Thin liquid crystal films

R. G. Horn, J. N. Israelachvili, E. Perez. Forces due to structure in a thin liquid crystal film. J Physique 42 39-52, 1981. 

Yoshida et al. recently disclosed an alternative method that allowed them to produce stable suspensions of gold nanoparticles (1-2 nm in diameter) in nematic liquid crystals [315]. They used a simple sputter deposition process, which allowed them to prepare thin liquid crystal films of well-dispersed gold nanoparticles in both 5CB and E47 (available from Merck) with a nanoparticle size depending on the used nematic liquid crystal. Unfortunately, the authors did not provide any details on whether the nanoparticles were capped with a ligand or bare, non-coated particles, which makes it difficult to assess and compare the reported thermal as well as electro-optic data. However, very similar effects were found as a result of nanoparticle doping, including lower nematic-to-isotropic phase transition temperatures compared to the used pure nematics as well as 10% lower threshold voltages at nanoparticle concentrations below 1 wt% [315]. 

Lavrentovich, O.D., Pergamenshchik, V.M. Patterns in thin liquid crystal films and the divergence ( surfacelike ) elasticity. In Kumar, S. (ed.) Liquid Crystals in the Nineties and Beyond S, pp. 251-298. World Scientific, Singapore (1995)... 

The mechanism of Mb Fe /Fe reduction in snrfactant films involves protonation coupled to electron transfer. Details were investigated in thin liquid crystal films of DDAB and phosphatidylcholines. F° of the heme Fe /Fe couple at... 

U. Delabre, C. Richard, Y. Yip Cheung Sang and A. M. Cazabat, Thin liquid crystal films in the nematic range of temperatures, Langmuir, 2613368 (20103. 

Nanorheology of thin liquid crystal film studied by shear force resonance method... 

Key words Nanorheology - shear force apparatus - surface forces - mechanical resonance - thin liquid crystal films - cyano-alkylbiphenyls... 

Fig. 2 Experimental data and theoretical fit illustrating the effect of the viscous friction in thin liquid crystal film of 4-cyano-4-hexyl-biphenyl (6CB) (a) Amplitude. The lines are theoretical fits by Eq. (1) (b) Phase delay of the output signal. The lines are drawn by Eq. (2) using the same parameters and the following values for the phase 9 (in deg) 0, 20, 30, 40, 40, 70. Liquid crystal volume 10 pi, input voltage Ui = 200 mV, cantilever spring constant K = 56.7 N/m, temperature 24 C assuring nematic state for 6CB...

Fig. 3 Apparent viscosity f of a thin liquid crystal film of 6CB versus the film thickness h. The viscosity is calculated by Eq. (3) at contact radius Rc = 0.0041 cm. The horizontal straight line marks the most probable literature value of f oo for a bulk sample. For comparison is plotted also the coefficient of viscous friction lc obtained from the data fit in Fig. 2a...

As an example of the application of the Jones matrices, the director configuration in a thin liquid crystal film of a SmC material has been investigated [34] by measuring... 

Idziak S H J ef a/1994 The x-ray surface forces apparatus structure of a thin smectic liquid crystal film under confinement Science 264 1915-8... 

T. Gruhn, M. Schoen. Microscopic structure of molecularly thin confined liquid-crystal films. Phys Rev E 55 2861-2875, 1997. 

Note In the off-state the electro-optical cell contains a thin film of a nematic liquid-crystal with mutually perpendicular directors at the upper and lower glass plates hence to reach the on-state the director performs a 90° twist over the thickness of the liquid crystal film. 

Moreover, segregation of nanoparticles at liquid crystal/substrate interfaces is also an effective way to promote or alter the alignment of thin nematic liquid crystal films. Depending on their concentration, size, and nature, several types of nanoparticles have been shown to induce homeotropic alignment as well as defects and remarkable defect patterns. 

Because of the limited magnitude of surface tension gradients and absence of electric double-layer effects, the stabilization of foams in nonpolar liquids requires other ways of retarding the thinning of foam lamellae. These include the high liquid-phase viscosity that has been discussed earlier and increased surface viscosity because of presence of highly viscous or even rigid liquid-crystal films. 

