arXiv subjects

Fernando Bresme

Publications and source records attributed to Fernando Bresme.

At least 19 recordsLinked to original sources

arXiv2026

Thermodiffusion in Aqueous Alkali Halide Solutions from Ambient to Supercooled Conditions: Ion-Specific, Structural, and Mass Effects

Thermodiffusion in aqueous electrolyte solutions exhibits complex dependencies on temperature, concentration, and salt composition, yet its microscopic origins remain incompletely understood. Here, we employ non-equilibrium molecular dynamics (NEMD) simulations to investigate thermal transport and thermodiffusion in aqueous alkali halide solutions over the temperature range 240-300 K at concentrations of 1 m and 4 m. Building on previous studies of NaCl and LiCl, we extend the analysis to systems containing K$^+$ and I$^-$ ions to assess ion-specific effects. Across all systems studied, the thermal conductivity decreases upon cooling and is generally reduced at higher salt concentration. The Soret coefficient generally increases with temperature, shifting the solutions from thermophilic behavior at low temperature toward more thermophobic behavior at high temperature. Clear ion-dependent trends are observed, with Na$^+$ and K$^+$ salts generally showing stronger thermophobic responses than Li$^+$ salts, especially in iodide solutions. We estimate that the shift in the inversion temperatures of the iodide salts relative to experiment corresponds to a small local offset of the effective heat of transport, 4-5 kJ/mol, showing that small changes in hydration thermodynamics or heat-mass coupling can strongly affect the sign change of the Soret coefficient. Structural analyses indicate that lower temperatures and lower concentrations favor more tetrahedrally ordered, LDL-like water environments, which are associated with enhanced thermophilicity. Analysis of inversion temperatures and mass effects further suggests that the heat of transport contains both structural and kinetic contributions. These findings provide molecular-level insight into the interplay between hydration structure, ionic mass, and thermodiffusive transport in aqueous electrolytes.

cond-mat.soft↗

arXiv2026

Statistical Mechanics of Density- and Temperature-Dependent Potentials: Application to Condensed Phases within GenDPDE

Coarse-grain Lagrangian methods, such as Dissipative Particle Dynamics ( Hoogerbrugge et al., EPL, 1992), are suitable for describing mesoscopic fluid systems that include thermal fluctuations. However, the realistic simulation of liquids using these methods represents a longstanding problem. In this work, we develop a local thermodynamic (LTh) model for the description of condensed phases within the framework of the Generalized Dissipative Particle Dynamics with Energy Conservation (GenDPDE) method (Bonet Avalos et al., PCCP 2019). Such a model is appropriate for the analysis of liquids, due to the explicit account of the thermal expansion coefficient and isothermal compressibility at the mesoscale. We demonstrate the accuracy of the LTh model by examining the thermodynamic properties of argon at both liquid and supercritical conditions, through equilibrium simulations performed around two key reference states (125.7 K, 85.31 MPa, 1419.7 kg/m3 for liquid Ar, and 418.8 K, 85.31 MPa, 695.99 kg/m3 for supercritical Ar). Remarkably, we show that the model is also valid over a range of thermodynamic conditions near the reference states, allowing a correct description of the physics of systems with spatial variations in density and temperature. We further derive analytical expressions for the macroscopic pressure and energy equations of state based on the model parameters, discussing their validity and limitations. We demonstrate that, even at the mean-field level, accurately capturing local particle arrangements is essential for predicting macroscopic thermodynamic properties from mesoscopic data. We also assess the applicability of the HNC approximation in predicting the radial distribution function of the GenDPDE system, exploring its strengths and limitations. With the LTh model, GenDPDE offers a dependable and versatile tool for analysing condensed phases through coarse-grain techniques.

cond-mat.soft↗

arXiv2026

Design Principles for Tailoring Heat Transport via Iris-Gated Core-Double-Shell Nanoparticles in the Context of Photothermal Therapies

