Nanoscale Wetting on Groove-Patterned Surfaces

Abstract

In this paper, nanoscale wetting on groove-patterned surfaces is thoroughly studied using molecular dynamics simulations. The results are compared with Wenzel's and Cassie's predictions to determine whether these continuum theories are still valid at the nanoscale for both hydrophobic and hydrophilic types of surfaces when the droplet size is comparable to the groove size. A system with a liquid mercury droplet and grooved copper substrate is simulated. The wetting properties are determined by measuring contact angles of the liquid droplet at equilibrium states. Correlations are established between the contact angle, roughness factor r, and surface fraction f. The results show that, for hydrophobic surfaces, the contact angle as a function of roughness factor and surface fraction on nanogrooved surfaces obeys the predictions from Wenzel's theory for wetted contacts and Cassie's theory for composite contacts. However, slight deviations occur in composite contacts when a small amount of liquid penetration is observed. The contact angle of this partial wetting cannot be accurately predicted using either Cassie's or Wenzel's theories. For hydrophilic surfaces, only wetted contacts are observed. In most cases, the resulting contact angles are found to be higher than Wenzel's predictions. At the nanoscale, high surface edge density plays an important role, which results in contact line pinning near plateau edges. For both hydrophobic and hydrophilic surfaces, substantial amount of anistropic spreading is found in the direction that is parallel to the grooves, especially at wetted or partially wetted contacts.

A study on the contact angles of a water droplet on smooth and rough solid surfaces

“…11 (a). Because the distribution of roughness on the solid surface is Gaussian, the difference in the contact area on the hydrophobic rough surface for different H σ val- However, as reported by Yang et al [34] and Yong and Zhang [35], the contact angle of liquid droplet does not follow Wenzel Eq. (14) due to the pinning effect in which the roughness plays a role of an energy barrier.…”

Tailoring Anisotropic Wettability Using Micro-pillar Geometry

“…Particularly, surfaces with periodic parallel grooves at different length scales, spaces, heights, etc. were extensively investigated in the literature [7][8][9][10][11][12][13][14][15][16][17].…”

Slip-stick wetting and large contact angle hysteresis on wrinkled surfaces

“…For example, a gradient in surface chemistry can allow a water droplet to flow uphill through wetting interactions [13]. Creating topographical patterns on surfaces is generally accomplished using photolithography [4,14], but this requires expensive toolsets; Moreover, a different mask is required for each pattern.…”