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.
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.
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%.
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