2010 10th International Conference on Laser and Fiber-Optical Networks Modeling, 2010
We report the spatial anisotropy of the acoustooptical figure of merit M 2 in LiNbO 3 :MgO and Ga... more We report the spatial anisotropy of the acoustooptical figure of merit M 2 in LiNbO 3 :MgO and GaP crystals. The analysis is based on the indicative surfaces being calculated for different geometries of the acoustooptical diffraction. Basing on these results the most efficient geometries of the acoustooptical cells made of these crystals are determined. It give a possibility to increase the application efficiency of LiNbO 3 :MgO and GaP crystals as sensitive elements for acoustooptical high-frequency control of powerful laser irradiation.
We present measurements of the complete spatiotemporal Fourier spectrum of Faraday waves. The Far... more We present measurements of the complete spatiotemporal Fourier spectrum of Faraday waves. The Faraday waves are generated at the interface of two immiscible index matched liquids of different density. By use of a light absorption technique we are able to determine the bifurcation scenario from the flat surface to the patterned state for each complex spatial and temporal Fourier component separately. The surface spectra at onset are found to be in good agreement with the predictions from the linear stability analysis. For the nonlinear state our measurements show in a direct manner how energy is transferred from lower to higher harmonics and we quantify the nonlinear coupling coefficients. Furthermore we find that the nonlinear coupling generates static components in the temporal Fourier spectrum leading thus to a contribution of a nonoscillating permanent sinusoidal deformed surface state. A comparison of hexagonal and rectangular patterns reveals that spatial resonance can give rise to a spectrum that violates the temporal resonance conditions given by the weakly nonlinear theory.
We report on measurements of the transverse magnetization of a ferrofluid rotating as a rigid bod... more We report on measurements of the transverse magnetization of a ferrofluid rotating as a rigid body in a constant magnetic field, H 0 , applied perpendicular to the axis of rotation. The rotation of the fluid leads to a nonequilibrium situation, where the ferrofluid magnetization M and the magnetic field within the sample, H, are no longer parallel to each other. The off-axis magnetization perpendicular to H 0 is measured as a function of both the applied magnetic field H 0 and the angular frequency ⍀. The latter ranges from a few hertz to frequencies well above a characteristic inverse Brownian relaxation time. Our experimental results strongly indicate that the transverse magnetization is caused only by a small fraction of the colloidal ferromagnetic particles. The effect of the polydispersity of the ferrofluid is discussed. Experimental results are compared to predictions based on several theoretical models. A single-time relaxation approach for the so-called effective field and a field-dependent Debye relaxation of M yield reasonably good shapes of the curves of transverse magnetization vs ⍀. However, like the other models, they overestimate their magnitudes.
Abstract: We present measurements on the spontaneous imbibition (capillary rise) of water, a line... more Abstract: We present measurements on the spontaneous imbibition (capillary rise) of water, a linear hydrocarbon (n-C16H34) and a liquid crystal (8OCB) into the pore space of monolithic, nanoporous Vycor glass (mean pore radius 5 nm). Measurements on the mass ...
We report on the capillary rise of water in three-dimensional networks of hydrophilic silica pore... more We report on the capillary rise of water in three-dimensional networks of hydrophilic silica pores with 3.5 nm and 5 nm mean radii respectively (porous Vycor monoliths). We find classical square root of time Lucas-Washburn laws for the imbibition dynamics over the entire capillary rise times of up to 16 h investigated. Provided we assume two preadsorbed, strongly bound layers of water molecules resting at the silica walls, which corresponds to a negative velocity slip length of -0.5 nm for water flow in silica nanopores, we can describe the filling process by a retained fluidity and capillarity of water in the pore center. This anticipated partitioning in two dynamic components reflects the structural-thermodynamic partitioning in strongly silica bound water layers and capillary condensed water in the pore center which is documented by sorption isotherm measurements.
We study the smectic director structure of the rodlike liquid crystal 4-n-dodecyl-4Ј-cyanobipheny... more We study the smectic director structure of the rodlike liquid crystal 4-n-dodecyl-4Ј-cyanobiphenyl ͑12CB͒ confined in cylindrical cavities of 200 nm diameter in porous alumina templates by means of combined broadband dielectric spectroscopy, optical birefringence, and neutron scattering measurements. We show that the collective molecular orientation differs between entering the smectic A phase upon cooling from the isotropic state and entering the same phase upon heating while melting the confined crystal. We discuss this collective molecular realignment in terms of a competition between weak planar anchoring at the p-Al 2 O 3 / 12CB interface and a preferred texture typical of the crystallization of rodlike molecules in nanochannels ͑Bridgman growth͒.
