Melt Dynamics Research Articles

“Free” oxide ions in silicate melts: Thermodynamic considerations and probable effects of temperature

Reactions of simple metal oxides with silica often describe the melt solution in terms of equilibria among three energetically distinct oxygen ion species: bridging oxygens connecting two network cations, non-bridging oxygens shared by one network cation and multiple modifier cations of lower charge and larger radius, and “free” oxide ion bonded only to modifiers. Interest in the role and concentration of “free” oxide ions has recently been renewed by spectroscopic studies that have directly determined its abundance in a few glassy and amorphous silicates with low silica contents and high cation field strengths, as well as being inferred to be present by less direct measurements of bridging/non-bridging oxygen ratios in more silica-rich compositions. Here we review and evaluate recent evidence on “free” oxide concentrations in silicate glasses and melts, and discuss the important clues about effects of composition that come from comparisons of heats of formation of silicates and measurements of oxide component activities: relative proportions of oxygen species are expected to depend strongly on the charge and size of the modifier cation and its degree of covalent/ionic bonding. We also present simple thermodynamic approximations that allow temperature effects on “free” oxide concentrations to be estimated. In most compositions, this relative high energy species is expected to become more abundant at high temperatures, and may reach quite significant concentrations (>10% of total oxygens?) in hot, ultramafic, Mg- and Fe-rich melts in nature. In contrast, in most common, glass-forming compositions the concentrations of “free” oxide ion are probably much lower, particularly on cooling to glass transition temperatures, but this species has been suggested to potentially have an important role in melt dynamics even at low abundances. In any case, further experimental determinations, and theoretical evaluations, of this somewhat elusive and controversial species are well warranted.

Transport properties and Stokes-Einstein relation in Al-rich liquid alloys.

We use ab initio molecular dynamics simulations to study the transport properties and the validity of the Stokes-Einstein relation in Al-rich liquid alloys with Ni, Cu, and Zn as alloying elements. First, we show that the composition and temperature dependence of their transport properties present different behaviors, which can be related to their local structural ordering. Then, we evidence that the competition between the local icosahedral ordering and the local chemical ordering may cause the breakdown of the Stokes-Einstein relation even in the liquid phase. We demonstrate that this breakdown can be captured by entropy-scaling relationships developed by Rosenfeld and using the two-body excess entropy. Our findings provide a unique framework to study the relation between structure, thermodynamics, and dynamics in metallic melts and pave the way towards the explanation of various complex transport properties in metallic melts.

Experiment and simulation study of laser dicing silicon with water-jet

Water-jet laser processing is an internationally advanced technique, which combines the advantages of laser processing with water jet cutting. In the study, the experiment of water-jet laser dicing are conducted with ns pulsed laser of 1064nm irradiating, and Smooth Particle Hydrodynamic (SPH) technique by AUTODYN software was modeled to research the fluid dynamics of water and melt when water jet impacting molten material. The silicon surface morphology of the irradiated spots has an appearance as one can see in porous formation. The surface morphology exhibits a large number of cavities which indicates as bubble nucleation sites. The observed surface morphology shows that the explosive melt expulsion could be a dominant process for the laser ablating silicon in liquids with nanosecond pulse laser of 1064nm irradiating. Self-focusing phenomenon was found and its causes are analyzed. Smooth Particle Hydrodynamic (SPH) modeling technique was employed to understand the effect of water and water-jet on debris removal during water-jet laser machining.

Melting Point and Solid–Liquid Coexistence Properties of α1 Isotactic Polypropylene as Functions of Its Molar Mass: A Molecular Dynamics Study

The melting point (Tm) of the α1 form of isotactic polypropylene (iPP) as a function of its molar mass was studied with molecular dynamics (MD) simulations. The 11mer, 32mer, and 47mer systems were simulated, with the latter displaying a Tm representative of the long-chain polymer. Adopting a methodology developed in previous work, composite (sandwich) configurations consisting of both melt and crystal subdomains in contact with each other were generated, and Tm was determined as that temperature where none of the phases in the sandwich grew at the expense of the other. To deal with the sluggish dynamics of solidification and melting, a constraining potential that drives chains toward helical conformations was added to the Hamiltonian, solid–liquid equilibration was achieved at high temperature in the presence of this potential, and Tm was ultimately obtained through gradual removal of the constraining potential in single-phase solid and liquid simulations by Gibbs–Duhem integration. The enthalpy differen...

