IUVSTA 15th International Vacuum Congress (IVC-15), AVS 48th International Symposium (AVS-48), 11th International Conference on Solid Surfaces (ICSS-11)
    Organic Films and Devices Thursday Sessions
       Session OF+TF-ThM

Paper OF+TF-ThM5
Viscoelastic Properties of Thin Liquid Crystal Films

Thursday, November 1, 2001, 9:40 am, Room 131

Session: Characterization of Organic Thin Films
Presenter: I. Zori@aa c@, Chalmers University of Technology, Sweden
Authors: I. Zori@aa c@, Chalmers University of Technology, Sweden
P. Borchard, Chalmers University of Technology, Sweden
T. Carlsson, Chalmers University of Technology, Sweden
B. Kasemo, Chalmers University of Technology, Sweden
Correspondent: Click to Email
Viscoelastic properties of thin liquid crystal (LC) films are strongly affected by the changes in orientational and/or translational order in the system. These changes may be induced via a temperature variation or by a presence of the two phase interface (e.g. a free surface or a LC-solid substrate interface). In this contribution we report the viscoelastic properties of thin (500-7000Å) 5CB films, spin coated on the Au electrode of the quartz crystal microbalance (QCM), with one free surface. The system (LC film) is exposed to a periodic shear force (at 5 MHz respectively 15MHz) and the changes in the QCM oscillator frequency, @DELTA@f, and dissipation factor, @DELTA@D, are measured as a function of temperature while the system undergoes a series of phase transitions (smectic-nematic-isotropic). Both @DELTA@f and @DELTA@D show unusual temperature dependence in the vicinity of the nematic-isotropic phase transition. Two approaches are used to deduce the temperature dependent viscoelastic coefficients from the measured frequency and dissipation factor changes. In both approaches the QCM is treated as a harmonic oscillator and the Navier Stokes equation is used to calculate the velocity profile in the viscous overlayer caused by the periodic shear. Once the velocity profile in the film is known, a frictional force causing a change in the oscillator frequency and dissipation, may be calculated. In the first case the overlayer is treated as a homogenous isotropic thin liquid film (Voight model) while in the second case a proper anisotropic structure of the LC film is taken into account in the hydrodynamic continuum approach (Leslie-Ericksen theory). We compare our results to generalized viscosities obtained using different methods.