Adaptive Two Capacitor Model to Describe Slide Electrification in Moving Water Drops

Triboelectric charging between solid surfaces is a well-known phenomenon. It is less obvious that a similar charge separation process can be observed in water drops moving on a hydrophobic surface [1-4]. In so-called slide electrification experiments, neutral water drops slide over a neutral hydrophobic surface, accumulating and leaving behind a net charge [2]. The accumulated drop charge for successive sliding drops decreases and eventually reaches a steady state. On hydrophobic/hydrophilic mixed surfaces, we could even observe a polarity flip in the drop charge upon decreasing the drop rate. To rationalize the charge separation process, we propose an electric circuit model based on capacitors and a voltage generated at the moving three-phase contact line. By introducing a voltage adaptation upon water contact, we can successfully describe drop charge experiments on many surfaces, including the drop-rate-dependent polarity flips. The characteristic adaptation times hint toward molecular processes affecting the solid-liquid charge separation. Thus, our adaptive two-capacitor model enables new insights into the molecular details of the charge separation mechanism.<br/><br/>[1] Yatsuzuka, K., Mizuno, Y., & Asano, K. (1994). Electrification phenomena of pure water droplets dripping and sliding on a polymer surface. <i>Journal of electrostatics</i>, <i>32</i>(2), 157-171.<br/>[2] Stetten, A. Z., Golovko, D. S., Weber, S. A., & Butt, H. J. (2019). Slide electrification: charging of surfaces by moving water drops. <i>Soft Matter</i>, <i>15</i>(43), 8667-8679.<br/>[3] Li, X., Bista, P., Stetten, A. Z., Bonart, H., Schür, M. T., Hardt, S., ... & Butt, H. J. (2022). Spontaneous charging affects the motion of sliding drops. <i>Nature Physics</i>, 1-7.<br/>[4] Bista, P., Stetten, A. Z., Wong, W. S., Butt, H. J., & Weber, S. A. (2022). Adaptive two capacitor model to describe slide electrification in moving water drops. <i>arXiv preprint arXiv:2202.03948</i>.