New two-dimensional friction force apparatus design for measuring shear forces at the nanometer scale

A device to study the friction of two molecularly smooth surfaces separated by an ultrathin liquid film is presented along with its design, calibration, and performance. The apparatus can move one of the surfaces and measure the friction force on the other one bidimensionally for both processes. A high mechanical impedance system $(104 N/m)$ measures continuous friction forces where only stick–slip was previously observed. The frequency and travel distance of the movement can be varied over a wide range (frequency from $10−4$ to 7 Hz and distance from 1 to 800 μm) to provide variations of the shear rate over seven orders of magnitude. The actual movement provided by piezoelectric bimorph drive can be affected by the friction forces and is measured by strain gauges. The friction forces are measured with an accuracy of ±2μN with a capacitance sensor. The mechanical design prevents the surfaces from rolling under force. The apparatus is tested with hexadecane. The potential applications of this apparatus and its limitations are discussed.