Direction Dependence of Adhesion Force for Droplets on Rough Substrates

Area of Science:

• Surface science
• Fluid dynamics
• Computational physics

Background:

• Determining solid surface free energy remains a challenge.
• Existing theories for droplet adhesion on rough surfaces focus on either contact area or contact line length.
• A unified understanding of droplet adhesion mechanisms is lacking.

Purpose of the Study:

• To unify existing theories on droplet adhesion to rough substrates.
• To investigate the influence of contact area versus contact line length on adhesion forces.
• To differentiate the dependencies of vertical and lateral adhesion forces.

Main Methods:

• Utilized lattice Boltzmann simulations to model droplet-surface interactions.
• Simulated droplet adhesion under different force measurement directions (vertical separation and lateral sliding).
• Analyzed the relationship between adhesion forces and geometric parameters (contact area, contact line length).

Main Results:

• Demonstrated that adhesion force dependence is directional.
• Vertical adhesion (pull-off) force is primarily governed by the contact area.
• Lateral adhesion force during sliding is dependent on the contact line length and independent of the contact area.

Conclusions:

• Successfully unified two leading theories of droplet adhesion.
• Provided a nuanced understanding of how contact geometry influences adhesion based on force direction.
• Established distinct dependencies for vertical and lateral adhesion forces on rough surfaces.