Tribocharging is the exchange of electrical charge between materials during contact.  Although ubiquitous in daily life and noted as early as Plato in ancient Greece, we know surprisingly little about the physics behind this effect.  For metals, it is known that charging depends on the work function—the same parameter involved in the photoelectric effect.  For insulators, this parameter isn’t relevant, and yet these materials charge more than metals.  Understanding tribocharging is extremely important as it plays a critical role in both technological and natural settings. Tribocharging is the backbone mechanism in next-generation devices such as triboelectric nanogenerators, which convert mechanical contacts into stored electrical energy (e.g. a device in your shoe that powers your phone with each step). More exotically, tribocharging may be the key ingredient that helps tiny dust particles in protoplanetary disks aggregate into ever larger and larger bodies in the millimeter to meter range, thus helping rocky planets such as the earth to form.

A major effort in our lab is to understand how tribocharging occurs for insulating materials.  Previous models proposed that trapped electrons—those caused by material defects—might be the culprit, but thermoluminescence data show that the effect can occur when these are not present (see our articles in Physical Review Letters and Nature Physics).  Recently, a new hypothesis has been put forth that proposes adsorbed surface water might play a key role.  We are forging ahead in the development of an array of techniques to test this hypothesis, and in doing so perhaps solve one of the oldest problems in physics.


Quantitatively consistent scale-spanning model for same-material tribocharging
Galien Grosjean, Sebastian Wald, Juan Carlos Sobarzo, and Scott Waitukaitis
Physical Review Materials 4, 082602 (2020).

Collisional charging of individual sub-millimeter particles:  usingultrasonic levitation to initiate and track charge transfer
Victor Lee, Nicole M. James, Scott Waitukaitis, and Heinrich Jaeger
Physical Review Materials 2, 035602 (2018).

The retention of dust in protoplanetary disks: evidence from agglomeratic olivine chondrules from the outer Solar System
Devin Schrader, Kazuhide Nagashima, Scott Waitukaitis, Jemma Davidson, Timothy McCoy, Harold Connoly and Dante Lauretta
Geochimica et Cosmochimica Acta 223, 405-421 (2018).

Direct observation of particle interactions and clustering in charged granular streams
Victor Lee, Scott Waitukaitis, Marc Miskin and Heinrich Jaeger
 Nature Physics 11, 733-737 (2015).

Size-dependent, same-material tribocharging in insulating grains     Scott Waitukaitis, Victor Lee, James Pierson, Steve Forman and Heinrich Jaeger
Physical Review Letters 112, 218001 (2014).

From nanoscale cohesion to macroscale entanglement: opportunities for designing granular behavior by tailoring grain shape and interactions
Heinrich Jaeger, Marc Miskin, and Scott Waitukaitis
Powders and Grains 1542, 3-6 (2013).

In situ granular charge measurement by free-fall videography   
Scott Waitukaitis and Heinrich Jaeger
Review of Scientific Instruments 84, 025104 (2013).