In this work, a generalized solution for the thermoelastic plane wave in a semi-infinite solid induced by pulsed laser heating is formulated in the form of Fourier series. The solution takes into account the non-Fourier effect in heat conduction and the coupling effect between temperature and strain rate, which play significant roles in ultra-short pulsed laser heating. Based on this solution, calculations are conducted to study stress waves induced by different laser parameters. It is found that with the same maximum surface temperature increase, a shorter pulsed laser induces a much stronger stress wave in a solid. The non-Fourier effect causes a higher surface temperature increase, but a weaker stress wave. The surface displacement accompanying thermal expansion shows a time delay to the laser pulse in femtosecond laser heating. On the contrary, surface displacement and heating occur simultaneously in nano- and picosecond laser heating. In femtosecond laser heating, results show that the coupling effect attenuates the stress wave and extends the duration of the stress wave. This may explain the minimal damage in ultra-short laser materials processing.

Ultrafast (or ultrashort) pulsed laser ablation of biological tissue has drawn much attention due to the minimal collateral damage caused by laser irradiation. Many clinical applications of ultrafast laser ablation have been proposed, including ophthalmology, dentistry, and neurosurgery (Kim et al., 1998). During ultrafast laser interaction with biological tissues, which are dielectric materials, multiphonon absorption occurs which enhances the absorption of the laser beam in tissue. The tissue can then be heated rapidly to a high temperature, causing evaporation and tissue removal.

This paper proposes a genetic algorithm to determine the optimal assembly sequence for a robotic autonomous assembly task. The coding rule of chromosomes, the selection, crossover and mutation operations are presented. In order to avoid invalid chromosomes, a modified crossover operation is suggested. In addition, a specific fitness table is created to evaluate a random assembly sequence. Some simulation results are employed to validate the effectiveness and accuracy of the proposed approach.

In this paper, boiling phenomena on a copper surface coated with superhydrophobic micropatterns have been investigated. The micropatterns consisted of chemically inert nanoparticles (PTFE) were in square patterns 180 μm on each side with contact angles above 150° for the superhydrophobic behavior. Boiling experiments were conducted on the patterned copper surface with or without degassing treatment prior to heating, from which bubbles were found to form only on the superhydrophobic sites. As controlled experiments, a uniformly-coated superhydrophobic surface as well as a bare copper surface was also tested in comparison.

In this paper, an atomic scale study is carried out to characterize the thermal transport in Si/Ge nanocomposites by using the molecular dynamics (MD) simulation. The influence of size, heat flux, interface, as well as voids, on thermal properties and the inner temperature profiles of nanowire composites are studied. The results show that the thermal conductivity of nanowire composites is much lower than that of alloy, which accounts mainly for ZT enhancement and owes a great deal to the effect of interface thermal resistance. The results also indicate that a nonfourier phenomena what we call “reflecting effect” is remarkable at the Si/Ge interface, and the thermal conductivity is also dependent slightly on the bulk temperature and the specified heat flux in the range of selected system sizes and temperatures. It is also investigated that how the thermal conductivity of Si 1−x Ge x composites changes with the atomic percentage x of germanium for wire dimension of L Si = 10 nm. Simulation results reveal that for a constant silicon wire dimension, the thermal conductivity of the Si/Ge nanocomposites increases with x . An attempt study on the influence of the voids on thermal conductivity shows that the thermal conductivity decreases with the void density. Most of the presented results from the simulations in this work come to favorable agreement with previous work, suggesting a good reliability of the present simulation method for further analysis of thermal transportation phenomena in nanocomposites and even more complex composites with pores, dislocations and impurities.

This work pioneers the atomistic modeling of the shock wave in picosecond laser-material interaction by simulating the material that is irradiated with a picosecond laser pulse (11.3 ps FWHM) in a 0.25 MPa background gas. The dynamic structure and mutual mass penetration between the plume and background gas are investigated in detail. In the shock wave the compressed ambient gas region has a very uniform temperature distribution while the temperature decreases from the front of the plume to its end. The group velocity of atoms in the shock wave front is much smaller than the shock wave propagation speed and experiences a fast decay due to momentum exchange with the ambient gas. Strong decay of the shock wave front temperature and pressure is observed while its density features much slower attenuation. An effective mixing length is designed to quantitatively evaluate the mutual mass penetration between the plume and background gas. This effective mixing length grows at a rate of ∼ 60 m/s. This fast mixing/mass penetration is largely due to the strong relative movement between the plume and the background gas. The MD results agree well with the analytical solution in terms of relating various shock wave strengths.