Optical birefringence measurements on a rod-like liquid crystal (8OCB), imbibed in silica channel... more Optical birefringence measurements on a rod-like liquid crystal (8OCB), imbibed in silica channels (7 nm diameter), are presented and compared to the thermotropic bulk behavior. The orientational and positional order of the confined liquid evolves continuously at the paranematic-to-nematic and sizeably broadened at the nematic-to-smectic order transition, resp., in contrast to the discontinuous and well-defined second-order character of the bulk transitions. A Landau-de-Gennes analysis reveals identical strengths of the nematic and smectic ordering fields (imposed by the walls) and indicates that the smectic order is more affected by quenched disorder (originating in channel tortuosity and roughness) than the nematic transition.
We report combined optical birefringence and neutron scattering measurements on the liquid crysta... more We report combined optical birefringence and neutron scattering measurements on the liquid crystal 12CB nanoconfined in mesoporous silicon layers. This liquid crystal exhibits strong nematicsmectic coupling responsible for a discontinuous isotropic-to-smectic phase transition in the bulk state. Confined in porous silicon, 12CB is subjected to strong anisotropic quenched disorder: a shortranged smectic state evolves out of a paranematic phase. This transformation appears continuous, losing its bulk first order character. This contrasts with previously reported observations on liquid crystals under isotropic quenched disorder. In the low temperature phase, both orientational and translational order parameters obey the same power-law.
We present high-resolution optical birefringence measurements upon sequential filling of an array... more We present high-resolution optical birefringence measurements upon sequential filling of an array of parallel-aligned nanochannels ͑14 nm mean diameter͒ with rodlike ͑acetonitrile͒ and disclike ͑hexafluoroben-zene͒ molecules. We will demonstrate that such birefringence isotherms, when performed simultaneously with optically isotropic and index-matched counterparts ͑neopentane and hexafluoromethane͒, allow one to characterize the orientational state of the confined liquids with a high accuracy as a function of pore filling. The pore condensates are almost bulklike, optically isotropic liquids. For both anisotropic species we find, however, a weak orientational order ͑of a few percent at maximum͒ upon film condensation in the monolithic mesoporous membrane. It occurs upon formation of the second and third adsorbed layer, only, and vanishes gradually upon onset of capillary condensation. Presumably, it originates in the breaking of the full rotational symmetry of the interaction potential at the cylindrical, free liquid-vapor interface in the film-condensed state rather than at the silica-liquid interface. This conclusion is corroborated by comparisons of our experimental results with molecular-dynamics simulations reported in the literature.
The European Physical Journal Special Topics, 2007
We present measurements on the capillary rise of water and two linear alkanes (n-C16H34, n-C24H50... more We present measurements on the capillary rise of water and two linear alkanes (n-C16H34, n-C24H50) in nanopores of monolithic Vycor glass (mean pore radius 5 nm). Measurements of the mass uptake as a function of time, m(t), are in good agreement with the Lucas-Washburn √ t -prediction typical of imbibition of liquids into porous hosts. The relative capillary rise velocities of the liquids investigated scale as expected from the bulk fluid parameters.
The optical birefringence of liquid n-hexane condensed in an array of parallel silica channels of... more The optical birefringence of liquid n-hexane condensed in an array of parallel silica channels of 7 nm diameter and 400 m length is studied as a function of filling of the channels via the vapor phase. By an analysis with the generalized Bruggeman effective-medium equation we demonstrate that such measurements are insensitive to the detailed geometrical ͑positional͒ arrangement of the adsorbed liquid inside the channels. However, this technique is particularly suitable to search for any optical anisotropies and thus collective orientational order as a function of channel filling. Nevertheless, no hints for such anisotropies are found in liquid n-hexane. The n-hexane molecules in the silica nanochannels are totally orientationally disordered in all condensation regimes, in particular, in the film growth as well as in the capillary-condensed regime. Thus, the peculiar molecular arrangement found upon freezing of liquid n-hexane in nanochannel confinement, where the molecules are collectively aligned perpendicularly to the channels' long axes, does not originate in any prealignment effects in the nanoconfined liquid due to capillary nematization.
