197 results found
Gittus OR, Bresme F, 2023, Mass dipole contribution to the isotopic Soret effect in molecular mixtures., J Chem Phys, Vol: 159
Temperature gradients induce mass separation in mixtures in a process called thermal diffusion and are quantified by the Soret coefficient ST. Thermal diffusion in fluid mixtures has been interpreted recently in terms of the so-called (pseudo-)isotopic Soret effect but only considering the mass and moment of inertia differences of the molecules. We demonstrate that the first moment of the molecular mass distribution, the mass dipole, contributes significantly to the isotopic Soret effect. To probe this physical effect, we investigate fluid mixtures consisting of rigid linear molecules that differ only by the first moment of their mass distributions. We demonstrate that such mixtures have non-zero Soret coefficients in contrast with ST = 0 predicted by current formulations. For the isotopic mixtures investigated in this work, the dependence of ST on the mass dipole arises mainly through the thermal diffusion coefficient DT. In turn, DT is correlated with the dependence of the molecular librational modes on the mass dipole. We examine the interplay of the mass dipole and the moment of inertia in defining the isotopic Soret effect and propose empirical equations that include the mass dipole contribution.
Jiang M, Chapman A, Olarte-Plata JD, et al., 2023, Controlling local thermal gradients at molecular scales with Janus nanoheaters, Nanoscale, Vol: 15, Pages: 10264-10276, ISSN: 2040-3364
The generation and control of heat transport with nanoparticles is an essential objective of thermoplasmonics. Janus nanoparticles consisting of dissimilar materials with contrasting interfacial Kapitza conductance provide a route to control heat transport at the nanoscale. Here we use the recently introduced Atomistic Nodal Approach to map the surface temperature and Kapitza conductance of Janus nanoparticles to individual atoms. We show that the transition in the thermal transport properties between the hydrophobic and hydrophilic interfaces is exceptionally abrupt, occurring over length scales below 1 nm. We demonstrate the generality of this result using coarse-grained and all-atom models of gold nanoparticles. Further, we show how this behaviour provides a route to sustain significant temperature differences, on the order of tens of degrees for μW heat rates, between adjacent molecular layers attached to heated gold nanoparticles. Our work provides fundamental insight into nanoscale heat transport and a principle to design heterogeneous Janus nanoparticles for thermal transport applications.
Kondrat S, Feng G, Bresme F, et al., 2023, Theory and simulations of ionic liquids in nanoconfinement., Chemical Reviews, Vol: 123, Pages: 6668-6715, ISSN: 0009-2665
Room-temperature ionic liquids (RTILs) have exciting properties such as nonvolatility, large electrochemical windows, and remarkable variety, drawing much interest in energy storage, gating, electrocatalysis, tunable lubrication, and other applications. Confined RTILs appear in various situations, for instance, in pores of nanostructured electrodes of supercapacitors and batteries, as such electrodes increase the contact area with RTILs and enhance the total capacitance and stored energy, between crossed cylinders in surface force balance experiments, between a tip and a sample in atomic force microscopy, and between sliding surfaces in tribology experiments, where RTILs act as lubricants. The properties and functioning of RTILs in confinement, especially nanoconfinement, result in fascinating structural and dynamic phenomena, including layering, overscreening and crowding, nanoscale capillary freezing, quantized and electrotunable friction, and superionic state. This review offers a comprehensive analysis of the fundamental physical phenomena controlling the properties of such systems and the current state-of-the-art theoretical and simulation approaches developed for their description. We discuss these approaches sequentially by increasing atomistic complexity, paying particular attention to new physical phenomena emerging in nanoscale confinement. This review covers theoretical models, most of which are based on mapping the problems on pertinent statistical mechanics models with exact analytical solutions, allowing systematic analysis and new physical insights to develop more easily. We also describe a classical density functional theory, which offers a reliable and computationally inexpensive tool to account for some microscopic details and correlations that simplified models often fail to consider. Molecular simulations play a vital role in studying confined ionic liquids, enabling deep microscopic insights otherwise unavailable to researchers. We describe the
Haimov E, Chapman A, Bresme F, et al., 2023, Addendum: Theoretical demonstration of a capacitive rotor for generation of alternating current from mechanical motion., Nature Communications, Vol: 14, Pages: 483-483, ISSN: 2041-1723
Saurabh S, Li Z, Hollowell P, et al., 2023, Structure and interaction of therapeutic proteins in solution: a combined simulation and experimental study, Molecular Physics, ISSN: 0026-8976
