Behavior of a Liquid Bridge between Nonparallel Hydrophobic Surfaces

Area of Science:

• Fluid dynamics
• Surface science
• Wettability studies

Background:

• Liquid bridges between nonparallel surfaces move along them.
• Movement direction is typically linked to surface wettability (hydrophilic vs. hydrophobic).
• Existing literature suggests hydrophobic surfaces cause outward movement from the cusp.

Purpose of the Study:

• To investigate factors beyond wettability influencing liquid bridge motion on hydrophobic surfaces.
• To introduce and define a new geometrical parameter: confinement (cf).
• To elucidate the role of confinement and contact angle hysteresis (CAH) in bridge displacement.

Main Methods:

• Experimental investigations.
• Numerical simulations.
• Analytical reasoning.
• Definition of confinement (cf) as a ratio of contact point distances to the cusp.

Main Results:

• Wettability is not the sole determinant for hydrophobic surfaces; confinement (cf) is crucial.
• Low cf (large surface separation) leads to movement towards the cusp via pinning/depinning.
• High cf (small surface separation) leads to movement away from the cusp.
• Contact angle hysteresis (CAH) fundamentally explains movement, especially with high cf.
• Ideal hydrophobic surfaces (no CAH) show stretching-induced movement towards the cusp and compression-induced movement away.

Conclusions:

• Liquid bridge motion on hydrophobic surfaces is complex, influenced by both confinement (cf) and contact angle hysteresis (CAH).
• The study redefines understanding of hydrophobic surface interactions with liquid bridges.
• Geometrical parameters like cf and dihedral angle are key for ideal hydrophobic surfaces.