The rational design of Janus nanostructures that combine efficient optical absorption with controlled thermal transport is essential for advancing plasmonic photothermal therapies and related applications. Here, we introduce a theoretical and computational framework to investigate core-double-shell nanoparticles and their asymmetric version, the iris-gated core-double-shell architecture. The optical response of the structures is first evaluated using generalized Mie theory and subsequently validated through FEM and FDTD simulations, ensuring a consistent description of their electromagnetic and thermal behavior. To systematically map the space of variables, we defined a multi-objective figure of merit that integrates absorption efficiency, absorption cross section, and polymer-layer thickness. Furthermore, we define a thermal gain parameter that quantifies energy densification and complements the analysis of thermal directionality. Our results reveal a near-optimal configuration with parameters ($r_c$, $\delta_{Au}$, $\delta_p$, $\theta$)=(36 nm, 5 nm, 40 nm, 70$\deg$), capable of producing a temperature rise of 20-23 $\deg$C, with 67% of the thermal fluz directed toward the upper hemispace and yielding 50% focusing enhancement relative to the symmetric case. This design preserves geometric simplicity and high symmetry while delivering robust thermal asymmetry, thereby facilitating experimental implementation. Beyond photothermal therapies, the proposed methodology constitutes a versatile platform for the rapid screening and optimization of layered nanostructures, adaptable to diverse materials, excitation wavelengths, and functional objectives in nanophotonics.

physics.optics↗

arXiv2025

Thermal Transport Anomalies of Electrolyte Solutions in the Water Supercooled Regime: Signatures of the Liquid-Liquid Water Phase Transition

Water exhibits remarkable anomalies when supercooled, attributed to a hypothesized liquid-liquid phase transition (LLPT) between low-density (LDL) and high-density (HDL) liquid phases. Using non-equilibrium molecular dynamics simulations, we explore thermal transport and coupled effects in supercooled NaCl and LiCl solutions (1-4 m, 200-300 K). At 1 m, thermal conductivity exhibits a pronounced minimum near 220 K, coinciding with maxima in isothermal compressibility and minima in the speed of sound, both of which are signatures of critical fluctuations. The anomalies progressively diminish with increasing salt concentration and vanish at 4 m, suggesting suppression of the LLPT. The Soret coefficient exhibits a striking behavior. Initially thermophobic at high temperatures (> 280 K), becoming thermophilic upon cooling, then reverting to thermophobic below 220 K. This behavior correlates with structural changes in the hydrogen-bond network of water. Specifically, we find that electrolyte solutions dominated by HDL structures, which are characterized by lower tetrahedral order, exhibit thermophobic behavior, whereas thermodynamic states dominated by LDL structures, with higher tetrahedral order, display thermophilic behavior. Furthermore, Seebeck coefficients exhibit sign reversals near 220-230 K, highlighting the thermoelectric sensitivity to structural transformations and temperature. These findings establish thermal transport as a sensitive probe of supercooled water, revealing that electrolyte solutions preserve the water's anomalies deep into the supercooled regime.

cond-mat.soft↗

arXiv2024

Redefining the dielectric response of nanoconfined liquids: insights from water

Recent experiments show that the relative dielectric constant $\epsilon$ of water confined to a film of nanometric thickness reaches a strikingly low value of 2.1, barely above the bulk's 1.8 value for the purely electronic response. We argue that $\epsilon$ is not a well-defined measure for dielectric properties at sub-nanometer scales due to the ambiguous definition of confinement width. Instead we propose the 2D polarizability $\alpha_{\perp}$ as the appropriate, well-defined response function whose magnitude can be directly obtained from both measurements and computations. Once the appropriate description is used, understanding the interplay between electronic and ionic contributions becomes critical, contrary to what is widely assumed. This highlights the importance of electronic degrees of freedom in interpreting the dielectric response of polar fluids under nanoconfinement conditions, as revealed by molecular dynamics simulations.

cond-mat.mes-hall↗

arXiv2024

Thermodynamic Origin of Water's Thermal Conductivity Maximum

The thermal conductivity of water features a maximum (TCM) as a function of temperature at constant pressure. By examining why molecular force fields succeed or fail to reproduce the maximum and interpreting our results using the Bridgman equation, we show that water's TCM is connected with its compressibility minimum. Using Stillinger-Weber potentials for tetrahedral liquids, we interpolate between the behaviour of simple liquids and highly tetrahedral materials such as carbon. Together with two vanishing limits at low/high tetrahedrality, we identify three regimes for the TCM: when it originates from either the compressibility minimum or density maximum, or both. Thus, the TCM exists in a "Goldilocks Zone" of tetrahedral order. We provide a thermodynamic explanation for the TCM of not only water, but tetrahedral liquids in general.

