12 results
Now showing1 - 10 of 12
- Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Spatial variation of molecular dynamics in the nanoconfined glass-former methanol(2014-06-12); ; ; We report filling-fraction-dependent dielectric measurements on methanol confined in an array of parallel-aligned channels of 8 nm diameter and 100 μm length in a monolithic mesoporous silica membrane. For channel fillings up to 43%, the channel walls are covered with films of average thickness from 0 to 1 nm. Upon filling, the average relaxation time decreases by three orders of magnitude, and the apparent glass-transition temperature T decreases from 99 to 90 K. The local relaxation times τ of the molecules added in each adsorption step even change by 3.5 orders of magnitude. Analyzing the distribution of relaxation times, we show that added molecules alter the dynamics of those previously deposited. The speeding up of molecular dynamics upon filling hints to spatial heterogeneity and is in accordance with two scenarios, which are experimentally undistinguishable: (i) heterogeneity of the inner channel surface, where molecules are at first attached to strong adsorption sites and only subsequently to weaker sites or on top of already deposited molecules, and (ii) an exponential decay of molecular relaxation times with increasing average distance z from the channel walls, that is, In(τ /τ ) exp(-z/ζ) holds. Following a theoretical model, we interpret ζ as a dynamical correlation length of the supercooled liquid. We find ζ ≈ 0.22 nm at T = 125 K, a value close to the molecular radius of methanol. The latter result corroborates molecular dynamics simulations reported in the literature.Publicationtype: Journal Article50 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Anomalous front broadening during spontaneous imbibition in a matrix with elongated pores(National Acad. of Sciences, 2012-06-26); ; ; ; During spontaneous imbibition, a wetting liquid is drawn into a porous medium by capillary forces. In systems with comparable pore length and diameter, such as paper and sand, the front of the propagating liquid forms a continuous interface. Sections of this interface advance in a highly correlated manner due to an effective surface tension, which restricts front broadening. Here we investigate water imbibition in a nanoporous glass (Vycor) in which the pores are much longer than they are wide. In this case, no continuous liquid-vapor interface with coalesced menisci can form. Anomalously fast imbibition front roughening is experimentally observed by neutron imaging. We propose a theoretical pore-network model, whose structural details are adapted to the microscopic pore structure of Vycor glass and show that it displays the same large-scale roughening characteristics as observed in the experiment. The model predicts that menisci movements are uncorrelated, indicating that despite the connectivity of the network the smoothening effect of surface tension on the imbibition front roughening is negligible. These results suggest a new universality class of imbibition behavior, which is expected to occur in any matrix with elongated, interconnected pores of random radii.Publicationtype: Journal Article137 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Orientational order in liquids upon condensation in nanochannels: an optical birefringence study on rodlike and disclike molecules in monolithic mesoporous silica(American Physical Society (APS), 2010-12-02); ; ; We present high-resolution optical birefringence measurements upon sequential filling of an array of parallel-aligned nanochannels (14~nm mean diameter) with rod-like (acetonitrile) and disc-like (hexafluorobenzene) molecules. We will demonstrate that such birefringence isotherms, when performed simultaneously with optically isotropic and index-matched counterparts (neopentane and hexafluoromethane), 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 bulk-like, optically isotropic liquids. For both anisotropic species we find, however, a weak orientational order (of a few percent at maximum) 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.Publicationtype: Journal Article13 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Collective molecular reorientation of a calamitic liquid crystal (12CB) confined in alumina nanochannels(American Physical Society (APS), 2010-07-15); ; ; ; We study the smectic director structure of the rodlike liquid crystal 4-n -dodecyl- 4′-cyanobiphenyl (12CB) 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 -Al2 O3 /12CB interface and a preferred texture typical of the crystallization of rodlike molecules in nanochannels (Bridgman growth).Publicationtype: Journal Article32 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Criticality of an isotropic-to-smectic transition induced by anisotropic quenched disorder(American Physical Society (APS), 2010-05-03); ; ; ; We report combined optical birefringence and neutron scattering measurements on the liquid crystal 12CB nanoconfined in mesoporous silicon layers. This liquid crystal exhibits strong nematic-smectic 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 short-ranged 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.Publicationtype: Journal Article28 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Liquid n-hexane condensed in silica nanochannels: a combined optical birefringence and vapor sorption isotherm studyThe optical birefringence of liquid n-hexane condensed in an array of parallel silica channels of 7nm diameter and 400 micrometer 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 (positional) 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 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 pre-alignment effects in the nanoconfined liquid due to capillary nematization.Publicationtype: Journal Article12 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Continuous paranematic-to-nematic ordering transitions of liquid crystals in tubular silica nanochannels(American Physical Society (APS), 2008-10-02); ; ; ; The optical birefringence of rod-like nematogens (7CB, 8CB), imbibed in parallel silica channels with 10 nm diameter and 300 micrometer length, is measured and compared to the thermotropic bulk behavior. The orientational order of the confined liquid crystals, quantified by the uniaxial nematic ordering parameter, evolves continuously between paranematic and nematic states, in contrast to the discontinuous isotropic-to-nematic bulk phase transitions. A Landau-de Gennes model reveals that the strength of the orientational ordering fields, imposed by the silica walls, is beyond a critical threshold, that separates discontinuous from continuous paranematic-to-nematic behavior. Quenched disorder effects, attributable to wall irregularities, leave the transition temperatures affected only marginally, despite the strong ordering fields in the channels.Publicationtype: Journal Article11 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Rheology of liquids in nanopores: a study on the capillary rise of water, n-Hexadecane and n-Tetracosane in mesoporous silicaWe 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.Publicationtype: Journal Article21 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Spontaneous imbibition of liquids into nanoporesWe present measurements on the spontaneous imbibition of water, a linear hydrocarbon (n-C15H34) and a liquid crystal (8OCB) into the pore space of monolithic, nanoporous Vycor glass (mean pore radius 5nm). Measurements of the mass uptake as a function of time, m(t), are in good agreement with the Lucas-Washbum √t -prediction typical of imbibition of liquids into porous hosts. The relative capillary rise velocities scale as expected from the bulk fluid parameters.Publicationtype: Conference Paper11 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Capillary sublimation of Ar in mesoporous glass(American Physical Society (APS), 2005-02-01); ; ; ; 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.Publicationtype: Journal Article18