Surface defects and structural transitions in very low anchoring energy nematic thin films

It has been discovered that the products of degradation of heated paper evaporated on glass constitute a low anchoring energy substrate for the nematic phase of MBBA (methoxy benzylidene butylaniline). The extrapolation length b=K/2W_s is of the order of 100 μm. In very thin specimens the anchoring is conical at low temperatures and becomes homeotropic at higher temperatures. A transition to (ordinary) strong anchoring conditions appears at still higher temperatures, presumably due to the appearance of a slight isotropic layer of MBBA at the interface. We have been interested in the study of defects typical of the weak anchoring phases, in specimens thinner than b. One observes the existence of Bloch walls. This typical defect allows for a direct measurement of the surface anchoring energy (W_s ∼ 10⁻⁴ erg/m²). Other defects are nuclei of strength ±1 (in repulsive or attractive interaction with the walls according to whether they are +1 or −1 nuclei), and reversing points which separate Bloch wall segments of opposite torsions. We demonstrate that Bloch walls constitute, in thin specimens (h<b) a favored configuration with respect to the more usual splitting in surface lines of these walls, and explain qualitatively the interactions between walls and nuclei. The stability of the different structural phases (conical and homeotropic) is related to the fact that at h<b the system is quasi‐two‐dimensional.