The aggregation of therapeutic proteins in solution has attracted significant interest, driving efforts to understand the relationship between microscopic structural changes and protein-protein interactions determining aggregation processes in solution. Additionally, there is substantial interest in being able to predict aggregation based on protein structure as part of molecular developability assessments. Molecular Dynamics provides theoretical tools to complement experimental studies and to interrogate and identify the microscopic mechanisms determining aggregation. Here we perform all-atom MD simulations to study the structure and inter-protein interaction of the Fab and Fc fragments of the monoclonal antibody (mAb) COE3. We unravel the role of ion-protein interactions in building the ionic double layer and determining effective inter-protein interaction. Further, we demonstrate, using various state-of-the-art force fields (charmm, gromos, amber, opls/aa), that the protein solvation, ionic structure and protein-protein interaction depend significantly on the force field parameters. We perform SANS and Static Light Scattering experiments to assess the accuracy of the different forcefields. Comparison of the simulated and experimental results reveal significant differences in the forcefields' performance, particularly in their ability to predict the protein size in solution and inter-protein interactions quantified through the second virial coefficients. In addition, the performance of the forcefields is correlated with the protein hydration structure.
Gittus OR, Bresme F, 2022, On the microscopic origin of Soret coefficient minima in liquid mixtures, Physical Chemistry Chemical Physics, Vol: 25, Pages: 1606-1611, ISSN: 1463-9076
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 at specific salt concentrations 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. Here, we report the existence of a minimum in liquid mixtures of non-polar liquids modelled as Lennard-Jones mixtures, demonstrating the generality of minima in the Soret coefficient. The minimum originates from a coincident minimum in the thermodynamic factor, and hence denotes a maximization of non-ideality mixing conditions. We rationalize the microscopic origin of this effect in terms of the atomic coordination structure of the mixtures.
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 experimentally measured pressures. We have therefore developed a novel experimental technique to image the nanoconfined 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. On the basis of the high-resolution experiments and modeling, 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.
Gonzalez-Colsa J, Franco A, Bresme F, et al., 2022, Janus-Nanojet as an efficient asymmetric photothermal source, Scientific Reports, Vol: 12, ISSN: 2045-2322
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. Interesting advantages can be taken from a thermal Janus effect for photoinduced hyperthermia cancer therapies. Such therapies have limitations associated to the heating control in terms of temperature stability and energy management. Single-material plasmonic nanoheaters have been widely used for cancer therapies, however, they are highly homogeneous sources that heat the surrounding biological medium isotropically, thus equally affecting cancerous and healthy cells. Here, we propose a prototype of a Janus-Nanojet heating unit based on toroidal shaped plasmonic nanoparticles able to efficiently generate and release local heat directionally under typical unpolarized illumination. Based on thermoplasmonic numerical calculations, we demonstrate that these Janus-based nanoheaters possess superior photothermal conversion features (up to ΔT≈35 K) and unique directional heating capacity, being able to channel up over 90% of the total thermal energy onto a target. We discuss the relevance of these innovative nanoheaters in thermoplasmonics, and hyperthermia cancer therapies, which motivate the development of fabrication techniques for nanomaterials.
Saurabh S, Kalonia C, Li Z, et al., 2022, Understanding the stabilizing effect of histidine on mAb aggregation: a molecular dynamics study., Molecular Pharmaceutics, Vol: 19, ISSN: 1543-8384
Histidine, a widely used buffer in monoclonal antibody (mAb) formulations, is known to reduce antibody aggregation. While experimental studies suggest a nonelectrostatic, nonstructural (relating to secondary structure preservation) origin of the phenomenon, the underlying microscopic mechanism behind the histidine action is still unknown. Understanding this mechanism will help evaluate and predict the stabilizing effect of this buffer under different experimental conditions and for different mAbs. We have used all-atom molecular dynamics simulations and contact-based free energy calculations to investigate molecular-level interactions between the histidine buffer and mAbs, which lead to the observed stability of therapeutic formulations in the presence of histidine. We reformulate the Spatial Aggregation Propensity index by including the buffer-protein interactions. The buffer adsorption on the protein surface leads to lower exposure of the hydrophobic regions to water. Our analysis indicates that the mechanism behind the stabilizing action of histidine is connected to the shielding of the solvent-exposed hydrophobic regions on the protein surface by the buffer molecules.
