Electric Field and Humidity Trigger Contact Electrification

Here, we study the old problem of why identical insulators can charge one another on contact. We perform several experiments showing that, if driven by a preexisting electric field, charge is transferred between contacting insulators. This transfer happens because the insulator surfaces adsorb small amounts of water from a humid atmosphere. We believe the electric field then separates positively from negatively charged ions prevailing within the water, which we believe to be hydronium and hydroxide ions, such that at the point of contact, positive ions of one insulator neutralize negative ions of the other one, charging both of them. This mechanism can explain for the first time the observation made four decades ago that wind-blown sand discharges in sparks if and only if a thunderstorm is nearby.

Popular Summary

“Contact electrification” refers to two contacting insulators acquiring electric charges when they are separated. The effects of this phenomenon are common in everyday life and include static electricity, which is responsible for the small shocks one sometimes receives from touching doorknobs. Contact electrification is thought to be responsible for the spectacular lightning seen in many volcanic dust plumes, as well as the spark-emitting wind-blown sand observed during a thunderstorm in a New Mexico desert in 1971. Although it has been studied since the times of ancient Greece, contact electrification remains poorly understood. We experimentally determine that electric fields and air humidity together trigger the mutual electrification of contacting insulators, and we describe a theoretical mechanism to explain our observations.

Contact electrification begins when the surfaces of the insulators adsorb small amounts of water from a humid atmosphere. Next, the electric field separates the charged ions in this water—presumably hydronium and hydroxide ions, although ions resulting from salts and gasses dissolved in water might also be involved. At the point of contact, positive ions from one insulator neutralize the negative ions of the other insulator, charging both of them. When the insulators are separated, one retains a more positive charge and the other possesses a more negative charge. We test this hypothesis by experimentally showing that charge can be transferred between glass beads surrounded by two charged metal electrodes. We suggest that the ions move within the water film, coating the insulators, or potentially hop from one water island to another on the insulator surface, aided by nearby water vapor. When the beads were baked to remove their surface water, they did not retain a charge after contacting each other or the electrodes. With regards to the sparks observed during a thunderstorm in New Mexico in 1971, we hypothesize that sandstorms in humid environments become strongly electrified as a result of multiple collisions between sand grains if, and only if, a thunderstorm is nearby because only a thunderstorm provides an electric field of sufficient strength to trigger this contact electrification mechanism.

This work clarifies our understanding of how insulators can charge one another upon contact.

Figure 1

Experiments within silicone oil: (a) Absolute value of the charges of a glass bead ($r=1\mathrm{mm}$) and a water drop ($r=1\mathrm{mm}$), bouncing between two high-voltage metal electrodes, after contact with the electrodes versus electric field strength (symbols). The charges are estimated from a balance between the viscous drag and Coulomb forces, while the solid line displays the theoretical charge of a perfectly conducting sphere ($r=1\mathrm{mm}$) [39]. (b) Positions relative to the cathode and charges of two glass beads colliding between two high-voltage metal electrodes ($E=2.5\mathrm{kV}/\mathrm{cm}$) before and after their collision ($Q_1+Q_2\approx q_1+q_2$).

Figure 2

Experiments within silicone oil: Four glass beads ($r=1\mathrm{mm}$) bouncing between two high-voltage metal electrodes ($E=2.5\mathrm{kV}/\mathrm{cm}$). The arrows indicate the locations of charge exchange during each bounce. All pictures show the view from the top of the cell.

Figure 3

Experiments within silicone oil: (a)–(c) Two contacting glass beads ($r=1\mathrm{mm}$) placed on fibers hanging down a bar. (a) Separation of the glass beads does not occur in the absence of an electric field. (b) Separation does occur under the influence of an electric field ($E=2.5\mathrm{kV}/\mathrm{cm}$) directed normal to the contact plane. (c) Separation does not occur under the influence of an electric field directed tangential to the contact plane. (d)–(f) Charge-transfer mechanism of hydrophilic particles in contact with each other. Note that the water film is amplified and drawn as continuous for illustration. The real water film is very thin and may not be continuous.

Figure 4

Experiments within silicone oil: Two contacting glass beads ($r=1\mathrm{mm}$) placed on fibers hanging down a bar. Measurements of the absolute value of the charge of the glass beads as a function of the electric field component in the direction normal to the contact plane after removal of one of the beads.

Figure 5

Difficulty of charge transfer (qualitatively) versus contact angle between a $1\text{−}\mu \mathrm{L}$ water drop and the insulator surface for several dielectric materials measured with an optical contact angle-measuring device. Inset: Number of monolayers of water molecules composing the water films on the surfaces of silicon oxide and PTFE, measured with an ellipsometer, versus air humidity.

Figure 6

Illustration of experiments 1, 2, and 3.

Figure 7

Illustration of experiment 4.

Figure 8

Sketch of glass-bead displacement due to electric field.

Figure 9

Charge of spherical beads of different dielectric materials after contact with the electrode.

Figure 10

(a)–(d) Charging mechanism of a hydrophobic particle in contact with the cathode.