DESIGN OF AN APPARATUS TO MEASURE THE SHEAR RESPONSE OF ULTRATHIN LIQUID-FILMS

Abstract

The design, calibration, and performance are described of an apparatus to study the shear response of ultrathin liquid films. The device, a modification of the surface forces apparatus, measures the resistance to shear of liquids confined between two atomically smooth solid surfaces. The surfaces are separated by distances on the order of the size of the liquid molecules (liquid film thickness < 10 nm). Shear forces with periodic time dependence are applied to one surface while the second is held fixed, and any motion so induced is analyzed to determine the behavior of the liquid film. The frequency and amplitude of the shear forces applied can be varied over a wide range (0.03-60 Hz frequency and 0.1-1000 nm amplitude) to achieve different values for the magnitude of the shear rate. The dynamic response of the device is linear in the applied force at a given frequency; nonetheless, nonlinear dependence of the liquid&apos;s shear resistance on the shear rate, net normal pressure, and film thickness can be observed with the technique. The mechanical and electrical characteristics of the device are modeled to gain insight into its behavior and facilitate analysis of the measured data. The central results of this approach are expressions for the magnitude of the shear rate and an effective friction coefficient of the liquid film in terms of easily measured electrical quantities. For convenience the friction coefficient is restated as an apparent dynamic viscosity in analogy to continuum hydrodynamics, but the validity of the approach does not depend on a particular understanding of the structure in the liquid layer. The applications and limitations of the device are discussed, as well as other potential uses to which the apparatus may be applied by rational extension to the approach presented.