The Delicate Electrostatic Dance of Falling Space Dust

In a nearly gravity-free environment the tiniest micron-scale flecks of matter undergo interactions mimicking the orbital relationships of the largest planets. This observation, which may offer crucial insights into the process of large-structure formation in the interstellar medium, comes courtesy of physicists at the University of Chicago via an innovative experiment using a high-speed video camera and streams of falling particles of fine dust. The resulting electrostatic dance can be seen below.

Note that these aren't "particles" in the sense of electrons and protons and quarks—instead, they're macro-scale (on the order of hundreds of micrometers) grains of zirconium dioxide–silicate, a somewhat toxic dielectric material with a number of uses in manufacturing and industry. So: silicon dust, essentially.

The task of the U of C researchers was to observe the electrostatic interactions between these dust grains. This is made difficult in large part due to the confounding influence of gravity, a force that acts to obscure the much smaller forces at work governing the interactions between the grains themselves. The answer was to use a three-meter desktop drop tower, a cylinder from which all of the air has been sucked out of it, and a high-speed video camera. The particles are dropped from the top of the vacuum cylinder while the camera falls at the same rate. The resulting video makes it seem like the particles aren't falling at all, but are instead suspended, dancing and interacting only with each other.

The U of C group's experiments are reported in a study recently published in Nature Physics.

The particle interactions the physicists observed is the result of tribocharging, which is the fancy name for the charge build-up that occurs when two different materials rub against each other in such a way as to allow charge to accumulate in one of them. This is just everyday static electricity. Socks on carpet.

"This type of charging among same-material bodies—tribocharging, as it's known—is itself still poorly understood," Frank Spahn and Martin Seiβ, astrophysicists at the University of Potsdam not directly affiliated with the U of C work, write in a Nature Physics commentary accompanying the U of C study. "It can cause unpredictable hazards like explosions in grain- or coal-processing facilities and is thought to be responsible for producing natural spectacles like the immense lightning occurring during sandstorms or volcanic eruptions. Even more exciting, it may also play a crucial role in planet formation."

"The observation alone would have constituted an experimental feat—an intriguing analogue to celestial gravitational dynamics," they add. "But the authors went a step further. They studied the grains' tendency to cluster due to mutual collisions, including electrostatic forces, as well as their dissolution in response to more energetic impacts."

Planetesimals, the kilometer-sized chunks of material that constitute "planetary embryos," have proved to be an elusive phenomena for astrophysicists to explain. While the tiniest sub-micrometer particles tend to adhere to each other, they lose this ability as they increase in size. So, at some point, it would seem that the process of planetary formation should hit a considerable roadblock. Tribocharging, it seems, offers a solution, a way for particles to attract and go on to become planets like Earth.

"Charged particles can become trapped in their mutual electrostatic energy well and aggregate via multiple bounces," the U of C physicists write. "This enables the initiation of clustering at relative velocities much larger than the upper limit for sticking after a head-on collision, a long-standing issue known from pre-planetary dust aggregation."