ISRAELACHVILI, Jacob: Direct Measurement of Adhesion Forces: Implications for Understanding Bioadhesion

Various techniques have recently been developed for accurately measuring adhesion forces and adhesion mechanisms of surfaces under various conditions, i.e., both in dry and humid air, and in organic liquids and aqueous solutions. These techniques, for example, the Surface Forces Apparatus (SFA), the JKR Apparatus (after the Johnson-Kendall-Roberts theory of adhesion mechanics and the Atomic Force Microscope (AFM) will be briefly described. Results on various surfaces, including smooth crystalline inorganic surfaces, rough surfaces, lipid monolayer and bilayer surfaces, and polymer and protein surfaces, show that adhesion processes can be very complex, involving time (hysteresis) effects, loading and unloading rate effects, previous history and memory effects, etc. As a consequence of these non-equilibrium effects the measured adhesion can be well below or well above the equilibrium (thermodynamic) value, often by many orders of magnitude. Reasons for these effects will be discussed with illustrations of where they arise and in what systems they may be expected to occur.