Melt Dynamics Research Articles

Role of Chain Length in the Formation of Frank-Kasper Phases in Diblock Copolymers.

The phase behavior of poly(styrene)-b-poly(1,4-butadiene) diblock copolymers with a polymer block invariant degree of polymerization N[over ¯]_{b}≈800 shows no evidence of Frank-Kasper phases, in contrast to low molar mass diblock copolymers (N[over ¯]_{b}<100) with the same conformational asymmetry. A universal self-concentration crossover parameter N[over ¯]_{x}≈400 is identified, directly related to the crossover to entanglement dynamics in polymer melts. Mean-field behavior is recovered when N[over ¯]_{b}>N[over ¯]_{x}, while complex low symmetry phase formation is attributed to fluctuations and space-filling constraints, which dominate when N[over ¯]_{b}<N[over ¯]_{x}.

MEMOS 3D modelling of ELM-induced transient melt damage on an inclined tungsten surface in the ASDEX Upgrade outer divertor

The first MEMOS 3D simulations of liquid metal motion on an inclined bulk tungsten sample transiently molten by edge-localized modes (ELMs) are reported. The exposures took place at the outer ASDEX-Upgrade divertor with the tungsten surface tangent intersecting the magnetic field at ∼ 18°. Simulations confirm that the observed poloidal melt motion is caused by the volumetric J × B force with J the bulk replacement current triggered by thermionic emission. The final erosion profile and total melt build up are reproduced by employing the escaping thermionic current dependence on the incident heat flux derived from dedicated particle-in-cell simulations. Modelling reveals that melt dynamics is governed by the volumetric Lorentz force, capillary flows due to thermal surface tension gradients and viscous deceleration. The effect of the evolving surface deformation, that locally alters the field-line inclination modifying the absorbed power flux and the escaping thermionic current, in the final surface morphology is demonstrated to be significant.

Polymer Chain Mobility under Shear-A Rheo-NMR Investigation.

The local dynamics in polymer melts and the impact of external shear in a Couette geometry have been investigated using rheological nuclear magnetic resonance (NMR). The spin-spin relaxation time, T2, which is sensitive to chain-segment motion, has been measured as a function of shear rate for two samples of poly(dimethylsiloxane). For the low-molecular-weight sample, a mono-exponential decay is observed, which becomes slightly faster with shear, indicating restrictions of the polymer chain motion. For the high-weight sample, a much faster bi-exponential decay is observed, indicative of entanglements. Both components in this decay become longer with shear. This implies that the free polymer segments between entanglements become effectively longer as a result of shear.

Heat flux analysis of Type-I ELM impact on a sloped, protruding surface in the JET bulk tungsten divertor

Tungsten (W) melting due to transient power loads, for example those delivered by edge localised modes (ELMs), is a major concern for next step fusion devices. A series of experiments has been performed on JET to investigate the dynamics of Type-I ELM-induced transient melting. Following initial exposures in 2013 of a W-lamella with sharp leading edge in the bulk W outer divertor, new experiments have been performed in 2016–2017 on a protruding W-lamella with a 15° slope, allowing direct and spatially resolved (0.85 mm/pixel) observation of the top surface using the IR thermography system viewing from the top of the poloidal cross-section. Thermal and IR analysis have already been conducted assuming the geometrical projection of the parallel heat flux on the W-lamellas, thus ignoring the gyro-radius orbit of plasma particles. Although it is well justified during L-mode or inter-ELM period, the hypothesis becomes questionable during ELM when the ion Larmor radius is larger. The goal of this paper is to extend the previous analysis based on the forward approach to the H-mode discharges and investigate in particular the gyro-radius effect during the Type-I ELMs, those used to achieve transient melting on the slope of the protruding W-lamella. Surface temperatures measured by the IR camera are compared with reconstructed synthetic data from 3D thermal modelling using heat loads derived from optical projection of the parallel heat flux (ignoring the gyro-radius orbit), 2D gyro-radius orbit and particle-in-cell (PIC) simulations describing the influence of finite Larmor-radius effects and electrical potential on the deposited power flux. Results show that the ELM power deposition behaves differently than the optical projection of the parallel heat flux, contrary to the L-mode observations, and may thus be due to the much larger gyro-orbits of the energetic ELM ions in comparison to L-mode or inter-ELM conditions.

