Contact angle of a nanodrop on a nanorough solid surface

Area of Science:

• Surface Science
• Materials Science
• Nanotechnology

Background:

• Understanding the contact angle of nanodrops on nanorough surfaces is crucial for applications in microfluidics and coatings.
• Existing models like Wenzel and Cassie-Baxter may not fully capture nanoscale wetting phenomena.

Purpose of the Study:

• To investigate the contact angle of cylindrical nanodrops on nanorough surfaces using density functional theory.
• To analyze the influence of surface roughness, fluid-surface interactions, and nanodrop size on wetting behavior.
• To evaluate the applicability of classical wetting models for nanorough surfaces.

Main Methods:

• Density functional theory (DFT) calculations were employed to simulate nanodrop behavior.
• Surface roughness was modeled using rectangular pillars with varying properties.
• Hydrophobic and hydrophilic substrates were considered, with distinct fluid-pillar and fluid-substrate interactions.

Main Results:

• For hydrophobic surfaces, the contact angle's dependence on roughness is not monotonic and is influenced by fluid-pillar interactions, pillar geometry, and nanodrop size.
• Hydrophilic surfaces exhibit a unique kink-like contact angle dependence on nanodrop size, attributed to shifts in the nanodrop's leading edges.
• Classical wetting models (Wenzel and Cassie-Baxter) fail to explain the observed contact angle behaviors on nanorough surfaces.

Conclusions:

• The contact angle on nanorough surfaces is a complex phenomenon influenced by multiple factors beyond simple roughness.
• New insights into nanoscale wetting are provided, highlighting the limitations of current theoretical models.
• This study necessitates a refined understanding of surface topography and fluid interactions for accurate wetting predictions at the nanoscale.