arXiv subjects

Alejandro D. Rey

Publications and source records attributed to Alejandro D. Rey.

At least 19 recordsLinked to original sources

arXiv2026

Atomistic Modeling of Methane and Carbon Dioxide Structure I Gas Hydrates Under Pressure: Guest Effects and Properties

Gas hydrates are potential candidates in future energy sources while simultaneously providing structures with extensive applications in carbon capture and storage, gas transport, and important separation processes. Prior research in the field considers the dynamics of the water molecule backbone in particular. We investigated the pressure-enthalpy landscape and mechanical stability envelope of sI methane and carbon dioxide hydrates simulated using DFT. We investigated the effect of the revPBE + DFT-D2 and the SCAN + rVV10 and their treatment of the exchange correlation interactions. We examined the zero pressure material properties, finding that revPBE comparatively underbinds the interactions, causing more flexible structures with large equilibrium volumes. Under pressure, the carbon dioxide molecule was found to align itself parallel to the hexagonal faces of the large cage despite the functional used. Additionally, the property differences are caused by the ability of the carbon dioxide molecule to rotate and disperse the changes in the energy landscape in ways that methane molecules cannot. This computational methodology describes the elastic stability of gas hydrate, marginal stability, and critical differences across important molecular interactions, confirming experimentally observed restrictions in guest molecule rotations and novel pressure behaviors under hydrostatic loads

physics.chem-ph↗

arXiv2025

Periodic feature characterization in nanostructured surfaces and emulsions

Understanding structure-function relationships is essential to advance the manufacturing of next-gen materials with desired properties and functionalities. Precise and rapid measurement of features like wrinkle size, droplet diameter, and surface roughness is essential to establishing such structure-function relationships. To this end, this work developed feature size and surface morphology characterizations through image analysis in Python and validated them with both synthetic and experimental images. Manual measurements of bio-based surfaces resulted in between 3.3% (N=50, visually simple) and 51.2% error (N=100, visually complex) compared to Python analysis results. This analysis was also used to accurately distinguish multiple feature size populations in a given image (which were missed entirely in manual measurements), and to determine the skewness and kurtosis of biological surfaces in a surface roughness map. This work contributes to a larger goal of developing a robust and computationally cheap platform to analyze complex materials to accelerate structure-function discovery.

physics.app-ph↗

arXiv2024

Simultaneous Crystallization Effects in Multiple Levitated Plasma-Functionalized Graphene Nanoflake Nanofluid Droplets

Acoustic levitation is a container-free method for examining novel crystallization effects, though liquid-to-solid phase change has seen little investigation for levitated nanofluids. Recent developments have allowed for examining the morphological and temperature evolution of multiple levitated nanofluid droplets freezing simultaneously. The fundamental effect of adding nanoparticles to a levitated crystallization system is crystal growth rate enhancement from improved mass transfer at the growing solid front. Nucleation times are unaffected as freezing is initiated by secondary ice nucleation particles (INPs). Instead, the enhancement produces higher instantaneous nucleation pressures and more cracking in the primary ice shell. In turn, more INPs are ejected, resulting in faster protrusion formation on the droplet surface (hastened further in systems containing adjacent droplets). The crystal matrix also includes more defects, resulting in liquid escaping and forming beads at the droplet base and optical clarity loss. During crystal decomposition, thermal gradients create convective currents dampened by the same transport phenomena that enhance crystal growth. Suspension loss after a crystallization-decomposition cycle reduced opacity and light absorbance such that the droplets were 62% closer in appearance to water. However, the non-isobaric, sample-encompassing cooling process resulted in smaller particle clusters than if the droplets were frozen on a solid surface.

physics.app-ph↗

arXiv2023

Dynamics of Acoustically Levitated Ice Impacts on Smooth and Textured Surfaces: Effects of Surface Roughness, Elasticity, and Structure

