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.