We explore the nematic ordering of the rodlike liquid crystals 5CB and 6CB, embedded into paralle... more We explore the nematic ordering of the rodlike liquid crystals 5CB and 6CB, embedded into parallel-aligned nanochannels in mesoporous silicon and silica membranes as a function of mean channel radius (4.7 R 8.3 nm), and, thus, geometrical confinement strength, by optical birefringence measurements in the infrared region. The orientational order inside the nanochannels results in an excess birefringence, which is proportional to the nematic order parameter. It evolves continuously on cooling with a precursor behavior, typical of a paranematic state at high temperatures. These observations are compared with the bulk behavior and analyzed within a phenomenological model. Such an approach indicates that the strength of the nematic ordering fields σ is beyond a critical threshold σ c = 1/2 that separates discontinuous from continuous paranematic-to-nematic behavior. In agreement with the predictions of the phenomenological approach, a linear dependency of σ on the inverse channel radius is found and we can infer therefrom the critical channel radii, R c , separating continuous from discontinuous paranematic-to-isotropic behavior, for 5CB (12.1 nm) and 6CB (14.0 nm). Our analysis suggests that the tangential anchoring at the channel walls is of similar strength in mesoporous silicon and mesoporous silica membranes. A comparison with the bulk phase behavior reveals that the nematic order in nanoconfinement is significantly affected by channel wall roughness, leading to a reduction of the effective nematic ordering.
Optical birefringence and light absorption measurements reveal four regimes for the thermotropic ... more Optical birefringence and light absorption measurements reveal four regimes for the thermotropic behavior of a nematogen liquid (7CB) upon sequential filling of parallel-aligned capillaries of 12 nm diameter in a monolithic, mesoporous silica membrane. No molecular reorientation is observed for the first adsorbed monolayer. In the film-condensed state (up to 1 nm thickness), a weak, continuous paranematic-to-nematic (P-N) transition is found, which is shifted by 10 K below the discontinuous bulk transition at T IN = 305 K. The capillary-condensed state exhibits a more pronounced, albeit still continuous P-N reordering, located 4 K below T IN . This shift vanishes abruptly upon complete filling of the capillaries. It could originate in competing anchoring conditions at the free inner surfaces and at the pore walls or result from the 10-MPa tensile pressure release associated with the disappearance of concave menisci in the confined liquid upon complete filling. The study documents that the thermo-optical properties of nanoporous systems (or single nanocapillaries) can be tailored over a surprisingly wide range simply by variation of the filling fraction with liquid crystals.
The condensation of Ar into mesoporous glass has been studied by adsorption isotherms and optical... more The condensation of Ar into mesoporous glass has been studied by adsorption isotherms and optical transmission for the solid state of the pore filling. The change of the adsorption isotherms with respect to the liquid regime is interpreted in terms of triple-point wetting. The transmission data suggest that the mechanism both of pore filling and emptying relies on advancing menisci quite in contrast to the liquid regime where this mode is reserved to pore emptying.
Using a high resolution optical polarimeter, we have measured the temperature dependence of the b... more Using a high resolution optical polarimeter, we have measured the temperature dependence of the birefringence and optical rotation in chiral smectic liquid crystals that exhibit antiferroelectric, ferroelectric, and intermediate phases. The temperature dependence of the magnitude of the tilt angle was determined from the birefringence of 4-͑1-methyl-heptyloxycarbonyl-phenyl͒ 4Ј-octylbiphenyl-4-carboxylate ͑MHPOBC͒ and of 4-͑1-methylheptyloxycarbonyl-phenyl͒ 4Ј-octylcarbonyloxybiphenyl-4-carboxylate ͑MHPOCBC͒. Both substances exhibit a crossover of the order parameter exponent from the classical value of ϭ0.5 close to the transition to the tricritical value ϭ0.25 far away. This stresses the importance of the sixth order terms in the Landau free-energy expansion for ferroelectric and antiferroelectric liquid crystals. In addition, a discontinuous behavior in the magnitude of the tilt is observed when crossing the smectic-C ␣ * -smectic-C* or smectic-C* -smectic-C A * transitions, whereas the smectic-A -smectic-C ␣ * transition is continuous. The simultaneously determined optical rotation is used to elucidate the structures and the nature of phase transitions. The results are well explained within the framework of a discrete phenomenological model with nearest and next nearest neighbor interactions between the smectic layers.