Influence of snowpack internal structure on snow metamorphism and melting intensity on Hansbreen, Svalbard

AbstractThis paper presents a detailed study of melting processes conducted on Hansbreen – a tidewater glacier terminating in the Hornsund fjord, Spitsbergen. The fieldwork was carried out from April to July 2010. The study included observations of meltwater distribution within snow profiles in different locations and determination of its penetration time to the glacier ice surface. In addition, the variability of the snow temperature and heat transfer within the snow cover were measured. The main objective concerns the impact of meltwater on the diversity of physical characteristics of the snow cover and its melting dynamics. The obtained results indicate a time delay between the beginning of the melting processes and meltwater reaching the ice surface. The time necessary for meltwater to percolate through the entire snowpack in both, the ablation zone and the equilibrium line zone amounted toc.12 days, despite a much greater snow depth at the upper site. An elongated retention of meltwater in the lower part of the glacier was caused by a higher amount of icy layers (ice formationsandmelt-freeze crusts), resulting from winter thaws, which delayed water penetration. For this reason, a reconstruction ofrain-on-snowevents was carried out. Such results give new insight into the processes of the reactivation of the glacier drainage system and the release of freshwater into the sea after the winter period.

An investigation of heat transfer and fluid flow on laser micro-welding upon the thin stainless steel sheet (SUS304) using computational fluid dynamics (CFD)

A transient three-dimensional model is numerically developed using the method of computational fluid dynamics (CFD) to characterize some thermal phenomena and characterization of heat transfer and fluid flow in laser micro-welding by considering the heat source and the material interaction leads to rapid heating, melting and thermal cycles in the heating zone. The application of developed thermal models has demonstrated that the laser parameters, such as laser power, scanning velocity and spot diameter, have considerable effects on the peak temperature and resulted weld pool. The heat source model is consisted of surface heat source and adaptive volumetric heat source that could be well represented the real laser welding as the heat penetrates into the material. In the computation of melt dynamics, mass conservation, momentum and energy equations have been considered to compute the effects of melt flow and the thermo-fluid energy heat transfer. The simulation results have been compared with two sets of experimental research to predict the weld bead geometry and solidification pattern, which laser welds are made on thin stainless steel sheet (SUS304). The shape comparison describes those parameters relevant to any changes in the temperatures and melt dynamics are of great importance on the heat distribution and formation of weld pool during laser micro-welding process. The fair agreement between simulated and experimental results, demonstrates the reliability of the computed model.

Multi-scale fluence-dependent dynamics of front-side femtosecond laser heating, melting and ablation of thin supported aluminum film

Optically thick, thermally thin aluminum film was homogeneously pumped by single IR femtosecond laser pulses at variable local fluences, resulting in its electronic excitation, electron-lattice energy transfer, melting, thermal expansion, ablative spallation and supercritical hydrodynamic expansion, probed by low-fluence UV femtosecond laser pulses at different – sub-picosecond, picosecond and sub-nanosecond – timescales. Fluence dependences of their corresponding key dynamic parameters – peak electronic temperature, internal melt temperature and pressure, volume energy density present in the film on different time scales upon different dissipative processes, spallation and phase explosion temperatures, their onset times, lift-off speeds and crater depths were measured for aluminum for the first time. These dependences indicate sub-nanosecond and picosecond onsets of spallation and phase explosion in the film, as well as their occurrence near the boiling and critical temperatures, respectively. Significant difference in energy density magnitudes, primarily absorbed from the laser radiation and subsequently deposited into electronic and ion sub-systems, was revealed and related preferably to prompt charge emission, reducing the peak transient electron temperature, and evaporative surface cooling of the molten material.

Glass structure, melt structure, and dynamics: Some concepts for petrology

![Figure][1] The thermodynamic and transport properties of the aluminosilicate melts at the heart of most magmatic processes vary in complex ways with composition, temperature, and pressure. Insights into these properties can come from information on the structure of the melts themselves, and more commonly from their glassy, quenched equivalents. Although most such connections remain qualitative or semi-quantitative, they are fundamentally important in interpretation of observations on igneous systems in nature and the laboratory, and in the formulation of physically accurate models. This review presents some of the important concepts of aluminosilicate glass and melt structure and dynamics that are most relevant to furthering our understanding of the igneous processes so central to how our planet has formed and evolved. The relationships among glasses, melts, and crystals are introduced. The structural underpinnings of temperature and pressure effects on melt free energies, densities, and viscosities, constraints on the extent of order/disorder among cations and anions in melts, why silica activity varies so strongly with composition, and how liquid-liquid phase separation can be understood, are discussed. Some simple, but useful, general views are presented on melt disorder and the shapes of liquidus surfaces (key to magmatic phase equilibria), as are links between atomic-scale dynamics and viscous flow and diffusion. [1]: pending:yes

