Contact charge can be gone in a blink of an eye
Everyone knows that shock after walking across a rug or rubbing a balloon on your hair and sticking it to a wall. This effect is called contact electrification, the process by which materials acquire electrical charge when they touch. Better known as ‘static electricity’, it is ubiquitous, and affects phenomena ranging from thunder formation to pollen sticking to the bee’s hair, yet still lacks a unified explanation. A common experimental approach to study the charge left after contact is to image the surface potential with Kelvin probe techniques, most notably Kelvin probe force microscopy (KPFM). In KPFM, a sharp metal tip scans slowly across the surface while recording a voltage caused by local electric charge. Depending on the scan size and resolution, a single image can take minutes to hours to acquire. These KPFM maps are widely used to argue that charge transfer is not homogeneous but instead occurs in spatially heterogeneous patterns. They have also been used to study how charge evolves over space and time, invoking mechanisms such as surface diffusion, surface drift, bulk drift, or combinations thereof. Recently, however, Pertl and Waitukaitis from the Institute of Science and Technology Austria have shed new light on the phenomenon.
In recent work published in Physical Review Letters, Pertl built an automated measurement platform that combines an AFM/KPFM instrument and a contact device consisting of PDMS countersample mounted on a linear actuator. Samples were prepared on gold-coated wafers so that the dielectric layer sits above a grounded electrode — a configuration that turns out to be central to the effect they observed. The protocol they used is straightforward and repeatable: measure the sample in a discharged state, contact it with the PDMS countersample, then return the sample to the same KPFM measurement location for post-contact imaging. Impressively, their experiments were completely automated to eliminate handling artefacts, and switching between the KPFM and contact steps could happen in as little as 30 seconds. This setup prevents accidental charging during handling and allows measurement of nearly all the charge left behind after contact.
Their data show that before contact, KPFM maps are uniform. After contact, however, a striking spatial gradient in the KPFM image is observed: the potential appears to decay in the slow-scan direction which, on first glance, could be mistaken for lateral heterogeneity. However, two careful observations challenge this interpretation. First, averaging the potential along scan lines and plotting versus time shows a clean and continuous decay toward the pre-contact baseline (for PDMS the characteristic relaxation time is ~90 s). Second, scanning a macroscopic sample with a millimetre-scale Kelvin probe reveals no lateral spreading of the charge whatsoever; the potential simply lowers in amplitude. Together these results point away from surface diffusion/drift and toward a bulk process.
These observations can be explained with a simple model. When surface charge is placed on the insulating layer, the resulting field between it and the back electrode drives a current through the bulk proportional to the material’s conductivity. This yields a decay of surface charge, with time constant τ=ϵrϵ0/c, where c is the bulk conductivity and ϵr the dielectric constant. Crucially, the back electrode is not a passive spectator: it provides the source that allows charges to nullify the surface field, producing a spatially uniform bulk discharge that can masquerade as an apparent scan-direction gradient in KPFM images.
To test the model they measured decay dynamics across multiple materials with very different bulk conductivities. The normalized decay curves vary systematically with conductivity: lower conductivity materials relax more slowly, and the measured time constants agree with values predicted from nominal literature conductivities and dielectric constants. These quantitative matches support the bulk-drift picture.
The implications are twofold. First, the results warn that KPFM voltage maps can be deceptive: time dependence and the presence of a grounded back electrode must be considered before attributing observed spatial patterns to transferred charge. Second, they re-orient attention toward bulk conductivity as key determinant of how long contact-generated charge remains observable at the surface. This does not negate all previous observations of lateral charge features (which can occur under other conditions, e.g., highly localized injection or humidity-driven surface transport), but it does urge care in interpretation and suggests that some reported dynamics may instead reflect bulk relaxation dynamics.
More broadly, the work provides a simple, testable framework for separating surface and bulk contributions in studies of contact electrification. Because τ depends only on material constants, the model gives clear experimental predictions and a pathway for reconciling apparently
contradictory reports in the literature. For practitioners using KPFM or SKP, the take-home is this: account for sample grounding and be aware of lateral and vertical time dependence before concluding that a particular surface charge pattern exists.