Through acoustically levitated ice formation and subsequent release onto a controlled area, this study introduces a third class of ice-countering system beyond de- and anti-icing: ice-impacting. By subjecting stainless steel 316 (SS), epoxy resin-coated (ER), and laser-textured (LT) surfaces with known surface roughness, hardness, and structural characteristics to 40 ice droplet impacts each, the effect on surface properties and their effect on solid-solid interfacial impact dynamics, in turn, was examined using a novel analysis framework based on fundamental conservation laws. For the velocities experienced in this study, the impacts did not affect the surface properties; they were consistent after each impact. Elasticity was the most significant factor in droplet behavior: the ER surface exhibited rebounding for 78% of impacts (important for moving surfaces). Surface roughness also played a role, particularly for droplets with rotational motion, as immobilization occurred for 66% of impacts on the rougher LT surface. However, the nanostructures on that textured surface resulted in droplet redirection perpendicular to the surface directionality (critical for stationary surfaces). In contrast, the other surfaces saw no change or no consistent change in rebound angle. Elasticity also affected momentum retention, where the ER surface had a translational restitution coefficient of 0.32 compared to 0.17 for the two stainless steel surfaces. Surface roughness was the predominant aspect of energy retention: the LT surface had a translational-to-rotational energy transfer coefficient of 0.07 (0.23 for the smoother surfaces), resulting in an overall energy retention coefficient of 0.09 compared to 0.28 for the SS and ER surfaces on average.

physics.app-ph↗

arXiv2023

Non-Einsteinian Viscosity Reduction in Boron Nitride Nanotube Nanofluids

(1) Introduction: Nanoparticles have multiple applications, including drug delivery systems, biosensing, and carbon capture. Non-Einstein-like viscosity reduction has been reported in nanoparticle-polymer blends at low nanoparticle concentrations. More recently, a similar non-Einsteinian viscosity reduction effect has been observed in aqueous ultra-low concentration carbon-based nanofluids. (2) Methods: We use a boron nitride nanotube functionalized with hydrophilic groups in rheological experiments to investigate the viscosity reduction in ultra-low concentration nanofluids (0.1-10 ppm). We measure the dynamic viscosity in an air atmosphere and methane (0-5 MPag) at low temperatures (0-10 C). (3) Results: A negligible effect on the temperature dependence of viscosity was found. Ultra-low concentrations of BNNT reduced the viscosity of the nanofluid by up to 29% at 10 ppm in the presence of methane. The results presented here were compared to similar studies on O-GNF and O-MWCNT nanofluids, which also reported significant viscosity reductions. (4) Conclusions: This work identified a non-Einsteinian viscosity reduction in BNNT nanofluids, which was exacerbated by methane dissolved in the nanofluid.

cond-mat.mtrl-sci↗

arXiv2023

Dynamic Viscosity of Methane Hydrate Systems from Non-Einsteinian, Plasma-Functionalized Carbon Nanotube Nanofluids

The viscosity of oxygen-functionalized multi-walled carbon nanotube (O-MWCNT) nanofluids was measured for concentrations from 0.1 to 10 ppm under conditions of 0 to 30 MPag pressures and 0 to 10 C temperatures. The presence of O-MWCNTs did not affect the temperature dependence of viscosity but did reduce the effective viscosity of solution due to cumulative hydrogen bond-disrupting surface effects, which overcame internal drag forces. O-MWCNTs added a weak pressure dependence to the viscosity of solution because of their ability to align more with the flow direction as pressure increased. In the liquid to hydrate phase transition, the times to reach the maximum viscosity were faster in O-MWCNT systems compared to the pure water baseline. However, the presence of O-MWCNTs limited the conditions at which hydrates formed as increased nanoparticle collisions in those systems inhibited the formation of critical clusters of hydrate nuclei. The times to viscosity values most relevant to technological applications were minimally 28.02 % (200 mPa s) and 21.08 % (500 mPa s) slower than the baseline, both in the 1 ppm system, even though all systems were faster to the final viscosity. This was attributed to O-MWCNT entanglement, which resulted in a hydrate slurry occurring at lower viscosity values.

physics.app-ph↗

arXiv2023

Molecular dynamics predictions of transport properties for carbon dioxide hydrates under pre-nucleation conditions using TIP4P/Ice water and EPM2, TraPPE, and Zhang carbon dioxide potentials

(1) Introduction: New technologies that leverage gas hydrates phenomena include carbon capture and sequestrations. These processes are often semi-continuous and require regulation of the system's flow properties for proper operation. Accurate computational models for the viscosity of carbon dioxide hydrate systems at pre-nucleation conditions can be important for process design and control of such technologies. (2) Methods: This work validates the viscosity predictions of molecular dynamics simulations using previously measured experimental data. The TIP4P/Ice force field was used to model water, while the EPM2, TraPPE, and Zhang force fields were used for carbon dioxide. The Green-Kubo and Einstein formulations of viscosity and diffusivity were used in this work. (3) Results: All force fields overpredicted viscosity when compared to experimental data, but EPM2 resulted in lower discrepancies. Additionally, EPM2 was determined to model molecular behavior expected from the macroscopic trends in viscosity with respect to temperature and pressure. (4) Conclusions: The EPM2 force field more accurately predicted the viscosity of carbon dioxide hydrates systems at pre-nucleation conditions relative to TraPPE and Zhang.

