Charging at a distance

It has been proposed [L. S McCarty and G. M Whitesides, Angew. Chem., Int. Ed. 47, 2188 (2008); Lee et al., Phys. Rev. Mater. 2, 035602 (2018)] that contact charging between insulators may be mediated by nonequilibrium transport of adsorbed ions. We remark here that if adsorbed ions transport charges between surfaces, they could equally transport charges across a single surface. We test this hypothesis by contacting initially neutral insulating spheres. We find that localized charge patterns appear very far (centimeters) from a point of contact. We visualize the charges, evaluate their spatial distributions, and discuss mechanisms and implications of this apparent action at a distance.

Figure 1

Contact charge visualizations. (a) Sketch of experiment in which a shaking ball repeatedly contacts a fixed ball. The balls are initially discharged (see text), and the top ball is then brought into contact at 10 Hz for 30 s via an external shaker. Note that by detaching the supporting wood post from the shaker, the top ball can be removed for examination without further contact. (b) Visualization is achieved by blowing bipolar toner (see text) on the balls. (c) Exposure to bipolar toner produces little sticking before shaking. (d) The shaken ball shows a nearly charge-free halo around the contact point (dashed circles), surrounded by charged regions. The orange balls are Mapol 3-star brand, hand sanded with 150- and then 320-grit sandpaper, and the white ball is Kevenz 3-star brand and is unsanded. The orange balls are mounted as depicted in (a); the white ball is pierced with a steel blade (crosshatched), that is connected to the wood post. (e) Remarkably, the opposite side of each ball, which was never contacted with anything after initial discharge, shows charge patterns [details of some of these in panel (f)]. Balls are cleaned with alcohol prior to each trial, and all snapshots are deblurred with an unsharp mask.

Figure 2

Discriminating net and dipole charge components. (a) Sketch of experiment, showing charged ball that is pulled a distance $r$ from a noncontact voltage probe. Data are collected for either a ball contacted at a point toward the probe (front contact) or perpendicular (top contact). (b) Control data (blue online) for a discharged ball. (c) Front contact data, including fits to net charge only (dashed, green online) and net plus dipole charge (solid, red online) models. (d) Top contact data, with fits and insets as in panel (c), plus a comparison cubic fit (white circles) described in text. Insets in all panels show additional trials with axes identical to main plots; balls in all cases are unsanded, and RH is above 40%. Balls are discharged with static eliminator (see text) prior to each trial.

Figure 3

Quantitative charge distributions. (a) Data collected from fits shown in Figs. 2 and 2. Coefficients are put into common units by dividing the dipole coefficient by the ball radius (see text). (b) Experiment in which hollow ball is contacted as in Fig. 2, and is then rotated at constant distance from voltage probe. The attached post is perpendicular to the sketch. (c) Voltage vs angle for orange ball contacted 20 times with white ball, fit with trial functions for a point surface charge plus an induced dipole (top), and for a point charge plus a uniform charge (bottom). (d) Replicates of the same experiment, including example at bottom showing positive charge at contact point and negative charge on opposite side of ball. (e) Resulting voltage vs angle data for balls used in Fig. 2; thick line is running average of 1/4 s of data. Orange ball is unsanded Mapol 3-star brand ball, white ball is unsanded Kevenz 3-star brand, and RH is above 40%. Balls are discharged with static eliminator prior to each trial.