Study of Brownian functionals for a Brownian process model of snow melt dynamics with purely time dependent drift and diffusion

In this paper, we investigate a Brownian motion (BM) with purely time dependent drift and diffusion by suggesting and examining several Brownian functionals, which characterize the stochastic model of water resources availability in snowmelt dominated regions with power law time dependent drift and diffusion. Snow melt process is modelled by a overdamped Langevin equation for a Brownian process with power law time dependent drift (μ(t) ~ qktα) and diffusion (D(t) ~ ktα) where they are proportional to each other. We introduce several probability distribution functions (PDFs) associated with such time dependent BMs. For instance, with initial starting value of snow amount H0, we derive analytical expressions for: (i) the PDF P(tf|H0) of the first passage time tf which specify the lifetime of such stochastic process, (ii) the PDF P(A|H0) of the area A till the first passage time and it provides us numerous valuable information about the average available water resources, (iii) the PDF P(M) associated with the maximum amount of available water M of the BM process before the complete melting of snow, and (iv) the joint PDF P(M;tm) of the maximum amount of available water M and its occurrence time tm before the first passage time. We further confirm our analytical predictions by computing the same PDFs with direct numerical simulations of the corresponding Langevin equation. We obtain a very good agreement of our theoretical predictions with the numerically simulated results. Finally, several nontrivial scaling behaviour in the asymptotic limits for the above mentioned PDFs are predicted, which can be verified further from experimental observation.

Mantle Source and Magmatic Evolution of the Dying Spreading Ridge in the South China Sea

AbstractThe genesis of seamount chains along fossil spreading ridges reflects the mantle melting dynamics of dying spreading center. The South China Sea developed a seamount chain along its fossil ridge shortly after cessation of spreading. International Ocean Discovery Program Expedition 349 has recovered three volcanic layers, a shallow volcanic breccia layer and two deeper (upper and lower sections) basalt layers, at Site U1431 near the fossil ridge of the South China Sea. Despite the mid‐ocean ridge basalt (MORB)‐like geochemistry of the lower and upper section basalts, they show distinct geochemical differences that indicate a change of mantle source and a transition from spreading to intraplate volcanism. While the lower section basalts could reflect the last stage volcanism before the cessation of ridge spreading, the upper section basalts with whole‐rock MgO up to ~20 wt.% were formed by intraplate volcanism with significant olivine accumulation once the spreading center had become inactive. The MORB type basalts at Site U1431 have Hawaii‐like high olivine Ni and Fe/Mn and low olivine Ca and Mn and bulk‐rock low CaO that are distinctly different from normal global MORBs, suggesting an anomalous mantle source in lithology. We suggest that an eclogite‐/pyroxenite‐rich component, possibly sourced from the Hainan hot spot, played a fundamental role in the transition from spreading to intraplate volcanism of the dying spreading ridge of the South China Sea.

Quantum nonlinear phononics route towards nonequilibrium materials engineering: Melting dynamics of a ferrielectric charge density wave

Negative nonlinear electron-phonon coupling involving an infrared-active phonon mode can lead to an instability towards the formation of a polar lattice distortion with ferrielectric (FE) moments accompanied by an electronic charge-density wave (CDW). Analyzing a small model system in and out of thermal equilibrium, we investigate the FE-CDW and its melting dynamics following an ultrashort laser pulse that drives the ionic dipoles. We observe nonequilibrium coherent phonon amplitude mode oscillations that soften towards the transition to the normal phase. Our case study serves as a first step towards a microscopic understanding of quantum nonlinear phononics as a basis for nonequilibrium control in quantum materials.