A Retrospective, Iterative, Geometry-Based (RIGB) tilt-correction method for radiation observed by automatic weather stations on snow-covered surfaces: application to Greenland

Abstract. Surface melt and mass loss of the Greenland Ice Sheet may play crucial roles in global climate change due to their positive feedbacks and large fresh-water storage. With few other regular meteorological observations available in this extreme environment, measurements from automatic weather stations (AWS) are the primary data source for studying surface energy budgets, and for validating satellite observations and model simulations. Station tilt, due to irregular surface melt, compaction and glacier dynamics, causes considerable biases in the AWS shortwave radiation measurements. In this study, we identify tilt-induced biases in the climatology of surface shortwave radiative flux and albedo, and retrospectively correct these by iterative application of solar geometric principles. We found, over all the AWS from the Greenland Climate Network (GC-Net), the Kangerlussuaq transect (K-transect) and the Programme for Monitoring of the Greenland Ice Sheet (PROMICE) networks, insolation on fewer than 40 % of clear days peaks within ±0.5 h of solar noon time, with the largest shift exceeding 3 h due to tilt. Hourly absolute biases in the magnitude of surface insolation can reach up to 200 W m−2, with respect to the well-understood clear-day insolation. We estimate the tilt angles and their directions based on the solar geometric relationship between the simulated insolation at a horizontal surface and the observed insolation by these tilted AWS under clear-sky conditions. Our adjustment reduces the root mean square error (RMSE) against references from both satellite observation and reanalysis by 16 W m−2 (24 %), and raises the correlation coefficients with them to above 0.95. Averaged over the whole Greenland Ice Sheet in the melt season, the adjustment in insolation to compensate station tilt is ∼ 11 W m−2, enough to melt 0.24 m of snow water equivalent. The adjusted diurnal cycles of albedo are smoother, with consistent semi-smiling patterns. The seasonal cycles and inter-annual variabilities of albedo agree better with previous studies. This tilt-corrected shortwave radiation data set derived using the Retrospective, Iterative, Geometry-Based (RIGB) method provide more accurate observations and validations for surface energy budgets studies on the Greenland Ice Sheet, including albedo variations, surface melt simulations and cloud radiative forcing estimates.

Investigation of the microstructure and surface morphology of a Ti6Al4V plate fabricated by vacuum selective laser melting

As an additive manufacturing technology, the selective laser melting (SLM) process is useful to directly form complicated shapes. The SLM process in a vacuum has been used to fabricate three-dimensional Ti6Al4V (Ti64) plates because this method can control the phase transformation. To investigate the laser melting and solidification dynamics, the formation of a Ti64 plate by SLM in a vacuum was captured by a high-speed video camera. Due to the effects of temperature and scanning speed on the phase transformation, the crystal orientation was evaluated with X-ray diffraction. A phase transformation of the crystal orientation occurred as the baseplate temperature was heated up from 50 to 150 °C.

Experimental Study and Simulation of the Structure-Phase Transitions in Deposited Ge Layers during Pulsed Laser Annealing

Ion-beam deposition of amorphous Ge layers on different substrates (silicon and quartz) has been performed. Deposited amorphous Ge layers were subjected to pulsed laser annealing (λ = 0.69 μm, τ = 80 ns). Simultaneously the optical probing of the Ge surface was carried out. The computer simulation of heating processes and phase transitions was performed taking into account the temperature dependences of film and substrates’ parameters and phase transition energies. The results of the dynamics of heating, melting, crystallization and plasma formation processes are well described by simulation data. It is shown that the threshold values for radiation power density and phase transition rates are determined mainly by thermophysical parameters of the substrates and thermal contact between Ge melt and substrate.

