Single-Collision Statistics Reveal a Global Mechanism Driven by Sample History for Contact Electrification in Granular Media

Models for same-material contact electrification in granular media often rely on a local charge-driving parameter whose spatial variations lead to a stochastic origin for charge exchange. Measuring the charge transfer from individual granular spheres after contacts with substrates of the same material, we find instead a “global” charging behavior, coherent over the sample’s whole surface. Cleaning and baking samples fully resets charging magnitude and direction, which indicates the underlying global parameter is not intrinsic to the material, but acquired from its history. Charging behavior is randomly and irreversibly affected by changes in relative humidity, hinting at a mechanism where adsorbates, in particular, water, are fundamental to the charge-transfer process.

Figure 1

Setup and protocol. (a) The setup consists of a Langevin transducer above the target substrate and electrode. The spherical particle levitates in the node of the acoustic standing wave. (b) Charge-exchanging contacts are initiated by briefly interrupting the acoustic field, causing the sphere to bounce exactly once on the surface before we “catch” it. (c) To measure charge, we frequency sweep a spatially uniform applied electric field through the sphere’s resonance and track its position with a high-speed camera. Fitting the acceleration to Eq. (1) yields the charge. (d) Trajectory of a charged sphere in response to a harmonic $E$-field with the discharger off ($t<0$), and then on ($t>0$); fitting for $t>0$ to an exponential gives a time constant of $\sim 8\mathrm{s}$. (e) The three tasks shown in (b)–(d), contact (i), sweep (ii), and discharge (iii), can be combined in different ways depending on the measurement mode. For further details on the setup and videos demonstrating contacts, charge measurement, and discharge, see the Supplemental Material [39].

Figure 2

Charge evolution. (a) The charge $Q$ of a sphere is measured after $n$ contacts with the substrate and with no discharge between, i.e., in charge evolution mode, for a total of 1000 bounces. The steady charging rate $Q^{\prime }$ indicates a global difference between this sphere and substrate pair. (b) Distribution of $Q^{\prime }$ measured for 25 sphere and subtrate pairs, which is centered on zero; this indicates that there is no systematic difference between spheres and substrates.

Figure 3

Charge distributions. (a) In charge distribution mode, the sphere charge after the first contact $Q_1$ is repeatedly measured for a given sphere and substrate pair by discharging the system between collisions. The median of the distribution can be either negative (pair no. 1), positive (pair no. 2), or close to zero (pair no. 3). With pair no. 3, the distribution is composed of two sets of measurements taken 56 hours apart (colored dark gray and light green), showing no significant drift of the distribution with time. Gaussian fits are shown only as a guide to the eye. (b) Between contacts, the substrate is moved along a square spiral. Plotting $Q_1$ vs the contact location on the substrate, no clear trend can be identified with either space or time.

Figure 4

Resetting charging via recleaning and rebaking. If we reclean and rebake a sphere and substrate pair, their charging behavior is “reset.” Every time, the median and width of the distribution is randomly altered; even the sign can be flipped. This indicates that the global charge-driving parameter is acquired, not intrinsic.

Figure 5

Charge shifts via humidity changes. Changing the RH of the experimental chamber (from 15% to 30%) while a sphere and substrate pair is present causes their charging distributions to shift. This shift can either (a) increase or (b) decrease the charging magnitude. Such shifts are not reversible, as shown in (b) where the RH was decreased back to 15%. These effects are similar to those observed after recleaning and rebaking, though no clear sign flips were observed.