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Molecular simulation of chevrons in confined smectic liquid crystals WEBSTER, R.

E., MOTTRAM, N. J. and CLEAVER, D. J. Available from Sheffield Hallam University Research Archive (SHURA) at: http://shura.shu.ac.uk/900/ This document is the author deposited version. You are advised to consult the publisher'

s version if you wish to cite from it. Published version WEBSTER, R. E., MOTTRAM, N. J. and CLEAVER, D. J. (2003). Molecular simulation of chevrons in confined smectic liquid crystals. Physical review E, 68, 021706. Copyright and re-use policy See http://shura.shu.ac.uk/information.html Sheffield Hallam University Research Archive http://shura.shu.ac.uk Molecular simulation of chevrons in con?ned smectic liquid crystals Richard E. Webster1 , Nigel J. Mottram2 and Douglas J. Cleaver1

1 Materials Research Institute She?eld Hallam University She?eld S1 1WB United Kingdom

2 Department of Mathematics University of Strathclyde Livingstone Tower Glasgow G1 1XH United Kingdom PACS numbers: 61.30.Cz, 61.20.Ja, 68.15.+e, 42.79.Kr April 24,

2003 1 Abstract Chevron structures adopted by con?ned smectic liquid crystals are investigated via molecular dynamics simulations of the Gay-Berne model. The chevrons are formed by quenching nematic ?lms con?ned between aligning planar substrates whose easy axes have opposing az- imuthal components. When the substrates are perfectly smooth, the chevron formed migrates rapidly towards one of the con?ning walls to yield a tilted layer structure. However, when substrate roughness is included, by introducing a small-amplitude modulation to the particle- substrate interaction well-depth, a symmetric chevron is formed which remains stable over su?ciently long runtimes for detailed structural information, such as the relevant order parameters and director orien- tation, to be determined. For both smooth and rough boundaries, the smectic order parameter remains non-zero across the entire chevron, implying that layer identity is maintained across the chevron tip. Also, when the surface-stabilised chevron does eventually revert to a tilted layer structure, it does so via surface slippage, such that layer integrity is maintained throughout the chevron to tilted layer relaxation pro- cess.

2 1 Introduction In the chevron structure formed by con?ned smectic liquid crystals (LCs), the molecular layers which traverse a cell in the more conventional bookshelf arrangement become distorted into a V-shape. The chevron structure was ?rst observed in a ferroelectric smectic C LC in an x-ray di?raction study by Rieker et al. [1], and con?rmed by a study of optical modes in a thin ferroelectric LC ?lm [2]. Subsequently, chevron structures were also found to be formed by con?ned smectic A LC'

s [3]. Due to its crucial role in the bistability of surface-stabilised ferroelectric LC devices, the chevron structure has been the focus of several theoretical and experimental studies. These have concluded that chevrons form due to the mismatch which develops between bulk and surface layer periodicities because of their very di?erent temperature dependencies [4]. The registry be- tween smectic layers and the adsorbing substrate is thought to be essentially frozen-in, a notion supported by the periodic stress oscillations measured by Cagnon and Durand on shearing a bookshelf smectic A cell [5]. Indeed, recent mesoscopic theoretical work [6] and a subsequent Monte Carlo simula- tion study [7] of such systems showed that concerted breaking and reforming of smectic layers takes place near the centre of a cell if a bookshelf-geometry con?ned smectic LC is sheared. The prevalence of chevron structures over