Monitoring the crystallization of water-saturated granitic melts in real time using the hydrothermal diamond anvil cell

Critical aspects of the crystallization dynamics of H2O-saturated melts of a typical granitic composition as well as granitic melts enriched in lithium (8800 ppm) were investigated in real-time experiments using the hydrothermal diamond anvil cell at 480–700 °C and 220–960 MPa. Complete crystallization of the charges was achieved within 5–118 h with average crystal growth rates ranging from 3 to 41 cm/year for quartz and from 18 to 58 cm/year for alkali feldspars, demonstrating that crystals formed from a silicate melt in the presence of a coexisting aqueous phase crystallize rapidly. The combination of substantial nucleation delays, low nucleation densities, and rapid growth rates for quartz and alkali feldspars led to the formation of euhedral megacrysts of these minerals in the vicinity of clusters comprised of much smaller muscovite or α-spodumene crystals. Subsolidus replacement processes initiated during crystal–fluid interactions after the silicate melt was consumed were directly observed in the experiments. The experimental results underscore the important role of water as a medium for the transport of essential elements such as Si, Al, Na, and K from the silicate melt to the newly formed crystals, and provide important insights into the crystallization of miarolitic pegmatites.

Warming in the Arctic Captured by productivity variability at an Arctic Fjord over the past two centuries.

Arctic fjords feature among some of the most climate-sensitive regions on the planet. The site of this study–Kongsfjorden–is one such fjord in which sedimentation and sediment geochemistry reflect climate-mediated changes in glacial melt and marine primary productivity. Here, we have shown that the fjord is particularly sensitive to the changing melt dynamics of the surrounding glaciers which are a direct consequence of warming/cooling in the region and is reflected in the productivity at the fjord. Warming increases meltwater influx into the fjord leading to enhanced turbidity which results in lower productivity. A multi-proxy study (sedimentary organic matter content, carbon and nitrogen isotope ratios, and microfossil abundance) using a 21 cm long sediment core from the Kongsfjorden helped us reconstruct warming driven melt-dynamics history for the past two centuries. Proxy data show a general decreasing trend in productivity along with a few excursions over the last two centuries. Warming driven glacial-melt dynamics appears to be the dominant control on productivity throughout the span of the core.

The influence of dynamic rheological properties on carbon fiber-reinforced polyetherimide for large-scale extrusion-based additive manufacturing

Printing high-performance thermoplastics on large scale extrusion-based additive manufacturing platforms requires stability over a range of processing conditions. However, studies on the melt dynamics and processing conditions of these thermoplastics in big area additive manufacturing (BAAM) are limited. This study characterizes the dynamic rheological behavior of polyetherimide (PEI), a high-performance thermoplastic, as well as carbon fiber (CF)-reinforced PEI composites as a BAAM feedstock material. The viscoelastic properties, such as the storage and loss moduli and complex viscosity, are investigated in relation to the BAAM extrusion process. The results show that CF-PEI composites behave like a viscous liquid during BAAM extrusion. The addition of CF to PEI enhances the shear thinning effect and significantly increases the complex viscosity (2.5× increase for 20% CF, and 3× for 30% CF). The increased viscosity increases the torque on the extruder, which may be alleviated by increasing the material processing temperature.

Evolution of foamed aluminum melt at high rate tension: A mechanical model based on atomistic simulations

Dynamics of foamed metal melt relates to the following problems: cavitations in melt at negative pressure; ablation and nanostructuring of the surface layer of materials under the action of powerful sub picosecond laser pulses; additive manufacturing; and production of solid foamed metals. We propose a mechanical model of the foamed melt evolution at the stage of bubble enlargement—after the completion of their nucleation and before the breaking of the inter-bubble walls. The foamed melt is considered as a system of strongly connected bubbles; their size variations are driven by surface forces. The interconnected system of equations for the second time derivatives of the bubble radii is derived by using the Lagrange formalism. The model is verified by comparison with the molecular dynamic (MD) simulation data for aluminum melt at high-rate uniform tension. An algorithm for searching of individual pores is developed and used to analyze the MD data. The size distribution of pores in the melt at the considered stage of enlargement is found to be close to the normal distribution on a finite interval. Evolution of the foamed melt structure is completely determined by the surface tension. The surface tension makes the largest bubbles growing and all the smaller bubbles collapsing, which leads to rapid reduction of the total number of bubbles. The foamed melt maintains negative pressure determined only by the size distribution of bubbles and the surface tension coefficient.

