Colliding Dust and the Sparks of Creation

Added: 2026-05-26

[00:00:06]Materials tend to exchange electric charge when they touch.

[00:00:11]This is contact electrification.

[00:00:15]We developed an acoustic levitation setup to measure this effect precisely.

[00:00:21]Using ultrasound, we can suspend a small oxide sphere above a target plate.

[00:00:28]We trigger collisions between the sphere and the plate by briefly interrupting the ultrasound, like so.

[00:00:38]This happened very fast, so let's slow it down using a high-speed camera.

[00:00:43]Both the sphere and the plate are made of silicon oxide.

[00:00:48]When we turn off the field, the sphere falls, bounces off the plate, then we catch it again using ultrasound.

[00:00:57]Through this collision, the sphere gained a tiny bit of electric charge.

[00:01:01]But how do we measure it?

[00:01:04]First, we apply an electric field and look at the response.

[00:01:13]The equation of motion is equivalent to a driven damped harmonic oscillator.

[00:01:20]We fit the force expressions and the acceleration data and extract the charge.

[00:01:27]We can now repeat these operations.

[00:01:30]Bounce, measure, then bounce, then measure, and obtain the charging behavior of a given pair.

[00:01:38]This sphere charged positive and this next one charged negative.

[00:01:45]If we repeat this for many samples, we see that about 50% charged positive and 50% charged negative.

[00:01:54]Paradoxically, this tells us that each sample behaves almost as a different material.

[00:02:01]How can this be?

[00:02:03]The answer lies in what's on the surface of the samples.

[00:02:08]Even in the cleanest environments, tiny amounts of carbon compounds adsorb on the surface.

[00:02:15]We have found that if we remove these compounds from a sample using heat or plasma, we can control the charging to the point where we can reverse the charging polarity in both directions.

[00:02:29]This even works between samples made of different oxides to the point where we can reverse their natural tendency to charge more positive or more negative.

[00:02:40]However, if we just wait a few hours, the carbon comes back and the effect on charging relaxes.

[00:02:50]Importantly, things don't quite revert to how they were before as the surface carbon is not in equilibrium.

[00:02:59]This shows the importance of the sample's history.

[00:03:03]Slight differences in conditions can lead to completely different outcomes.

[00:03:08]Ultimately, a slight imbalance in adsorbed carbon is enough to break symmetry between otherwise identical samples leading to contact electrification.