Materials science. Melting colloidal crystals from the inside out (original) (raw)

2012

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Abstract

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The research explores internal melting mechanics in colloidal crystals, contrasting it with familiar external melting processes. By utilizing laser techniques, the study finds that large-scale particle agitation is primarily responsible for this internal melting, debunking traditional theories that emphasize crystalline defect formation. This work informs future colloidal experiments and deepens our understanding of phase transitions.

In-situ observations of solutal melting using laser scanning confocal microscopy: The Cu/Ni model system

Solutal melting was investigated in-situ by mean of high temperature laser scanning confocal microscopy. This technique enabled us to track the motion of the solid-liquid interface in order to determine the evolution of the interfacial velocity. The Cu-Ni binary system was chosen as a model case and concentric samples were fabricated from both pure metals. Two holding temperatures above the melting point of Cu were investigated, i.e., 1115 and 1145. As the average composition of the mounted samples was chosen to lie within the solid solution region, the reaction occurred via the following steps: i) thermal melting of Cu, ii) solutal melting of Ni, iii) resolidification.

Melting and Premelting : Background and Evidence

2006

When the free surfaces of most solids approach their bulk melting temperatures from below, the molecular structure of the material gives way to a disordered structure with some attributes of both the solid and liquid phases. When the temperature is sufficiently close to that of bulk transition, the surface melts and literally flows as a viscous fluid. This phenomenon, called interfacial premelting, lies at the heart of the microscopic theory of melting of solid matter, and captures the interest of condensed matter physicists and physical chemists alike. The process is ubiquitous and responsible for a wide range of consequences in materials with biological, geophysical, and technological significance. Because such systems are often exposed to spatial or temporal variations in thermodynamic forcing, there are a host of fluid mechanical phenomena that result from this underlying melting behavior. The fluid dynamics of unfrozen surfaces holds clues for understanding the bulk behavior of...

The dynamics of melt and shear localization in partially molten aggregates

Nature, 2006

The volcanoes that lie along the Earth's tectonic boundaries are fed by melt generated in the mantle. How this melt is extracted and focused to the volcanoes, however, remains an unresolved question. Here we present new theoretical results with implications for melt focusing beneath mid-ocean ridges. By modelling laboratory experiments 1,2 , we test a formulation for magma dynamics and provide an explanation for localized bands of high-porosity and concentrated shear deformation observed in experiments. These bands emerge and persist at 158-258 to the plane of shear. Past theoretical work on this system predicted the emergence of melt bands 3,4 but at an angle inconsistent with experiments. Our results suggest that the observed band angle results from a balance of porosity-weakening and strain-rate-weakening deformation mechanisms. Lower band angles are predicted for greater strainrate weakening. From these lower band angles, we estimate the orientation of melt bands beneath mid-ocean ridges and show that they may enhance magma focusing toward the ridge axis.

Melting Driven by Particle Size Dispersity: A Study in Two Dimensions

We study the e ect of particle size-dispersity on solids and melting of solids by molecular dynamics simulation on a two-dimensional size-dispersed Lennard-Jones system. We ÿnd that size-dispersity disfavours solidiÿcation and on increasing the dispersity, the solid 'melts', at a critical dispersity, to a liquid. The solid-liquid transition depends on the density -the transition is continuous through the 'hexatic' phase at low densities and abrupt ÿrst order transition at higher densities. We ÿnd that size-dispersity creates topological defects in the close-pack solid structure which destroy the crystalline order in solids. .in (P. Ray) 0378-4371/99/$ -see front matter c 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 -4 3 7 1 ( 9 9 ) 0 0 1 5 1 -X

