  The charging and dissolution of mineral surfaces in contact with flowing
liquids are ubiquitous in nature, as most minerals in water spontaneously
acquire charge and dissolve. Mineral dissolution has been studied extensively
under equilibrium conditions, even though non-equilibrium phenomena are
pervasive and substantially affect the mineral-water interface. Here we
demonstrate using interface-specific spectroscopy that liquid flow along a
calcium fluoride surface creates a reversible, spatial charge gradient, with
decreasing surface charge downstream of the flow. The surface charge gradient
can be quantitatively accounted for by a reaction-diffusion-advection model,
which reveals that the charge gradient results from a delicate interplay
between diffusion, advection, dissolution, and desorption/adsorption. The
underlying mechanism is expected to be valid for a wide variety of systems,
including groundwater flows in nature and microfluidic systems.
