174 results found
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.
Gonzalez MA, Bresme F, 2020, Membrane-ion interactions modify the lipid flip-flop dynamics of biological membranes: a molecular dynamics study, The Journal of Physical Chemistry B: Biophysical Chemistry, Biomaterials, Liquids, and Soft Matter, Vol: 124, Pages: 5156-5162, ISSN: 1520-5207
The asymmetric distribution of lipids in the cell membrane is maintained by protein transporters, and in the absence of proteins, by spontaneous flip-flop of lipids that involve the formation of transient pores in the membrane. The composition of the cell membranes influences the metabolism of the cell by modulating the function of transmembrane proteins, and flip-flop processes are therefore of key importance. Membranes are in direct contact with aqueous solutions, containing ions of different compositions that interact with the lipid head groups or cross the cell membrane through transmembrane channels. The impact of the ions on the lipid flip-flop rates is, however, an outstanding question. Here, we show that the flip-flop rate slows down significantly with the increasing valence of the cation, from minutes for monovalent cations (Na+, K+) to hours for divalent (Mg2+, Ca2+) or days for trivalent cations (Yb3+).
Zheng L, Rucker M, Bultreys T, et al., 2020, Surrogate models for studying the wettability of nanoscale natural rough surfaces using molecular dynamics, Energies, Vol: 13, ISSN: 1996-1073
A molecular modeling methodology is presented to analyze the wetting behavior of natural surfaces exhibiting roughness at the nanoscale. Using atomic force microscopy, the surface topology of a Ketton carbonate is measured with a nanometer resolution, and a mapped model is constructed with the aid of coarse-grained beads. A surrogate model is presented in which surfaces are represented by two-dimensional sinusoidal functions defined by both an amplitude and a wavelength. The wetting of the reconstructed surface by a fluid, obtained through equilibrium molecular dynamics simulations, is compared to that observed by the different realizations of the surrogate model. A least-squares fitting method is implemented to identify the apparent static contact angle, and the droplet curvature, relative to the effective plane of the solid surface. The apparent contact angle and curvature of the droplet are then used as wetting metrics. The nanoscale contact angle is seen to vary significantly with the surface roughness. In the particular case studied, a variation of over 65° is observed between the contact angle on a flat surface and on a highly spiked (Cassie–Baxter) limit. This work proposes a strategy for systematically studying the influence of nanoscale topography and, eventually, chemical heterogeneity on the wettability of surfaces.
Olarte-Plata JD, Brekke-Svaland G, Bresme F, 2020, The influence of surface roughness on the adhesive interactions and phase behavior of suspensions of calcite nanoparticles, Publisher: ROYAL SOC CHEMISTRY
Olarte-Plata JD, Bresme F, 2020, Orientation of Janus particles under thermal fields: The role of internal mass anisotropy., Journal of Chemical Physics, Vol: 152, Pages: 204902-204902, ISSN: 0021-9606
Janus particles (JPs) are a special kind of colloids that incorporate two hemispheres with distinct physical properties. These particles feature a complex phase behavior, and they can be propelled with light by heating them anisotropically when one of the hemispheres is metallic. It has been shown that JPs can be oriented by a homogeneous thermal field. We show using multiscale simulations and theory that the internal mass gradient of the JPs can enhance and even reverse the relative orientation of the particle with the thermal field. This effect is due to a coupling of the internal anisotropy of the particle with the heat flux. Our results help rationalize previous experimental observations and open a route to control the behavior of JPs by exploiting the synergy of particle-fluid interactions and particle internal mass composition.
Olarte-Plata JD, Brekke-Svaland G, Bresme F, 2020, The influence of surface roughness on the adhesive interactions and phase behavior of suspensions of calcite nanoparticles., Nanoscale, Vol: 12, Pages: 11165-11173, ISSN: 2040-3364
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.
