A surface forces platform for dielectric measurements

Methods are described to implement dielectric spectroscopy (frequency range 10−1–106 Hz) within a surface forces apparatus by using as electrodes silver sheets on the backside of mica. These methods are applied to study the competitive effects of surface alignment, confinement, and shear field on 5CB (5-cyanobiphenyl), a nematic liquid crystal at the experimental temperature of 25 °C. In the planar orientation, films could be squeezed to a minimum thickness of ≈5 Å, the molecule’s thickness. In the perpendicular (homeotropic) orientation, films could be squeezed to ≈25 Å, the expected thickness of the head-to-tail 5CB dimer. It was difficult to discuss responses at f>105 Hz quantitatively because the peak was not visible in the experimental frequency window. Nonetheless, the onset of the relaxation mode for the planar oriented molecules appears at higher frequency than for the homeotropic orientation. A slower relaxation mode, peaked at f≈10 Hz, was assigned to electrode polarization due to the mobility of trace ions within the 5CB samples although these samples were >99.7% pure. The peak frequency was a factor of 3 slower with homeotropic than planar alignment and, in both cases, independent of film thickness except when the film thickness exceeded 10 μm. This was explained using a simple model based on the assumption that trace ions move to oppositely charged electrodes to form electric double layers. A small influence of shear on the dielectric response was observed but only when the dielectric response was measured at the same frequency as the large-amplitude shear deformation. Also described is the use of capacitance to measure force–distance profiles.