Chemical Reaction Evolving on a Droplet.

A high-speed camera was used to investigate the early stage of a chemical reaction within a few milliseconds. We focus on the process of color change caused by a droplet containing a pH indicator when impinging on the surface of alkaline solution. Contrary to our expectation, this reaction starts along the equatorial line, and not at the protruding edge of the droplet, where it first touches the reaction partner. Small vertical fingers emerge from the front line within 1.5 ms. The results suggest that the observed deformation of the droplet and heat diffusion play major roles during this early reaction stage. Our investigations contribute to the understanding of short-term transport processes across interfaces, including the onset of unstable behavior of reaction fronts.