Fundamental study of surface charging in solid-liquid systems
Creators: Chen,Zhixiang
Publisher: name: University of Alberta Library
Year: 2022

<p>Surface charging in solid-liquid systems is an elementary process influencing the performance of various industrial processes such as colloidal production, oil transportation, green energy harvesting and self-power sensor fabrication. The surface charging of solids in various liquid systems is possibly influenced by ionization and electron transfer, although this assumption has not been verified. <br>
The focus of this thesis is the fundamental understanding of surface charging between a solid-nonpolar solvent or solid-water systems. A unique method was developed based on a custom designed Triboelectric Nanogenerator (TENG). Two surfaces of the TENG’s triboelectric materials, water and Teflon, possess opposite charges, which create an electric potential difference between the electrodes as they separate. Connecting an impedance causes a current to flow between the electrodes, which screens up the electric field created by the separated charged surfaces. Bringing the two surfaces back into contact results in a change in the potential difference between the two electrodes, causing a backward flow of current. This alternative current (AC) can be observed when the cycle is repeated. Experimental parameters can be flexibly adjusted, including liquid and surface properties.<br>
Through the design of the surface microstructure, we revealed the influence of the curvature on surface charging in the liquid-solid made TENG. The convex surface (negative curvature) is more conducive to the electron transfer with liquid drops than that of the concave surface (positive curvature). A statistical trend was found in which a smaller curvature would typically lead to a higher charging rate of negative charges after contact electrification (CE). After contact separation, the charge transfer from the surface to the atmosphere followed an exponential decay at a fixed temperature. In contrast with the curvature effect on the triboelectric charge generation, the charges on the concave surface were more likely to be emitted into the atmosphere than those on the convex surface. Based on these experimental results, we propose a curvature-dependent charge transfer model for distinct materials CE by introducing curvature-induced energy shifts of the surface states. <br>
The solid surface chemical properties are then changed by trapping a layer of lubricant oil at the solid porous medium surface, leading to superhydrophobic feature and having low friction to water droplets. Increasing the lubricant volume caused a sharp decrease in the surface charging capability. This is most likely because the water-lubricant-infused solid CE is a combination of water-lubricant and water-solid CEs, whereas the triboelectric charges generated from the liquid-liquid CE are weaker than a water drop impacting on a solid. Theoretical calculations and atomic force microscope (AFM) measurements showed that a thin layer of oil (20 nm in thickness) on the porous solid surface was enough to preserve the advantage of low contact hysteresis if the moving velocity of the droplet on the surface surpassed a threshold (e.g., 0.3 mm/s). Importantly, we integrated the slippery lubricant-infused porous surface (SLIPS) with transistor-inspired architecture to produce a robust single-electrode triboelectric nanogenerator (SLIPS-SE-TENG) that could harvest triboelectric energy in all kinds of weather. This SLIPS-SE-TENG enhanced the instantaneous short-circuit current (~3 μA) by two orders of magnitude over an equivalent device (~0.02 μA) that has been previously reported in the literature.</p>