In this work, extensive equilibrium molecular dynamics simulations are conducted to study the shear viscosity of nanocolloidal dispersion. Strong oscillation of the pressure tensor autocorrelation function is observed. The computational domain contains solvent of liquid argon at 143.4 K and spherical particles with volume fraction of 3%. By studying the effect of the particle size, particle density, and acoustic impedance, it is found for the first time that the stress wave scattering/reflecting at the liquid-particle interface due to acoustic mismatch plays a critical important role in the oscillation of pressure tensor autocorrelation function. The Brownian motion/vibration of solid particles is considered to have little effect on the oscillation of pressure tensor autocorrelation function curve except the frequency. And when the particle size is comparable with the wavelength of stress wave, the diffraction of stress wave happens at the interface that will also weaken the oscillation of pressure tensor autocorrelation function.

In this work, a transient photon-electro-thermal (TPET) technique based on step laser heating and electrical thermal sensing is developed to characterize the thermophysical properties of one-dimensional micro/nanoscale conductive and nonconductive wires. In this method, the to-be-measured thin wire/tube is suspended over two electrodes and is irradiated with a step CW laser beam. The laser beam induces a transient temperature rise in the wire/tube, which will lead to a transient change of its electrical resistance. A DC current is applied to the sample, and the resulting transient voltage variation over the wire is measured and used to extract the thermophysical properties of the sample. A 25.4-μm thick Pt wire is used as the reference sample to verify this technique. Sound agreement is obtained between the measured thermal diffusivity and the reference value. Applying the TPET technique, the thermal diffusivity of conductive single-wall carbon nanotube (SWCNT) bundles and nonconductive cloth fibers is measured. For nonconductive wires, a thin (∼nm) metallic film is coated on the outside of the wire for electrical thermal sensing. The measured thermal diffusivity for the SWCNT bundle is 2.53×10 −5 m 2 /s, much less than the thermal diffusivity of graphite in the layer direction. For microscale cloth fibers, our experiment shows its thermal diffusivity is at the level of 10 −7 m 2 /s.

In this work, the dynamics of the shock wave in laser-ablated argon plume with its evolution through the background gas is explored at the atomic level. Molecular Dynamics (MD) simulations have been conducted which give the insight into atomistic scale interaction and correlation effects of the propagating shock wave in the background medium. The supersonic shock wave front carries inherent sharp increase in density, temperature, and pressure. These thermodynamic parameters of the expanding shock wave are evaluated with emphasis on the kinetics of the shock wave front. The position of the shock wave front has been defined and determined over nanoseconds. Extensive research is elaborated upon to study the inside structural evolution of the shock wave and the effect of optical absorption depth.

This paper presents the results from molecular dynamics simulations that are performed to explore the properties of the shock wave during laser-assisted near field surface nanostructuring. A quasi-three dimensional model is constructed to study systems consisting of over 2 million atoms. This work includes studies on the velocity as well as pressure evolution of shock wave front with respect to different solid/gas molecular mass ratios and different ambient gas densities. The limitation on shock wave formation under the same laser fluence is also investigated. The results show that lower ratio of the solid/gas molecular weight weakens the strength of the shock wave during the nanostructuring process. Additionally, the formation and attenuation of the shock wave under different ambient gas conditions is studied in substantial detail.

In this work, molecular dynamics simulation is performed to explore the long-time (up to 5 ns) behavior of argon crystal in surface nanostructuring with a laser-assisted scanning tunneling microscope. The surface nanostructuring region is limited to tens of nanometers in diameter although the simulated systems are much larger (more than 300,000 of atoms inside). This study is focused on the long-time solidification and crystallization procedure, which is driven by the heat conduction in the material. The effect of the computational domain on the final nanostructuring result is studied in detail. Different laser fluences are used in the simulation to explore how and to what extent the energy input affects the dimension and profile of the surface nanostructure. Meanwhile, spot-like structural defects in the sub-surface region are investigated during the solidification. Additionally, the evolutions of surface melting depth and lowest position in the whole process reveal the impact of different laser fluence on surface nanostructuring from another aspect.

A laser flash technique was applied to measure the thermal diffusivity along a multi-walled carbon nanotube (CNT) array in temperature range of −55∼200 °C. In the measurements, a nanosecond pulsed laser was used to realize noncontact heating and the temperature variations were recorded by an infrared detector. The experimental results show that the thermal diffusivity of the CNT array increases slightly with temperature in the −55∼70 °C temperature range and exhibits no obvious change in the −75∼200 °C temperature range. The CNT array has much larger thermal diffusivity than several known excellent thermal conductors, reaching about 4.6 cm 2 s −1 at room temperature. The mean thermal conductivity (λ) of individual CNTs was further estimated from the thermal diffusivity, specific heat ( Cp ), and density (ρ) by using the correlation of λ = αρ C p . The thermal conductivity of individual CNTs increases smoothly with the temperature increase, reaching about 750 Wm −1 K −1 at room temperature.