Use of a hydrologic mixing model to examine the roles of meltwater, precipitation and groundwater in the Langtang River basin, Nepal

AbstractUnderstanding the hydrology of glacierized catchments is an important step in assessing the vulnerability of water resources to a changing climate. While there have been increased efforts recently to understand the dynamics of Asia's cryosphere, glacier melt dynamics and hydrograph separation of river discharge are open questions. A multi-year, multi-seasonal dataset of water chemistry from the Langtang Valley, Nepal, is used to explore water sources and flow paths that contribute to Langtang River discharge. Differences in hydrochemistry of samples from debris-free Khimsung Glacier and debris-covered Lirung Glacier demonstrate the effect of debris cover on glacier outflow. Additional data show seasonal transitions in the composition of Langtang River discharge. End-member mixing analysis (EMMA) using geochemical and isotopic tracers suggests that reacted meltwater contributes the majority of flow during most of the year, with the exception of the summer when unreacted meltwater and precipitation dominate streamflow. We hypothesize our dataset is missing characteristic monsoon water and utilize a Late May river sample as a proxy for precipitation-influenced groundwater in the EMMA. Results offer insight into the plausibility of flow sources and pathways in the basin.

A new insight into the conformation and melt dynamics of hydrogenated polybutadiene as revealed by computer simulations.

Extensive molecular dynamics simulations of the macromolecular conformation and the melt dynamics for model polymers of different molecular weights have been carried out. The selected models are hydrogenated polybutadienes with a 2% content of ethyl branches and linear polyethylene. It will be shown that the density and chain stiffness are clearly affected by both the molecular weight and the presence of ethyl branches. Furthermore, the results obtained from the simulations on the molecular size and, more remarkably, chain dynamics, perfectly match the neutron scattering experiments performed by Zamponi et al. in hydrogenated polybutadienes. We observe a clear chain contraction and a slow dynamics for the hydrogenated polybutadiene with respect to the linear chain of the same molecular length. Using the Likhtman-McLeish definitions, the obtained values of the entanglement relaxation time (τe) and the tube diameter (a) are found to be in agreement with the available experimental data (by rheology and neutron spin echo) as well as with those obtained by the simulations. Finally, a very good agreement of diffusion coefficients as a function of the molecular weight between simulations and experiments is observed. Therefore, there exists a clear difference between the results obtained for branched and linear polyethylene, accounting for a definitive effect of the short chain branching on the conformational properties and the melt dynamics of polyolefins.

Open boundary molecular dynamics of sheared star-polymer melts.

Open boundary molecular dynamics (OBMD) simulations of a sheared star polymer melt under isothermal conditions are performed to study the rheology and molecular structure of the melt under a fixed normal load. Comparison is made with the standard molecular dynamics (MD) in periodic (closed) boxes at a fixed shear rate (using the SLLOD dynamics). The OBMD system exchanges mass and momentum with adjacent reservoirs (buffers) where the external pressure tensor is imposed. Insertion of molecules in the buffers is made feasible by implementing there a low resolution model (blob-molecules with soft effective interactions) and then using the adaptive resolution scheme (AdResS) to connect with the bulk MD. Straining with increasing shear stress induces melt expansion and a significantly different redistribution of pressure compared with the closed case. In the open sample, the shear viscosity is also a bit lowered but more stable against the viscous heating. At a given Weissenberg number, molecular deformations and material properties (recoverable shear strain and normal stress ratio) are found to be similar in both setups. We also study the modelling effect of normal and tangential friction between monomers implemented in a dissipative particle dynamics (DPD) thermostat. Interestingly, the tangential friction substantially enhances the elastic response of the melt due to a reduction of the kinetic stress viscous contribution.

Capillary filling dynamics of polymer melts in nanopores: experiments and rheological modelling

The spontaneous capillarity-driven flow behaviors of PE melts through AAO nanopores suggest that the zero-shear-rate viscosity of the PE melts decreases, which can be predicted by a modified Lucas–Washburn law considering the polymer rheological model.

Isosteric and fluorescent DNA base pair formed by 4-amino-phthalimide and 2,4-diaminopyrimidine: melting, structure, and THz polar solvation dynamics.

An artificial base pair in the center of a duplex DNA oligomer, formed by 2,4-diaminopyrimidine and fluorescent 4-aminophthalimide C-nucleosides, is characterized spectroscopically, with a view towards its use in femtosecond solvation dynamics. Quantum-chemical calculations predict H-bonding energy equivalent to A:T. UV-vis absorption spectra provide insight into local melting at the 4-aminophthalimide modification site. Increase of temperature to nearly the melting temperature of the duplex leads to better hybridisation of the fluorescent nucleoside, contrary to native base pairs. This unusual observation is explained by the NMR solution structure of the duplex. Two conformations are adopted by the artificial pair due to backbone constraints, having either two or one interbase hydrogen bonds. In the latter, hydrogen bonding sites remain accessible for water solvation. The time-resolved dynamic Stokes' shift of 4-aminophthalimide fluorescence is consistent with that of a mixture of a slow and fast species. From the observations, the optimal linkage between 4-aminophthalimide and 2-deoxyribose for fitting into the duplex B-DNA structure is deduced.

