https://www.sciencedaily.com/releases/2021/02/210226121303.htm

Jeff Moran, an assistant professor of mechanical engineering in the Volgenau School of Engineering, and colleagues from the University of Washington in Seattle studied self-propelled half-platinum/half-gold rods that "swim" in water using hydrogen peroxide as a fuel. The more peroxide there is, the faster the swimming; without peroxide in pure water, the rods don't swim.

In this work, they set out to understand what happens when these artificial microswimmers are placed in a fluid reservoir containing a gradient of hydrogen peroxide--lots of peroxide on one side, not much on the other side.