Formation of adhesive contacts: spreading versus dewetting

Area of Science:

• Physics
• Materials Science
• Fluid Dynamics

Background:

• Understanding the mechanics of soft materials interacting with surfaces is crucial in various applications.
• The behavior of liquid films at interfaces during contact is complex and depends on multiple factors.

Purpose of the Study:

• To investigate the contact mechanics of a soft rubber bead pressed against a hydrophobic glass plate through a water droplet.
• To characterize the different regimes of contact zone evolution based on approach velocity.

Main Methods:

• Utilized a "wet" Jomini, Reynolds, and Kawai (JKR) set-up with a soft bead (radius Rb) and a hydrophobic glass plate.
• Employed high-speed camera imaging to observe the dynamic growth of the contact zone.
• Analyzed the influence of approach velocity (V) on the liquid film behavior and bead deformation.

Main Results:

• Observed two distinct contact regimes dependent on approach velocity (V).
• At high V, a liquid film squeezed and dewetted, leading to dry contact nucleation and growth.
• At low V, the bead remained nearly spherical, with spreading governed by a characteristic time (τ ≈ ηRb²/F) and a balance of mechanical and viscous forces.

Conclusions:

• The study reveals distinct mechanisms of contact formation for soft beads on wet hydrophobic surfaces.
• The observed spreading behavior at low velocities can be explained by a balance between global mechanical and viscous forces.
• The findings provide insights into liquid-mediated adhesion and deformation of soft materials.