X-ray determination of the molecular tilt and layer fluctuation profiles of freely suspended liquid-crystal films

Tweet, D. J.; Hołyst, Robert; Swanson, Brian D.; Stragier, H.; Sorensen, L. B.

doi:10.1103/physrevlett.65.2157

Cited by 100 publications

(27 citation statements)

References 13 publications

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“…This requires a special experimental geometry. 16 The fact that the higher temperature anomaly increases in size relative to the one occurring at a lower temperature provides additional support for our association of the former with surface hexatic transition (see the evolution in Fig. 1).…”

Section: Smectic-a-hexatic-b Transitionsupporting

confidence: 54%

Heat Capacity Investigations of Extremely Thin Liquid-Crystal Free-Standing Films

Huang

Geer

Stoebe

1992

Molecular Crystals and Liquid Crystals Science and Technology.

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A high-resolution differential ac free-standing film calorimeter has been constructed in our laboratories. We have employed this unique calorimeter to investigate the evolution of the temperature dependence of heat-capacity anomalies through the smectic-A-hexatic-B-crystal-E transition sequence of one liquid crystal compound as the f i l m thickness is reduced from a few hundred to just three molecular layers. The existence of free surfaces in the free-standing liquid-crystal films has a very strong effect on the development of the molecular order through the smectic-A-hexatic-B transition and other unexpected results related to the crystal-E order.

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Section: Smectic-a-hexatic-b Transitionsupporting

confidence: 54%

Heat Capacity Investigations of Extremely Thin Liquid-Crystal Free-Standing Films

Huang

Geer

Stoebe

1992

Molecular Crystals and Liquid Crystals Science and Technology.

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“…However, unambiguous electrondensity profiles providing proof of such liquid layering have not yet been experimentally determined. More recently, synchrotron X-ray diffraction from various single solid-liquid interfaces and free surfaces revealed structural ordering [6][7][8][9][10][11]. A recent structural investigation of liquid within a nanometre-sized gap reported thickness quantization between silicon surfaces at externally forced surface separations [12].…”

mentioning

confidence: 99%

X-ray reflectivity reveals equilibrium density profile of molecular liquid under nanometre confinement

Perret¹,

Nygård²,

Satapathy³

et al. 2009

Europhys. Lett.

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-A silane (tetrakis(trimethylsiloxy)silane) has been confined within a space of a few molecular diameters (9Å) between two atomically flat opposing mica membranes. The liquid's electron density profile along the confinement direction has been determined by synchrotron X-ray reflectivity for film thicknesses of 8.58 and 11.22 nm. We find the liquid's molecules to be strongly layered at layer distances significantly larger than the effective molecular diameter. The considerable free volume enables the confined liquid to retain its liquid properties.A liquid which is confined within a slit or pore exhibits properties markedly different from the bulk liquid if it is confined within a space of only a few molecular diameters in size [1]. Such "extreme confinement" imposes a structural ordering to the liquid, which in turn will affect many of the liquid's properties. A considerable part of confined-fluids research has been concerned with measurements of the normal and lateral forces acting between two approaching surfaces in a liquid medium. Typically, oscillatory forces are observed having a periodicity of just below one molecular diameter and an exponential decay of comparable length scale [2][3][4][5]. Discrete steps in film thickness are observed, which are commonly interpreted as layering transitions through expulsions of single molecular layers. However, unambiguous electrondensity profiles providing proof of such liquid layering have not yet been experimentally determined. More recently, synchrotron X-ray diffraction from various single solid-liquid interfaces and free surfaces revealed structural ordering [6][7][8][9][10][11]. A recent structural investigation of liquid within a nanometre-sized gap reported thickness quantization between silicon surfaces at externally forced surface separations [12].(a) E-mail: friso.vanderveen@psi.ch Using X-ray reflectivity (XRR) [13] as a function of perpendicular momentum transfer q ⊥ we have determined the electron density profile along the confinement direction of the spherical, non-polar liquid tetrakis(trimethylsiloxy)silane (TTMSS) having a molecular diameter of 9.0Å [8]. The liquid was confined between flat surfaces and the gap width between the surfaces was allowed to equilibrate in the absence of external forces. The confining walls were formed by a pair of (001)-oriented single-crystal surfaces of muscovite mica. The confinement configuration can be regarded as a free-standing single crystal which is interrupted by an ultrathin film of fluid. The advantage of using a crystal lies in its known structure and in its smooth surface provided it is free of atomic steps. The latter is a prerequisite for resolving distinct electron density peaks in the confined liquid. Figure 1 shows a schematic of the confinement geometry and the molecular structure of the mica crystal and TTMSS. Muscovite mica H 2 KAl 3 (SiO 4 ) 3 is a stack of aluminum silicate sheets separated by sheets of potassium ions. The muscovite (001) planes are easily cleaved due to the absence of cova...

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“…The layers therefore act as barriers to the hexatic fluctuations along thc film normal and allow the transition to progress in a series of discrete steps. The ordering effects of the surface tension inherent ill the liquid crystal/vapor interface have been clearly demonstrated by an X-ray diffi'action study [10]. The authors lbund that the molecular fluctuation arnplitudes incrcased sharply as a function of increasing depth into thc film, strongly supporting the hypothesis that the ordering originates at the free surfaces.…”

Section: Resultsmentioning

confidence: 92%

Layer-by-layer transitions in liquid crystals

et al. 1995

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Employing our free-standing fihn calc~rilnetric system, we have identified three remarkable layer-by-klycr transiticms in scver+,l liquid crystal COlllpottnds. Each of these Iransiticms can be well described by the simple pc~wer-law fc, rm: L= L,,t '. The layerirtg transitions Ibund near the smectic-A hexatic-B and smectic-A crystaI-B transitions are well characterized by the expcment v m I 3. This vahle is consistent with rnodels bz, sed on a van tier Waals-like dc, minant intermolecuhlr interaction. Another novel layer-by-k, yer thin,ling traqsition has been discovered abc, ve the bulk smectic-A isotropic transition of a perIluorim, tcd liquid cryst~,l compotmd. The value of the exponent obtained. v ~ 3 4. c~,nnot be easily explained using familiar models.KEY WORDS: free-standing film: heat capacity: I~tyer-b.v-layer transiticm: liquid crystal: smeclic-A hexalic-B transiticm.

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X-ray determination of the molecular tilt and layer fluctuation profiles of freely suspended liquid-crystal films

Cited by 100 publications

References 13 publications

Heat Capacity Investigations of Extremely Thin Liquid-Crystal Free-Standing Films

Heat Capacity Investigations of Extremely Thin Liquid-Crystal Free-Standing Films

X-ray reflectivity reveals equilibrium density profile of molecular liquid under nanometre confinement

Layer-by-layer transitions in liquid crystals

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