J. K. Chen

Dual-phase lag effects on thermal damage to biological tissues caused by laser irradiation

A dual-phase lag (DPL) bioheat conduction model, together with the broad beam irradiation method and the rate process equation, is proposed to investigate thermal damage in laser-irradiated biological tissues. It is shown that the DPL bioheat conduction model could predict significantly different temperature and thermal damage in tissues from the hyperbolic thermal wave and Fouriers heat conduction models. It is also found that the DPL bioheat conduction equations can be reduced to the Fourier heat conduction equations only if both phase lag times of the temperature gradient (tau(T)) and the heat flux (tau(q)) are zero. This is different from the DPL model for pure conduction materials, for which it can be reduced to the Fouriers heat conduction model provided that tau(q)=tau(T). Effects of laser parameters and blood perfusion on the thermal damage simulated in tissues are also studied. The result shows that the overall effects of the blood flow on the thermal response and damage are similar to those of the time delay tau(T).

Non-Fourier Heat Conduction Effect on Laser-Induced Thermal Damage in Biological Tissues

To ensure personal safety and improve treatment efficiency in laser medical applications, one of the most important issues is to understand and accurately assess laser-induced thermal damage to biological tissues. Biological tissues generally consist of nonhomogeneous inner structures, in which heat flux equilibrates to the imposed temperature gradient via a relaxation phenomenon characterized by a thermal relaxation time. Therefore, it is naturally expected that assessment of thermal damage to tissues could be inaccurate when a classical bioheat conduction model is employed. However, little attention has been given to studying the impact of the bioheat non-Fourier effect. In this article, a thermal wave model of bioheat transfer, together with a seven-flux model for light propagation and a rate process equation for tissue damage, is presented to investigate thermal damage in biological tissues. It is shown that the thermal damage assessed with the thermal wave bioheat model may differ significantly from that assessed with the classical bioheat model. Without including the bioheat non-Fourier effect, the assessment of thermal damage to biological tissue may not be reliable.

Optical properties and thermal response of copper films induced by ultrashort-pulsed lasers

A critical point model with three Lorentzian terms for interband transition was proposed to describe temperature-dependent reflectivity (R) and absorption coefficient (α) for copper irradiated by ultrashort-pulsed lasers of wavelength 200–1000 nm. After validated with experimental data at room temperature, it was incorporated into a two-temperature model to study ultrafast laser-material interactions. The dynamic changes of optical properties R and α, distributions of laser heat density, electron and lattice temperature, and phase changes of a copper film were investigated. Comparing with the experimental data of average absorption showed that the proposed two-temperature model together with the critical point model can simulate satisfying results for temperature-dependent R and α. The drastic changes in R and α could alter laser energy deposition in a heated target, leading to different thermal responses than those predicted with constant R and α at room temperature.

Integrated continuum-atomistic modeling of nonthermal ablation of gold nanofilms by femtosecond lasers

Ultrafast nonthermal ablation of gold nanofilms is studied using a combined two-temperature model and molecular dynamics method. The results show that for thinner films the tensile stress is directly reversed from the initially generated compressive stress. For thicker films, on the other hand, the tensile stress wave is reflected from the irradiated surface. The key driving force for ultrafast nonthermal material ablation is conventional thermal stress, instead of the hot electron blast force.

Numerical Simulation of Thermal Damage to Living Biological Tissues Induced by Laser Irradiation Based on a Generalized Dual Phase Lag Model

A generalized dual phase lag (DPL) bioheat model based on the nonequilibrium heat transfer in living biological tissues is applied to investigate thermal damage induced by laser irradiation. Comparisons of the temperature responses and thermal damages between the generalized and classical DPL bioheat model, derived from the constitutive DPL model and Pennes bioheat equation, are carried out in this study. It is shown that the generalized DPL model could predict significantly different temperature and thermal damage from the classical DPL model and Pennes bioheat conduction model. The generalized DPL equation can reduce to the classical Pennes heat conduction equation only when the phase lag times of temperature gradient (τ T ) and heat flux vector (τ q ) are both zero. The effects of laser parameters such as laser exposure time, laser irradiance, and coupling factor on the thermal damage are also studied.