physics.app-ph↗

arXiv2023

TinyLev Acoustically Levitated Water: Direct Observation of Collective, Inter-Droplet Effects through Morphological and Thermal Analysis of Multiple Droplets

Initially, the acoustic field forced the droplets into an oblate spheroid shape, though the counteracting force of the cooling stream caused them to circularize. Droplet geometry was thus the net result of streaming forces and surface tension at the acoustic boundary layer/air-liquid interface. Nucleation was determined to be neither homogeneous nor heterogeneous but secondary, and thus dependent on the cooling rate and not on the degree of supercooling. It was likely initiated by aerosolized ice particles from the air or from droplets that had already nucleated and broken up. The latter secondary ice production process resulted in multi-drop systems with statistically identical nucleation times. Notably, this meant that the presence of interfacial rupture at an adjacent droplet could influence the crystallization behaviour of another. After the formation of an initial ice shell around the individual droplets, dendritic protrusions grew from the droplet surface, likely seeded by the same ice particles that caused nucleation, but at a quasi-liquid layer. When freezing was complete, it was determined that the frozen core had undergone a volumetric expansion of 30.75%, compared to 9% for pure, sessile water expansion. This significantly greater expansion may have resulted from entrained air bubbles at the inner solid-liquid interface and oscillations at the moving phase boundary caused by changes in local acoustic forces. Soon after melting began, acoustic streaming, the buoyancy of the remaining ice, and convective currents caused by an inner thermal gradient and thermocapillary effects along the air-liquid interface, all contributed to the droplet spinning about the horizontal axis.

physics.app-ph↗

arXiv2023

Effects of Poly(vinylpyrrolidone) on the Dynamic Viscosity of Methane Hydrate Systems at High-Pressure Driving Forces: Investigation of Concentration, Molecular Weight, and Shear Rate

The viscosity of methane hydrate slurries with poly(vinylpyrrolidone) (PVP) at 700 and 7000 ppm by weight, molecular weights of 40,000 (PVP40) and 360,000 (PVP360) Da, and shear rates of 400 and 80 1/s, were measured in a high-pressure rheometer with pressures up to 30 MPag and compared to pure water systems. The additives successfully reduced the formation of high-viscosity slurries, but at low concentrations were incapable of delaying hydrate agglomeration at the late growth stage. The average relative time required for PVP40 solutions at 700 ppm to grow to 50 mPa.s was 1.9 times the water reference value, but only 1.2 times to reach 200 mPa.s. Improved inhibition was observed for the higher concentration and higher molecular weight sets, where the relative time to reach 50 mPa.s were 8.2 and 2.6 times the water reference value, respectively. While the additives demonstrated anti-nucleation properties and suppressed crystal growth initially, they accelerated the hydrate clusters agglomeration rate, and potentially weakened the hydrate mechanical properties.

physics.app-ph↗

arXiv2023

Interfacial Effects during Phase Change in Multiple Levitated Tetrahydrofuran Hydrate Droplets

In this study, using direct digital and infrared imaging techniques, the freezing of up to three simultaneous THF hydrate droplets was investigated for the first time. Nucleation was initiated at the aqueous solution-air interface. Two pseudo-heterogeneous mechanisms created additional nucleation interfaces: one from cavitation effects entraining microbubbles and another from subvisible ice particles, also called hydrate nucleating particles (HNPs), impacting the droplet surface. For systems containing droplets in both the second and third positions, nucleation was statistically simultaneous between all droplets. This effect may have been caused by the high liquid-solid interfacial pressures that developed at nucleation, causing some cracking in the initial hydrate shell around the droplet and releasing additional HNPs (now of hydrate) into the air. During crystallization, the THF hydrate droplets developed a completely white opacity, termed optical clarity loss or OCL. It was suggested that high hydrate growth rates within the droplet resulted in the capture of tiny air bubbles within the solid phase. In turn, light refraction through many smaller bubbles resulted in the OCL. These bubbles created structural inhomogeneities, which may explain how the volumetric expansion of the droplets upon complete solidification was 23.6% compared with 7.4% in pure, stationary THF hydrate systems. Finally, the thermal gradient that developed between the top and bottom of the droplet during melting resulted in a surface tension gradient along the air-liquid interface. In turn, convective cells developed within the droplet, causing it to spin rapidly about the horizontal axis.

