56 research outputs found
Thermotropic Nematic and Smectic Order in Silica Glass Nanochannels
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.Comment: 3 pages, 4 figure
Application of retardation-modulation polarimetry in studies of nanocomposite materials
We demonstrate an application of retardation-modulation polarimetry in
studies of nanocomposite materials. Molecular ordering is explored on both
nonchiral and chiral liquid crystals (LCs) in the bulk state and embedded into
parallel-arrays of cylindrical channels of alumina or silica membranes of
different channel sizes (12-42 nm). Two arms polarimetry serves for
simultaneous measurements of the birefringence retardation and optical activity
characterizing, respectively, orientational molecular ordering and chiral
structuring inside nanochannels.Comment: Conference article, 5 pages, 5 figure
Capillary rise of water in hydrophilic nanopores
We report on the capillary rise of water in three-dimensional networks of
hydrophilic silica pores with 3.5nm and 5nm 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 16h
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 -0.5nm 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.Comment: 4 pages, 3 figure
Thermotropic nematic order upon nano-capillary filling
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=305K. The
capillary-condensed state exhibits a more pronounced, albeit still continuous
P-N reordering, located 4 K below T_IN. This shift vanishes abruptly on
complete filling of the capillaries, which we tentatively trace to a 10 MPa
tensile pressure release associated with the disappearance of concave menisci
in the confined liquid.Comment: 6 pages, 3 figure
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