Solid-liquid contact electrification in sliding drops

Abstract

A surface in contact with a liquid is known to form an electric double layer, EDL, consisting of fixed charges at the surface and a diffuse layer in the liquid. Recently, it has been observed that the moving contact line atop a hydrophobic surface can deposit some of these charges from the EDL onto the surface, leading to charge separation, also known as contact charge separation. The charge separation between the inclined hydrophobic surface and the sliding drops is known as slide electrification, where moving drops leave behind some surface charge. In doing so, the drops accumulate an equal and opposite charge. Lately, such charge separation processes have been utilized for energy harvesting. Furthermore, it has been shown that these separated charges can influence the motion of the sliding drop as well as the dynamic contact angle. Nevertheless, the physical understanding of the charge separation process is not complete.

This dissertation introduces two novel experimental setups to determine the relevant physical parameters in slide electrification. The super probe method measures the drop potential of sliding drops after a certain slide length. Using this setup, we quantify the drop potential dependence on slide length and drop interval. The subsurface probe method measures the capacitive current resulting from charge separation. This allows us to quantify the charge separation at the receding contact line and measure surface neutralization/discharge time, which refers to the time it takes for the surface to recover by losing charges between drops, either through the substrate or the surrounding environment.

Moreover, based on the experimental observations, we develop a comprehensive model to describe the behaviour of multiple drops sliding over an inclined hydrophobic surface. The model explores the physical and chemical processes involved in building the EDL and reveals the relationship between the EDL and the slide electrification. We demonstrate that charge separation occurs at the receding contact line and derive a simple relationship illustrating its dependence on the drop charge. We demonstrate that as the drop slides down the substrate, the drop's charge compensates for the surface charge in the EDL. When the drop charge fully compensates for the surface charge, the drop potential can reach up to kilovolts. Our model reveals that this drop potential is an amplification of the surface/zeta potential.

Furthermore, we show that the substrate on which the hydrophobic surface is coated, along with the surrounding environment, plays a vital role in the surface neutralization time. We provide a method to determine this surface neutralization time and incorporate it into the model to describe the drop charge dependence in the drop interval.

The physical insights into slide electrification from this work offer a new window into the EDL at the solid-liquid interface. Similarly, a better understanding of surface neutralization helps improve the conversion of a sliding drop's kinetic energy into electrical energy.