pH drives electron density fluctuations that enhance electric field-induced liquid flow

Liquid flow along a charged interface is commonly described by a classical continuum theory, which assumes ionic groups as responsible, uniformly distributed point charge carriers. Within this framework, electric field-induced flow (electro-osmosis or -phoresis) arises from charged ionic groups at the surface that interact with an external field. Changing bulk pH doubles the electrophoresis and osmosis by changing the number of surface charges. Here, we challenge this idea, combining measurements of hydrophobic nanodroplet velocity in water under an external field, all-optical surface charge density, and surface molecular structure measurements with quantum level computations. We show that classical continuum theory fails under the general case that the pH of the solution is varied and that the resulting increase in mobility originates from charge density fluctuations. The force that propels the droplets originates from gradients in the fluctuating electron polarizability that interact with the external electric field. Basic solutions display hydroxide-induced and electric field-enhanced charge asymmetry. This doubles the solutions’ charge conductivity, manifesting as a change in the solutions’ polarizability. The electromagnetic energy gradient responsible for the propelling force is proportional to the solutions’ polarizability, which leads to the remarkable influence of bulk pH on electrophoresis (and -osmosis). This general mechanism deeply impacts a plethora of processes in biology, chemistry, and nanotechnology and provides a unified explanation of how pH influences hydrodynamics.