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ILCC Edinburgh, July 2002 VAN DER WAALS INTERACTION AND STABILITY OF MULTILAYERED LIQUID-CRYSTALLINE SYSTEMS dr. Andreja [ arlah Univerza v Ljubljani Fakulteta za matematiko in fiziko, Oddelek za fiziko

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THERMODYNAMIC ORIGIN OF INTERACTION BETWEEN MACROSCOPIC BODIES modified free energy of the system interaction disjoining pressure

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structural force fluctuation-induced force van der Waals force DISJOINING PRESSURE

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structural force fluctuation-induced force van der Waals force Orientational wetting - heterophase systems surface wetting layer nonhomogeneous degree of order logarithmic divergence of the wetting layer thickness high ordering powerhigh disordering power due to modified average order

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structural force due to modified average order DISJOINING PRESSURE Hybrid nematic system

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structural force fluctuation-induced force van der Waals force DISJOINING PRESSURE structural force fluctuation-induced force van der Waals force due to modified spectrum of fluctuations of the order parameter surface-induced modification of the average order changed potential for fluctuations boundary conditions

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fluctuation-induced force due to modified spectrum of fluctuations DISJOINING PRESSURE Orientational wetting - fluctuations of the degree of order

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character range correlation length boundary conditions fluctuation-induced force due to modified spectrum of fluctuations DISJOINING PRESSURE finite short range exponential decay infinite long range power-law decay strong-weak repulsive strong-strong weak-weak attractive

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structural force fluctuation-induced force van der Waals force DISJOINING PRESSURE due to interaction of fluctuating dipoles (permanent or instantaneous) in molecules

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VAN DER WAALS INTERACTION BETWEEN ANISOTROPIC BODIES vdW interaction between macroscopic bodies due to changed spectrum of electromagnetic field modes due to changed occupancy of states anisotropic vs. isotropic bodies different boundary conditions different penetration depth analytical calculation possible only for uniaxial symmetry thickness dependent

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VAN DER WAALS FORCE IN ANISOTROPIC MEDIA Hamaker constant for uniaxial media the relevant parameters are instead of averages

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(b) change of the character of the van der Waals interaction at the ordering transition ordered state isotropic state repulsive interactionattractive interaction 5CB at room temperature ! VAN DER WAALS FORCE at the ordering transition

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RENORMALIZATION OF AVERAGE ORDER DUE TO THE VAN DER WAALS INTERACTION different symmetry of isotropic and nematic phase nonzero van der Waals force acting on the wetting layer change of the critical exponent

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(IN)STABILITY OF THIN LIQUID DEPOSITIONS wrinkling of the free liquid surface due to thermal fluctuations enhanced fluctuations decreased fluctuations stable film decomposition of film into drops I. Dreven { ek (2000) F. Vandenbrouck, et al., PRL 82, 2693 (1999)

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substrate: PVCN on BK7 glass LC evaporation about 40 minutes LC evaporation about 3 hours [I. Dreven { ek (2000)] (IN)STABILITY OF THIN LIQUID DEPOSITIONS the same result also for quartz and sapphire substrates U N S T A B L E

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solid substrate-LC-air additional layer of water on the solid substrate unstability of thin LC films on different solid substrates due to thin layer ( ~ 0.3 nm) of water VAN DER WAALS FORCE stability of thin depositions usually, in experimental set-up the solid substrate is covered with a natural oxide layer, adsorbed hydrocarbons, water, etc. unstable usually, in experimental set-up the solid substrate is covered with a natural oxide layer, adsorbed hydrocarbons, water, etc. unstable film stable films ?

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SUMMARY & CONCLUSIONS the effect of anisotropy of dielectric permittivity of media on the magnitude and character of the van der Waals interaction determination of the Hamaker constant for the van der Waals interaction between uniaxial media neglecting dielectric and optical anisotropy can yield wrong character of the interaction renormalization of phenomenological description of the heterophase system important for the interpretation of stability of thin films characterized by highly anisotropic arrangement importance of additional layers

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