Experimental solidification of an andesitic melt by cooling

Chemical Geology, 2011

Solidification experiments at (a) five different cooling rates (25, 12.5, 3, 0.5 and 0.125°C/min) between 1300 and 800°C, and (b) variable quenching temperatures (1100, 1000, 900 and 800°C) at a fixed cooling rate of 0.5°C/min were performed on an andesitic melt (SiO 2 = 58.52 wt.% and Na 2 O + K 2 O = 4.43 wt.%) at air conditions from high superheating temperature. The results show that simultaneous and duplicated experiments with Pt-wire or Pt-capsule produce identical run-products. Preferential nucleation on Ptcontainers or bubbles is lacking. Plagioclase and Fe-Ti oxide crystals nucleate firstly from the melt. Clinopyroxene crystals form only at lower cooling rates (0.5 and 0.125°C/min) and quenching temperatures (900 and 800°C). At higher cooling rates (25, 12.5 and 3°C/min) and quenching temperature (1100°C), plagioclase and Fe-Ti oxide crystals are embedded in a glassy matrix; by contrast, at lower cooling rates (0.5 and 0.125°C/min) and below 1100°C they form an intergrowth texture. The crystallization of plagioclase and Fe-Ti oxide starts homogeneously and then proceeds by heterogeneous nucleation. The crystal size distribution (CSD) analysis of plagioclase shows that crystal coarsening increases with decreasing cooling rate and quenching temperature. At the same time, the average growth rate of plagioclases decreases from 2.1 × 10 − 6 cm/s (25°C/min) to 5.7 × 10 − 8 cm/s (0.125°C/min) and crystals tend to be more equant in habit. Plagioclases and Fe-Ti oxides depart from their equilibrium compositions with increasing cooling rate; plagioclases shift from labradorite-andesine to anorthite-bytownite. Therefore, kinetic effects due to cooling significantly change the plagioclase composition with remarkable petrological implications for the solidification of andesitic lavas and dikes. The glass-forming ability (GFA) of the andesitic melt has been also quantified in a critical cooling rate (R c ) of~37°C/min. This value is higher than those measured for latitic (R c~1°C /min) and trachytic (R c b 0.125°C/min) liquids demonstrating that little changes of melt composition are able to significantly shift the initial nucleation behavior of magmas and the following solidification paths.

Homogeneous nucleation of colloidal melts under the influence of shearing fields

Journal of Physics: Condensed Matter, 2004

We study the effect of shear flow on homogeneous crystal nucleation, using Brownian Dynamics simulations in combination with an umbrella sampling like technique. The symmetry breaking due to shear results in anisotropic radial distribution functions. The homogeneous shear rate suppresses crystal nucleation and leads to an increase of the size of the critical nucleus. These observations can be described by a simple, phenomenological extension of classical nucleation theory. In addition, we find that nuclei have a preferential orientation with respect to the direction of shear. On average the longest dimension of a nucleus is along the vorticity direction, while the shortest dimension is preferably perpendicular to that and slightly tilted with respect to the gradient direction.

An experimental and numerical study of surface tension-driven melt flow

2008

To determine the role of surface tension-driven melt migration in planetary bodies, we investigated the effect of static annealing on the evolution of melt-rich bands in partially molten samples. In shear deformation experiments, deviatoric stress causes melt to segregate; when the stress is removed, surface tension causes the melt to relax back to a homogeneous distribution. Samples composed of 76 vol.% olivine + 20 vol.% chromite + 4 vol.% MORB were deformed to shear strains of~3.5 at 1523 K, 300 MPa and shear stresses of 20 to 55 MPa. After deformation, the samples were statically annealed for 0, 10, or 100 h. During annealing, melt transport driven by surface tension occurs, but takes place much more slowly than flow driven by deviatoric stress. Finite difference numerical simulations were performed of surface tension-driven melt flow resisted by viscous deformation of the olivine matrix. These models best reproduce the distribution of melt in the annealed samples when the solid viscosity η s = 1.7 ± 0.5 × 10 12 Pa s with n = 2.4 ± 0.3 and b = 9000 ± 1900 in the expression for permeability κ = ϕ n d 2 / b where d is grain size. The large value of b compared with estimates from geometrical models is probably due to clogging of the melt tubes by the secondary solid phase (chromite). Redistribution of melt by surface tension is likely to be the dominant process in small (~10 km radius) planetesimals in the absence of convection or impact-induced deformation. However, this redistribution process is sufficiently slow that large bodies of localized melt (magma chambers) are likely to develop.

