No abstract

Despite the fact that quantitative experimental data have been available for more than forty years now, nematoacoustics still poses intriguing theoretical and experimental problems. In this paper, we prove that the main observed features of acoustic wave propagation through a nematic liquid crystal cell - namely, the frequency-dependent anisotropy of sound velocity and acoustic attenuation - can be explained by properly accounting for two fundamental features of the nematic response: anisotropy and relaxation. The latter concept - new in liquid crystal modelling - provides the first theoretical explanation of the structural relaxation process hypothesised long ago by Mullen and co-workers [Mullen et al., Phys. Rev. Lett., 1972, 28, 799]. We compare and contrast our proposal with an alternative theory where the liquid crystal is modelled as an anisotropic second-gradient fluid.

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I have been puzzled for a long time by the unnatural divide between the theory of bulk growth-strikingly underdeveloped-and that for surface growth-much better developed, along apparently independent lines. Recent advances in growth mechanics (DiCarlo and Quiligotti, 2002) make it now possible to subsume growth phenomena of both kinds under one and the same format, where surface growth is obtained as an infinitely intense bulk growth confined in a layer of vanishingly small thickness. This has allowed me to recover the results collected in Gurtin (2000) from the standpoint of DiCarlo and Quiligotti (2002). In particular, I am able to construe Gurtin's technique of referential control volumes that evolve in time as a special application of the principle of virtual power. 1 This is not at all obvious, since I can change my mind. But in this respect I did not. My coauthor did (Quiligotti, 2002), on the (not minor) issue of invariance requirements, convinced by an argument intimated by Green and Naghdi (1971) and spelled out in Casey and Naghdi (1980, 1981), Casey (1987), and elsewhere. For reasons I expound elsewhere (DiCarlo, in preparation), I find that old argument faulty and misleading.