Hideo Fujimura

Analysis on wetting and local dynamic properties of single water droplet on a polarized solid surface: A molecular dynamics study

Molecular dynamics simulations of single water droplets on a solid surface were carried out in order to investigate the effects that the Coulomb interaction between liquid and solid molecules has on wetting behavior by appending vertical electric polarization on a solid surface. The water droplet became more wettable both on upward and downward polarized surfaces, although structures of the adsorption layer appearing near the solid surface were clearly different, and the relation between droplet contact angle and surface polarization was also different for upward and downward polarization directions. The probability density distribution of molecular orientation around the adsorption layer indicated that preferable water molecule orientations varied largely by the surface polarization, and the rotational mobility around the preferable orientations was also affected. The dynamic property due to this rotational mobility was clearly captured by means of distribution of rotational diffusion coefficient, which potentially corresponded to local viscosity distribution.

Molecular dynamics analysis on wetting and interfacial properties of water-alcohol mixture droplets on a solid surface.

Molecular dynamics simulations of single water, water-methanol, or water-IPA (isopropyl-alcohol) mixture droplets on a solid surface were performed with various mixture ratios. An increase in alcohol fraction generally gave an increase in droplet wettability. Both methanol and IPA molecules showed a strong preference to gather at various interfaces, with methanol molecules also showing a tendency to diffuse into the droplet bulk. Specific interfacial tensions were investigated using quasi-one-dimensional simulation systems, and liquid-vapor and solid-liquid interfacial tensions were found to decrease greatly due to the presence of interfacial alcohol, while solid-vapor interfacial tensions were proved to have little influence on wettability. Youngs relation was found to hold quantitatively well for both water-methanol and water-IPA droplets. The validity of using Bakkers equation on solid-liquid interfaces was also investigated, and it was shown that for tightly spaced crystal surfaces, the introduced uncertainly is small.

Molecular dynamics analysis of multiphase interfaces based on in situ extraction of the pressure distribution of a liquid droplet on a solid surface

Molecular dynamics simulations of a nanoscale liquid droplet on a solid surface are carried out in order to examine the pressure tensor field around the multiphase interfaces, and to explore the validity of Youngs equation. By applying the virial theorem to a hemicylindrical droplet consisting of argon molecules on a solid surface, two-dimensional distribution of the pressure tensor is obtained. Tensile principal pressure tangential to the interface is observed around the liquid-vapor transition layer, while both tensile and compressive principal pressure tangential to the interface exists around the solid-liquid transition layer due to the inhomogeneous density distribution. The two features intermix inside the overlap region between the transition layers at the contact line. The contact angle is evaluated by using a contour line of the maximum principal pressure difference. The interfacial tensions are calculated by using Bakkers equation and Young-Laplace equation to the pressure tensor distribution. The relation between measured contact angle and calculated interfacial tensions turns out to be consistent with Youngs equation, which is known as the description of the force balance at the three-phase interface.

Molecular Analysis on the Dynamic Properties of Water Droplet at Solid-Liquid Interface Based on MD Simulations

Molecular dynamics simulations of single water or water-IPA (Isopropyl-alcohol) mixture droplets on a solid surface were performed. The interaction for solid-water molecules was modeled as the combination of L-J and Coulomb potentials, and the effects of Coulomb interaction were independently investigated by appending arbitrary electric charge as a parameter on the solid surface. The water droplet became more wettable both with positive and negative surface charges as the absolute value of the electric charge increased, and the cosine of the contact angle was roughly a linear function of the absolute value of the electric charge although the correlation was obviously different between positive and negative charge values. Multiple molecular orientations seemed possible as local equilibrium states near the adsorption layer on negatively charged surface, and the mean rotational diffusion was higher there. On the other hand, mean rotational diffusion was reduced in the vicinity of the positively charged surface.Copyright