Calorimetric Study of Nematic Prewetting

– We report specific-heat measurements for a series of liquid crystals imbedded in a porous cylindrical geometry. Above the nematic-to-isotropic transition and dependent on nematic width (or chain length), the specific heat shows a small peak. In analogy to known ellipsometry results, the peak is believed to be the signature of a nematic prewetting transition. The behavior of liquid crystals (LC) at interfaces has been extensively researched due to the fundamental phenomena present and the possible impact on technological applications. Typical studies have determined the orientational anchoring and/or wetting behavior of LC at solid substrates or at the air interface [1–3]. Using ellipsometry on planar LC films, Lucht et al. [4–6] investigated the wetting behavior at the free surface, above the nematic (or smecticA) to isotropic phase transition for several LC series. Above the smectic-A–to–isotropic (AI) transition of non-polar compounds they observed either i) complete wetting, or ii) partial wetting with pretransitional increase of surface order, or iii) partial or non-wetting without any pretransitional increase. They reported the first observation of a prewetting transition in a LC system 0.3K above the bulk nematic-to-isotropic (NI) transition for a 1mm thick planar film formed from the 10.O.8 compound [5]. The nematic prewetting signature arose from a homeotropically aligned, 12 nm thick, orientationally ordered layer. Broadly, the study of wetting and prewetting transitions has grown enormously in recent years. A review by Bonn and Ross [7] summarizes experimental observations of wetting and prewetting, recaps some major theoretical predictions, and provides references to earlier review articles on the subject. The primary observations of prewetting transitions have been in superfluid helium and in binary mixtures [8]. Note that a calorimetry study for LC in a porous interconnected medium [9] showed two specific-heat peaks, one of which disappeared after long-term annealing; those results were not cast in a wetting language. We report specific-heat studies as a function of temperature for members of the LC series m.O.8 confined to the submicron pores of Anopore membranes. Our goal was to determine if wetting (prewetting) yielded a thermodynamic signature due to the onset of an increase in orientational order. We chose a confined system for the following reasons. If our experimental sample were in the same geometry as [4, 5], one would have to extract the contribution to the specific heat due to a few nanometers thick layer of LC out of a one millimeter thick