An analysis of the lattice dynamics of Cs 2 HgCl 4 crystals and their relation to the pressure-in... more An analysis of the lattice dynamics of Cs 2 HgCl 4 crystals and their relation to the pressure-induced proper ferroelastic instability is reported using a semiempirical rigid-ion model in the quasiharmonic approximation. The model parameters ͑effective radii and charges of ions͒ have been adjusted to satisfy the equilibrium configuration of the observed structure in the normal phase. The influence of unharmonicity has been introduced into the lattice dynamics simulation indirectly through the changes of the lattice parameters that occur under the action of temperature and hydrostatic pressure. Such a simple model reproduces the lattice instability leading to the appearance of the proper ferroelastic phase. It has been shown that the lattice loses its stability with respect to the transverse-acoustical mode TA Z (kʈb) whereas the soft optical mode (B 3g symmetry at k ϭ0) remains stable. This is in agreement with the phenomenological theory. The rigid-ion model also reproduces the line of the proper ferroelastic phase transitions in the P,T-phase diagram which is close to the experimentally observed phase diagram.
The elastic dynamics of AgNa(NO 2 ) 2 crystals around the proper ferroelectric phase transition a... more The elastic dynamics of AgNa(NO 2 ) 2 crystals around the proper ferroelectric phase transition at T C ϭ38°C was investigated by ultrasonic ( f ϭ10 MHz) and dynamic mechanical analysis ͑fϭ0.6-50 Hz͒ techniques . The system represents a unique example of a ferroelectric crystal with extremely slow dielectric relaxation. Due to this reason it reveals a substantially different elastic behavior in megahertz and hertz frequency regions. We describe our data using a phenomenological model that includes both the effect of order parameter ͑polarization͒ relaxation with a characteristic time p and thermal ͑entropy͒ relaxation with a characteristic time th .
Very recently we presented puzzling results of diffuse neutron scattering experiments on KSCN and... more Very recently we presented puzzling results of diffuse neutron scattering experiments on KSCN and RbSCN. The data yield an increase of the diffuse intensity with increasing temperature below T c , whereas the width remains constant. Using molecular dynamics and 3D Monte Carlo simulations, we have shown that below T c the width of the correlation functions can be stabilized by strain fields originating from the order parameter strain interactions. Here we construct a novel analytic model which predicts the existence of a second characteristic length scale and explains the suppression of the growth of precursor clusters by the influence of inhomogeneous strain fields.
We present measurements of the complete spatio-temporal Fourier spectrum of Faraday waves. The Fa... more We present measurements of the complete spatio-temporal Fourier spectrum of Faraday waves. The Faraday waves are generated at the interface of two immiscible index matched liquids of different density. By use of a new absorption technique we are able to determine the bifurcation scenario from the flat surface to the patterned state for each complex spatial and temporal Fourier component separately. The measured surface spectra at onset are in good agreement with the predictions from a linear stability analysis. For the nonlinear state our measurements show in a direct manner how energy is transferred from lower to higher harmonics and we quantify the nonlinear coupling coefficients. Furthermore we find that the nonlinear coupling generates constant components in the spatial Fourier spectrum leading to a contribution of a non oscillating permanent sinusoidal deformed surface state. A comparison of a hexagonal and a rectangular pattern reveals that spatial resonances only can give rise to a spectrum that violates the temporal resonance conditions given by the weakly nonlinear theory.