Bresme F, Olarte-Plata JD, Chapman A, et al., 2022, Thermophoresis and thermal orientation of Janus nanoparticles in thermal fields, The European Physical Journal E: soft Matter and Biological Physics, Vol: 45, ISSN: 1292-8941
Thermal fields provide a route to control the motion of nanoparticles and molecules and potentially modify the behaviour of soft matter systems. Janus nanoparticles have emerged as versatile building blocks for the self-assembly of materials with novel properties. Here we investigate using non-equilibrium molecular dynamics simulations the behaviour of coarse-grained models of Janus nanoparticles under thermal fields. We examine the role of the heterogeneous structure of the particle on the Soret coefficient and thermal orientation by studying particles with different internal structures, mass distribution, and particle–solvent interactions. We also examine the thermophoretic response with temperature, targeting liquid and supercritical states and near-critical conditions. We find evidence for a significant enhancement of the Soret coefficient near the critical point, leading to the complete alignment of a Janus particle in the thermal field. This behaviour can be modelled and rationalized using a theory that describes the thermal orientation with the nanoparticle Soret coefficient, the mass and interaction anisotropy of the Janus nanoparticle, and the thermal field’s strength. Our simulations show that the mass anisotropy plays a crucial role in driving the thermal orientation of the Janus nanoparticles.
González-Colsa J, Olarte-Plata JD, Bresme F, et al., 2022, Enhanced thermo-optical response by means of anapole excitation., Journal of Physical Chemistry Letters, Vol: 13, Pages: 6230-6235, ISSN: 1948-7185
High refractive index (HRI) dielectric nanostructures offer a versatile platform to control the 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. We show that a gold ring nanostructure, a priori nonresonant, can be turned into a resonant unit by just filling its hole with an HRI material supporting anapole modes, resulting in a more efficient nanoheater able to amplify the photothermal response of the bare nanoring. As proof of concept, we perform a detailed study of the thermoplasmonic response of a gold nanoring used as heating source and a silicon disk, designed to support anapole modes, located in its center acting as an anapolar resonator. Furthermore, we utilize the anapole excitation to easily shift the thermal response of these structures from the shortwave infrared range to the near-infrared range.
Bresme F, Kornyshev AA, Perkin S, et al., 2022, Electrotunable friction with ionic liquid lubricants, Nature Materials, Vol: 21, Pages: 848-858, ISSN: 1476-1122
Room-temperature ionic liquids and their mixtures with organic solvents as lubricants open a route to control lubricity at the nanoscale via electrical polarization of the sliding surfaces. Electronanotribology is an emerging field that has a potential to realize in situ control of friction—that is, turning the friction on and off on demand. However, fulfilling its promise needs more research. Here we provide an overview of this emerging research area, from its birth to the current state, reviewing the main achievements in non-equilibrium molecular dynamics simulations and experiments using atomic force microscopes and surface force apparatus. We also present a discussion of the challenges that need to be solved for future applications of electrotunable friction.
