Liquid Flow Reversibly Creates a Macroscopic Surface Charge Gradient

TL;DR

This study shows that liquid flow over mineral surfaces can reversibly create a spatial surface charge gradient, influenced by complex reaction-diffusion-advection processes, with implications for natural and microfluidic systems.

Contribution

It demonstrates for the first time that liquid flow induces a reversible surface charge gradient on mineral surfaces, explained by a reaction-diffusion-advection model.

Findings

Flow creates a reversible surface charge gradient.

The charge gradient is explained by a reaction-diffusion-advection model.

The mechanism applies broadly to natural and microfluidic systems.

Abstract

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.