|Ph.D Student||Kovalchuk Kogan Tatiana|
|Subject||Optical Breakdown in Liquid Suspensions and its Analytical|
|Department||Department of Chemistry||Supervisor||PROF. Israel Schechter|
|Full Thesis text|
Micro- and nano-particulates are present in all environmental aquatic systems. They have a high surface to mass ratio and therefore they possess a large sorption capacity for pollutants and microbial water impurities. Therefore it is important to identify and quantify waterborne particulates. Laser-induced Breakdown Detection (LIBD) is an excellent method for analysis of particulate matter in air and might be very useful also for aquatic particles. Unfortunately, so far direct application of this method to water did not result in adequate analytical performance. It is hoped that further understanding of the breakdown in water might solve this problem. In the present work we applied several optical techniques for investigation laser breakdown in water.
Laser breakdown in water, induced by nanosecond pulses of 1064 nm wavelength, was studied. The time dependent structure and physical properties of the breakdown were measured at high temporal and spatial resolutions, using Mach-Zehnder interferometry, shadow and Schlieren diagnostic techniques. These techniques allowed for monitoring the formation of micro bubbles, their expansion and the evolution of the associated shockwaves. Several micro plasma balls create a spark column. The micro plasma balls evolve with time and they bring about concentric micro-bubbles and spherical shockwaves. These structures and their time-evolution have been measured by interferometric techniques. In addition, the radial distributions of the density between the bubbles and the corresponding spherical shockwave front were estimated for a series of time. The main ﬁndings in this work is the discrete nature of the plasma column, which lasts up to 100 ns. In all previous reports the plasma was considered as a continuous object. Measurements of the spark columns as a function of energy suggest a new deﬁnition of breakdown threshold in liquids.
We were interested in testing the application of LIBD to identification the nature of common waterborne particulates. We found that multivariate analysis of the Laser-induced Breakdown Spectroscopy (LIBS) signals can be used for differentiation between various groups of particles.
We also investigated the underwater interaction of 1064 nm laser radiation with metal targets. This is relevant to understanding the efficiency of nano-particle generation in water. The underwater processes of laser beam - metal target interaction were visualized and measured using Mach-Zehnder interferromerty. The interferograms allow for characterizing the shockwaves and the pressure behind their front. The size distribution of the produced nano-particles was evaluated from UV absorption spectra and from SEM images.