Tascini AS, Wang S, Seddon JM, et al., 2020, Fats’ love–hate relationships: a molecular dynamics simulation and hands-on experiment outreach activity to introduce the amphiphilic nature and biological functions of lipids to young students and the general public, Journal of Chemical Education, Vol: 97, Pages: 1360-1367, ISSN: 0021-9584
Lipids are fundamental components of biological organisms and have important applications in the pharmaceutical, food, and cosmetics industries. Thus, it is important that young students and the general public properly understand the basic properties of lipids and how these relate to their biological and industrial roles. Here, we use molecular dynamics computer simulations and a simple, safe, and inexpensive popular hands-on activity, to communicate to participants why and how lipid molecules play a fundamental role in all living organisms and in our bodies. The activity is called “Fats’ Love–Hate Relationships”, to highlight how the different parts of amphiphilic lipids interact with water. This “love–hate relationship” is vital to the biological functions of lipids and drives the formation of lipid structures that can be visualized at molecular scale with the computer simulations. The participants were encouraged to investigate the interactions between milk lipids and soap surfactants, creating beautiful complex artwork that they could then take home. The hands-on activity was accompanied by a video of a molecular simulation that illustrates milk–soap interactions at a molecular scale and helps to explain how the amphiphilicity of lipids creates the beautiful artwork at a molecular level. The outreach activity has been performed in science festivals and in classrooms and has been well received by participants of all ages with multiple learner comprehension levels (primary and secondary school students and the general public). By combining molecular simulation, explanations of the amphiphilic structure of the lipids, and an engaging hands-on activity, we explained how lipids interact with water and surfactants and inspired discussions on the link between the structure of the lipids and their biological function, namely, their structural and protective roles as a key component of cell membranes.
Muller E, Trusler J, Bresme F, et al., 2020, Employing SAFT coarse grained force fields for the molecular simulation of thermophysical and transport properties of CO2 – n-alkane mixtures, Journal of Chemical and Engineering Data, Vol: 65, Pages: 1159-1171, ISSN: 0021-9568
We report an assessment of the predictive and correlative capability of the SAFT coarse-grained force field as applied to mixtures of CO2 with n-decane and n-hexadecane. We obtain the pure and cross-interaction parameters by matching simulations to experimental phase equilibrium behavior and transfer these parameters to predict shear viscosities. We apply both equilibrium (based on the Green–Kubo formulation) and nonequilibrium (based on the application of an external force to generate an explicit velocity field) algorithms. Single- and two-site models are explored for CO2, and while for volumetric properties both models provide good results, only the model that aligns with the molecular shape is found to be robust when describing highly asymmetric binary mixtures over wide ranges of temperature and pressure. While the models provide good quantitative predictions of viscosity, deviations among the algorithms and with experimental data are encountered for binary mixtures involving longer chain fluids, and in particular at high-pressure and low-temperature states.
Di Lecce S, Kornyshev AA, Urbakh M, et al., 2020, Electrotunable lubrication with ionic liquids: the effects of cation chain length and substrate polarity., ACS Applied Materials and Interfaces, Vol: 12, Pages: 4105-4113, ISSN: 1944-8244
Electrotunable lubrication with ionic liquids (ILs) provides dynamic control of friction with the prospect to achieve superlubrication. We investigate the dependence of the frictional and structural forces with 1-n,2-methyl-imidazolium tetrafluoroborate [C n MIM]+[BF4]- (n = 2, 4, 6) ILs as a lubricant on the molecular structure of the liquid, normal load, and polarity of the electrodes. Using non-equilibrium molecular dynamics simulations and coarse-grained force-fields, we show that the friction force depends significantly on the chain length of the cation. ILs containing cations with shorter aliphatic chains show lower friction forces, ∼40% for n = 2 as compared to the n = 6 case, and more resistance to squeeze-out by external loads. The normal load defines the dynamic regime of friction, and it determines maxima in the friction force at specific surface charges. At relatively low normal loads, ∼10 MPa, the velocity profile in the confined region resembles a Couette type flow, whereas at high loads, >200 MPa, the motion of the ions is highly correlated and the velocity profile resembles a "plug" flow. Different dynamic regimes result in distinctive slippage planes, located either at the IL-electrode interface or in the interior of the film, which ultimately lead, at high loads, to the observation of maxima in the friction force at specific surface charge densities. Instead, at low loads the maxima are not observed, and the friction is found to monotonously increase with the surface charge. Friction with [C n MIM]+[BF4]- as a lubricant is reduced when the liquid is confined between positively charged electrodes. This is due to better lubricating properties and enhanced resistance to squeeze out when the anion [BF4]- is in direct contact with the electrode.
Fajardo OY, Di Lecce S, Bresme F, 2020, Molecular dynamics simulation of imidazolium CnMIM-BF4 ionic liquids using a coarse grained force-field., Physical Chemistry Chemical Physics, Vol: 22, Pages: 1682-1692, ISSN: 1463-9076
Ionic liquids feature thermophysical properties that are of interest in solvents, energy storage materials and tunable lubrication applications. Here we use new Coarse Grained (CG) models to investigate the structure, dynamics and interfacial properties of the [C2-8MIM][BF4] family of ionic liquids (ILs). The simulated equation of state and diffusion coefficients are in good agreement with experimental data and with all-atom force-fields. We quantify the nano-structure and liquid-vapour interfacial properties of the ILs as a function of the size of the imidazolium cation. The computational efficiency of the CG models enables the simulation of very long time scales (100's of nanoseconds), which are needed to resolve the dynamic and interfacial properties of ILs containing cations with long aliphatic chains. For [C>4MIM] [BF4] the break in symmetry associated to the liquid-vapour interface induces nanostructuring of polar and non-polar domains in the direction perpendicular to the interface plane, with the inhomogeneous regions penetrating deep inside the bulk liquid, typically 5 nm for C8MIM cations.
