Chie C. Poon

Optical probing of the temperature transients during pulsed-laser induced boiling of liquids

The thermodynamics of the rapid boiling of a liquid on a solid surface heated by an excimer laser pulse is studied experimentally. The dynamics of bubble nucleation, growth, and collapse is detected by probing the optical specular reflectance. The transient temperature field is measured by monitoring the reflectance of a thin film with calibrated optical properties. The metastability behavior of the liquid and the criterion for the liquid‐vapor phase transition in nanosecond time scale are obtained for the pressure from 1 atmosphere to 3.3 MPa.

Optical and Acoustic Study of Nucleation and Growth of Bubbles at a Liquid-Solid Interface Induced by Nanosecond-Pulsed-Laser Heating

The dynamics of liquid-vapor phase-change in the nanosecond time-scale induced by pulsed-laser heating of a liquid on a solid sample is studied by means of optical reflectance and scattering measurements, and the piezoelectric detection technique. The liquids studied include water, ethanol, methanol, IsoproPropyl Alcohol (IPA), and mixtures of water and IPA. The threshold fluence for nucleation is determined with high accuracy using the optical and acoustic signals. Heat diffusion calculations performed for the threshold fluences indicate that the liquids are sufficiently superheated before nucleation sets on. The transient optical reflectance signal is analyzed by an effective-medium theory to provide bubble-growth kinetics, so that the bubble-growth velocity for the test liquids could be estimated. In addition, it is observed that, following the thermally induced nucleation, repetitive acoustic cavitation at the surface of the solid sample occurs, with a time interval related to the speed of sound in the liquid.

LASER-ASSISTED SURFACE MODIFICATION OF THIN CHROMIUM FILMS

The objective is to investigate a process by which micrometer scale topographical changes are produced on thin chromium films using a pulsed Nd:YLF laser. The surface of chromium films is altered through laser-induced solid-liquid phase transformation and fluid flow. Experimental parametric studies are conducted to correlate the laser parameters with the topography of the laser irradiated surfaces. Experimental and analytical work is also performed to study the transport phenomena involved in the process. A numerical finite-element analysis is used to simulate the transient field variables. A nanosecond-time-resolution, fast photography system is constructed to capture the phase change and the fluid flow occurring at the target surface. The experimental and the numerical studies showed that the surface topography change was caused by the laser-induced surface-tension-driven flow, and the recoil pressure due to surface evaporation had negligible effect on the topography variation.

Laser-assisted microscale deformation of stainless steels and ceramics

This work investigates deformation of stainless steel and ceramic specimens with a precision on the order of submicrometers by use of a pulsed laser beam as the energy source. Such a technique is useful, for example, in a process of removing distortions on magnetic head components for a better contact between the magnetic disk head and the hard disk surface. Experiments are conducted to study the bending behavior of stainless steel and ceramics due to laser irradiation. A pulsed Nd:YLF laser beam is used to scan over the specimen to create out-of-plane deformation. The amount of deformation from each laser scan is correlated with various laser and processing parameters. A theoretical model of the laser deformation process is presented based on thermo-elasticity/plasticity. The laser deformation process is explained as a result of the laser-induced non-uniform distribution of the residual strain. Numerical simulations are carried out to estimate the laser-induced temperature field, the residual stress field, and the amount of deformation of the specimen. These theoretical studies help to understand the complex phenomena involved in the pulsed laser deformation process.

Optical probing of the temperature and pressure transients at a liquid/solid interface due to pulsed laser-induced vaporization

The transient temperature and pressure field development in the excimer laser-induced vaporization of liquids in contact with a solid surface is studied. A thin silicon film, which has temperature-dependent optical properties, is embedded between an absorbing chromium film and a transparent fused quartz substrate. Static reflectivity measurement is performed to determine the thin film optical properties at elevated temperatures. The transient backward reflectance responses from the silicon layer are compared with heat transfer modeling results. The backward reflectance probe is not affected by the creation of bubbles and is successfully employed for the first time to measure non-intrusively the temperature development during the rapid vaporization process. The optical reflectance probes are applied from the front-side and back-side of the sample simultaneously to monitor the dynamic bubble nucleation behavior and transient temperature development, respectively, at various ambient pressures using a high- pressure cell. The investigation on the effect of ambient pressure on the bubble nucleation threshold combined with the surface temperature measurement determines the thermodynamic state of the superheated metastable liquid at the interface and subsequently the explosion pressure.

Laser-assisted surface modification of thin chromium films

The objective of this work is to develop a novel process by which topographical changes are produced on a micrometer scale using a pulsed Nd:YLF laser, and to investigate the energy transfer and fluid flow phenomena involved in the process. The surface of thin chromium films is altered through the laser- induced phase transformation and fluid flow. Experimental parametric studies were conducted to correlate the laser parameters with the topography of the laser irradiated surfaces. Experimental and analytical work were also performed to study the transport phenomena involved in the process. A numerical finite element analysis was carried out to simulate the transient field variables. A nanosecond-time resolution, fast photography system was constructed to capture the phase change and the fluid flow occurring at the target surface. Comparison between the numerical and the experimental data helps to understand the mechanisms of the process as well as to develop a controlled surface modification technique.

Memory-Effect on Acoustic Cavitation

The formation of bubbles at a liquid-solid interface due to acoustic cavitation depends particularly on the preconditions of the interface. Here, it is shown that following laser- induced bubble formation at the interface the acoustic cavitation efficiency is strongly enhanced. Optical reflectance measurements reveal that this observed enhancement of acoustic cavitation due to preceding laser-induced bubble formation, which could be termed as a memory effect, decays in a few hundred microseconds. By performing a double-pulse experiment using two excimer lasers the influence of process parameters, such as liquid temperature and salt concentration, on the temporal decay of the memory effect has been studied. An analysis of the experimental results by a diffusion model is presented.