Travelling waves in two-dimensional smectic-C domains
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THE EUROPEAN PHYSICAL JOURNAL E
Travelling waves in two-dimensional smectic-C domains ? J. Claret1,3 , J. Crusats2 , R. Albalat1,3 , J. Ign´es-Mullol1,3,a , H. Mart´ınez-Seara1 , R. Reigada1 , and F. Sagu´es1 1 2 3
Departament de Qu´ımica F´ısica, Universitat de Barcelona, Mart´ı i Franqu´es 1, 08028 Barcelona, Catalonia, Spain Departament de Qu´ımica Org` anica, Universitat de Barcelona, Mart´ı i Franqu´es 1, 08028 Barcelona, Catalonia, Spain Nanoscience and Nanotechnology Institute, Universitat de Barcelona, Barcelona, Catalonia, Spain Received 31 May 2006 and Received in final form 13 October 2006 / c EDP Sciences / Societ` Published online: 27 November 2006 – ° a Italiana di Fisica / Springer-Verlag 2006 Abstract. Continued irradiation of smectic-C–like domains of photosensitive Langmuir monolayers from azobenzene derivatives induces the nucleation and propagation of orientational travelling waves as observed with Brewster angle microscopy (BAM). BAM image analysis has allowed to identify different dynamical behaviors involving the generation and propagation of such waves. A model based on the coupling between an orientational and a composition field proposes a scenario for dynamic self-assembly that accounts for most of the observed phenomena, and allows to pinpoint the relevance of boundary defects in wave-emitting structures. PACS. 68.18.-g Langmuir-Blodgett films on liquids – 81.16.Dn Self-assembly – 89.75.Fb Structures and organization in complex systems
1 Introduction Monomolecular films of amphiphiles at the air/water interface (Langmuir monolayers) are well-known model systems to study static self-assembling, resulting in ordered textures and phase transitions in a two-dimensional environment [1]. In a soft-matter context, however, the interest is being more and more focussed on dynamic selfassembling [2], a perspective from which Langmuir monolayers are expected to be also worth studying [3, 4]. In this regard, it has been recently reported that by spreading the trans isomer of a photoisomerizable azobenzene derivative [5], (4-[4-[(4-octylphenyl)azo]phenoxy]butanoic acid), 8Az3COOH hereforth, on a water subphase at appropriate temperature and surface pressure, one may prepare a photosensitive and labile smectic-C–like mesophase [6, 7], suitable to study dynamical patterns as a response to modest external perturbations [8]. The dynamics of the system can be monitored in real time by means of Brewster angle microscopy (BAM) [9, 10]. BAM imaging of these monolayers reveals the existence of a texture with long-range orientational order, which progressively coarsens into a stripe pattern with 2π-wide modulation of the azimuthal field [11]. Previous work by Tabe et al. [12, 13] reported changes in the stripe-like pattern under polarized illumination, ? Supplementary material in the form of video files available from the Journal web page at http://dx.doi.org/10.1140/epje/i2006-10049-5 a e-mail: [email protected]
and reported qualitatively the propagation of small spots of perturbed molecular orientations. We
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