Surface force measurements provide direct experimental access to the forces acting between surfaces and molecules. The surface force apparatus (SFA) was developed by Israelachvili and Tabor and measures the force between two molecularly smooth surfaces as a function of separation with angstrom resolution. It has been fundamental in establishing the DLVO theory of colloidal stability and understanding intermolecular interactions.
The SFA uses two crossed cylinders of atomically smooth mica. The separation distance is measured by multiple beam interferometry using fringes of equal chromatic order. White light passed through the mica surfaces produces interference fringes whose wavelengths depend on the separation and refractive index of the medium between the surfaces. The force is determined from the deflection of a double-cantilever spring supporting the lower surface.
Van der Waals forces are always present between surfaces in vacuum and in media. The Hamaker constant describes the magnitude of van der Waals interactions. In liquids, the van der Waals force can be repulsive when the dielectric properties of the medium are intermediate between those of the surfaces.
Electrostatic double-layer forces arise from charged surfaces in electrolyte solutions. The surface charge is neutralized by counterions forming a diffuse layer. The overlap of diffuse layers when surfaces approach produces repulsion in aqueous systems. The Debye length characterizes the range of electrostatic forces and depends on ionic strength.
DLVO theory combines van der Waals attraction and electrostatic repulsion to explain colloidal stability. The primary minimum at close approach leads to irreversible aggregation, while the secondary minimum at larger separation may produce reversible flocculation. Schulze-Hardy rules describe the effect of electrolyte valence on critical coagulation concentration.
Beyond DLVO forces, additional interactions are measured in specific systems. Hydration forces arise from water structuring near hydrophilic surfaces. Hydrophobic interactions drive the attraction between non-polar surfaces in water and are implicated in protein folding. Steric forces from adsorbed polymers and biomolecules provide stabilization. Bridging forces occur when polymers adsorb on both surfaces.
The atomic force microscope also measures surface forces including adhesion, friction, and single-molecule interactions. Force spectroscopy with functionalized tips measures specific ligand-receptor interactions with picoNewton sensitivity.
