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, calibratio n, and performance. The apparatus can move one of the surfaces and measure the friction force on the other one bidimensionally for both processes. A h igh mechanical impedance system (10(4) N/m) measures continuous friction fo rces where only stick-slip was previously observed. The frequency and trave l distance of the movement can be varied over a wide range (frequency from 10(-4) to 7 Hz and distance from 1 to 800 mum) 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 accurac y of +/-2 muN 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 disc ussed. (C) 2001 American Institute of Physics.