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Molecular dynamics simulations describing the equilibrium shape of a nanodroplet located on the solid substrate are presented for the cases of a “cylindrical water droplet” on silicon substrates. Several examples of the structuration of the solid substrate surface are simulated, i.e.: atomistic flat substrate and substrates with ordered nanopillars and nanopores. The adhesives forces between molecules of the substrate and the fluid are modified to change the wettability. Three wetting configurations are considered in this work for the smooth surface: (i) hydrophilic (0 = 30∘), (ii) hydrophobic (0 = 136∘), and (iii) an intermediate regime (0 = 80∘). Further, the dependence of the wetting angle as a function of the surface state is studied in details for the above-mentioned configurations., Робота присвячена дослiдженню рiвноважної форми нанокраплi, розташованої на твердiй наноструктурованiй пiдкладинцi, зокрема, було розглянуто випадок “цилiндричної краплi води” на кремнiєвiй пiдкладинцi. Було змодельовано декiлька варiантiв структурування поверхнi твердої пiдкладинки: атомiстично плоска пiдкладинка та пiдкладинка з упорядкованими наностовпчиками та нанопорами. Сили адгезiї мiж молекулами субстрату та рiдини були модифiкованi вiдповiдним чином для того, щоб варiювати змочуванiсть. У данiй роботi було розглянуто три конфiгурацiї змочування поверхнi: 1) гiдрофiльний (0 = 30∘), 2) гiдрофобний (0 = 136∘), та 3) промiжний режим (0 = 80∘). В подальшому залежнiсть кута змочування вiд функцiї стану поверхнi детально вивчалась для вищезгаданих конфiгурацiй.

International audience

The information regarding the wetting characteristic is important in various aspects of surface science. One of the markers that more frequently uses for describing of wetting characteristics is the wetting angle. Even an estimation of the wetting at the macroscale is a tricky issue, because of the hysteresis between receding and advancing values presence. In the same time, the situation is more complicated for a nanoscopic droplet due to the tangible thickness of the interphase region, what causes an uncertainty of the dividing surface determination, as a consequence it causes an uncertainty of the contact angle determination. In this work using methods molecular dynamics we performed simulations of the cylindrical nanodprolet and the layer of the argon fluid. The density maps and the maps of tension tensor were also measured. Basing on the analysis of the maps within the framework of the Gibbs approach, the equimolar surfaces and the surfaces of tension on the liquid-gas interface and the liquid-solid interface was determined. For the cylindrical nanodroplet, the equilibrium contact wetting angles formed by the corresponding dividing surfaces were estimated. The measured angles differ by more than 10%.

The paper is devoted to the study of the evaporation of water sessile drops resting on a smooth surface consisting of a vanadium dioxide thin film (∼150 nm) covering a sapphire substrate. Vanadium dioxide is considered because of the promising applications it is expected to have in new technologies for heat waste recuperation. Water drops are deposited on the substrate heated at different temperatures from 20 to 80°C, and the time dependence of the droplet shape is registered with the use of shadow imaging. This reveals that the three-phase contact line remains pinned during most of the drop lifetime, while the wetting contact angle as well as the drop volume are continuously changing. With the help of image processing, the vaporization rate of the drop is calculated. It is found that it is almost constant with time, and increases exponentially with the temperature of the substrate, well in agreement with theoretical models developed to describe the evaporation of drops with a pinned contact line.

We present an efficient technique for the evaluation of the Gibbs adsorption of a liquid on a solid substrate. The behavior of a water nanodroplet on a silicon surface is simulated with molecular dynamics. An external field with varying strength is applied on the system to tune the solid-liquid interfacial contact area. A linear dependence of droplet's volume as a function of the contact area is observed. We introduce a modified Young-Laplace equation to explain the influence of the Gibbs adsorption on the nanodroplet volume contraction. Fitting of the molecular dynamics results with the analytical approach allows us to evaluate the number of atoms per unit area adsorbed on the substrate, which quantifies the Gibbs adsorption. Thus, a threshold of a droplet size is obtained, for which the impact of the adsorption is crucial. For instance, a water droplet with 5 nm radius has 3% of its molecules adsorbed on silicon substrate, while for droplets less than 1 nm this amount is more than 10%. The presented results could be beneficial for the evaluation of the adsorption impact on the physical-chemical properties of nanohybrid systems with large surface-to-volume ration.

International audience; Spherical and cylindrical water droplets on silicon surface are studied to tune the silicon–oxygen interaction. We use molecular dynamics simulations to estimate the contact angle of two different shaped droplets. We found that the cylindrical droplets are independent of the line tension as their three phases curvature is equal zero. Additionally, we compare an analytical model, taking into account or not the Tolman length and we show that for spherical small size droplets, this length is important to be included, in contrast to cylindrical droplets in which the influence of the Tolman length is negligible. We demonstrate that the usual convenient way to exclude linear tension in the general case can give wrong results. Here, we consider cylindrical droplets, since their contact angle does not depend on the droplet size in the range of few to 10ths of nanometres. The droplets are stabilised due to the periodic boundary conditions. This allows us to propose a new parameterisation for nanoscale droplets, which is independent the size of the droplets or its shape, minimising at the same time the calculation procedure. With the proposed methodology, we can extract the epsilon parameter of the interaction potential between a liquid and a solid from the nanoscaled molecular simulation with only as input the macrosized experimental wetting angle for a given temperature.

We report on the size dependence of the surface tension of a free surface of an isotropic fluid. The size dependence of the surface tension is evaluated based on the Gibbs-Tolman-Koenig-Buff equation for positive and negative values of curvatures and the Tolman lengths. For all combinations of positive and negative signs of curvature and the Tolman length, we succeed to have a continuous function, avoiding the existing discontinuity at zero curvature (flat interfaces). As an example, a water droplet in the thermodynamical equilibrium with the vapor is analyzed in detail. The size dependence of the surface tension and the Tolman length are evaluated with the use of experimental data of the International Association for the Properties of Water and Steam. The evaluated Tolman length of our approach is in good agreement with molecular dynamics and experimental data.