We studied the microscopic polymer conformations in the droplet detachment process of an elastic ... more We studied the microscopic polymer conformations in the droplet detachment process of an elastic semi-dilute polyelectrolytic Xanthan solution by measuring the instantaneous birefringence. As in earlier studies, we observe the suppression of the finite time singularity of the pinch-off process and the occurrence of an elastic filament. Our microscopic measurements reveal that the relatively stiff Xanthan molecules are already significantly pre-stretched to about 90 % of their final extension at the moment the filament appears. At later stages of the detachment process, we find evidence of a concentration enhancement due to the elongational flow. 83.80.Rs, 47.20.Gv,
2010 10th International Conference on Laser and Fiber-Optical Networks Modeling, 2010
We report the spatial anisotropy of the acoustooptical figure of merit M 2 in LiNbO 3 :MgO and Ga... more We report the spatial anisotropy of the acoustooptical figure of merit M 2 in LiNbO 3 :MgO and GaP crystals. The analysis is based on the indicative surfaces being calculated for different geometries of the acoustooptical diffraction. Basing on these results the most efficient geometries of the acoustooptical cells made of these crystals are determined. It give a possibility to increase the application efficiency of LiNbO 3 :MgO and GaP crystals as sensitive elements for acoustooptical high-frequency control of powerful laser irradiation.
We present measurements of the complete spatiotemporal Fourier spectrum of Faraday waves. The Far... more We present measurements of the complete spatiotemporal Fourier spectrum of Faraday waves. The Faraday waves are generated at the interface of two immiscible index matched liquids of different density. By use of a light absorption technique we are able to determine the bifurcation scenario from the flat surface to the patterned state for each complex spatial and temporal Fourier component separately. The surface spectra at onset are found to be in good agreement with the predictions from the linear stability analysis. For the nonlinear state our measurements show in a direct manner how energy is transferred from lower to higher harmonics and we quantify the nonlinear coupling coefficients. Furthermore we find that the nonlinear coupling generates static components in the temporal Fourier spectrum leading thus to a contribution of a nonoscillating permanent sinusoidal deformed surface state. A comparison of hexagonal and rectangular patterns reveals that spatial resonance can give rise to a spectrum that violates the temporal resonance conditions given by the weakly nonlinear theory.
We report on measurements of the transverse magnetization of a ferrofluid rotating as a rigid bod... more We report on measurements of the transverse magnetization of a ferrofluid rotating as a rigid body in a constant magnetic field, H 0 , applied perpendicular to the axis of rotation. The rotation of the fluid leads to a nonequilibrium situation, where the ferrofluid magnetization M and the magnetic field within the sample, H, are no longer parallel to each other. The off-axis magnetization perpendicular to H 0 is measured as a function of both the applied magnetic field H 0 and the angular frequency ⍀. The latter ranges from a few hertz to frequencies well above a characteristic inverse Brownian relaxation time. Our experimental results strongly indicate that the transverse magnetization is caused only by a small fraction of the colloidal ferromagnetic particles. The effect of the polydispersity of the ferrofluid is discussed. Experimental results are compared to predictions based on several theoretical models. A single-time relaxation approach for the so-called effective field and a field-dependent Debye relaxation of M yield reasonably good shapes of the curves of transverse magnetization vs ⍀. However, like the other models, they overestimate their magnitudes.
Abstract: We present measurements on the spontaneous imbibition (capillary rise) of water, a line... more Abstract: We present measurements on the spontaneous imbibition (capillary rise) of water, a linear hydrocarbon (n-C16H34) and a liquid crystal (8OCB) into the pore space of monolithic, nanoporous Vycor glass (mean pore radius 5 nm). Measurements on the mass ...
We report on the capillary rise of water in three-dimensional networks of hydrophilic silica pore... more We report on the capillary rise of water in three-dimensional networks of hydrophilic silica pores with 3.5 nm and 5 nm mean radii respectively (porous Vycor monoliths). We find classical square root of time Lucas-Washburn laws for the imbibition dynamics over the entire capillary rise times of up to 16 h investigated. Provided we assume two preadsorbed, strongly bound layers of water molecules resting at the silica walls, which corresponds to a negative velocity slip length of -0.5 nm for water flow in silica nanopores, we can describe the filling process by a retained fluidity and capillarity of water in the pore center. This anticipated partitioning in two dynamic components reflects the structural-thermodynamic partitioning in strongly silica bound water layers and capillary condensed water in the pore center which is documented by sorption isotherm measurements.
We study the smectic director structure of the rodlike liquid crystal 4-n-dodecyl-4Ј-cyanobipheny... more We study the smectic director structure of the rodlike liquid crystal 4-n-dodecyl-4Ј-cyanobiphenyl ͑12CB͒ confined in cylindrical cavities of 200 nm diameter in porous alumina templates by means of combined broadband dielectric spectroscopy, optical birefringence, and neutron scattering measurements. We show that the collective molecular orientation differs between entering the smectic A phase upon cooling from the isotropic state and entering the same phase upon heating while melting the confined crystal. We discuss this collective molecular realignment in terms of a competition between weak planar anchoring at the p-Al 2 O 3 / 12CB interface and a preferred texture typical of the crystallization of rodlike molecules in nanochannels ͑Bridgman growth͒.