Chapman A, Bresme F, 2022, Polarisation of water under thermal fields: the effect of the molecular dipole and quadrupole moments., Physical Chemistry Chemical Physics, Vol: 24, Pages: 14924-14936, ISSN: 1463-9076
The investigation of the behaviour of water under thermal fields is important to understand thermoelectricity of solutions, aqueous suspensions, bioelectric effects or the properties of wet materials under spatially inhomogeneous temperature conditions. Here we discuss the response of bulk water to external thermal fields using non-equilibrium molecular dynamics simulations, and five widely used forcefields: TIP4P/2005, TIP4P/2005f, OPC, SPC/E and TIP3P. These models all show the thermal polarisation (TP) effect in bulk water, namely the build-up of an electrostatic field induced by the temperature gradient. The strength of this effect is ∼0.1-1 mV K-1 at near-standard conditions for all forcefields, supporting the generality of TP. Moreover, all the models predict a temperature inversion of the polarisation field, although the inversion temperatures vary significantly across different models. We rationalise this result by deriving theoretical equations that describe the temperature inversion as a balance of the isobaric thermal expansion, dipole orientation in the thermal field and the ratio of the molecular dipole/quadrupole moments. Lower ratios lead to higher inversion temperatures. Based on our results, we conclude that the accuracy of the forcefields describing the TP effect decreases as, TIP4P/2005 ∼ TIP4P/2005f ∼ OPC > SPC/E > TIP3P. At coexistence conditions, the inversion temperature is expected to be around 400 K. Furthermore, we establish a correlation between the TP inversion temperature and the temperature corresponding to the minimum of the liquid-vapour surface potential of water.
Olarte-Plata JD, Bresme F, 2022, Thermal conductance of the water-gold interface: the impact of the treatment of surface polarization in non-equilibrium molecular simulations., Journal of Chemical Physics, Vol: 156, Pages: 204701-204701, ISSN: 0021-9606
Interfacial thermal conductance (ITC) quantifies heat transport across material-fluid interfaces. It is a property of crucial importance to study heat transfer processes at both macro- and nanoscales. Therefore, it is essential to accurately model the specific interactions between solids and liquids. Here, we investigate the thermal conductance of gold-water interfaces using polarizable and non-polarizable models. Both models have been fitted to reproduce the interfacial tension of the gold-water interface, but they predict significantly different ITCs. We demonstrate that the treatment of polarization using Drude-like models, widely employed in molecular simulations, leads to a coupling of the solid and liquid vibrational modes that give rise to a significant overestimation of the ITCs. We analyze the dependence of the vibrational coupling with the mass of the Drude particle and propose a solution to the artificial enhancement of the ITC, preserving at the same time the polarization response of the solid. Based on our calculations, we estimate ITCs of 200 MW/(m2 K) for the water-gold interface. This magnitude is comparable to that reported recently for gold-water interfaces [279 ± 16 MW/(m2 K)] using atomic fluctuating charges to account for the polarization contribution.
Hernández-Muñoz J, Bresme F, Tarazona P, et al., 2022, Bending modulus of lipid membranes from density correlation functions., Journal of Chemical Theory and Computation, Vol: 18, Pages: 3151-3163, ISSN: 1549-9618
The bending modulus κ quantifies the elasticity of biological membranes in terms of the free energy cost of increasing the membrane corrugation. Molecular dynamics (MD) simulations provide a powerful approach to quantify κ by analyzing the thermal fluctuations of the lipid bilayer. However, existing methods require the identification and filtering of non-mesoscopic fluctuation modes. State of the art methods rely on identifying a smooth surface to describe the membrane shape. These methods introduce uncertainties in calculating κ since they rely on different criteria to select the relevant fluctuation modes. Here, we present a method to compute κ using molecular simulations. Our approach circumvents the need to define a mesoscopic surface or an orientation field for the lipid tails explicitly. The bending and tilt moduli can be extracted from the analysis of the density correlation function (DCF). The method introduced here builds on the Bedeaux and Weeks (BW) theory for the DCF of fluctuating interfaces and on the coupled undulatory (CU) mode introduced by us in previous work. We test the BW-DCF method by computing the elastic properties of lipid membranes with different system sizes (from 500 to 6000 lipid molecules) and using coarse-grained (for POPC and DPPC lipids) and fully atomistic models (for DPPC). Further, we quantify the impact of cholesterol on the bending modulus of DPPC bilayers. We compare our results with bending moduli obtained with X-ray diffraction data and different computer simulation methods.