Pivnic K, Bresme F, Kornyshev AA, et al., 2019, Structural forces in mixtures of ionic liquids with organic solvents, Langmuir: the ACS journal of surfaces and colloids, Vol: 35, Pages: 15410-15420, ISSN: 0743-7463
Using molecular dynamics simulations, we study the impact of electrode charging and addition of solvent (acetonitrile, ACN) on structural forces of the BMIM PF6 ionic liquid (IL) confined by surfaces at nanometer separations. We establish relationships between the structural forces and the microscopic structure of the confined liquid. Depending on the structural arrangements of cations and anions across the nanofilm, the load-induced squeeze-out of liquid layers occurs via one-layer or bilayer steps. The cations confined between charged plates orient with their aliphatic chain perpendicular to the surface planes and link two adjacent IL layers. These structures facilitate the squeeze-out of single layers. For both pure IL and IL-ACN mixtures, we observe a strong dependence of nanofilm structure on the surface charge density, which affects the simulated pressure–displacement curves. Addition of solvent to the IL modifies the layering in the confined film. At high electrode charges and high dilution of IL (below 10% molar fraction), the layered structure of the nanofilm is less well defined. We predict a change in the squeeze-out mechanism under pressure, from a discontinuous squeeze-out (for high IL concentrations) to an almost continuous one (for low IL concentrations). Importantly, our simulations show that charged electrodes are coated with ions even at low IL concentrations. These ion-rich layers adjacent to the charged plate surfaces are not squeezed out even under very high normal pressures of ∼5 GPa. Hence, we demonstrate the high performance of IL–solvent mixtures to protect surfaces from wear and to provide lubrication at high loads.
Bresme F, Vesovic V, Bataller H, et al., 2019, Topical issue on thermal non-equilibrium phenomena in soft matter, The European Physical Journal E: soft Matter and Biological Physics, Vol: 42, ISSN: 1292-8941
Zheng L, Trusler JPM, Bresme F, et al., 2019, Predicting the pressure dependence of the viscosity of 2,2,4-trimethylhexane using the SAFT coarse-grained force field, Fluid Phase Equilibria, Vol: 496, Pages: 1-6, ISSN: 0378-3812
This work is framed within AIChE's 10th Industrial Fluid Properties Simulation Challenge, with the aim of assessing the capability of molecular simulation methods and force fields to accurately predict the pressure dependence of the shear viscosity of 2,2,4-trimethylhexane at 293.15 K (20 °C) at pressures up to 1 GPa. In our entry for the challenge, we employ coarse-grained intermolecular models parametrized via a top-down technique where an accurate equation of state is used to link the experimentally-observed macroscopic volumetric properties of fluids to the force-field parameters. The state-of-the-art version of the statistical associating fluid theory (SAFT) for potentials of variable range as reformulated in the Mie incarnation is employed here. The potentials are used as predicted by the theory, with no fitting to viscosity data. Viscosities are calculated by molecular dynamics (MD) employing two independent methods; an equilibrium-based procedure based on the analysis of the pressure fluctuations through a Green-Kubo formulation and a non-equilibrium method where periodic perturbations of the boundary conditions are employed to simulate experimental shear stress conditions. There is an indication that, at higher pressures, the model predicts a solid phase (freezing) which we believe to be an artefact of the simplified molecular geometry used in the modelling. A comparison (made after disclosure of the experimental data) show that the model consistently underpredicts the viscosity by about 30%, but follows the pressure dependency accurately.
Gittus OR, Olarte-Plata JD, Bresme F, 2019, Thermal orientation and thermophoresis of anisotropic colloids: the role of the internal composition, European Physical Journal E, Vol: 42, ISSN: 1292-8941
The drift motion experienced by colloids immersed in a fluid with an intrinsic temperature gradient is referred to as thermophoresis. An anisotropic mass distribution inside colloidal particles imparts a net orientation with respect to the applied thermal field, and in turn alters the thermophoretic response of the colloids. This rectification of the rotational Brownian motion is called thermal orientation. To explore the sensitivity of the thermal orientation effect with the internal composition of colloids, we investigate the thermophoretic response of rod-like colloids in the dilute regime, targeting different internal mass distributions. We derive phenomenological equations to model the dependence of the Soret coefficient with degree of mass anisotropy and test these equations using non-equilibrium molecular dynamics simulations. Using both theory and simulation, we show that the average orientation and the Soret coefficients of the colloids can depend significantly on the internal mass distribution. This observation suggests an approach to identify internal colloidal compositions using thermal gradients as sensing probes.