In this work, an equilibrium technique is developed to study the thermal transport in nanomaterials. By directly tracking the relaxation behavior of energy carriers, the developed technique is able to determine the effect of boundary scattering on thermal transport. Since no temperature differential across the material is required to determine its thermal conductivity, the developed technique is applicable to nanomaterials of different shapes and capable of capturing the anisotropic nature of the thermal transport inside. Applying this technique, the thermal transport in several typical nanomaterials—nanofilms, square and round nanowires, and spherical and cubic nanoparticles are studied in detail. A strong anisotropic nature of thermal transport in nanomaterials is observed. For nanofilms and nanowires, the thermal conductivity in the restricted directions (thickness and radial) is smaller than that in the unrestricted direction. This anisotropic nature is more obvious and important when the characteristic size of nanomaterials becomes comparable to or smaller than the mean free path of energy carriers. Our results comparison shows that with the same characteristic size, the shape of the cross section of nanowires has appreciable effect on the thermal transport in the axial direction. For spherical and cubic nanoparticles, little difference is observed between their thermal conductivities.

In this work, thermal transport in nanocrystalline materials is studied using large-scale equilibrium molecular dynamics (MD) simulation. Nanocrystalline materials with different grain sizes are studied to explore how and to what extent the size of nanograins affects the thermal conductivity and specific heat. Substantial thermal conductivity reduction is observed and the reduction is stronger for nanocrystalline materials with smaller grains. On the other hand, the specific heat of nanocrystalline materials shows little change with the grain size. The simulation results are compared with the thermal transport in individual nanograins based on MD simulation. Further discussions are provided to explain the fundamental physics behind the observed thermal phenomena in this work.

In this work, nonequilibrium molecular dynamics is used to predict the thermal conductivity of nanoscale thin silicon films in the thickness direction. Recently developed environment-dependent interatomic potential for silicon, which offers considerable improvement over the more common Stillinger-Weber potential, is used. Silicon films of various thicknesses are modeled to establish the variation of thermal conductivity with the film thickness. The obtained relationship between the thermal conductivity and the film thickness is compared with the results of the Lattice Boltzmann method, and sound agreement is observed.

In this work, large-scale molecular dynamics simulation is conducted to explore nanoscale manufacturing with laser-assisted scanning tunneling microscope. Employing a super parallel computer, more than 100 million atoms are modeled to provide substantial details about how the localized thermal and mechanical perturbations result in surface nanostructures. It is found that thermal equilibrium cannot be established due to the small number of atoms. Extremely localized stress accumulation beneath the sample surface results in an explosion of the melted/vaporized material, leaving a nanoscale hole on the sample surface. Normal and shear stress development is observed. Stress propagation in space is strongly influenced by the anisotropic nature of the crystal. The high pressure in the melted/vaporized region pushes the melt adjacent to the solid to move, thereby forming a protrusion at the edge of the hole. More importantly, visible structural destruction is observed in the region close to the bottom of the sample. These destructions are along the direction of 45 degrees with respect to the axial direction, and are attributed to the strong tensile stress. Atomic dislocation is observed in the destructed regions.

Molecular Dynamics (MD) and Finite Difference (FD) methods are applied to investigate femtosecond laser ablation of copper. Laser induced heat transfer, melting, vaporization, and material ablation are studied. Phase change relevant parameters, such as the velocity of solid-liquid and liquid-vapor interfaces are reported. It is shown that results of MD and FD are in good agreement before strong material removal occurs. However, only MD is capable of capturing the ablation phenomena accurately.

Nanoparticles formed in short pulsed laser materials interaction have strong effects on laser micro-machining, material surface processing, and thin film deposition. In this work, Molecular Dynamics (MD) simulations are conducted to attain physical origins and the nature of nanoparticle formation in picosecond laser materials interaction. The MD simulation reveals that nanoparticles originate from an intense vapor phase explosion process occurring after laser heating. This phase explosion is driven by an accumulated high pressure of the order of 30 MPa in the near surface region. It is observed that nanoparticles start coming out after laser heating as a result of the after-heating intense phase explosion. Also observed is that the pressure in nanoparticles undergoes a reduction because of the temperature decrease and atom re-construction in space. In addition, saturation of the nanoparticle yield is observed when the laser fluence reaches a certain level, which is a result of limited heat penetration depth during laser heating.