Optical birefringence measurements on a rod-like liquid crystal (8OCB), imbibed in silica channel... more Optical birefringence measurements on a rod-like liquid crystal (8OCB), imbibed in silica channels (7 nm diameter), are presented and compared to the thermotropic bulk behavior. The orientational and positional order of the confined liquid evolves continuously at the paranematic-to-nematic and sizeably broadened at the nematic-to-smectic order transition, resp., in contrast to the discontinuous and well-defined second-order character of the bulk transitions. A Landau-de-Gennes analysis reveals identical strengths of the nematic and smectic ordering fields (imposed by the walls) and indicates that the smectic order is more affected by quenched disorder (originating in channel tortuosity and roughness) than the nematic transition.
We report combined optical birefringence and neutron scattering measurements on the liquid crysta... more We report combined optical birefringence and neutron scattering measurements on the liquid crystal 12CB nanoconfined in mesoporous silicon layers. This liquid crystal exhibits strong nematicsmectic coupling responsible for a discontinuous isotropic-to-smectic phase transition in the bulk state. Confined in porous silicon, 12CB is subjected to strong anisotropic quenched disorder: a shortranged smectic state evolves out of a paranematic phase. This transformation appears continuous, losing its bulk first order character. This contrasts with previously reported observations on liquid crystals under isotropic quenched disorder. In the low temperature phase, both orientational and translational order parameters obey the same power-law.
We present high-resolution optical birefringence measurements upon sequential filling of an array... more We present high-resolution optical birefringence measurements upon sequential filling of an array of parallel-aligned nanochannels ͑14 nm mean diameter͒ with rodlike ͑acetonitrile͒ and disclike ͑hexafluoroben-zene͒ molecules. We will demonstrate that such birefringence isotherms, when performed simultaneously with optically isotropic and index-matched counterparts ͑neopentane and hexafluoromethane͒, allow one to characterize the orientational state of the confined liquids with a high accuracy as a function of pore filling. The pore condensates are almost bulklike, optically isotropic liquids. For both anisotropic species we find, however, a weak orientational order ͑of a few percent at maximum͒ upon film condensation in the monolithic mesoporous membrane. It occurs upon formation of the second and third adsorbed layer, only, and vanishes gradually upon onset of capillary condensation. Presumably, it originates in the breaking of the full rotational symmetry of the interaction potential at the cylindrical, free liquid-vapor interface in the film-condensed state rather than at the silica-liquid interface. This conclusion is corroborated by comparisons of our experimental results with molecular-dynamics simulations reported in the literature.
The European Physical Journal Special Topics, 2007
We present measurements on the capillary rise of water and two linear alkanes (n-C16H34, n-C24H50... more We present measurements on the capillary rise of water and two linear alkanes (n-C16H34, n-C24H50) in nanopores of monolithic Vycor glass (mean pore radius 5 nm). Measurements of the mass uptake as a function of time, m(t), are in good agreement with the Lucas-Washburn √ t -prediction typical of imbibition of liquids into porous hosts. The relative capillary rise velocities of the liquids investigated scale as expected from the bulk fluid parameters.
The optical birefringence of liquid n-hexane condensed in an array of parallel silica channels of... more The optical birefringence of liquid n-hexane condensed in an array of parallel silica channels of 7 nm diameter and 400 m length is studied as a function of filling of the channels via the vapor phase. By an analysis with the generalized Bruggeman effective-medium equation we demonstrate that such measurements are insensitive to the detailed geometrical ͑positional͒ arrangement of the adsorbed liquid inside the channels. However, this technique is particularly suitable to search for any optical anisotropies and thus collective orientational order as a function of channel filling. Nevertheless, no hints for such anisotropies are found in liquid n-hexane. The n-hexane molecules in the silica nanochannels are totally orientationally disordered in all condensation regimes, in particular, in the film growth as well as in the capillary-condensed regime. Thus, the peculiar molecular arrangement found upon freezing of liquid n-hexane in nanochannel confinement, where the molecules are collectively aligned perpendicularly to the channels' long axes, does not originate in any prealignment effects in the nanoconfined liquid due to capillary nematization.
