Observation of chemical reactions induced by impact of a droplet

Abstract

In order to investigate, how a chemical reaction starts and develops, we select the moment of the impact of a droplet falling into a reactive solution and observe the initial stage of the reaction with a high-speed camera. As examples, we use the pH indicator system with bromothymol blue (BTB) reaction, as well as the Belousov-Zhabotinsky (BZ) reaction forming an excitable medium, in which superthreshold disturbance propagates as an excitation wave and thus gives rise to a reaction-diffusion structure. Focusing on the BTB solution, we observe the color change caused by the droplet containing a pH indicator when impinging on the surface of alkaline solution. Contrary to our expectation, this reaction starts at the equatorial line, and not at the protruding edge of the droplet, where it first gets into touch with its reaction partner. Small vertical fingers emerge from the front line within 1.5 ms. Some arguments make it is likely that heat diffusion is responsible for the finger formation. For the BZ reaction, where due to a redox reaction the color changes from red to blue and vice versa, we do not observe this color change in our experiment. However, the effects on the drop shape (from spherical to ellipsoidal) is the same as observed in the BTB solution. Independently of the chemical systems, a thin needle-like tip developed from the protruding edge within about 300 μs after the drop has touched the solution surface. The emergence of this thin pin cannot be due to chemical processes, but to the physical impact of the droplet.