Heterogeneous nucleation of colloidal melts under the influence of shearing fields

Journal of Physics: Condensed Matter, 2004

The flash melting of chondrules: an experimental investigation into the melting history and physical nature of chondrule precursors

Geochimica et Cosmochimica Acta, 1998

Constraints placed on chondrule formation have largely been generated from experiments which use a long duration, below liquidus isothermal melting (minutes to hours) rather than a short duration, above liquidus flash melting event (seconds to minutes). In this paper we examine how a short duration, superliquidus heat pulse can produce chondrule textures. By incompletely melting material with a type of flash melting we show that the maximum temperature limit of chondrule formation was approximately 2100°C, almost 400°C higher than previously constrained. Previous experiments also have not studied the effect of variations in precursor grain size on the formation of chondrule textures. For this reason we simultaneously investigate the effect of variations in the grain size of a starting composition on the formation of chondrule textures. We show how MgO-rich (Type IA) chondrules and other fine-grained chondrules could only have been formed from the incomplete melting of a rather uniformly grain sized precursor of less than 63m. Because fine-grained, MgO-rich chondrules have the some of the highest chondrule liquidus temperatures, we proposed that these types of textures define a minimum melting temperature for chondrule formation.

An experimental study of grain scale melt segregation mechanisms in two common crustal rock types

Journal of Metamorphic Geology, 2002

Creation of pathways for melt to migrate from its source is the necessary first step for transport of magma to the upper crust. To test the role of different dehydration-melting reactions in the development of permeability during partial melting and deformation in the crust, we experimentally deformed two common crustal rock types. A muscovite-biotite metapelite and a biotite gneiss were deformed at conditions below, at and above their fluid-absent solidus. For the metapelite, temperatures ranged between 650 and 800 uC at P c =700 MPa to investigate the muscovite-dehydration melting reaction. For the biotite gneiss, temperatures ranged between 850 and 950 uC at P c =1000 MPa to explore biotite dehydration-melting under lower crustal conditions. Deformation for both sets of experiments was performed at the same strain rate (e . ) 1.37310 x5 s x1 . In the presence of deformation, the positive DV and associated high dilational strain of the muscovite dehydration-melting reaction produces an increase in melt pore pressure with partial melting of the metapelite. In contrast, the biotite dehydration-melting reaction is not associated with a large dilational strain and during deformation and partial melting of the biotite gneiss melt pore pressure builds more gradually. Due to the different rates in pore pressure increase, melt-enhanced deformation microstructures reflect the different dehydration melting reactions themselves. Permeability development in the two rocks differs because grain boundaries control melt distribution to a greater extent in the gneiss. Muscovite-dehydration melting may develop melt pathways at low melt fractions due to a larger volume of melt, in comparison with biotite-dehydration melting, generated at the solidus. This may be a viable physical mechanism in which rapid melt segregation from a metapelitic source rock can occur. Alternatively, the results from the gneiss experiments suggest continual draining of biotite-derived magma from the lower crust with melt migration paths controlled by structural anisotropies in the protolith.

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Theory of Shear-Induced Melting of Colloidal Crystals

Physical Review Letters, 1986

We propose a theory of shear-induced melting of colloidal crystals based on a nonequilibrium generalization of first-order freezing theory. The results agree qualitatively with experiment.

Melting of a colloidal crystal

Physica A: Statistical Mechanics and its Applications

A melting transition for a system of hard spheres interacting by a repulsive Yukawa potential of DLVO form is studied. To find the location of the phase boundary, we propose a simple theory to calculate the free energies for the coexisting liquid and solid. The free energy for the liquid phase is approximated by a virial expansion. The free energy of the crystalline phase is calculated in the spirit of the Lenard-Jonnes and Devonshire (LJD) theory. The phase boundary is found by equating the pressures and chemical potentials of the coexisting phases. When the approximation leading to the equation of state for the liquid breakes down, the first order transition line is also obtained by applying the Lindemann criterion to the solid phase. Our results are then compared with the Monte Carlo simulations.

Contribution to the theory of melting

Physical Review B, 1990

An ensemble of particles with repulsive Yukawa-type interaction is solved at high dimension. The fluid exhibits a new static singularity at density (p/T)", which characterizes the supercooledfluid branch and the glass transition; at equilibrium the system crystalhzes at p(p". Thus, a unified picture of crystallization, supercooled fluid, glass formation, and melting is discovered. The theory remains exact for arbitrary potential as p~p"and agrees qualitatively with experiments.

