How foam-like is the shear-induced lamellar phase of an ionic liquid crystal?

Abstract

In a recent article [M.H. Godinho et al., Liq. Cryst. 35, 103 (2008)] we reported that sheared films of two n-alkylimidazolium salts exhibit liquid crystalline behaviour below their bulk equilibrium freezing temperature. The resulting morphologies are strongly reminiscent of two-dimensional liquid foams: the materials partition into dark domains (cells) separated by brighter (birefringent) walls, which are approximately arcs of circle and meet at vertices (“Plateau borders”) with three or more sides. Here we investigate whether they satisfy known quantitative results for foams [see, e.g. D. Weaire and S. Hutzler, The Physics of Foams (Oxford University Press, Oxford, 1999)]. We find that: (i) where three walls meet, they do so at approximately 120° angles, for all times studied; (ii) Lewis's law of linear relation between cell area and number of sides is approximately satisfied at late times; (iii) the morphological patterns coarsen in time, both T1 and T2 processes are observed and, at late times, evolution is consistent with von Neumann's law; and (iv) relatively large numbers of 5-sided cells survive up to fairly late times. Results (i) and (iii) suggest that surface tension may play a key role in determining the physics of this system, as it does in low-viscosity liquid foams.