A Computational Model for Flexoelectricity-Driven Contact Electrification

Abstract

Recent theoretical studies show that nanoscale contact on dielectric substrates can induce flexoelectric polarization large enough to drive electron transfer. This has been supported by experimental evidence, indicating that contact electrification is inherently a coupled electromechanical phenomenon. In this work, we develop a computational model for flexoelectricity-driven contact electrification that integrates finite-deformation flexoelectricity with contact mechanics and physically motivated charge transfer. A tunneling transparency function is introduced to regulate the interfacial channel based on the WKB approximation, capturing the irreversible charge trapping during unloading. Three contact scenarios are investigated with specific hypotheses for charge transfer: unbiased metal-dielectric contact driven by surface polarization, biased contact restricted to carriers of a single polarity, and dielectric-dielectric contact where surface states with finite capacity limit the transferable charge. The model is compared with atomic force microscopy measurements on PMMA and PDAP substrates under both biased and unbiased conditions.For contact between identical dielectric materials, we show that geometric asymmetry in surface curvature is sufficient to induce charge separation, with polarity reversal occurring at a critical surface wavenumber. Three-dimensional simulations on random rough surfaces reproduce the mosaic charge distributions observed experimentally, confirming that contact-induced local strain gradient heterogeneity can generate spatially non-uniform charge patterns without introducing any material inhomogeneity.

Figures (28)

• Figure 1: Tunneling transparency factor $\mathcal{T}$ as a function of normal gap distance $g_N$, with $\ell_q = 0.24$ nm and $\Delta_g = 0.012$ nm.
• Figure 2: Schematic of charge transfer in unbiased metal-dielectric contact: (a) large gap with closed tunneling channel; (b) loading with flexoelectric polarization driving charge transfer; (c) unloading with partial charge backflow and edge freezing; (d) complete separation with frozen residual charge.
• Figure 3: Schematic of charge transfer in biased metal-dielectric contact (negative tip bias): (a) initial contact with localized high charge density; (b) further contact with distributed lower charge density.
• Figure 4: AFM contact configuration: (a) experimental setup; (b) axisymmetric computational mesh with spherical tip and dielectric substrate.
• Figure 5: Contact radius as a function of contact force for PMMA substrate, compared with classical Hertz theory.

Theorems & Definitions (1)

• Remark