Anchoring of Liquid Crystal and Dynamics of Molecular Exchange Between Adsorbed LC Film and the Bulk

Download or read book Anchoring of Liquid Crystal and Dynamics of Molecular Exchange Between Adsorbed LC Film and the Bulk written by Rui Guo and published by . This book was released on 2008 with total page 74 pages. Available in PDF, EPUB and Kindle. Book excerpt: One of the goals of this dissertation project is to understand the anchoring and reorientation, or gliding of the nematic director on a solid substrate. It has been recognized for a long time that a thin adsorbed layer of LC at the subtrate plays an important role in anchoring and the surface memory effect. The model due to Romanenko, et al., which explained the changes in director orientation, postulates that it occurs through continuous changes in the angular distribution function of adsorbed molecules and gradual change in its orientation mediated by adsorption/desoprtion (AD) of liquid crystal (LC) molecules at the interface. Anchoring properties and the AD of molecules between bulk and anisotropically adsorbed LC film have been studied. The adsorbed LC film was prepared by cooling a homogeneously aligned LC cell from the isotropic phase in the presence of a magnetic field parallel to the surface of ITO substrate. The changes in anchoring energy with time and the temperature at which the LC film is annealed, allow an estimation of the activation energy of desorption of LC molecules from ITO surface, E=0.55 eV. The results suggest that hydrogen bonds are responsible for the adsorption of LC molecules onto the substrate. Relationships between the anchoring energy, the direction of easy axis, angular distribution of the adsorbed molecules, and changes in their angular distribution due to AD, are explained using Maier-Saupe theory and Rapini-Papoular surface potential. The second goal is to better understand the AD process. Thin cells are prepared with rubbed polymer alignming surface and two nematic liquid crystals are selected with very different pretilt angles at this polymer's surface. The cells filled with one liquid crystal were prepared to later be replaced by a second LC having a very different pretilt angle, leaving the adsorbed layers intact. The adsorption of second LC molecules and desorption of first LC at the substrate change with time, the composition of the adsorbed layer, and thus, the pretilt angle. The final equilibrium pretilt angle depends on the net concentration of the two liquid crystals in the cell. Time dependence of the molecular pretilt angle after refilling the cell with second LC is measured and found to depend on the characteristic molecular exchange rate and on the coefficients of desorption of the first LC. Experimental results provide an important insight into the molecular exchange between solid surface and bulk fluid.