Steplike Behavior in Grain-Size-Dependent Optical Emission of Plasma Induced by Laser-Ablating Granular Material

Optical emission of plasma induced by a pulsed laser ablating the surface of a randomly packed granular (RPG) material in air is investigated experimentally, taking sieved copper microspheres with discrete diameters, d, ranging from 49 to 390 \textmu{}m as examples. We find a steplike phenomenon (also called a critical-like phenomenon) in the dependence of plasma emission on the grain size: when d is less than a critical size, only weak emissions are detected; however, when d exceeds the critical size, the emissions abruptly become at least 5 times stronger. Such a phenomenon can be explained by considering the RPG material as a non-Newtonian fluid with a yield stress and an effective viscosity. Specifically, in the range of the grain size above (below) the critical value, the RPG target has a yield stress larger (smaller) than the shock pressure imparted by the energetic processes of plasma generation and expansion, and thus, behaves like an elastic solid (a viscous fluid) to assist (impede) the formation of the plasma with high temperature and high density as an optical emission source for spectrochemical analysis. This work not only has significance in assessing a lower size limit for direct multielement analysis of RPG material using laser-induced breakdown spectroscopy (LIBS), but also shows that the mechanical characteristics of RPG material may be probed using LIBS.