Generic Rules for Distinguishing Autophoretic Colloidal Motors.

Distinguishing the operating mechanisms of nano- and micromotors powered by chemical gradients, i.e. "autophoresis", holds the key for fundamental and applied reasons. In this article, we propose and experimentally confirm that the speeds of a self-diffusiophoretic colloidal motor scale inversely to its population density but not for self-electrophoretic motors, because the former is an ion source and thus increases the solution ionic strength over time while the latter does not. They also form clusters in visually distinguishable and quantifiable ways. This pair of rules is simple, powerful, and insensitive to the specific material composition, shape or size of a colloidal motor, and does not require any measurement beyond typical microscopy. These rules are not only useful in clarifying the operating mechanisms of typical autophoretic micromotors, but also in predicting the dynamics of unconventional ones that are yet to be experimentally realized, even those involving enzymes.