physics.app-ph↗

arXiv2023

Non-Einsteinian Viscosity Behavior in Plasma-Functionalized Graphene Nanoflake Nanofluids and their Effect on the Dynamic Viscosity of Methane Hydrate Systems

Water's viscosity dependence on pressure was also not affected by O-GNFs, except at 10 ppm, where the shuttle effect may have increased the presence of hydrophobic methane bubbles in the solution. Under high pressure, the relative viscosity of the system remained non-Einsteinian at all temperatures except 2C. This may have been because the density anomaly of water was shifted to a colder temperature as the hydrogen bonding network was weaker. The phase transition from liquid to hydrate was identical to that of pure water, indicating that the presence of different stages of growth was not affected by the presence of O-GNF. However, the times to reach a maximum viscosity were faster in O-GNF systems compared to pure water. This said, the hydrate formation limitations inherent to the measurement system were not overcome by the presence of O-GNFs. The times to application-relevant viscosity values were maximized in the 1 ppm system at 49.75 % (200 mPa.s) and 31.93 % (500 mPa.s) faster than the baseline. Therefore, the presence of O-GNFs allowed for shorter times to desired viscosities and at lower driving forces than the baseline, improving the viability of the hydrate technologies to which they can be added.

physics.app-ph↗

arXiv2023

Dynamic Viscosity of Methane and Carbon Dioxide Hydrate Systems from Pure Water at High-Pressure Driving Forces

The viscosity of methane and carbon dioxide hydrate systems were measured using a high-pressure rheometer up to 30 MPag. Where hydrate formation was not detected, the effect of temperature on the viscosity was one order of magnitude higher than the pressure effect on viscosity in most of the experimental pressure range (-0.048 mPa s/C at 1 MPag and 0.009 mPa s/MPag at 2C). The pressure effect on the viscosity of carbon dioxide systems where no hydrate formation was observed was up to one order of magnitude higher than that of the methane systems, due to carbon dioxide's higher solubility in water. Novel rheological phases diagrams were developed to further characterize the gas hydrate systems. Several systems with high driving forces for hydrate formation (2.07 MPag to 4.1 MPag) did not form gas hydrates. System limitations to the formation of hydrates were categorized as kinetic, mass diffusion, and/or heat of crystallization effects.

physics.app-ph↗

arXiv2013

Textures in Polygonal Arrangements of Square Nanoparticles in Nematic Liquid Crystals Matrices

A systematic analysis of defect textures in facetted nanoparticles with polygonal configurations embedded in a nematic matrix is performed using the Landau-de Gennes model, homeotropic strong anchoring in a square domain with uniform alignment in the outer boundaries. Defect and textures are analyzed as functions of temperature T, polygon size R, and polygon number N. For nematic nanocomposites, the texture satisfies a defect charge balance equation between bulk and surface (particle corner) charges. Upon decreasing the temperature, the central bulk defects split and together with other -1/2 bulk defects, are absorbed by the nanoparticle's corners. Increasing the lattice size decreases confinement and eliminates bulk defects. Increasing the polygon number increases the central defect charge at high temperature and the number of surface defects at lower temperatures. The excess energy per particle is lower in even than in odd polygons, and it is minimized for a square particle arrangement. These discrete modeling results show for first time that even under strong anchoring, defects are attached to particles as corner defects, leaving behind a low energy homogeneous orientation field that favors nanoparticle ordering in nematic matrices. These new insights are consistent with recent thermodynamic approaches to nematic nanocomposites that predict the existence of novel nematic/crystal phases and can be used to design nanocomposites with orientational and positional order.

cond-mat.mtrl-sci↗

arXiv2013

A good and computationally efficient polynomial approximation to the Maier-Saupe nematic free energy