Discussion on local equilibrium at solid/liquid interfaces during melting

Scripta Materialia - SCRIPTA MATER, 2002

Melting experiments were recently used to support the assumption of considerable deviation from local equilibrium. It is now claimed that the observed velocities of melting were low enough to allow diffusion in the solid to give a well developed spike of the solute in front of the migrating interface making a strong deviation from local equilibrium unlikely.

Experimental and theoretical constraints on melt distribution in crustal sources: the effect of crystalline anisotropy on melt interconnectivity

Chemical Geology, 1995

In partially molten systems, the equilibrium distribution of melt at the grain scale is governed by the principle of interfacial energy minimization. In ideal sources (i.e. partially molten rocks that are monomineralic, have single-valued solid-liquid and solid-solid interfacial energies, and are subject to hydrostatic stress) the wetting angle 0 is known to be a unique characteristic which specifies the melt configuration for a given melt fraction. Crustal rocks cannot be modelled as ideal sources because of their polymineralic nature, the moderate to high anisotropy of interfacial energies which characterizes common refractory minerals, and the possible presence of a crystallographic preferred orientation. That partially molten crustal rocks depart from ideal sources is documented by a series of highP,high-T experiments illustrating the textural relationships of biotite and amphibole with silicic melts. The melt distributions observed in these experiments differ significantly from those expected in ideal sources: (1) crystal-melt interfaces are commonly planar, rational faces rather than smoothly curved, irrational surfaces; and (2) the concept of a unique wetting angle does not hold as shown in the biotite-silicic melt system. These textural features demonstrate that anisot.ropy of crystal-melt interfacial energy is a factor of primary importance in modelling the grain-scale distribution of partial melts. The petrological implications of our study are the following: (1) At high degrees of anisotropy and low melt fractions, melt is predicted to form isolated, plane-faced pockets at grain comers. The overall shape of these pockets, and therefore the value of the connectivity threshold & are expected to be very sensitive to the ratio of solid-solid to solid-liquid interfacial energies, ySS/ yS, (& is the melt fraction at which melt interconnectivity is established). Melt pockets with low volume-to-surface ratio, and low (but non-constant) wetting angles should prevail at high ySyss/-yS,, resulting in very low values of 4c (< 1 to a few vol%). Higher values of &,, a high volume-to-surface ratio of melt pockets, and high wetting angles are expected at low ySS/ yS,. (2) The wetting angle at hornblende-hornblende-melt junctions, at 1200 MPa-975°C is 25". A review of existing data indicates that quartz-melt and feldspar-melt wetting angles are also low to moderate (12-60"). A very low value of 4c should, therefore, be the general rule during crustal anatexis. In particular, a connectivity threshold lower than 34 ~01% is predicted for partially molten amphibolite. (3) In biotite-rich rock-types, such as melanosomes in migmatites, the combination of a pronounced crystalline anisotropy and a marked preferred orientation of mica flakes leads to a very low permeability (normal to layering). Biotite-rich melanosomes should therefore impedt: chemical interactions between neighbouring leucosomes and mesosomes.

Melting by temperature-modulated calorimetry

Thermochimica Acta, 1998

WeII-crystallized macromolecules melt irreversibly due to the need of molecular nucleation. while small molecules melt reversibly as long as crystal nuclei are present to assist crystallization.

Reentrant melting of two-dimensional colloidal systems

Journal of Physics: Condensed Matter, 2000

When a two-dimensional colloidal suspension of highly charged particles is subjected to a periodic one-dimensional (1D) light field, a liquid-solid transition can be induced. This phase transition is well known as light-induced freezing. However, upon further increase of the intensity, the crystal is found to remelt (laser-induced melting) to a 1D liquid. This reentrance behaviour is in good agreement with predictions based on Monte Carlo simulations. We suggest explaining this intriguing phenomenon in terms of particle fluctuations which tend to stabilize the crystalline phase.

Materials science. Melting colloidal crystals from the inside out (original) (raw)

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