Using laser-induced breakdown spectroscopy to characterize the surface mechanical properties of soft materials

A method for characterizing the surface mechanical properties of soft materials via the laser-induced breakdown spectroscopy (LIBS) technique was investigated experimentally. The experiment was performed using five typical soft samples prepared by random packings of (72 +/- 20) mu m copper grains into various bulk packing fractions ranging from 0.593 to 0.623. Such sample preparations lead to the five samples having only dozens of Pa mechanical strengths and about a 6% difference in mechanical properties (rigidity specifically involved here) between adjacent samples. It was found that LIBS measurements are successful in monitoring such small mechanical differences among these samples, implying that the method investigated here is feasible and has a favorable sensitivity. It was also found that atomic spectral lines obtained from LIBS measurements play important roles in establishing a mechanical indicator of soft materials, instead of the ionic lines reported frequently in the LIBS literature regarding the characterization of solid materials' mechanical properties. Combining the plasma parameters calculated through spectroscopic analysis, we interpreted the findings by introducing a detailed surface absorption mechanism during interaction between the plasma and the soft surface after the end of a laser pulse. This study also provides some guidelines on how to choose specific analytical lines at which LIBS as an analytical technique to quantify elements embedded in soft materials is viable without considering the difference in surface mechanical properties.