Approximate Solution for the Spreading of a Droplet on a Smooth Solid Surface

Characterization of watermarks formed in nano-carpet effect

In the spreading of a water droplet on an aligned silicon nanorod array surface, a precursor rim was detected moving ahead of the contact line. In this process, nanorods were bundled by the capillary force to form clusters, and a watermark developed on the surface after water evaporated. The size of the watermark, R(p)max, corresponding to the maximum radius of the precursor rim, followed a simple power law relationship with the volume of water droplet omega, R(p)max proportional to omega(beta). The scaling exponent beta increased when the nanorod height decreased, but all in the vicinity of 1/3. This behavior was attributed to the competition of evaporation and spreading of a water droplet during the spreading process. The size of the bundled nanorod cluster formed by the capillary force not only depended on the nanorod height but also on the location in the watermark. The cluster size almost remained as a constant near the center, and then it decreased with the distance from the center. This phenomenon can be qualitatively interpreted through the change of the total free energy during the precursor invading the nanorod array, by considering the contribution from the mechanical energy change due to the bending and clustering of nanorods.

Ion redistribution and meniscus stability at Langmuir monolayer deposition

Considering the deposition of charged Langmuir monolayers, it is necessary to take into account electrostatic interactions between the monolayer and the substrate surface. These interactions depend on the nature of ionizable surface groups and on the ionic composition of the subphase (e.g. pH, multivalent counterions, indifferent electrolytes). The non-uniformity of electric, concentration and hydrodynamic velocity fields leads to the formation of ion concentration profiles near the three-phase contact line during the deposition. This effect is similar to the effect of concentration polarization in membranes and electrode systems. The ions redistribution results in change of the monolayer ionization, adhesion work, dynamic contact angle, and, therefore, in the change of morphology, composition and structure of the deposited monolayer. With increasing withdrawal velocity the meniscus can become unstable because of concentration polarization in the solution.

Modeling drop shape on contaminated surfaces or surfaces with physical structures

Surface contaminants are commonly found on films. They get transferred to the surface from incompletely cured silicone liners on the films or owing to migration of additives to the surface from within the film. During the process of ink jet printing (a noncontact printing process), surface contamination affects the shape of the drops (causing the formation of fingers and crescents) and hence image quality. This study uses modeling methods to examine how such surface contamination affects the drops shapes. Subsequently, it models the effect of surface structures (pits) on the drop shape. This study explores how image quality can be controlled in the presence of surface contamination and surface structures.

The Influence of Surfactants on the Hydrodynamics of Surface Wetting

The hydrodynamic model of steady wetting developed by Boender et al. is extended to include the effect of a (nonionic) surfactant in the limiting case of negligible diffusion and low concentrations, confining attention to steady wetting between parallel plates. The approximation that the meniscus inclination becomes equal to the static contact angle at a distance from the solid of the order of a molecular dimension is extended to take account of the local surfactant concentration, making use of Young's law. A second inner boundary condition, provided by a surfactant balance at the contact line, places a restriction on the speed at which the interface is shed, leading to surfactant accumulation and partial or almost total immobilization of the interface which reduces the wetting speed. Under certain conditions, this immobilization is self-stabilizing, leading to hysteresis effects. Both these effects and the reduced wetting speed correspond with results reported in the literature. Copyright 1998 Academic Press.