Numerical Simulation of Random Packing of Spherical Particles for Powder-Based Additive Manufacturing

Powder-based additive manufacturing is an efficient and rapid manufacturing technique because it allows fabrication of complex parts that are often unobtainable by traditional manufacturing processes. A better understanding of the packing structure of the powder is urgently needed for the powder-based additive manufacturing. In this study, the sequential addition packing algorithm is employed to investigate the random packing of spherical particles with and without shaking effect. The 3D random packing structures are demonstrated by illustrative pictures and quantified in terms of pair distribution function, coordination number, and packing density. The results are presented and discussed aiming to produce the desirable packing structures for powder-based additive manufacturing.

Ultrafast melting and resolidification of gold particle irradiated by pico- to femtosecond lasers

Ultrafast melting and resolidification of a submicron gold particle subject to pico- to femtosecond laser pulse are studied in this paper. The nonequilibrium heat transfer in the electrons and lattice is described using a two-temperature model, and the locations of the solid-liquid interface are determined using an interfacial tracking method. The interfacial velocity, as well as elevated melting temperature and depressed solidification temperature, is obtained by considering the interfacial energy balance and nucleation dynamics. The results showed that the maximum melting depth, peak interfacial temperature, and velocity increase with the decreasing particle size and pulse width or with the increasing laser fluence.

Ultrashort laser pulse energy deposition in metal films with phase changes

Four optical models of reflectivity and absorption coefficient are investigated in this letter. After compared with existing experimental data, the extended Drude model is incorporated into a two-temperature model to simulate laser energy deposition and thermal response, including solid–liquid and liquid–vapor phase change, in a gold film irradiated by a femtosecond laser pulse. Dynamic reflectivity and absorption coefficient should be employed in modeling ultrafast laser heating except for very low laser fluencies.

Inverse Heat Conduction Using Measured Back Surface Temperature and Heat Flux

In the high-energy laser heating of a target, the temperature and heat flux at the heated surface are not directly measurable, but they can be estimated by solving an inverse heat conduction problem based on the measured temperature and/or the heat flux at the accessible (back) surface. In this study, the one-dimensional inverse heat conduction problem in a finite slab is solved by the conjugate gradient method, using measured temperature and heat flux at the accessible (back) surface. Simulated measurement data are generated by solving a direct problem, in which the front surface of the slab is subjected to high-intensity periodic heating. Two cases are simulated and compared, with the temperature or heat flux at the heated front surface chosen as the unknown function to be recovered. The results show that the latter choice (i.e., choosing back surface heat flux as the unknown function) can give better estimation accuracy in the inverse heat conduction problem solution. The front surface temperature can be computed with high precision as a by-product of the inverse heat conduction problem algorithm. The robustness of this inverse heat conduction problem formulation is tested by different measurement errors and frequencies of the input periodic heating flux.

An atomic-level study of material ablation and spallation in ultrafast laser processing of gold films

Ablation and spallation of gold thin films by an ultrashort laser pulse are simulated by using an integrated two-temperature model and molecular dynamics method with inclusion of the hot electron blast force. The simulation results show that the ultrafast laser-induced nonthermal ablation and spallation both are essentially due to dynamic tensile stress that exceeds the local material strength. It is also demonstrated that a simultaneous use of femtosecond and picosecond laser pulses could induce spallation without causing undesired front-side damage to the film. This may be suited to peel an ultrathin film (1 μm in thickness or less) from its substrate or to improve the material removal rate of precise laser processing.Ablation and spallation of gold thin films by an ultrashort laser pulse are simulated by using an integrated two-temperature model and molecular dynamics method with inclusion of the hot electron blast force. The simulation results show that the ultrafast laser-induced nonthermal ablation and spallation both are essentially due to dynamic tensile stress that exceeds the local material strength. It is also demonstrated that a simultaneous use of femtosecond and picosecond laser pulses could induce spallation without causing undesired front-side damage to the film. This may be suited to peel an ultrathin film (1 μm in thickness or less) from its substrate or to improve the material removal rate of precise laser processing.