cond-mat.soft↗

arXiv2023

The impact of the interfacial Kapitza resistance on colloidal thermophoresis

Thermal gradients impart thermophoretic forces on colloidal particles, pushing colloids towards cold or hot regions, a phenomenon called thermophoresis. Current theoretical approaches relate the Soret coefficient to local changes in the interfacial tension around the colloid, which lead to fluid flow around the colloid surface. The Kapitza resistance, a key variable in the description of interfacial heat transport, is an experimentally accessible property that modifies interfacial thermal fields. Here, we introduce a theoretical approach that describes colloid thermophoretic forces by incorporating explicitly Kapitza resistance effects. Our formulation can be used to monitor the dependence of thermophoresis on the interfacial thermal resistance. We show that the resistance modifies the thermal field around the colloids and identify experimental conditions where the Kapitza resistance influences the thermophoretic forces. We validate our theoretical approach by implementing a non-equilibrium molecular dynamics model of a colloid suspended in a solvent.

cond-mat.soft↗

arXiv2022

On the microscopic origin of Soret coefficient minima in liquid mixtures

Temperature gradients induce mass separation in mixtures in a process called thermodiffusion and quantified by the Soret coefficient. The existence of minima in the Soret coefficient of aqueous solutions was controversial until fairly recently, where a combination of experiments and simulations provided evidence for the existence of this physical phenomenon. However, the physical origin of the minima and more importantly its generality, e.g. in non-aqueous liquid mixtures, is still an outstanding question. We report the existence of a minimum in liquid mixtures of non-polar liquids modelled as Lennard-Jones mixtures, demonstrating the generality of this phenomenon. The Soret coefficient minimum originates from a coincident minimum in the thermodynamic factor, and hence denotes a maximimzation of non-ideality mixing conditions. We explain the microscopic origin of this effect in terms of the atomic coordination structure of the mixtures.

cond-mat.soft↗

arXiv2022

Enhanced thermo-optical response by means of anapole excitation

High refractive index dielectric nanostructures offer a versatile platform to control light-matter interaction at the nanoscale as they can easily support electric and magnetic modes with low losses. An additional property that makes them extraordinary is that they can support low radiative modes, so-called anapole modes. In this work, we propose a spectrally tunable anapole nanoheater based on the use of a dielectric anapole resonator able to amplify ten-fold the thermal response of a plasmonic nanoheater. This would allow the use of lower light intensities to achieve striking heating effects. As proof of concept, we perform a detailed study of the thermo-plasmonic response of a gold nanoring used as heating source and a silicon disk, designed to support anapole modes, located in its center acting as anapolar resonator. Furthermore, we utilize the anapole excitation to easily shift the thermal response of these structures from SWIR to the NIR range.

physics.optics↗

arXiv2021

Efficient Asymmetric photothermal source: Designing a heating Janus-Nanojet

Janus particles have flourished as subject of intensive research due to their synergetic properties and their promising use in different fields, especially in biomedicine. The combination of materials with radically different physical properties in the same nanostructure gives rise to the so-called Janus effects, allowing phenomena of a contrasting nature to occur in the same architecture. In particular, interesting advantages can be taken from a thermal Janus effect for photoinduced hyperthermia cancer therapies. Such therapies still have limitations associated to the heating control in terms of temperature stability and energy management. While previous studies have shown that some plasmonic single-material nanoparticles are somehow effective at killing cancer cells, it is necessary to investigate alternative plasmonic Janus nanoheaters to improve the heating efficiency and thermal control, mainly because the widespread single-material nanoheaters are highly homogeneous sources of heat, which implies that the surrounding biological medium is isotropically heated, equally affecting cancerous and healthy cells. A detailed thermoplasmonic study of the thermal Janus effect is still missing. Here we perform such study and demonstrate that doughnut-based Janus nanoparticles exhibit an outstanding photothermal control under practical illumination conditions, i.e., unpolarized light. Furthermore, we present novel and effective Janus nanoparticle designs that possess superior photothermal conversion features and unique directional heating capacity, being able to channel up to 91% of the total thermal energy onto a target. We discuss the implications of these innovative nanoparticles with regards to thermoplasmonics hyperthermia cancer therapy.