Jiang M, Olarte-Plata JD, Bresme F, 2022, Heterogeneous thermal conductance of nanoparticle-fluid interfaces: An atomistic nodal approach, Journal of Chemical Physics, Vol: 156, ISSN: 0021-9606
The Interfacial Thermal Conductance (ITC) is a fundamental property of materials and has particular relevance at the nanoscale. The ITC quantifies the thermal resistance between materials of different compositions or between fluids in contact with materials. Furthermore, the ITC determines the rate of cooling/heating of the materials and the temperature drop across the interface. Here, we propose a method to compute local ITCs and temperature drops of nanoparticle–fluid interfaces. Our approach resolves the ITC at the atomic level using the atomic coordinates of the nanomaterial as nodes to compute local thermal transport properties. We obtain high-resolution descriptions of the interfacial thermal transport by combining the atomistic nodal approach, computational geometry techniques, and “computational farming” using non-equilibrium molecular dynamics simulations. We use our method to investigate the ITC of nanoparticle–fluid interfaces as a function of the nanoparticle size and geometry, targeting experimentally relevant structures of gold nanoparticles: capped octagonal rods, cuboctahedrons, decahedrons, rhombic dodecahedrons, cubes, icosahedrons, truncated octahedrons, octahedrons, and spheres. We show that the ITC of these very different geometries varies significantly in different regions of the nanoparticle, increasing generally in the order face < edge < vertex. We show that the ITC of these complex geometries can be accurately described in terms of the local coordination number of the atoms in the nanoparticle surface. Nanoparticle geometries with lower surface coordination numbers feature higher ITCs, and the ITC generally increases with the decreasing particle size
Gonzalez-Colsa J, Serrera G, Maria Saiz J, et al., 2022, Gold nanodoughnut as an outstanding nanoheater for photothermal applications, Optics Express, Vol: 30, Pages: 125-137, ISSN: 1094-4087
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 theoretical study comparing the thermoplasmonic response of the most effective nanoparticle geometries up to now with a doughnut-shaped nanoparticle. We numerically demonstrate that the latter exhibits a superior tunable photothermal response in practical illumination conditions (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 nanodoughnuts are the best nanoheaters in our set of structures, with an average temperature increment 40% higher than the second best nanoheater (nanodisk). 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.
Olarte-Plata JD, Bresme F, 2022, The impact of the thermostats on the non-equilibrium computer simulations of the interfacial thermal conductance, Molecular Simulation, Vol: 48, Pages: 87-98, ISSN: 0892-7022
Non-equilibrium molecular dynamics simulations have expanded our ability to investigate interfacial thermal transport and quantify the interfacial thermal conductance (ITC) across solid and fluid interfaces. NEMD studies have highlighted the importance of interfacial degrees of freedom and the need to include effects beyond traditional theoretical methods that rely on bulk properties. NEMD simulations often use explicit hot and cold thermostats to set up thermal gradients. We analyse here the impact of the thermostat on the calculated ITC of the gold-water interface. We employ a polarisable model for gold based on Drude oscillators. We show that the ‘local’ Langevin thermostat modifies the vibrational density of states of the polarisable solid, resulting in ITCs that depend very strongly on the damping constant of the thermostat. We report an increase of the ITC of up to 40% for short damping times. Damping times longer than the characteristic heat flux relaxation time of the solid lead to converging ITCs. In contrast, the ITCs obtained with global canonical velocity rescale thermostats are independent of the damping time but lead to a break of equipartition for Drude particles. Setting individual thermostats for the core and shell sites in the Drude particle solves this problem.
Olarte-Plata JD, Gabriel J, Albella P, et al., 2021, Spatial control of heat flow at the nanoscale using Janus Particles., ACS Nano, Vol: 16
Janus nanoparticles (JNPs) feature heterogeneous compositions, bringing opportunities in technological and medical applications. We introduce a theoretical approach based on nonequilibrium molecular dynamics simulations and heat transfer continuum theory to investigate the temperature fields generated around heated spherical JNPs covering a wide range of particle sizes, from a few nm to 100 nm. We assess the performance of these nanoparticles to generate anisotropic heating at the nanoscale. We demonstrate that the contrasting interfacial thermal conductances of the fluid-material interfaces arising from the heterogeneous composition of the JNPs can be exploited to control the thermal fields around the nanoparticle, leading to a temperature difference between both sides of the nanoparticle (temperature contrast) that is significant for particles comprising regions with disparate hydrophilicity. We illustrate this idea using coarse-grained and atomistic models of gold nanoparticles with hydrophobic and hydrophilic coatings, in water. Furthermore, we introduce a continuum model to predict the temperature contrast as a function of the interfacial thermal conductance and nanoparticle size. We further show that, unlike homogeneous nanoparticles, the interfacial fluid temperature depends on the interfacial thermal conductance of Janus nanoparticles.