Quantifying the effects of freeze-thaw transitions and snowpack melt on land surface albedo and energy exchange over Alaska and Western Canada

Variations in land surface albedo and snow-cover strongly impact the global biosphere, particularly through the snow-albedo feedback on climate. The seasonal freeze-thaw (FT) transition is coupled with snowpack melt dynamics and strongly impacts surface water mobility and the energy budget in the northern (≥45°N) arctic and boreal region (ABR). However, understanding of the regional variation in snowmelt and its effect on the surface energy budget are limited due to sparse in situ measurements of these processes and environmental constraints on effective monitoring within the ABR. In this study, we combined synergistic observations from overlapping satellite optical-infrared and microwave sensor records to quantify the regional patterns and seasonal progression in wet snow conditions during the spring snowmelt and autumn snow accumulation periods across Alaska and western Canada. The integrated satellite record included daily landscape FT status from AMSR microwave brightness temperature retrievals; and snow-cover extent, black sky albedo and net shortwave solar radiation (Rsnet) derived from MODIS and AVHRR observations. The integrated satellite records were analyzed with in situ surface air temperature and humidity observations from regional weather stations over a two-year study period (2015–2016) overlapping with the NASA ABoVE (Arctic Boreal Vulnerability Experiment). Our results show a large (79%) mean decline in land surface albedo between dry snow and snow-free conditions during the spring (March–June) and autumn (August–November) transition periods. Onset of diurnal thawing and refreezing of the surface snow layer and associated wet snow conditions in spring contributed to an approximate 25% decrease in snow cover albedo that extended over a seven to 21 week snowpack depletion period. The lower wet snow albedo enhances Rsnet by approximately 74% (9–10 MJ m−2 d−1) relative to dry snow conditions, reinforcing snowmelt and surface warming, and contributing to growing season onset and activation of biological and hydrological processes in the ABR. These results contribute to better understanding of snow albedo feedbacks to Arctic amplification, and the representation of these processes in global Earth system models.

Molecular Dynamics of Polyfarnesene

This paper represents the first comprehensive report of the molecular weight dependence of the dynamics of polyfarnesene (PF), utilizing oscillatory shear rheology and broadband dielectric spectroscopy. Extended PF chain conformations arising from tightly packed C11/C13 pendant groups reduce the probability of chain entanglements and lead to Rouse-like melt dynamics up to a critical molecular weight ∼105 g/mol. At higher molecular weights, PF behaves as an entangled polymer melt. Dielectric spectroscopy measurements establish PF as a type-A polymer, whose normal mode relaxation is strongly dependent on molecular weight, providing a compliment to melt rheology for the exploration of PF global chain dynamics.

Continuous-time random-walk approach to supercooled liquids: Self-part of the van Hove function and related quantities.