We explore the nematic ordering of the rodlike liquid crystals 5CB and 6CB, embedded into paralle... more We explore the nematic ordering of the rodlike liquid crystals 5CB and 6CB, embedded into parallel-aligned nanochannels in mesoporous silicon and silica membranes as a function of mean channel radius (4.7 R 8.3 nm), and, thus, geometrical confinement strength, by optical birefringence measurements in the infrared region. The orientational order inside the nanochannels results in an excess birefringence, which is proportional to the nematic order parameter. It evolves continuously on cooling with a precursor behavior, typical of a paranematic state at high temperatures. These observations are compared with the bulk behavior and analyzed within a phenomenological model. Such an approach indicates that the strength of the nematic ordering fields σ is beyond a critical threshold σ c = 1/2 that separates discontinuous from continuous paranematic-to-nematic behavior. In agreement with the predictions of the phenomenological approach, a linear dependency of σ on the inverse channel radius is found and we can infer therefrom the critical channel radii, R c , separating continuous from discontinuous paranematic-to-isotropic behavior, for 5CB (12.1 nm) and 6CB (14.0 nm). Our analysis suggests that the tangential anchoring at the channel walls is of similar strength in mesoporous silicon and mesoporous silica membranes. A comparison with the bulk phase behavior reveals that the nematic order in nanoconfinement is significantly affected by channel wall roughness, leading to a reduction of the effective nematic ordering.
Optical birefringence and light absorption measurements reveal four regimes for the thermotropic ... more Optical birefringence and light absorption measurements reveal four regimes for the thermotropic behavior of a nematogen liquid (7CB) upon sequential filling of parallel-aligned capillaries of 12 nm diameter in a monolithic, mesoporous silica membrane. No molecular reorientation is observed for the first adsorbed monolayer. In the film-condensed state (up to 1 nm thickness), a weak, continuous paranematic-to-nematic (P-N) transition is found, which is shifted by 10 K below the discontinuous bulk transition at T IN = 305 K. The capillary-condensed state exhibits a more pronounced, albeit still continuous P-N reordering, located 4 K below T IN . This shift vanishes abruptly upon complete filling of the capillaries. It could originate in competing anchoring conditions at the free inner surfaces and at the pore walls or result from the 10-MPa tensile pressure release associated with the disappearance of concave menisci in the confined liquid upon complete filling. The study documents that the thermo-optical properties of nanoporous systems (or single nanocapillaries) can be tailored over a surprisingly wide range simply by variation of the filling fraction with liquid crystals.
The condensation of Ar into mesoporous glass has been studied by adsorption isotherms and optical... more The condensation of Ar into mesoporous glass has been studied by adsorption isotherms and optical transmission for the solid state of the pore filling. The change of the adsorption isotherms with respect to the liquid regime is interpreted in terms of triple-point wetting. The transmission data suggest that the mechanism both of pore filling and emptying relies on advancing menisci quite in contrast to the liquid regime where this mode is reserved to pore emptying.
Using a high resolution optical polarimeter, we have measured the temperature dependence of the b... more Using a high resolution optical polarimeter, we have measured the temperature dependence of the birefringence and optical rotation in chiral smectic liquid crystals that exhibit antiferroelectric, ferroelectric, and intermediate phases. The temperature dependence of the magnitude of the tilt angle was determined from the birefringence of 4-͑1-methyl-heptyloxycarbonyl-phenyl͒ 4Ј-octylbiphenyl-4-carboxylate ͑MHPOBC͒ and of 4-͑1-methylheptyloxycarbonyl-phenyl͒ 4Ј-octylcarbonyloxybiphenyl-4-carboxylate ͑MHPOCBC͒. Both substances exhibit a crossover of the order parameter exponent from the classical value of ϭ0.5 close to the transition to the tricritical value ϭ0.25 far away. This stresses the importance of the sixth order terms in the Landau free-energy expansion for ferroelectric and antiferroelectric liquid crystals. In addition, a discontinuous behavior in the magnitude of the tilt is observed when crossing the smectic-C ␣ * -smectic-C* or smectic-C* -smectic-C A * transitions, whereas the smectic-A -smectic-C ␣ * transition is continuous. The simultaneously determined optical rotation is used to elucidate the structures and the nature of phase transitions. The results are well explained within the framework of a discrete phenomenological model with nearest and next nearest neighbor interactions between the smectic layers.
