How Surfactants Affect Droplet Wetting on Hydrophobic Microstructures

Area of Science:

• Surface Science
• Physical Chemistry
• Materials Science

Background:

• Surfactants are amphiphilic molecules that adsorb at interfaces.
• Superhydrophobic surfaces rely on the Cassie-Baxter (CB) wetting state for functionality.
• Surfactants can destabilize the CB state, leading to the Wenzel (W) state.

Purpose of the Study:

• To quantitatively understand how surfactants alter wetting states and contact angles on superhydrophobic surfaces.
• To investigate the influence of surface roughness and solid fraction on surfactant-induced wetting transitions.
• To develop a thermodynamic model predicting wetting behavior and a strategy for stable CB states.

Main Methods:

• Experimental measurements of contact angles and wetting states for aqueous drops with varying surfactant concentrations on microstructured hydrophobic surfaces.
• Theoretical analysis using free energy calculations to model wetting state stability.
• Systematic variation of surface roughness (r) and solid fraction (ϕ).

Main Results:

• At low surfactant concentrations (C), drops exhibited either the Wenzel (W) or Cassie-Baxter (CB) state, with CB angles predictable by a thermodynamic model.
• At high surfactant concentrations (C), all drops transitioned to the Wenzel (W) state.
• Theoretical free energy analysis confirmed that the W state is always preferred, while the CB state is metastable at low C.

Conclusions:

• Surfactant concentration is a critical factor in determining the wetting state of drops on superhydrophobic surfaces.
• The transition from CB to W state is driven by surfactant adsorption and associated free energy changes.
• A predictive framework and design principles are provided for achieving stable CB states in applications requiring superhydrophobicity.