Mechanical Measurements on Langmuir Films Using a High-Q Torsional Oscillator

The mechanical shear elasticity has been measured for a monolayer Langmuir film of the phospholipid dipalmitoylphosphatidylcholine (DPPC). These measurements were performed as a function of the surface pressure in the vicinity of the "liquid-condensed" to "solid" transition which is characterized by a "kink" in the pressure-area isotherm as well as the onset of positional ordering in x-ray structural measurements. Our preliminary results indicate that a shear elasticity develops in the "solid" phase of magnitude 200 ergs/cm sup 2. This value is comparable with the thermodynamic mechanical stability limit for a two dimensional crystal.