physics.optics↗

arXiv2021

Gold nanodoughnut as an outstanding nanoheater for photothermal applications

Photoinduced hyperthermia is a cancer therapy technique that induces death to cancerous cells via heat generated by plasmonic nanoparticles. While previous studies have shown that some nanoparticles can be effective at killing cancer cells under certain conditions, there is still a necessity (or the need) to improve its heating efficiency. In this work, we perform a detailed thermoplasmonic study comparing the most effective nanoparticle geometries up to now with a doughnut-shaped nanoparticle, demonstrating that the latter exhibits a superior tunable photothermal response in practical illumination conditions, i.e., unpolarized light. Furthermore, we show that nanoparticle heating in fluidic environments, i.e., nanoparticles undergoing Brownian rotations, strongly depends on the particle orientation with respect to the illumination source. We conclude that the heating performance of nanodoughnuts is outstanding, with a temperature increment 35% higher than the second best nanoheater (nanodisks). Furthermore, nanodoughnuts feature a weak dependence on orientation, being therefore, ideal candidates for photothermal therapy applications. Finally, we present a designing guide, covering a wide range of toroid designs, which can help on its experimental implementation.

physics.optics↗

arXiv2021

Limits to crystallization pressure

Crystallization pressure drives deformation and damage in monuments, buildings and the Earth's crust. Even though the phenomenon has been known for 170 years there is no agreement between theoretical calculations of the maximum attainable pressure and that found experimentally. We have therefore developed a novel experimental technique to image the nano-confined crystallization process while controlling the pressure and applied it to calcite. The results show that displacement by crystallization pressure is arrested at pressures well below the thermodynamic limit. We use existing molecular dynamics simulations and atomic force microscopy data to construct a robust model of the disjoining pressure in this system and thereby calculate the absolute distance between the surfaces. Based on the high resolution experiments and modelling we formulate a novel mechanism for the transition between damage and adhesion by crystallization that may find application in Earth and materials sciences and in conservation of cultural heritage.

cond-mat.mtrl-sci↗

arXiv2021

The nonlocal dielectric response of water in nanoconfinement

Recent experiments reporting a very low dielectric permittivity for nanoconfined water have renewed the interest to the structure and dielectric properties of water in narrow gaps. Here, we describe such systems with a minimal Landau-Ginzburg field-theory composed of a nonlocal bulk-determined term and a local water-surface interaction term. We show how the interplay between the boundary conditions and intrinsic bulk correlations encodes dielectric properties of confined water. Our theoretical analysis is supported by molecular dynamics simulations and comparison with the experimental data.

physics.chem-ph↗

arXiv2020

Non-equilibrium molecular dynamics and continuum modelling of transient freezing of atomistic solids

In this work we investigate the transient solidification of a Lennard-Jones liquid using non-equilibrium molecular dynamics simulations and continuum heat transfer theory. The simulations are performed in slab-shaped boxes, where a cold thermostat placed at the centre of the box drives the solidification of the liquid. Two well-defined solid fronts propagate outwards from the centre towards the ends of the box until solidification is completed. A continuum phase change model that accounts for the difference between the solid and the liquid densities is formulated to describe the evolution of the temperature and the position of the solidification front. Simulation results for a small and a large nanoscale system, of sizes $30.27$\,nm and $60.54$\,nm, are compared with the predictions of the theoretical model. Following a transient period of $\sim$20-40 ps and a displacement of the solidification front of 1-2.5 nm we find that the simulations and the continuum theory show good agreement. We use this fact to combine the simulation and theoretical approaches to design a simple procedure to calculate the latent heat of the material. We also perform simulations of the homogeneous freezing process, i.e. in the absence of a temperature gradient and at constant temperature, by quenching the liquid at supercooled temperatures. We demonstrate that the solidification rate of homogenous freezing is much faster than the one obtained under a thermal gradient for systems of the same size subject to the same thermostat temperature. Our study and conclusions should be of general interest to a wide range of atomistic solids.

cond-mat.mes-hall↗

arXiv2020

The influence of surface roughness on the adhesive interactions and phase behavior of suspensions of calcite nanoparticles

