The microphysics of ash tribocharging: New insights from laboratory experiments

The spectacular lightning strokes observed during eruptions testify to the enormous potentials that can be generated within plumes. Related to the charging of individual ash particles, large electric fields and volcanic lightning have been observed at Eyjafjallajokull, Redoubt, and Sakurajima, among other volcanoes. A number of mechanisms have been proposed for plume electrification, including charging from the brittle failure of rock, charging due to phase change as material is carried aloft, and triboelectric charging, also known as contact charging. While the first two mechanisms (fracto-emission and volatile charging) have been described by other authors (James et al, 2000 and McNutt et al., 2010, respectively), the physics of tribocharging--charging related to the collisions of particles--of ash are still relatively unknown.

Because the electric fields and lightning present in volcanic clouds result from the multiphase dynamics of the plume itself, understanding the electrodynamics of these systems may provide a way to detect eruptions and probe the interior of plumes remotely. In the present work, we describe two sets of experiments designed to explore what controls the exchange of charge during particle collisions. We employ natural material from Colima, Mt. Saint Helens, and Tungurahua.

Our experiments show that the magnitude and temporal behavior of ash charging depend on a number of factors, including particle size, shape, chemistry, and collisional energy. The first set of experiments were designed to determine the time-dependent electrostatic behavior of a parcel of ash. These experiments consist of fluidizing an ash bed and monitoring the current induced in a set of ring electrodes. As such, we are able to extract charging rates for ash samples driven by different flow rates. The second experimental setup allows us to measure how much charge is exchanged during a single particle-particle collision. Capable of measuring charges as small as 1 fC, this device allows us to methodically to characterize charges on particles with diameters down to 100 microns. Employing this instrument, we quantify the effect of particle pre-charging, mineralogy, and impact energy on the charge exchange between two colliding particles.