Di Lecce S, Bresme F, 2019, Soret coefficients and thermal conductivities of alkali halide aqueous solutions via non-equilibrium molecular dynamics simulations, Molecular Simulation, Vol: 45, Pages: 351-357, ISSN: 0892-7022
Thermal gradients induce thermodiffusion, the Ludwig–Soret effect, which might be exploited in biotechnological, chemical, micro and nanofluidic applications. It has been shown that thermodiffusion depends very sensitively on the nature of the solute, suggesting that water–solute interactions play an important role in determining the preference of solutes to move towards hot or cold regions. Here we employ non-equilibrium molecular dynamics computations to gain insight into the role of water–ion interactions on the strength and sign of the Soret coefficient of alkali halide solutions. By performing simulations with different force-fields, we draw conclusions on the dependence of the thermodiffusive response with the properties of the first hydration shell of the ions. We further compute the thermal conductivity of aqueous solutions. State-of-the art force-fields reproduce the decrease of the thermal conductivity with increasing salt concentration when the thermal conductivity of pure water is corrected to match experimental data.
Tascini AS, Noro MG, Seddon JM, et al., 2019, Mechanisms of lipid extraction from skin lipid bilayers by sebum triglycerides, Physical Chemistry Chemical Physics, Vol: 21, Pages: 1471-1477, ISSN: 1463-9076
The skin surface, our first barrier against the external environment, is covered by the sebum oil, a lipid film composed of sebaceous and epidermal lipids, which is important in the regulation of the hydration level of our skin. Here, we investigate the pathways leading to the transfer of epidermal lipids from the skin lipid bilayer to the sebum. We show that the sebum triglycerides, a major component of sebum, interact strongly with the epidermal lipids and extract them from the bilayer. Using microsecond time scale molecular dynamics simulations, we identify and quantify the free energy associated with the skin lipid extraction process.
Olarte-Plata JD, Bresme F, 2019, Theoretical description of the thermomolecular orientation of anisotropic colloids, Physical Chemistry Chemical Physics, Vol: 21, Pages: 1131-1140, ISSN: 1463-9076
Thermal fields bring new opportunities to manipulate colloidal suspensions. Mass anisotropy inside the colloid leads to the thermal orientation effect and to a non-monotonic dependence of the thermophoretic force with the mass of the colloid. We show here that the thermal orientation of these anisotropic colloids can be described using the von Mises probability distribution. We derive equations that link the orientation to the internal degrees of freedom of the colloid, and test these equations using both atomistic and mesoscopic stochastic rotation dynamics simulations. Our approach can be used to describe the thermophoretic response of anisotropic colloids as a function of their size and composition.
Bresme F, Robotham O, Chio W-IK, et al., 2018, Debye screening, overscreening and specific adsorption in solutions of organic ions, Physical Chemistry Chemical Physics, Vol: 20, Pages: 27684-27693, ISSN: 1463-9076
Tetrabutylammonium (TBA) and tetraphenylborate (TPB) ions dissolved in dichloroethane (DCE) are widely used in electrochemistry of liquid–liquid interfaces. Unlike alkali halide solutions in water, TBA–TPB–DCE solutions feature large organic ions and a solvent with a dielectric constant almost one order of magnitude lower than that of water. This is expected to dramatically amplify the impact of ionic correlations in the properties of the solution. Here we report atomistic simulations of TBA–TPB–DCE solutions and analyze ion correlations, clustering, and charge screening effects. We target concentrations in the range of 0.01–0.25 molal (m), hence exploring concentration regimes typical for many experimental investigations. We show that the transition from monotonic to oscillatory decay of the charge density, which signals the onset of strong ion correlations, takes place in this concentration interval, leading to overscreening effects. Furthermore, we investigate the distribution and adsorption of ions at the DCE–air interface. Unlike what is observed for small inorganic ions in water at similar concentrations, we find that TPB and TBA ions accumulate near the DCE surface, resulting in significant interfacial clustering and adsorption at concentrations ∼0.25 m. TPB ions adsorb more strongly leading to free energy wells of ∼1–2 kBT. The adsorption modifies significantly the electrostatic potential of the DCE–air interface, which undergoes a shift of 0.2 V in going from pure DCE to TBA–TPB–DCE solutions at 0.25 m.
