Electrostatic charging when conductive liquids dewet solid surfaces
Aaron D. Ratschow1, Lisa S. Bauer1, Pravash Bista2, Stefan A. L. Weber2,3, Hans-Jürgen Butt2, Steffen Hardt1
(1 Technische Universität Darmstadt, Darmstadt, Germany, 2 Max Planck Institute for Polymer Research, Mainz, Germany, 3 Johannes Gutenberg Universtität, Mainz, Germany)
Spontaneous charge accumulation in sliding water drops is ubiquitous in nature and is caused by charge separation at the moving, dewetting contact line. Notably, this charge separation mechanism occurs in conductive liquids whenever they dewet solid surfaces and can lead to electrostatic hazards in the process industries. Despite the growing number of experimental investigations in recent years, the physical mechanism behind the charging remains poorly understood. We identify the origin of charge separation as the dewetting of the immobilized part of the electric double layer (EDL) by the moving contact line. This layer of physically or chemically bound surface charges depends strongly on the local EDL structure, which is disturbed by the vicinity of the gas-liquid interface and the flow in the liquid. We summarize the physics of charge separation in a simple to use analytical model that predicts parametric dependencies on surface chemistry, wetting, and liquid properties. The results agree well with our experiments and numerical simulations and uncover increasing charge separation with increasing dynamic contact angle and counterintuitively with decreasing contact line velocity. Our findings have immediate implications for plant safety, for example for condensation or batch processes where surfaces are frequently dewetted.