges from hours to less than a second primarily as a function of surfactant concentration. When the retraction is asymmetric, it results in drop motion, and when it is symmetric, the mass of the drop collects itself on its spot. This phenomenon, which was predicted theoretically in 2014, is apparently a general one for drops with surfactants; however, other factors, such as evaporation and contamination, prevented its observance so far.