Deuteron NMR Study of the Isotropic-Smectic A transition in Liquid-Crystal-Aerosil Dispersions

ORAL

Abstract

In this work, we present a deuteron nuclear magnetic resonance (NMR) study of quenched disorder effects on the 12CB liquid crystal (LC) upon dispersion of silica nano particles (type A-300): hydrophilic silica spheres of diameter 7nm and surface area S=300m$^{2}$/g, with hydroxyl groups covering their surface. The LC-aerosil dispersions form a gel (network) if the aerosil density exceeds the gelation (percolation) threshold. The hydroxyl groups on the surface and the polar nature of the LC, likely yield a homeotropic alignment at the silica surfaces. For low densities of aerosil in the dispersions and while cooling the sample, the LC director in void volume is parallel to the external NMR field; a well defined and aligned LC configuration is established. When a complete silica network finally forms, if the dispersion orientation with respect to the NMR field is changed, a few silica links are broken by the field, re-aligning some LC molecules; effectively, the aerosil locks-in the LC configuration which exhibits a P$_{2}$ (Cos$\Theta )$ dependence. The external field anneals the random disorder introduced by aerosil up to a certain density beyond which, disordering effects dominate; for aerosil densities exceeding $\rho _{S} \quad \approx $ 0.055~g/cm$^{3 }$the NMR spectrum is a powder-pattern representing an isotropic distribution of smectic domains. The occurrence of quenching of 12CB Sm-A phase at $\rho _{S} \quad \approx $ 0.055~g/cm$^{3}$, is roughly one order of magnitude less than the density required for quenching of 8CB [1] [1] T. Jin and D. Finotello, \textit{Phys. Rev}. \textbf{E 69}, 041704 (2004); \textit{Phys. Rev Lett}. \textbf{86}, 818 (2001).

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Authors

  • Vishal Pandya

    Plasma Dynamics Corp., Department of Chemistry, University of Calabria, P. Bucci-15c, 87036 Rende (CS), ITALY, Air Force Institute of Technology, Gothenburg University, Sweden, Denison University, Department of Physics, University of Notre Dame, Department of Physics, John Carroll University, Department of Chemistry, The University of Akron, Department of Phyiscs, The University of Akron, Faculty of Chemical Technology, University of Pardubice, Czech Republic, Johns Hopkins University, Baltimore MD, Kent State University, Kent OH, University of Iowa, Physics Dept., Cleveland State University, Ohio State University, Rochester Institute of Technology, The Ohio State University, University of Akron, NASA Glenn Research Center, National Center For Space Exploration Research, Penn State College of Medicine, Department of Physics, Kent State University

  • Vishal Pandya

    Plasma Dynamics Corp., Department of Chemistry, University of Calabria, P. Bucci-15c, 87036 Rende (CS), ITALY, Air Force Institute of Technology, Gothenburg University, Sweden, Denison University, Department of Physics, University of Notre Dame, Department of Physics, John Carroll University, Department of Chemistry, The University of Akron, Department of Phyiscs, The University of Akron, Faculty of Chemical Technology, University of Pardubice, Czech Republic, Johns Hopkins University, Baltimore MD, Kent State University, Kent OH, University of Iowa, Physics Dept., Cleveland State University, Ohio State University, Rochester Institute of Technology, The Ohio State University, University of Akron, NASA Glenn Research Center, National Center For Space Exploration Research, Penn State College of Medicine, Department of Physics, Kent State University