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Thermo-osmotic flows - flows generated in micro and nanofluidic systems by thermal gradients - could provide an alternative approach to harvest waste heat. However, such use would require massive thermo-osmotic flows, which are up to now only predicted for special and expensive materials. There is thus an urgent need to design affordable nanofluidic systems displaying large thermo-osmotic coefficients. In this paper we propose a general model for thermo-osmosis of aqueous electrolytes in charged nanofluidic channels, taking into account hydrodynamic slip, together with the different solvent and solute contributions to the thermo-osmotic response. We apply this model to a wide range of systems, by studying the effect of wetting, salt type and concentration, and surface charge. We show that intense thermo-osmotic flows can be generated using slipping charged surfaces. We also predict for intermediate wettings a transition from a thermophobic to a thermophilic behavior depending on the surface charge and salt concentration. Overall, this theoretical framework opens an avenue for controlling and manipulating thermally induced flows with common charged surfaces and a pinch of salt., 7 pages, 3 figures

Among all fluids, water has always been of special concern for scientists from a broad variety of research fields due to its rich behavior. In particular, some questions remain unanswered nowadays concerning the temperature dependence of bulk and interfacial transport properties of supercooled and liquid water, e.g. regarding the fundamentals of the violation of the Stokes-Einstein relation in the supercooled regime or the subtle relation between structure and dynamical properties. Here we investigated the temperature dependence of the bulk transport properties from ab initio molecular dynamics based on density functional theory, down to the supercooled regime. We determined from a selection of functionals, that SCAN better describes the experimental viscosity and self-diffusion coefficient, although we found disagreements at the lowest temperatures. For a limited set of temperatures, we also explored the role of nuclear quantum effects on water dynamics using ab initio molecular dynamics that has been accelerated via a recently introduced machine learning approach. We then investigated the molecular mechanisms underlying the different functionals performance and assessed the validity of the Stokes-Einstein relation. We also explored the connection between structural properties and the transport coefficients, verifying the validity of the excess entropy scaling relations for all the functionals. These results pave the way to predict the transport coefficients from the radial distribution function, helping to develop better functionals. On this line, they indicate the importance of describing the long-range features of the radial distribution function., Comment: 10 pages, 5 figures

The discovery of ultrastable glasses raises novel challenges about glassy systems. Recent experiments studied the macroscopic devitrification of ultrastable glasses into liquids upon heating but lacked microscopic resolution. We use molecular dynamics simulations to analyze the kinetics of this transformation. In the most stable systems, devitrification occurs after a very large time, but the liquid emerges in two steps. At short times, we observe the rare nucleation and slow growth of isolated droplets containing a liquid maintained under pressure by the rigidity of the surrounding glass. At large times, pressure is released after the droplets coalesce into large domains, which accelerates devitrification. This two-step process produces pronounced deviations from the classical Avrami kinetics and explains the emergence of a giant lengthscale characterizing the devitrification of bulk ultrastable glasses. Our study elucidates the nonequilibrium kinetics of glasses following a large temperature jump, which differs from both equilibrium relaxation and aging dynamics, and will guide future experimental studies.

Nanofluidics shows great promise for energy conversion and desalination applications. The performance of nanofluidic devices is controlled by liquid-solid friction, quantified by the Navier friction coefficient (FC). Despite decades of research, there is no well-established generic framework to determine the frequency dependent Navier FC from atomistic simulations. Here, we have derived analytical expressions to connect the Navier FC to the random force autocorrelation on the confining wall, from the observation that the random force autocorrelation can be related to the hydrodynamic boundary condition, where the Navier FC appears. The analytical framework is generic in the sense that it explicitly includes the system size dependence and also the frequency dependence of the FC, which enabled us to address (i) the long-standing plateau issue in the evaluation of the FC and (ii) the non-Markovian behavior of liquid-solid friction of a Lennard-Jones liquid and of water on various walls and at various temperatures, including the supercooled regime. This new framework opens the way to explore the frequency dependent FC for a wide range of complex liquids., Comment: 5 pages, 3 figures

