Measurement of the viscoelastic properties of thin surfactant films

Abstract

The lateral viscoelastic properties of mica surfaces bearing deposited films of phospholipids have been determined. These experiments have been conducted using a modified form of the mica surface forces apparatus, SFA. In this experiment a monolayer of a phospholipid is deposited onto two mica surfaces from a Langmuir–Blodgett deposition trough. The two surfaces are then placed in contact in the SFA. A small displacement, 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 output signals, the viscoelastic properties of the molecular films of the phospholipids may be determined. The results show that all the films were more elastic than viscous, a consequence of the high surface pressures at which the films were transferred from the Langmuir trough, and that the elastic modulus decreased with increasing load applied to the two surfaces.