We investigate the impact of nanoparticle roughness on the phase behaviour of suspensions in models of calcium carbonate nanoparticles. We use a Derjaguin approach that incorporates roughness effects and interactions between the nanoparticles modelled with a combination of DLVO forces and hydration forces, derived using experimental data and atomistic molecular dynamics simulations, respectively. Roughness effects, such as atomic steps or terraces appearing in mineral surfaces result in very different effective inter-nanoparticle potentials. Using stochastic Langevin Dynamics computer simulations and the effective interparticle interactions we demonstrate that relatively small changes in the roughness of the particles modify significantly the stability of the suspensions. We propose that the sensitivity of the phase behavior to the roughness is connected to the short length scale of the adhesive attraction arising from the ordering of water layers confined between calcite surfaces. Particles with smooth surfaces feature strong adhesive forces, and form gel fractal structures, while small surface roughness, of the order of atomic steps in mineral faces, stabilize the suspension. We believe that our work helps to rationalize the contrasting experimental results that have been obtained recently using nanoparticles or extended surfaces, which provide support for the existence of adhesive or repulsive interactions, respectively. We further use our model to analyze the synergistic effects of roughness, pH and ion concentration on the phase behavior of suspensions, connecting with recent experiments using calcium carbonate nanoparticles.

cond-mat.soft↗

arXiv2019

Molecular Dynamics Simulation of Imidazolium CnMIM-BF4 Ionic Liquids using a Coarse Grained Force-Field

Ionic Liquids feature thermophysical properties that are of interest in solvents, energy storage materials and tenable lubrication applications. Recently, a series of coarse grained (CG) models was developed to investigate 1-ethyl-3-methylimidazolium tetrafluoroborate [C2MIM][BF4] and 1-butyl-3-methylimidazolium hexafluorophosphate [C4MIM][PF6] ionic liquids. Building on these CG models we derive force-fields to investigate the [C_{2-8}MIM] [BF4] family of ionic liquids, as a starting step for systematic investigations of lubrication under nanoconfinement conditions. The simualted equation of state and diffusion coefficients are in good agreement with experimental data and with all-atom force fields. We use these model to analyze the nano nanostructuring of ILs characteristic of cations with longer aliphatic chains as well as the ILs liquidvapour interfacial structure. The CG nature of these models enables the simulation of very long time scales, which are needed to computer reliable results of dynamic and interfacial properties. For [C>4MIM] [BF4] the break in symmetry associated to the liquid-vapor interface induces nanostructuring in polar and non-polar domains in the direction perpendicular to the interface plane, hence mimicking the behavior observed in the bulk phases.

cond-mat.soft↗

arXiv2017

Deconstructing temperature gradients across fluid interfaces: the structural origin of the thermal resistance of liquid-vapor interfaces

The interfacial thermal resistance determines condensation-evaporation processes and thermal transport across material-fluid interfaces. Despite its importance in transport processes, the interfacial structure responsible for the thermal resistance is still unknown. By combining non-equilibrium molecular dynamics simulations and interfacial analyses that remove the interfacial thermal fluctuations we show that the thermal resistance of liquid-vapor interfaces is connected to a low density fluid layer that is adsorbed at the liquid surface. This thermal resistance layer (TRL) defines the boundary where the thermal transport mechanism changes from that of gases (ballistic) to that characteristic of dense liquids, dominated by frequent particle collisions involving very short mean free paths. We show that the thermal conductance is proportional to the number of atoms adsorbed in the TRL, and hence we explain the structural origin of the thermal resistance in liquid-vapor interfaces.

cond-mat.soft↗

arXiv2017

A computational approach to calculate the heat of transport of aqueous solutions

Thermal gradients induce concentration gradients in alkali halide solutions, and the salt migrates towards hot or cold regions depending on the average temperature of the solution. This effect has been interpreted using the heat of transport, which provides a route to rationalize thermophoretic phenomena. Early theories provide estimates of the heat of transport at infinite dilution. These values are used to interpret thermodiffusion (Soret) and thermoelectric (Seebeck) effects. However, accessing heats of transport of individual ions at finite concentration remains an outstanding question both theoretically and experimentally. Here we discuss a computational approach to calculate heats of transport of aqueous solutions at finite concentrations, and apply our method to study lithium chloride solutions at concentrations $>0.5$~M. The heats of transport are significantly different for Li$^+$ and Cl$^-$ ions, unlike what is expected at infinite dilution. We find theoretical evidence for the existence of minima in the Soret coefficient of LiCl, where the magnitude of the heat of transport is maximized. The Seebeck coefficient obtained from the ionic heats of transport varies significantly with temperature and concentration. We identify thermodynamic conditions leading to a maximization of the thermoelectric response of aqueous solutions.

physics.chem-ph↗