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Evolution of the cavity in a particle dispersion triggered by laser-induced breakdown

Focusing a laser beam to a spot within particle-laden air flow can cause laser-induced breakdown, which generates a spherically expanding shockwave and ensuing hot gas vortex (HGV). This can cause an initially uniform spatial distribution of buoyant particles to be scattered non-homogeneously, creating a particle void region (or cavity). High-speed schlieren imaging has been applied to investigate the propagation of this shockwave and deformation of the HGV. Evolution of the particle distribution has been captured by a high-speed camera. It has been found that the cavity evolves over three temporal phases: expansion, distortion, and separation. The cavity is first created as the shockwave expels the particles in the radial direction. Next, the cavity is distorted by the HGV and then separates into smaller cavities before finally disappearing due to mixing from the HGV. The temporal and spatial characteristics of the cavity and the mechanism by which it changes in each phase are discussed. Experiments were conducted at three different breakdown energy of 15, 49, and 103 mJ. Propagation speed of the shockwave, and size and strength of the HGV are found to be the main factors controlling this phenomenon.