The movement of pepper on water when soap is added is a classic everyday demonstration of the Marangoni effect driven by changes in surface tension, and no formal citation is needed for this basic physical observation.
The claim describes a simple, widely observed physical phenomenon (the pepper-and-soap surface tension experiment) that functions as common knowledge. None of the provided papers directly study or test pepper on water, but rather cover complex engineering, fluid dynamics, and biological applications of the Marangoni effect. Therefore, this falls under the COMMON_KNOWLEDGE category.