E. Perret

Confinement-induced orientational alignment of quasi-2D fluids

We have developed a unique approach for studying the ensemble-averaged nearest-neighbor coordination of confined fluids by combining small-angle X-ray scattering and phase-retrieval–based X-ray diffraction from fluid-filled nanofluidic channel arrays. We apply the method to a charge-stabilized quasi–two-dimensional colloidal fluid (particle diameter 48 nm), focusing on the structural transition from a monolayer to a bilayer with increasing fluid film thickness. In contrast to theoretical work on the paradigmatic hard-sphere fluid, we find unambiguous experimental evidence for orientational alignment of fluids in extreme confinement.

Buckling and layering transitions in confined colloids

We report layering transitions within a charged silica colloidal dispersion confined by two opposite like-charged dielectric walls. The ensemble- averaged concentration profiles of the colloids (radius 60 +/- 2 nm) along the confinement direction have been determined using synchrotron X-ray diffraction from microfluidic arrays of channels of different widths. For small channel widths up to a critical value of 300 nm, the channel can accommodate just one layer of colloids which is stabilized against buckling by the confining charged walls. For channel widths larger than this critical value, a buckling of the single layer is observed. These phenomena are explained using a theoretical analysis of buckling instabilities due to Chou and Nelson, and a value for the charge density on the stabilizing charged walls is derived. At still larger channel widths a sequence of complex layering transitions is observed which involve the splitting and merging of individual layers. Copyright (C) EPLA, 2009

Molecular liquid under nanometre confinement: density profiles underlying oscillatory forces

Ultrathin (<12 nm) films of tetrakis(trimethyl)siloxysilane (TTMSS) have been confined by atomically flat mica membranes in the presence and absence of applied normal forces. When applying normal forces, discrete film thickness transitions occur, each involving the expulsion of TTMSS molecules. Using optical interferometry we have measured the step size associated with a film thickness transition (7.5 Å for compressed, 8.4 Å for equilibrated films) to be smaller than the molecular diameter of 9.0 Å. Layering transitions with a discrete step size are commonly regarded as evidence for strong layering of the liquids molecules in planes parallel to the confining surfaces and it is assumed that the layer spacing equals the measured periodicity of the oscillatory force profile. Using x-ray reflectivity (XRR), which directly yields the liquids density profile along the confinement direction, we show that the layer spacing (10-11 Å) proves to be on average significantly larger than both the step size of a layering transition and the molecular diameter. We observe at least one boundary layer of different electron density and periodicity than the layers away from the surfaces.

X-ray reflectivity theory for determining the density profile of a liquid under nanometre confinement

The determination of out-of-plane density profiles of a confined molecular liquid by synchrotron X-ray reflectivity is presented.

Grating-based holographic X-ray diffraction: theory and application to confined fluids

A grating-based holographic X-ray diffraction technique has been developed for reconstructing density profiles of nano-scale fluids confined in channel arrays. Within this approach, the reference wave is due to diffraction from the fabricated channel array, whereas the object wave is generated by the confinement-induced ordering of the fluid. The ensemble-averaged density profile of the fluid across the confining channel, which constitutes a weak phase object, is then determined in a model-independent manner from the interference between the reference and object waves by direct Fourier inversion. The validity of the linear holographic approach and its connection to the autocorrelation function, the inclusion of channel tapering, and volume-diffraction effects are discussed in detail.

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

A silane (tetrakis(trimethylsiloxy)silane) has been confined within a space of a few molecular diameters (9 A) between two atomically flat opposing mica membranes. The liquids 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 liquids 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.

Surface-specific ordering of reverse micelles in confinement

We have applied holographic X-ray diffraction from fluid-filled channel arrays for model-independent density reconstruction of spherical AOT/water/isooctane reverse micelles (average diameter σ≃12–13 nm) confined between planar surfaces. We find the confinement-induced ordering of the reverse micelles to strongly depend on the surface potential of the confining surfaces: for hydrophilic surfaces we find diffuse monolayers centered at 13 ± 3 nm away from the solid–fluid interface, while for hydrophobic surfaces we observe close-packed monolayers at the solid–fluid interface.

Self-Consistent Algorithm for Calibrating Spectrometers to Picometer Accuracy over the Entire Wavelength Range

Spectrometer calibration accuracies are of high importance for a wide range of applications. Typically, one calibrates the spectrometer with a calibration lamp, providing distinct and well-defined calibration lines. However, for small spectral ranges, where only two calibration lines are present, the calibration becomes inaccurate. We present a high-precision nonlinear wavelength calibration method, which is based on two or more reference lines from a calibration lamp. The additional key element introduced is a Fabry-Perot multilayer structure that yields multiple sharp transmission maxima of similar intensity over the full spectrometer range under broad-band illumination (e.g., white-light source). An iterative algorithm is put forward to obtain a self-consistent calibration of picometer precision over the full spectrometer range. In regions distant from calibration lines the accuracy is enhanced by at least a factor of two compared to conventional methods.

Structure of confined fluids by x-ray interferometry using diffraction gratings

We develop a novel method for structure determination of confined fluids using diffraction-grating-based x-ray interferometry.Within this approach, diffraction from a microfluidic array, which acts both as confinement and transmission diffraction grating, provides the reference wave, whereas the density modulations of the confined fluid, acting as a weak phase object, generate the object wave. The ensemble-averaged density profile of the fluid perpendicular to the confining channel is then unambiguously obtained from the interference between the reference and object waves by direct Fourier inversion.

Diffraction gratings as small-angle X-ray scattering calibration standards

It is shown that diffraction gratings can be used as accurate momentum-transfer calibration standards in small-angle X-ray scattering experiments. For demonstration purposes, a silicon diffraction grating with a period of 400 nm is used. The data exhibit 50 diffraction peaks evenly distributed in the momentum-transfer range q = 0.0–0.8 nm−1, a regime that is not accessible using the traditional silver behenate standard.