It is Separation, Not Contact: Electrification at Water-Hydrophobe Interfaces during Wetting-Dewetting Cycles

When water contacts hydrophobic materials─such as air, hydrocarbons, or fluorocarbons-the interface acquires charge, yet how dynamic wetting-dewetting governs this electrification remains largely unexplored. Here, using controlled pipetting experiments with hydrophobic capillaries, we show that electrification at water-hydrophobe interfaces is governed by liquid-solid separation rather than contact formation. By systematically varying liquid uptake and release rates over 3 orders of magnitude, we find that the charge transferred during a pipetting cycle depends nonlinearly on the velocity and acceleration of the receding liquid meniscus, while the advancing (uptake) motion contributes negligibly. High-resolution charge measurements reveal that, although net charge is conserved, the charge generated during liquid release in a given cycle directly influences the charge acquired during liquid uptake in the subsequent cycle. These observations uncover a previously unrecognized intercycle coupling in water-hydrophobe electrification and demonstrate that charge conservation is more appropriately described across successive wetting-dewetting cycles rather than strictly within an individual cycle. This intercycle formulation accurately captures charge balance under dynamically varying flow conditions and resolves apparent inconsistencies observed when release rates are changed between cycles. These findings hold across hydrophobic capillaries with negative, near neutral, and positive surface charge densities. Thus, our report establishes liquid-solid separation kinetics as the dominant control parameter for electrification at water-hydrophobe interfaces and highlight the inherently history-dependent nature of interfacial charging. These insights advance the fundamental understanding of water-hydrophobe electrification and have implications for droplet-based technologies, micro- and nanofluidics, and liquid-handling processes.