Introduction: The prevalence of heart failure is on the rise. In its advanced stage (AHF) the condition manifests with incapacitating symptoms. Exacerbations, frequent admissions, and high morbidity and mortality represent conditions amenable to palliative care. A cardiology palliative care (CPC) program was designed to monitor symptoms, deepen communication, establish advanced directives, and adjust therapeutic efforts. The goal of the study was to compare the care of patients who died from AHF before and after CPC program implementation. Material and methods: This was an observational, comparative study. CPC was offered to patients with AHF, and these subjects were compared to a control group without CPC because of unavailability (preCPC). In the statistical analysis categorical variables were evaluated using non-parametric tests. Continuous variables were compared using the Mann-Whithey U-test. Results: Seventy-seven patients were included in the preCPC group, and 65 in the CPC group. They all had similar demographic characteristics: age (median), 75 years; male gender, 70 %. A decrease in duration (27.3 vs. 7.2; p < 0.001) and in number of hospital admissions (2.64 vs. 1.51; p < 0.001) was found in the CPC group as compared to the control group. Invasive procedures within the final 5 days of life predominated in the preCPC group; assisted mechanical ventilation (52.7 % vs. 7.8 %; p < 0.001), hemodialysis (41 % vs. 5.5 %; p < 0.001), cardiopulmonary resuscitation (88.2 % vs. 11 %; p < 0.001), central vascular access (75.7 % vs. 26.5 %, p < 0.01), and use of inotropics (70.5 % vs. 20 %; p < 0.001). Non-invasive, comfort interventions predominated in the CPC group: palliative sedation (3 % vs. 25 %), opioid use (35 % vs. 58.3 %), deactivation of implantable cardiac defibrillator (0 vs. 37,5 %), and do-not-resuscitate orders (3 % vs. 77 %; p < 0.001). Mortality in critical care settings predominated in the preCPC group (51 % vs. 26 %; p < 0.005). Discussion: CPC improves quality of care for patients with AHF, and ensures adequacy of therapeutic efforts, thus preventing disproportionate, unnecessary interventions.

The search for materials with high thermal resistance has promising applications in thermoelectric devices and boiling crisis retardation. In this paper, we study the interfacial heat transfer between water and gold, nanostructuring the gold surface and coating it with graphene. By trapping air (or vacuum in our simulations) between graphene and the nanopatterned surface, we observe a considerable increase in the interfacial resistance compared to the planar gold situation, which is shown to scale with the effective graphene–gold contact surface for both monolayer and multilayer graphene. With the massive thermal resistances we predict (up to 200 nm in terms of Kapitza length), the system proposed here represents a robust alternative to superhydrophobic Cassie materials. Moreover, since the low thermal conductance is achieved primarily due to geometry (vacuum trapping), it is straightforward to extend our results to any material with a structure equivalent to that of the nanopatterned gold wall considered here.

Nanofluidics is an emerging field offering innovative solutions for energy harvesting and desalination. The efficiency of these applications depends strongly on liquid-solid slip, arising from a favorable ratio between viscosity and interfacial friction. Using molecular dynamics simulations, we show that wall slip increases strongly when water is cooled below its melting point. For water on graphene, the slip length is multiplied by up to a factor of five and reaches $230$nm at the lowest simulated temperature, $T \sim 225$K; experiments in nanopores can reach much lower temperatures and could reveal even more drastic changes. The predicted fast increase in water slip can also be detected at supercoolings reached experimentally in bulk water, as well as in droplets flowing on anti-icing surfaces. We explain the anomalous slip behavior in the supercooled regime by a decoupling between viscosity and bulk density relaxation dynamics, and we rationalize the wall-type dependency of the enhancement in terms of interfacial density relaxation dynamics. By providing fundamental insights on the molecular mechanisms of hydrodynamic transport in both interfacial and bulk water in the supercooled regime, this study is relevant to the design of anti-icing surfaces and it also paves the way to explore new behaviors in supercooled nanofluidic systems., Comment: 8 pages, 4 figures

Cecilia Herrero, Takeshi Omori, Yasutaka Yamaguchi, and Laurent Joly, "Shear force measurement of the hydrodynamic wall position in molecular dynamics", The Journal of Chemical Physics 151, 041103 (2019) https://doi.org/10.1063/1.5111966., Flows in nanofluidic systems are strongly affected by liquid-solid slip, which is quantified by the slip length and by the position where the slip boundary condition applies. Here, we show that the viscosity, slip length, and hydrodynamic wall position (HWP) can be accurately determined from a single molecular dynamics (MD) simulation of a Poiseuille flow, after identifying a relation between the HWP and the wall shear stress in that configuration. From this relation, we deduce that in gravity-driven flows, the HWP identifies with the Gibbs dividing plane of the liquid-vacuum density profile. Simulations of a generic Lennard-Jones liquid confined between parallel frozen walls show that the HWP for a pressure-driven flow is also close to the Gibbs dividing plane (measured at equilibrium), which therefore provides an inexpensive estimate of the HWP, going beyond the common practice of assuming a given position for the hydrodynamic wall. For instance, we show that the HWP depends on the wettability of the surface, an effect usually neglected in MD studies of liquid-solid slip. Overall, the method introduced in this article is simple, fast, and accurate and could be applied to a large variety of systems of interest for nanofluidic applications.

Se presenta un modelo en dos pasos que predice la probabilidad de presencia de necromasa en pinares de repoblación y rebollares del norte de España y cuantifica el área basimétrica de los árboles muertos en pie y el volumen de los árboles caídos. Por otra parte se presenta un modelo lineal que cuantifica la fijación de carbono en la necromasa caída en el suelo.