Gittus OR, Bresme F, 2021, Thermophysical properties of water using reactive force fields., Journal of Chemical Physics, Vol: 155, Pages: 1-28, ISSN: 0021-9606
The widescale importance and rich phenomenology of water continue to motivate the development of computational models. ReaxFF force fields incorporate many characteristics desirable for modeling aqueous systems: molecular flexibility, polarization, and chemical reactivity (bond formation and breaking). However, their ability to model the general properties of water has not been evaluated in detail. We present comprehensive benchmarks of the thermophysical properties of water for two ReaxFF models, the water-2017 and CHON-2017_weak force fields. These include structural, electrostatic, vibrational, thermodynamic, coexistence, and transport properties at ambient conditions (300 K and 0.997 g cm-3) and along the standard pressure (1 bar) isobar. Overall, CHON-2017_weak predicts more accurate thermophysical properties than the water-2017 force field. Based on our results, we recommend potential avenues for improvement: the dipole moment to quadrupole moment ratio, the self-diffusion coefficient, especially for water-2017, and the gas phase vibrational frequencies with the aim to improve the vibrational properties of liquid water.
Di Lecce S, Kornyshev AA, Urbakh M, et al., 2021, Structural effects in nanotribology of nanoscale films of ionic liquids confined between metallic surfaces, Physical Chemistry Chemical Physics, Vol: 23, Pages: 22174-22183, ISSN: 1463-9076
Room Temperature Ionic Liquids (RTILs) attract significant interest in nanotribology. However, their microscopic lubrication mechanism is still under debate. Here, using non-equilibrium molecular dynamics simulations, we investigate the lubrication performance of ultra-thin (<2 nm) films of [C2MIM]+ [NTf2]− confined between plane-parallel neutral surfaces of Au(111) or Au(100). We find that films consisting of tri-layers or bilayers, form ordered structures with a flat orientation of the imidazolium rings with respect to the gold surface plane. Tri-layers are unstable against loads >0.5 GPa, while bi-layers sustain pressures in the 1–2 GPa range. The compression of these films results in monolayers that can sustain loads of several GPa without significant loss in their lubrication performance. Surprisingly, in such ultra-thin films the imidazolium rings show higher orientational in-plane disorder, with and the rings adopting a tilted orientation with respect to the gold surface. The friction force and friction coefficient of the monolayers depends strongly on the structure of the gold plates, with the friction coefficient being four times higher for monolayers confined between Au(100) surfaces than for more compact Au(111) surfaces. We show that the general behaviour described here is independent of whether the metallic surfaces are modelled as polarizable or non-polarizable surfaces and speculate on the nature of this unexpected conclusion.
Haimov E, Chapman A, Bresme F, et al., 2021, Theoretical demonstration of a capacitive rotor for generation of alternating current from mechanical motion, Nature Communications, Vol: 12, Pages: 3678-3678, ISSN: 2041-1723
Innovative concepts and materials are enabling energy harvesters for slower motion, particularly for personal wearables or portable small-scale applications, hence contributing to a future sustainable economy. Here we propose a principle for a capacitive rotor device and analyze its operation. This device is based on a rotor containing many capacitors in parallel. The rotation of the rotor causes periodic capacitance changes and, when connected to a reservoir-of-charge capacitor, induces alternating current. The properties of this device depend on the lubricating liquid situated between the capacitor’s electrodes, be it a highly polar liquid, organic electrolyte, or ionic liquid – we consider all these scenarios. An advantage of the capacitive rotor is its scalability. Such a lightweight device, weighing tens of grams, can be implemented in a shoe sole, generating a significant power output of the order of Watts. Scaled up, such systems can be used in portable wind or water turbines.
