MEASUREMENT OF THE VISCOELASTIC PROPERTIES OF THIN SURFACTANT FILMS

The lateral viscoelastic properties of mica surfaces bearing deposited films of phospholipids have been determined. These experiments have b een conducted using a modified form of the mica surface forces apparat us, SFA. In this experiment a monolayer of a phospholipid is deposited onto two mica surfaces from a Langmuir-Blodgett deposition trough. Th e two surfaces are then placed in contact in the SFA. A small displace ment, typically 100 nm, is then applied in a sinusoidal manner to one of the surfaces through a piezoelectric bimorph. A second bimorph acts a receiver. From the amplitudes and phase angle of the input and outp ut signals, the viscoelastic properties of the molecular films of the phospholipids may be determined. The results show that all the films w ere more elastic than viscous, a consequence of the high surface press ures at which the films were transferred from the Langmuir trough, and that the elastic modulus decreased with increasing load applied to th e two surfaces.