Computational Investigation of Synchronized Multibeam Strategies for the Selective Laser Melting Process

The selective laser melting process features a nearly incomparable freedom of design. But its potential is still limited due to remaining porosity, cracking, distortion, low build-up rates and a limited range of materials. While there is some progress in process control and multiple parallel scan fields to tackle these issues, the potential of synchronized multibeam strategies has not yet been investigated. The presented synchronized multibeam approach is characterized by two widely overlapping scan fields fed by two independent laser sources that can be controlled to work in a synchronized manner with or without a defined offset. This allows a selective manipulation of the local temperature field and thus of melt pool dynamics, the temperature gradients and cooling rates, which are all influencing the processes’ porosity, cracking and distortion behavior. Therefore the influences of these strategies on the melt pool dimensions and dynamics as well as the temperature gradients are investigated in this work.

Combining snowpack modeling and terrestrial laser scanner observations improves the simulation of small scale snow dynamics

Summary Accurately determining the snowpack distribution in mountain areas is complex because of the difficulty of establishing over large areas the spatial distribution of all variables that define the state of the snowpack at any particular time. In this study we used distributed snowpack simulations that were corrected throughout the snow season using snow depth distributions measured using a terrestrial laser scanner (TLS). This enabled us to obtain a more realistic view of the small scale spatial evolution of the mass and depth of the snowpack. Several TLS snow depth data acquisitions were made during the 2012, 2013 and 2014 snow seasons in a small catchment (55 ha) located in the central Spanish Pyrenees. The Crocus snowpack model was used to simulate the snowpack evolution on a 5-m grid, based on downscaled meteorological variables obtained using SAFRAN reanalysis. The simulation was stopped when a snow depth distribution map measured using the TLS was available, and the modeled snow depth distribution was adjusted to match the observed snow depth. The snow simulation was then restarted and run until the next TLS acquisition was available. Prior to matching the simulation and observational data, both snowpack distributions were compared. The results for the three snow seasons showed an improvement in the snowpack simulation, especially with respect to simulating the influence of small scale topographic effects on the observed snowpack distribution, and also the timing of snow melt dynamics.

Laser induced morphological and optical properties changes in Au doped aluminum oxide and silicon oxide thin films

Results on laser processing of thin oxide films containing gold are presented. The Au doped aluminum oxide and silicon oxide thin films are fabricated by pulsed laser deposition using targets composed by areas of gold and the oxide material. The as fabricates samples are processed by laser pulses with broad range of parameters – pulse duration, wavelength, fluence and pulse number. The changes of the morphology and optical properties after the laser irradiation are studied. It is found that laser irradiation with both nanosecond and femtosecond pulses may induce clear color change of the samples, which is accompanied by modification of the film structure. Nanosecond laser pulse treatment results in disintegration of the film induced by melting and surface tension forces. The process is accompanied by formation of nanoparticles at the edges of the surface modifications. At higher fluences nanosecond pulse irradiation leads to formation of wires composed of oxide material and gold nanoparticles. The femtosecond laser pulses induce a homogeneous decomposition of the film, as melt dynamics is not observed. The decomposition is however accompanied by color change of the irradiated area and clear dip in the transmission spectra. The dynamics of the film decomposition is discussed. The conducted study can be applied in fabrication of optical elements and in plasmonics.

Melting dynamics of short dsDNA chains in saline solutions.

DNA melting has attracted much attention due to its importance in understanding the life-reproduction and metabolism and in the applications of modern DNA-based technologies. While numerous works have been contributed to the determination of melting profiles in diverse environments, the understanding of DNA melting dynamics is still limited. By employing three-site-per-nucleotide (3SPN) double-stranded DNA (dsDNA) model, we here demonstrate the melting dynamics of an isolated short dsDNA under different conditions (different temperatures, ionic concentrations and DNA chain lengths) can be accessed by coarse-grained simulation studies. We particularly show that at dilute ionic concentration the dsDNA, regardless being symmetric or asymmetric, opens at both ends with roughly equal probabilities, while at high ionic concentration the asymmetric dsDNA chain opens at the A-T-rich end. The comparisons of our simulation results to available data are discussed, and overall good agreements have been found.