Monet G, Berthoumieux H, Bresme F, et al., 2021, Nonlocal Dielectric Response of Water in Nanoconfinement, PHYSICAL REVIEW LETTERS, Vol: 126, ISSN: 0031-9007
Font F, Micou W, Bresme F, 2021, Non-equilibrium molecular dynamics and continuum modelling of transient freezing of atomistic solids, International Journal of Heat and Mass Transfer, Vol: 164, Pages: 1-11, ISSN: 0017-9310
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 ~ 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 associated to the liquid-solid phase transition. 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, for comparable temperature conditions, the solidification rate of homogenous freezing is much faster than the one obtained under a thermal gradient. Our study and conclusions should be of general interest to a wide range of atomistic solids.
Galteland O, Bresme F, Hafskjold B, 2020, Solvent-mediated forces between ellipsoidal nanoparticles adsorbed at liquid-vapor interfaces., Langmuir: the ACS journal of surfaces and colloids, Vol: 36, Pages: 14530-14538, ISSN: 0743-7463
Classical capillary theory predicts that a non-neutrally wetting ellipsoidal particle adsorbed at a liquid-vapor interface will deform the interface. The deformation gives rise to anisotropic capillary forces of a quadrupolar nature that induce strong directionality in the particle interactions. Here, we investigate the interactions between nanoparticles with characteristic lengths of 1-5 nm. We show that the near-field interactions are dominated by solvent-mediated forces, which arise from the fluid packing between the nanoparticles and direct nanoparticle-nanoparticle interactions. The solvent-mediated forces are two orders of magnitude larger than the estimated capillary force. We find that interacting ellipsoidal nanoparticles adsorbed at the liquid-vapor interface have a larger repulsion in the depletion region than the nanoparticles submerged in a dense bulk phase and argue that this is because of a negative line tension associated with the three-phase line.
Pivnic K, Bresme F, Kornyshev AA, et al., 2020, Electrotunable friction in diluted room temperature ionic liquids: implications for nanotribology, ACS Applied Nano Material, Vol: 3, Pages: 10708-10719, ISSN: 2574-0970
Using nonequilibrium molecular dynamics (NEMD) simulations, we study the mechanism of electrotunable friction in the mixture of a room temperature ionic liquid (RTIL), BMIM PF6, and an organic solvent, acetonitrile. The dilution itself helps to reduce the viscosity and thereby reduce the viscous contribution to friction. At the same time, we find that under nanoscale confinement conditions, diluted RTIL solutions, of just ∼10% molar fraction, still feature a remarkable variation of the friction force with the electrode surface charge density, not weaker than had been earlier shown for nanoconfined pure RTILs. In both classes of systems the electrotunable friction response is due to accumulation of counterions at charged surfaces. For both diluted mixtures and pure RTILs, the friction force is minimal for uncharged surfaces and it increases with surface charge of either sign but only in the range of low and moderate surface charges (16–32 μC/cm2). At higher surface charges (43–55 μC/cm2), the effect is different: in the pure RTIL, the friction force continues to increase with the surface charge, while in the diluted RTIL mixture it features a maximum, with a reduction of friction with the increasing surface charge. This contrasting behavior is explained by the difference in the slip conditions found for the pure and the diluted RTIL solutions in contact with highly charged surfaces. Overall, we demonstrate that nanoscale films of diluted mixtures of RTIL provide lower friction forces than the pure RTIL films, preserving at the same time a significant electrotunable response when the liquids are confined between symmetrically charged surfaces. Nanoconfinement between asymmetrically charged surfaces leads to a reduction of friction compared to the symmetric case, with a concomitant decrease in the range of friction variation with the surface charge density. Our results highlight the potential of diluted RTIL mixtures as cost-effective electrotunab
Gittus OR, Albella P, Bresme F, 2020, Polarization of acetonitrile under thermal fields via non-equilibrium molecular dynamics simulations., Journal of Chemical Physics, Vol: 153, Pages: 204503-204503, ISSN: 0021-9606
We show that thermal gradients polarize liquid and supercritical acetonitrile. The polarization results in a stationary electrostatic potential that builds up between hot and cold regions. The strength of the field increases with the static dielectric constant or with decreasing temperature. At near standard conditions, the thermal polarization coefficient is ∼-0.6 mV/K, making it possible to induce significant electrostatic fields, ∼103 V/m, with thermal gradients ∼1 K/μm. At supercritical conditions, ∼600 K and 0.249 g/cm3 (the critical isochore), the electrostatic field is of the same order, despite the low dielectric constant of the fluid. In this case, the electrostatic field is determined by the enhanced rotational diffusion of the molecules and stronger cross-coupling between heat and polarization fluxes. We show that the coupling between the heat and polarization fluxes influences the thermal conductivity of acetonitrile, which becomes a worse heat conductor. For the thermodynamic states investigated in this work, the thermal polarization effect leads to a ∼2%-5% reduction in thermal conductivity.
