ADHESION OF SURFACTANT MEMBRANE COVERED DROPLETS - SPECIAL FEATURES AND CURVATURE ELASTICITY EFFECTS

Adhesion between liquid droplets covered by tightly condensed surfactant membranes is described to first order by equilibrium equations analogous to equations for adhesion of immiscible liquid droplets. However, tension and contact angles are not fixed properties of the material surfaces. Contact angles are determined by geometric constraints of the membrane capsules (i.e. surface areas and enclosed volumes) and to a small extent by surface area compressibility. Tensions take on values as required to balance the variational change in free energy of contact formation with a "mechanical leverage" specified by contact angles. Higher order correction leads to a "line-tension" or "edge-energy" contribution which results from the non-zero bending stiffness of a membrane. The elastic bending energy in the region of large membrane curvature at the perimeter of the contact zone creates an energy/length given approximately by the geometric mean of the adhesion energy w̃ (per unit area) and the bending stiffness B, i.e. $ ̃(w̃B) sol1 2 . The dimension scale over which this effect is significant is approximated by the reciprocal of the curvature at the edge of the contact zone, i.e. (B/w̃) sol1 2. In the condensed state, the bending stiffness for surfactant bilayers is on the order 10-12 erg; thus, the edge-energy effect is expected to be prominent when vesicle dimensions are 10-4 cm or less. Direct measurements of adhesion between two surfactant bilayer vesicles with diameters on the order of 10-3 cm have verified the first order mechanical analysis without any indication of edge-energy effects when adhesion-energies are > 10-3 erg cm-2. For adhesion-energies below 10-3 erg cm-2, adhesive contacts are stabilized by weak entropy-driven tensions plus the edge energy due to membrane bending. In this low-tension regime ( < 10-3 dyn cm-1), fluctuations in the surface contours of large membrane capsules become macroscopically observable and lead to diminished areas of adhesive contact. Entropy confinement, which produces the tension, arises because membrane conformational fluctuations are restricted by the constraints of fixed surface area and capsule volume. © 1990.