A new computational strategy is proposed to approximate, with a simple but accurate expression, the Maier- Saupe free energy for nematic order. Instead of the traditional approach of expanding the free energy with a truncated Taylor series, we employ a least-squares fitting to obtain the coefficients of a polynomial expression. Both methods are compared, and the fitting with at most five polynomial terms is shown to provide a satisfactory fitting, and to give much more accurate results than the traditional Taylor expansion. We perform the analysis in terms of the tensor order parameter, so the results are valid in uniaxial and biaxial states.

cond-mat.soft↗

arXiv2013

Modeling Complex Liquid Crystals Mixtures: From Polymer Dispersed Mesophase to Nematic Nanocolloids

Liquid crystals are synthetic and biological viscoelastic anisotropic soft matter materials that combine liquid fluidity with crystal anisotropy and find use in optical devices, sensor/actuators, lubrication, super-fibers. Frequently mesogens are mixed with colloidal and nanoparticles, other mesogens, isotropic solvents, thermoplastic polymers, cross-linkable monomers, among others. This comprehensive review present recent progress on meso and macro scale thermodynamic modelling, highlighting the (i) novelties in spinodal and binodal lines in the various phase diagrams, (ii) the growth laws under phase transitions and phase separation, (iii) the ubiquitous role of metastability and its manifestation in complex droplet interfaces, (iv) the various spinodal decompositions due to composition and order fluctuations, (v) the formation of novel material architectures such as colloidal crystals, (vi) the particle rich phase behaviour in liquid crystal nanocomposites, (vii) the use of topological defects to absorb and organize nanoparticles, and (viii) the ability of faceted nanoparticles to link into strings and organize into lattices. Emphasis is given to highlight dominant mechanisms and driving forces, and to link them to specific terms in the free energies of these complex mixtures. The novelties of incorporating mesophases into blends, solutions, dispersions and mixtures is revealed by using theory, modelling, computation, and visualization.

cond-mat.soft↗

arXiv2013

Thermodynamic Modelling of Phase Equilibrium in Nanoparticles-Nematic Liquid Crystals Composites

In this work, a theoretical study of phase equilibrium in mixtures of a calamitic nematic liquid crystal and hard spherical nanoparticles is presented. A mean-field thermodynamic model is used, where the interactions are considered to be proportional to the number of contacts, which in turn are proportional to the areas and area fractions of each component. It is shown that, as the radius of the particle is increased, the effect of the particles on the isotropic-nematic transition is less pronounced, and that for large radius the miscibility increases as the particle radius increases.

cond-mat.mtrl-sci↗

arXiv2013

Multiple interfaces in diffusional phase transitions in binary mesogen-non-mesogen mixtures undergoing metastable phase separations

Theory and simulations of simultaneous chemical demixing and phase ordering are performed for a mixed order parameter system with an isotropic-isotropic (I-I) phase separation that is metastable with respect to an isotropic-nematic (I-N) phase ordering transition. Under certain conditions, the disordered phase transforms into an ordered phase via the motion of a double front containing a metastable phase produced by I-I demixing, a thermodynamically driven mechanism not previously reported. Different kinetic regimes are found depending on the location of the initial conditions in the thermodynamic phase diagram and the ratio between diffusional and nematic phase ordering mobilities. For a diffusional process, depending if the temperature is above or below the critical co-dissolution point, an inflection point or a phase separation takes place in the depletion layer. This phase separation leads to the formation of a second interface where the separation of the two metastable isotropic phases grows monotonically with time. The observed deviations from the typical Fickian concentration profiles are associated with strong positive deviations of the mixture from ideality due to couplings between concentration and nematic ordering. Although systems of interest include liquid crystalline nanocomposites, this novel mechanism may apply to any mixture that can undergo an order-disorder transition and demix.

cond-mat.soft↗

arXiv2013

Dynamics of transient metastable states in mixtures under coupled phase ordering and chemical demixing

We present theory and simulation of simultaneous chemical demixing and phase ordering in a polymer-liquid crystal mixture in conditions where isotropic-isotropic phase separation is metastable with respect to isotropic-nematic phase transition. In the case the mechanism is nucleation and growth, it is found that mesophase growth proceeds by a transient metastable phase that surround the ordered phase, and whose lifetime is a function of the ratio of diffusional to orientational mobilities. In the case of spinodal decomposition, different dynamic regimes are observed depending on the mobility ratio: metastable phase separation preceding phase ordering, phase ordering preceding phase separation, or simultaneous phase ordering and phase separation. Not only the overall dynamics but also the final structure of the material can be different for each kinetic regime.

cond-mat.soft↗