Zhang H, Chen S, Guo Z, et al., 2018, Contact line pinning effects influence determination of the line tension of droplets adsorbed on substrates, Journal of Physical Chemistry C, Vol: 122, Pages: 17184-17189, ISSN: 1932-7447
The precise determination of the line tension of sessile droplets still represents a major challenge. At present, the estimates of the line tension from contact angle measurements can differ by 4–5 orders of magnitude. Here we show that the pinning effect of the droplet contact line caused by the substrate inhomogeneities influences the apparent contact angle of the droplet, affecting the determination of the line tension via the corrected Young’s equation. We introduce the contribution of pinning effects into the Gibbs free energy differential and derive a modified version of the Young’s equation. Using classical density functional theory, we isolate the line tension and the pinning force contributions for substrates with different heterogeneity. The pinning effect leads to metastability of wetting states and influences the contact angle, hence introducing errors in the estimation of line tensions using the traditional analysis of contact angles, based on the modified Young’s equation.
Font F, Bresme F, 2018, Transient melting at the nanoscale: a continuum heat transfer and nonequilibrium molecular dynamics approach, The Journal of Physical Chemistry C, Vol: 122, Pages: 17481-17489, ISSN: 1932-7447
Transient melting is an ubiquitous phenomenon in nature, which plays an increasingly important role in the processing of nanomaterials. A sound theoretical description of this process is therefore important, both from fundamental and applied points of view. We present a numerical study of transient melting in simple atomic solids using both, continuum theory based on the heat diffusion equation and transient nonequilibrium molecular dynamics simulations. We show that continuum theory provides an accurate description of relevant properties, temperature relaxation, time-dependent internal energy, and dynamics of the melting front. However, deviations between the continuum approach and the molecular dynamics simulations are observed in picosecond time scales depending on the initial temperature used to melt the solid. These deviations are due to the emergence of new time scales associated with the activated character of the melting process. Consistently with this notion, we observe that the closer the initial temperature to the melting temperature of the solid, the longer the time it takes for the system to converge to the continuum solution. For systems investigated here we find a delay in the recovery of the continuum solution of ≲5 to ≲80 ps for initial temperatures between 40 and 25% above the melting temperature of the solid, respectively. We find that the combination of continuum theory and molecular dynamics simulations provides a useful approach to quantify the temperature relaxation and the melting temperature of materials using short molecular dynamics trajectories.
Olarte-Plata J, Rubi JM, Bresme F, 2018, Thermophoretic torque in colloidal particles with mass asymmetry, Physical Review E, Vol: 97, ISSN: 1539-3755
We investigate the response of anisotropic colloids suspended in a fluid under a thermal field. Using nonequilibrium molecular dynamics computer simulations and nonequilibrium thermodynamics theory, we show that an anisotropic mass distribution inside the colloid rectifies the rotational Brownian motion and the colloids experience transient torques that orient the colloid along the direction of the thermal field. This physical effect gives rise to distinctive changes in the dependence of the Soret coefficient with colloid mass, which features a maximum, unlike the monotonic increase of the thermophoretic force with mass observed in homogeneous colloids.
Olarte-Plata JD, Bresme F, 2018, Microscopic relationship between colloid-colloid interactions and the rheological behaviour of suspensions: a molecular dynamics-stochastic rotation dynamics investigation, MOLECULAR PHYSICS, Vol: 116, Pages: 2032-2040, ISSN: 0026-8976
We investigate the dependence of the shear viscosity of suspensions of spherical colloids as a function of the volume fraction of the suspension, the colloid–colloid interactions and the shear rate. We couple molecular dynamics to describe the motion of the colloids with stochastic rotation dynamics (MD–SRD) for the fluid environment by means of stochastic collisions, in order to incorporate hydrodynamics effects leading to non-newtonian responses. The shear viscosity is computed using non-equilibrium simulations by imposing explicit velocity gradients. The impact of the colloid–colloid interactions is examined by modelling the inter-colloid pair potential with a repulsive power law, that allows interpolating different degrees of colloidal softness. The general rheological behaviour of our suspensions can be described with a Krieger–Dougherty like equation, which must be corrected to account for the variations in the maximum packing fraction and non-equilibrium effects arising from the flux of momentum imposed to the suspension, which appear when varying the softness of the inter-colloidal interactions. We further show evidence for non-newtonian behaviour at high Péclet numbers, characterised both by shear thinning and shear thickening, and thus demonstrate these effects can be successfully captured using MD–SRD methods.
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