Shear resonance measurement on structuring of liquids confined between mica surfaces

The behavior of liquid molecules confined in nanometer-scale spaces is quite different from that in the bulk. The structuring of liquids confined between solid surfaces was investigated using a shear force resonance method, which was developed in order to examine in detail the ordering behavior of liquid molecules confined between two mica surfaces. The frequency and the amplitude of the resonance peak are highly sensitive to the long-range order and properties of the confined sample, affording information about the structuring, and frictional and lubrication properties of the liquids from the nanometer thickness to the bulk. This measurement was used to study the properties of octamethylcyclotetrasiloxane (OMCTS) confined between mica surfaces, and was compared with those of 4-cyano-4′-hexylbiphenyl, a liquid crystal molecule. Both liquids are known to exhibit an oscillation force. The resonance peak intensity measured in the presence of OMCTS showed no decrease down to the surface distance of 5 nm upon compression, and abruptly disappeared at 4.6 nm. This means that the resonance peak intensity was damped by the abrupt increase in the viscosity of OMCTS induced by the confinement. Broad and weak resonance peaks centered at angular frequencies of 250-305 s^-1 were observed while further decreasing the distance, indicating the enhanced structuring of OMCTS between the surfaces. Unlike 4-cyano-4′-hexylbiphenyl, finally, OMCTS was completely removed from the gap, and a resonance peak similar to that of mica contact in air appeared.