Small-scale roughness entraps water and controls underwater adhesion

Area of Science:

• Materials Science
• Surface Science
• Adhesion Science

Background:

• Underwater adhesion is crucial for various applications, including biological adhesives, tire traction, haptics, and health monitoring devices.
• A fundamental understanding of how trapped water affects interfacial bonding is lacking, hindering progress in controlling underwater adhesion.

Purpose of the Study:

• To investigate the role of nanoscale surface features in controlling underwater adhesion.
• To elucidate the mechanisms by which trapped water influences adhesion during both contact approach and separation.

Main Methods:

• Utilized well-characterized polycrystalline diamond surfaces with nanoscale features.
• Employed soft, non-hysteretic, low-surface energy elastomers for contact studies.
• Measured adhesion forces during both approach and retraction phases of contact.

Main Results:

• Observed a reduction in adhesion during the approach phase.
• Measured four times higher adhesion during retraction compared to the thermodynamic work of adhesion.
• Demonstrated that nanoscale surface features govern adhesion by entrapping water during approach and influencing polymer deformation during retraction.

Conclusions:

• The incompressibility and inextensibility of trapped water, along with the work required to deform the polymer around water pockets, explain the counterintuitive increase in adhesion during retraction.
• This research provides a new scientific basis for tailoring surface topography to enhance underwater adhesion, distinct from viscoelastic effects.