Observation of thickness quantization in liquid films confined to molecular dimension

Seeck, O. H.; Kim, Hyun-Jung; Lee, Dong Ryeol; Shu, Deming; Kaendler, I. D.; Basu, J. K.; Sinha, Sunil K.

doi:10.1209/epl/i2002-00274-6

Cited by 66 publications

(50 citation statements)

References 16 publications

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“…X-ray reflectivity has also been used to determine the fluid density distribution in confinement. The results directly confirm the layered structure of the confined fluids, but could not indicate or refute a first-order phase transition [30,31].…”

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confidence: 59%

Density and Phase State of a Confined Nonpolar Fluid

Kienle

Kuhl

2016

Phys. Rev. Lett.

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Measurements of the mean refractive index of a spherelike nonpolar fluid, octamethytetracylclosiloxane (OMCTS), confined between mica sheets, demonstrate direct and conclusive experimental evidence of the absence of a first-order liquid-to-solid phase transition in the fluid when confined, which has been suggested to occur from previous experimental and simulation results. The results also show that the density remains constant throughout confinement, and that the fluid is incompressible. This, along with the observation of very large increases (many orders of magnitude) in viscosity during confinement from the literature, demonstrate that the molecular motion is limited by the confining wall and not the molecular packing. In addition, the recently developed refractive index profile correction method, which enables the structural perturbation inherent at a solid-liquid interface and that of a liquid in confinement to be determined independently, was used to show that there was no measurable excess or depleted mass of OMCTS near the mica surface in bulk films or confined films of only two molecular layers.

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confidence: 59%

Density and Phase State of a Confined Nonpolar Fluid

Kienle

Kuhl

2016

Phys. Rev. Lett.

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“…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.…”

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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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“…Those molecules closest to the surface form the best-defined layer but the number of layers is believed to extend several molecular dimensions, to a distance normal to the surface approximately the correlation length of the bulk fluid. The layered structures on opposed surfaces interfere with one another when the spacing between two solids is less than twice this length [1]. Questions about confined fluid structure involve deep scientific puzzles.…”

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How Confined Lubricants Diffuse During Shear

et al. 2004

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The translational diffusion of a fluorescent dye embedded at a dilute concentration in a confined fluid was compared at rest and during shear. The fluid, octamethylcyclotetrasiloxane (OMCTS), was confined between step-free muscovite mica to thickness 3-4 layers. Fluorescence correlation spectroscopy showed that the time scales of intensity-intensity autocorrelation functions were essentially the same during shear and at rest, except they were faster during shear by a factor of 2 to 5. This dynamical probe of how liquids order in molecularly thin films fails to support the hypothesis that shear produced a melting transition. DOI: 10.1103/PhysRevLett.93.236105 PACS numbers: 68.35.Af, 68.08.-p A curious fact about liquids is that they wrap themselves around solid surfaces to form layers; the simple reason is that space must be filled. Those molecules closest to the surface form the best-defined layer but the number of layers is believed to extend several molecular dimensions, to a distance normal to the surface approximately the correlation length of the bulk fluid. The layered structures on opposed surfaces interfere with one another when the spacing between two solids is less than twice this length [1]. Questions about confined fluid structure involve deep scientific puzzles. On the applied side, they also pertain directly to understanding the physics of porous media, of dense suspensions of colloidal particles in fluids, and of friction and lubrication.Here we employ a fluorescence-based method to contrast the rate of Brownian diffusion during shear and at rest. Previously we introduced a method of few-molecule fluorescence to study confined fluids [2,3] but that study was restricted to fluids at rest. Here we describe what happens while sliding one surface past the other. The premise was that if the hypothesis of shear melting held, shear should induce speed up of the translational diffusion of a nonadsorbing fluorescent dye.The fluid, containing dilute dye, was confined to a thickness of 3-4 molecular dimensions between parallel single crystals of muscovite mica. A large amount of prior study probed the friction of such systems. The hypothesis of a shear-induced phase transition, from solid to fluid, has been advanced based on the observation that, in most such experiments, static friction gives way to kinetic friction in molecularly thin liquid films [4]. The generality of this interpretation has been cast into doubt by the observation that fluids confined between rough metal surfaces behave similarly [5] and especially by recent experiments showing that the solidity of molecularly thin films in mica-based experiments depends on a particular method of surface preparation [6,7]. As arguments about experimental protocols are surely of no interest outside a specialized community, it is opportune to address the important question of order in confined fluids by an independent technique. Fluorescence correlation spectroscopy (FCS) is usually used to study molecules dissolved in bulk solution [8,9]. As implement...

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Observation of thickness quantization in liquid films confined to molecular dimension

Cited by 66 publications

References 16 publications

Density and Phase State of a Confined Nonpolar Fluid

Density and Phase State of a Confined Nonpolar Fluid

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

How Confined Lubricants Diffuse During Shear

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