From equilibrium molecular dynamics (MD) simulations of a bead-spring model for short-chain glass-forming polymer melts we calculate several quantities characterizing the single-monomer dynamics near the (extrapolated) critical temperature [Formula: see text] of mode-coupling theory: the mean-square displacement g0(t), the non-Gaussian parameter [Formula: see text] and the self-part of the van Hove function [Formula: see text] which measures the distribution of monomer displacements r in time t. We also determine these quantities from a continuous-time random walk (CTRW) approach. The CTRW is defined in terms of various probability distributions which we know from previous analysis. Utilizing these distributions the CTRW can be solved numerically and compared to the MD data with no adjustable parameter. The MD results reveal the heterogeneous and non-Gaussian single-particle dynamics of the supercooled melt near [Formula: see text]. In the time window of the early [Formula: see text] relaxation [Formula: see text] is large and [Formula: see text] is broad, reflecting the coexistence of monomer displacements that are much smaller ("slow particles") and much larger ("fast particles") than the average at time t, i.e. than [Formula: see text]. For large r the tail of [Formula: see text] is compatible with an exponential decay, as found for many glassy systems. The CTRW can reproduce the spatiotemporal dependence of [Formula: see text] at a qualitative to semiquantitative level. However, it is not quantitatively accurate in the studied temperature regime, although the agreement with the MD data improves upon cooling. In the early [Formula: see text] regime we also analyze the MD results for [Formula: see text] via the space-time factorization theorem predicted by ideal mode-coupling theory. While we find the factorization to be well satisfied for small r, both above and below [Formula: see text] , deviations occur for larger r comprising the tail of [Formula: see text]. The CTRW analysis suggests that single-particle "hops" are a contributing factor for these deviations.

Structure and Dynamics of a Graphene Melt.

We explore the structural and dynamic properties of bulk materials composed of graphene nanosheets using coarse-grained molecular dynamics simulations. Remarkably, our results show clear evidence that bulk graphene materials exhibit a fluid-like behavior similar to linear polymer melts at elevated temperatures and that these materials transform into a glassy-like "foam" state at temperatures below the glass-transition temperature ( Tg) of these materials. Distinct from an isolated graphene sheet, which exhibits a relatively flat shape with fluctuations, we find that graphene sheets in a melt state structurally adopt more "crumpled" configurations and correspondingly smaller sizes, as normally found for ordinary polymers in the melt. Upon approaching the glass transition, these two-dimensional polymeric materials exhibit a dramatic slowing down of their dynamics that is likewise similar to ordinary linear polymer glass-forming liquids. Bulk graphene materials in their glassy foam state have an exceptionally large free-volume and high thermal stability due to their high Tg (≈ 1600 K) as compared to conventional polymer materials. Our findings show that graphene melts have interesting lubricating and "plastic" flow properties at elevated temperatures, and suggest that graphene foams are highly promising as high surface filtration materials and fire suppression additives for improving the thermal conductivities and mechanical reinforcement of polymer materials.

A detailed study on melting dynamics influenced by the pulse laser-induced transient heating

This paper presents a detailed study on the melting dynamics induced by the pulse laser heat transfer process. A numerical model has been developed in an open-source based computational fluid dynamics (CFD) platform, known as OpenFOAM. Upon development, its predictions have been compared with the experimental results of nanosecond pulse laser annealing of silicon with a pulse duration of 30 ns and wavelength (λ) of 308 nm, in which an excellent agreement with the experimental results for both of the melt depth and the melting duration has been observed. In the second part of this study, the numerical model has been implemented to explain the pulse laser-induced melting dynamics of silicon wafers irradiated with KrF excimer laser, which is similar to the cases used for processing of industry compatible photovoltaic solar cells. Since the results explain how thermal process, induced by the transient energy, controls the propagation of melting dynamics, this contribution can be used as a guide for designing new laser systems as well as optimisation of a chosen application.

Effect of molecular weight, branching and temperature on dynamics of polypropylene melts at very high shear rates

Dynamics of linear polypropylene (L-PP) and long-chain branched polypropylene (LCB-PP) miscible blends, having weight average molecular weight between 64 and 78 kg/mol, was investigated via high shear rate rheology. Results obtained were compared with the corresponding data for L-PP. High-shear rate secondary Newtonian plateaus, η∞, were identified at three different temperatures for well entangled L-PP/LCB-PP blends above shear rates of 2·106 1/s and their dependence on weight average molecular weight, Mw, was successfully related as η∞(T)=K∞(T)·Mwn with the exponent n = 1.010. Interestingly, the temperature dependant proportionality constant K∞ was found to be about 10–20% lower for the blend in comparison with the pure L-PP whereas the parameter n was found to be the same for both systems. The average values of high-shear rate flow activation energy, E∞, for the blends was found to be slightly lower than for pure L-PP and comparable with low-shear rate flow activation energy of PP like oligomer squalane (C30H62; 2,6,10,15,19,23-hexamethyltetracosane). This suggests that polymer chains are fully disentangled at very high shear rates and chain branching can enhance the flow in this regime due to smaller coil size and higher availability of the free volume (i.e. lower monomeric friction coefficient) in comparison with their linear counterparts.