Di Lecce S, Albrecht T, Bresme F, 2020, Taming the thermodiffusion of alkali halide solutions in silica nanopores, Nanoscale, Vol: 12, Pages: 23626-23635, ISSN: 2040-3364
Thermal fields give rise to thermal coupling phenomena, such as mass and charge fluxes, which are useful in energy recovery applications and nanofluidic devices for pumping, mixing or desalination. Here we use state of the art non-equilibrium molecular simulations to quantify the thermodiffusion of alkali halide solutions, LiCl and NaCl, confined in silica nanopores, targeting diameters of the order of those found in mesoporous silica nanostructures. We show that nanoconfinement modifies the thermodiffusion behaviour of the solution. Under confinement conditions, the solutions become more thermophilic, with a preference to accumulate at hot sources, or thermoneutral, with the thermodiffusion being inhibited. Our work highlights the importance of nanoconfinement in thermodiffusion and outlines strategies to tune mass transport at the nanoscale, using thermal fields.
Di Lecce S, Kornyshev AA, Urbakh M, et al., 2020, Lateral ordering in nanoscale ionic liquid films between charged surfaces enhances lubricity., ACS Nano, Vol: 14, Pages: 13256-13267, ISSN: 1936-0851
Electric fields modify the structural and dynamical properties of room temperature ionic liquids (RTILs) providing a physical principle to develop tunable lubrication devices. Using nonequilibrium molecular dynamics atomistic simulations, we investigate the impact of the composition of imidazolium RTILs on the in-plane ordering of ionic layers in nanogaps. We consider imidazolium cations and widely used anions featuring different molecular structures, spherical ([BF4]-), elongated surfactant-like ([C2SO4]-), and elongated with a more delocalized charge ([NTf2]-). The interplay of surface charge, surface polarity, and anion geometry enables the formation of crystal-like structures in [BF4]- and [NTf2]- nanofilms, while [C2SO4]- nanofilms form disordered layers. We study how the ordering of the ionic liquid lubricant in the nanogap affects friction. Counterintuitively, we find that the friction force decreases with the ability of the RTILs to form crystal-like structures in the confined region. The crystallization can be activated or inhibited by changing the polarity of the surface, providing a mechanism to tune friction with electric fields.
Carter JW, Gonzalez MA, Brooks NJ, et al., 2020, Flip-flop asymmetry of cholesterol in model membranes induced by thermal gradients, Soft Matter, Vol: 16, Pages: 5925-5932, ISSN: 1744-683X
Lipid asymmetry is a crucial property of biological membranes and significantly influences their physical and mechanical properties. It is responsible for maintaining different chemical environments on the external and internal surfaces of cells and organelles and plays a vital role in many biological processes such as cell signalling and budding. In this work we show, using non-equilibrium molecular dynamics (NEMD) simulations, that thermal fields can induce lipid asymmetry in biological membranes. We focus our investigation on cholesterol, an abundant lipid in the plasma membrane, with a rapid flip-flop rate, significantly influencing membrane properties. We demonstrate that thermal fields induce membrane asymmetry with cholesterol showing thermophobic behaviour and therefore accumulating on the cold side of the membrane. This work highlights a possible experimental route to preparing and controlling asymmetry in synthetic membranes.
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