Liquid-film Motor: Physical Mechanism

A liquid film that is under the action of two electric forces, an external electric field parallel to the film and a lateral voltage difference applied to both edges of the film, exhibits a universal rotational flow. In this article, we revisit this phenomena by considering an idealized so-called liquid-film motor and provide a theoretical description of the underlying physical mechanism that is responsible for the rotation. In this theory, the external electric filed induces a non-uniform distribution of free charges on the film then the internal field, resulted mainly from the voltage difference, will exert forces on these charges and subsequently induce a rotational flow in the ambient fluid. We show, how the fields contribute in developing a universal flow pattern.