Experimental Demonstration and Quantification of Electrostatic Lofting of Dust Clumps

Electrostatic lofting has been observed in the lab and has long been hypothesized to occur on airless planetary bodies. Combined with solar radiation pressure, electrostatic lofting could serve as a loss mechanism for ~100 micron-scale (and smaller) particles on small bodies. Outlying results in prior experimental investigations of electrostatic lofting have been attributed to the variation in cohesion acting on particles of varying sizes and shapes as well as the possible lofting of clumps of particles, rather than single particles. Prior analytical work has suggested that it is easier to detach clumps of small, cohesive particles than individual particles. We built an experimental teststand that electrostatically lofts clumps of 0.2-0.3 mm silicon microspheres in vacuum and images their trajectories. Electrostatic lofting is induced by a novel charging method in which the fringing fields of a plate biased to 10kV accelerate and deflect electrons around the plate edges toward the microspheres underneath. The same biased plate also produces an electric field of 870kV/m that detaches sufficiently charged clumps from the surface and accelerates them upward. A high-speed camera images the lofted clumps at 945fps in order to obtain their size and velocity. The initial clump detachment gives an impulsive force balance that will allow us to solve for cohesion by taking the difference of the electric field force (controlled by the biased plate potential and height) and gravity. These experimental results show the preferential detachment of clumps of particles, rather than individual particles, for small cohesive powders. We observed detachment of linear, 2- to 5-particle columnar chains (Fig. 1). Upon detachment, the center of mass and electric dipole moment of the chains were not aligned with the gravity and electric field vectors, respectively. Therefore, the restoring torques imparted on the chains caused them to oscillate about these field vectors and follow a parabolic trajectory. Despite this wobble, the chains remained intact due to cohesion. The terminal altitudes of the lofted chains were independent of the number of constituent particles (i.e. independent of mass) and thus depended more on accumulated surface charge density. The clumps broke apart upon re-impact. Aside from lofted clumps, equally interesting were several multi-particle chains that formed on the surface of the pile, but did not detach. In addition to these observations, we introduce the experimental setup, structure and magnitude of the electric field, and present the sizes and velocities of the lofted clumps observed.