Magnetic Orientation of 4-cyano-4′-pentylbiphenyl Liquid Crystal at Its Interface with Aqueous Solution

We observed the magnetic orientation of 4-cyano-4'-pentyl biphenyl (5CB), which is a nematic liquid crystal (LC) at room temperature, in contact with the aqueous solution of surfactant. We found that the orientation of 5CB molecule at the interface can be controlled by an external magnetic field. The liquid-crystal thin layer was confined with a copper specimen grid for TEM and a glass coated with octadecyltrichlorosilane, which induced a homeotropic alignment of 5CB. The LC thin layer was immersed in a cetyltrimethylammonium bromide (CTAB) solution. A magnetic field was applied to the 5CB layer with a pair of permanent magnets having a gap of 1 cm. The magnetic-field strength on the sample was controlled by adjusting the position of the magnets. The orientation behavior of the LC was observed via the change of the linear birefringence with a polarized microscope with a crossed-nicols condition. As the magnetic field gradually increased, an orientation change of LC suddenly took place, that is, the Frederiks transition was observed. The anchoring energy was estimated to be < 10(-6) Jm(-2) based on the threshold field of the Frederiks transition. This value is much smaller than that at the interface of the solid substrate in LC device (<10(-4) Jm(-2)). Furthermore, the orientation angle of 5CB could be estimated from the anchoring energy and the retardation of LC thin layer.