With respect to the interfacial structure of ionic liquids with solids, the molecular layering and local order in thin liquid crystalline films of [RMIM][PFis] ionic liquids with long alkyl chains (e.g., R = dodecyl and octadecyl) on solid silicon supports have been studied using X-ray reflectivity [45]. Here, thin films of the crystalline ionic liquids with a thickness of 100-210 A were deposited on polished and cleaned silicon wafers by initial spin-coating of a solution of the ionic liquids in methanol followed by heating the samples into the isotropic liquid state and crystallization. 

Japan, Mar. 19-20, 2002, ed. M. Iwamoto, K. Kaneto and S. Mashiko, Elsevier Science B.V., Amsterdam, Netherlands, 2003 R 249 A. Sugimura and G.R. Luckhurst, Explanation of the Static and Dynamic Director Orientation in Thin Nematic Liquid Crystal Films Using Deuterium NMR Spectroscopy , p. 313... 

Freely-suspended Films of Polymeric Liquid Crystals. The stabilization of freely-suspended films by using polymeric liquid crystals is obviously interesting and has been attempted previously. Unfortunately it seems to be extremely difficult to polymerize films of liquid crystalline monomers as these films were reported to always break during polymerization. It seems to be equally difficult to fabricate FS-films of polymeric liquid crystals in their smectic A and smectic C phases, most likely due to their enhanced viscosities. However, if one heats slightly into the isotropic phase it is possible to spread a film across an aperture which thins out to form a truly freely-suspended liquid crystal film after cooling into the smectic phases (57). Films of this type are homeotropic in the smectic A phase and show birefringence when cooled to the ferroelectric smectic C ... 

E. Grelet, H. Bock, Control of the orientation of thin open supported columnar liquid crystal films by the kinetics of growth. Europhys. Lett. 73, 712-718 (2006)... 

Comparisons must take into account that at the present level of analysis, the kinetic constants are reproducible to roughly + 20%. In thin Mb films, values of k follow the trend DHP > DMPC > DDAB, but the dependence is not strong (Table 1). Similarly, the dependence of k on surfactant type was weak for thick liquid crystal films of most of the surfactants in Fig. 2 at pH 5.5 [24]. The dependence of k° on electrode material measured for thick films was also weak, and we can conclude only that Pt, PG > Au, ITO (Table 2). Surfactant films on ITO are much less stable than on the other electrodes [32], so that metal and carbon electrodes are preferable except for spectroelectrochemistry. 

We now consider the Mueller matrix of a uniformly twisted nematic (or cholesteric) liquid crystal. The problem can be simplified if we consider the Stokes vector and the Mueller matrix in the local frame x y, in which the liquid crystal director lies along the x axis. Divide the liquid crystal film into N thin slabs. The thickness of each slab is dz = h/N, where h is the thickness of the liquid crystal film. The angle between the liquid crystal director of two neighboring slabs is dxff = qdz, where q is the twisting rate. The retardation angle of a slab is liT = k Andz. If the... 

Abstract The structure and mechanics of very thin hquid crystal films depend on the intermolecular interactions in confined dimensions. The rheology of such films has been investigated by a shear force apparatus constructed as an attachment to the surface forces apparatus. The novelty of this method is that the rheological parameters are extracted from the amplitude and the phase of the output signal as a function of the resonance frequency. The apparent viscosity of the liquid crystal film is calculated from the damping coefficient by using a simple theoretical model. The viscosity of nanometer thin films of 4-cyano-4-... 

It could happen, on the contrary, that a solid substrate promotes local nematic ordering in the isotropic phase which amounts to setting ()n-7i)<0 in Eq. (13) (this was experimentally observed for mica surfaces in lyotropic nematics [15]). The nematic phase may then form above T i for thin enough liquid crystal films. This is the so-called capillary condensation. 

The free-standing film is a very interesting physical system, in which the sample thickness can very easily be varied from two to a few hundred molecular layers. It can be used to test the theories of two-dimensional melting, the evolution of phase transitions as a function of the dimensionality of the system, and to investigate substrate-free, two-dimensional transitions and the effect of free surfaces [119]. In order to analyze the small heat capacity anomalies associated with phase transitions in very thin films, a new AC calorimeter technique has been set up [123]. Thus detailed calorimetric studies of free-standing liquid crystal films have been performed near the SmBhex-SmA transition of several compounds [ 124-126]. 

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