Running droplet of interfacial chemical reaction flow

Hydrofluoric acid (HF) etching of a silicon surface is demonstrated as an efficient means to create an interfacial chemical reaction flow, thus leading to self-propelled water-droplet motion. Before and after HF etching, the silicon surface exhibits a significant free energy change, represented by the increase of water contact angle from 0 to 60°. This favors self-propelled HF droplet motion with high-speed and long-distance, and in particular enables uphill motion. Even for a HF droplet 10 microliters in volume, vertical climbing along silicon strips is permitted. By investigating the temperature-dependent motion velocity, it confirms that the velocity is in positive proportion to the HF reaction rate.