An analysis of the lattice dynamics of Cs 2 HgCl 4 crystals and their relation to the pressure-in... more An analysis of the lattice dynamics of Cs 2 HgCl 4 crystals and their relation to the pressure-induced proper ferroelastic instability is reported using a semiempirical rigid-ion model in the quasiharmonic approximation. The model parameters ͑effective radii and charges of ions͒ have been adjusted to satisfy the equilibrium configuration of the observed structure in the normal phase. The influence of unharmonicity has been introduced into the lattice dynamics simulation indirectly through the changes of the lattice parameters that occur under the action of temperature and hydrostatic pressure. Such a simple model reproduces the lattice instability leading to the appearance of the proper ferroelastic phase. It has been shown that the lattice loses its stability with respect to the transverse-acoustical mode TA Z (kʈb) whereas the soft optical mode (B 3g symmetry at k ϭ0) remains stable. This is in agreement with the phenomenological theory. The rigid-ion model also reproduces the line of the proper ferroelastic phase transitions in the P,T-phase diagram which is close to the experimentally observed phase diagram.
The elastic dynamics of AgNa(NO 2 ) 2 crystals around the proper ferroelectric phase transition a... more The elastic dynamics of AgNa(NO 2 ) 2 crystals around the proper ferroelectric phase transition at T C ϭ38°C was investigated by ultrasonic ( f ϭ10 MHz) and dynamic mechanical analysis ͑fϭ0.6-50 Hz͒ techniques . The system represents a unique example of a ferroelectric crystal with extremely slow dielectric relaxation. Due to this reason it reveals a substantially different elastic behavior in megahertz and hertz frequency regions. We describe our data using a phenomenological model that includes both the effect of order parameter ͑polarization͒ relaxation with a characteristic time p and thermal ͑entropy͒ relaxation with a characteristic time th .
Very recently we presented puzzling results of diffuse neutron scattering experiments on KSCN and... more Very recently we presented puzzling results of diffuse neutron scattering experiments on KSCN and RbSCN. The data yield an increase of the diffuse intensity with increasing temperature below T c , whereas the width remains constant. Using molecular dynamics and 3D Monte Carlo simulations, we have shown that below T c the width of the correlation functions can be stabilized by strain fields originating from the order parameter strain interactions. Here we construct a novel analytic model which predicts the existence of a second characteristic length scale and explains the suppression of the growth of precursor clusters by the influence of inhomogeneous strain fields.
We present measurements of the complete spatio-temporal Fourier spectrum of Faraday waves. The Fa... more We present measurements of the complete spatio-temporal Fourier spectrum of Faraday waves. The Faraday waves are generated at the interface of two immiscible index matched liquids of different density. By use of a new absorption technique we are able to determine the bifurcation scenario from the flat surface to the patterned state for each complex spatial and temporal Fourier component separately. The measured surface spectra at onset are in good agreement with the predictions from a linear stability analysis. For the nonlinear state our measurements show in a direct manner how energy is transferred from lower to higher harmonics and we quantify the nonlinear coupling coefficients. Furthermore we find that the nonlinear coupling generates constant components in the spatial Fourier spectrum leading to a contribution of a non oscillating permanent sinusoidal deformed surface state. A comparison of a hexagonal and a rectangular pattern reveals that spatial resonances only can give rise to a spectrum that violates the temporal resonance conditions given by the weakly nonlinear theory.
We studied the microscopic polymer conformations in the droplet detachment process of an elastic ... more We studied the microscopic polymer conformations in the droplet detachment process of an elastic semi-dilute polyelectrolytic Xanthan solution by measuring the instantaneous birefringence. As in earlier studies, we observe the suppression of the finite time singularity of the pinch-off process and the occurrence of an elastic filament. Our microscopic measurements reveal that the relatively stiff Xanthan molecules are already significantly pre-stretched to about 90 % of their final extension at the moment the filament appears. At later stages of the detachment process, we find evidence of a concentration enhancement due to the elongational flow. 83.80.Rs, 47.20.Gv,
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Papers by A. Kityk