Topological Phases of Chalcogenide Glasses Encoded in the Melt Dynamics

Select Raman scattering and modulated differential scanning calorimetry (DSC) experiments on specially homogenized binary GexSe100–x and GexS100–x chalcogenide glasses, supported by ab initio MD simulations are undertaken. In both chalcogenides, the glasses exhibit a super‐strong behavior (with a melt fragility index, m(x) &lt; 20) in the 20(1)% &lt; x &lt; 26(1)% composition range, defining the fragility window, and with m(x) &gt; 20 as one goes away from the fragility window. Remarkably, molecular dynamics (MD) simulations confirm the existence of the fragility window. Furthermore, in both chalcogenide glasses, the enthalpy of relaxation at Tg, ΔHnr(x), becomes minuscule (≈0) in the 20(1)% &lt; x &lt; 26(1)% range, defining the reversibility window, and with ΔHnr(x) increasing by an order magnitude abruptly with x outside that window. Thus, super‐strong melts formed in the fragility window give rise to isostatically rigid glasses in the reversibility window and fragile melts formed at x &lt; 20(1)% and at x &gt; 26(1)% give rise to flexible‐ and stressed‐rigid glasses, respectively. Melt dynamics for the examined glass systems unequivocally encode the nature of the glass topological phases.

Wetting, infiltration and interaction behavior of CMAS towards columnar YSZ coatings deposited by plasma spray physical vapor

Thermal barrier coatings (TBCs) produced by electron beam physical vapor deposition (EB-PVD) or plasma spray (PS) usually suffer from molten calcium-magnesium-alumino-silicate (CMAS) attack. In this study, columnar structured YSZ coatings were fabricated by plasma spray physical vapor deposition (PS-PVD). The coatings were CMAS-infiltrated at 1250 °C for short terms (1, 5, 30 min). The wetting and spreading dynamics of CMAS melt on the coating surface was in-situ investigated using a heating microscope. The results indicate that the spreading evolution of CMAS melt can be described in terms of two stages with varied time intervals and spreading velocities. Besides, the PS-PVD columnar coating (∼100 μm thick) was fully penetrated by CMAS melt within 1 min. After the CMAS attack for 30 min, the original feathered-YSZ grains (tetragonal phase) in both PS-PVD and EB-PVD coatings were replaced by globular shaped monoclinic ZrO2 grains in the interaction regions.

Development and validation of a stabilized immersed boundary CFD model for freezing and melting with natural convection

Numerous processes in the automotive, additive manufacturing or energy storage industries require an accurate prediction of the solidification (freezing) and melting (thawing) dynamics of substances. The numerical modeling of these phase changes is highly complex because it includes sharp moving interfaces and strong discontinuities in the material properties. This complexity is often exacerbated by the occurrence of natural convection, which induces a strong coupling between the motion of the liquid and the position of the solid–liquid interface. This leads to strongly coupled non-linear thermo-fluid problems which have to be solved in complex geometries.In this work, we introduce two novel stabilized finite element models to predict the phase change with natural convection. The first model uses a more classical viscosity approach to impose stasis in the solid region whereas the second one is based on an immersed boundary formulation to accurately describe the solid–fluid interface.The efficiency of the stabilization is first demonstrated by studying the Stefan problem. The two approaches to impose stasis are then compared using 2D test cases before they are both used to study melting in a rectangular (2D) and prismatic (3D) cavity. Significant differences are observed in the flow profiles and the solid–liquid interface position between the 2D and the 3D simulations.