X-ray imaging of powder particles driven from a surface by a nearby electrostatic discharge

Electrostatic discharges (ESD) have recently been used as an experimental technique to generate laboratory-scale shockwaves measured in millimeters and microseconds. This makes studies of interactions between shocks and particulates readily accessible. Here, the interaction between ESD-generated mild shockwaves and a set of particles in the micrometer size range was studied using X-ray imaging for the purpose of experimental validation of particle lifting models. The use of x-ray imaging enabled observations of early motion patterns of the moving particles, which would have been obscured by the bright emission of the spark plasma with corresponding optical methods. X-ray images of moving powder particles of oxides of silicon, titanium, and bismuth, with sizes ranging from 2 to 100 µm with spherical and irregular shapes, were recorded using a high-speed camera at the Cornell High Energy Synchrotron Source FAST beamline. Trajectories and shapes of individual lifted particles and particle agglomerates were recorded, and velocity distributions were obtained. Average particle velocities are consistent with earlier experimental extrapolations and theoretical predictions. Finer powders were observed to form vertical columnar patterns while being lifted from the substrate. Such patterns are expected to lead to formation of transient agglomerates. For all powders, few faster-moving and rapidly lifted particles were detected. For larger powders, such faster-moving particles were observed to be molten. Late-time agglomeration was also observed for the lifted powders of Bi₂O₃ from the videos and confirmed by examination of the powder removed from the substrate after being exposed to ESD.