MOLECULAR MECHANISMS ASSOCIATED WITH ADHESION AND CONTACT-ANGLE HYSTERESIS OF MONOLAYER SURFACES

Experiments were carried out on a variety of surfactant-coated mica surfaces using the surface forces apparatus technique and contact angle measurements. The experiments were designed to clarify the molecular mechanisms underlying adhesion hysteresis (during loading-unloading cycles) and contact angle hysteresis (of advancing/receding liquids), and to explore any possible relationship between these two energy-dissipating phenomena. We found that hysteresis effects are not simply due to surface imperfections, such as roughness or chemical heterogeneity. Even surfaces that are initially smooth and chemically homogeneous can exhibit large adhesion and contact angle hysteresis effects. Our results indicate that, for such surfaces, hysteresis arises because of molecular rearrangements occurring at solid-solid or solid-liquid interfaces after they have come into contact. This results in a lower surface free energy during the approach of two surfaces (or during spreading) than during separation (or retraction). We have studied a number of factors that enhance hysteresis: (i) increasing the freedom of the surface molecules to reorder, (ii) increasing the load and time surfaces are allowed to remain in contact, and (iii) increasing the rate of separation (or retraction). These findings highlight the inherent nonequilibrium nature of most loading-unloading and wetting-dewetting cycles and suggest ways for reducing the energy-dissipating hysteresis associated with such processes. Our results further indicate that the adhesion or pull-off force F between two curved surfaces of radius R is related to the surface energy-gamma by the Johnson-Kendall-Roberts theory, for example, F = 3-pi-R-gamma for a sphere on a flat surface, but only when the separation occurs under equilibrium conditions. Preliminary results also indicate a correlation between adhesion hysteresis and friction/stiction.