Kim Nygård

Salt-induced changes of colloidal interactions in critical mixtures

We report on salt-dependent interaction potentials of a single charged particle suspended in a binary liquid mixture above a charged wall. For symmetric boundary conditions (BC) we observe attractive particle-wall interaction forces which are similar to critical Casimir forces previously observed in salt-free mixtures. However, in case of antisymmetric BC we find a temperature-dependent crossover from attractive to repulsive forces which is in strong contrast to salt-free conditions. Additionally performed small-angle X-ray scattering experiments demonstrate that the bulk critical fluctuations are not affected by the addition of salt. This suggests that the observed crossover can not be attributed to critical Casimir forces alone. Instead our experiments point towards a possible coupling between the ionic distributions and the concentration profiles in the binary mixture which then affects the interaction potentials in such systems.

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.

Nonequilibrium Phases of Nanoparticle Langmuir Films

We report on an in-situ observation of the colloidal silver nanoparticle self-assembly into a close-packed monolayer at the air/water interface followed by a 2D to 3D transition. Using the fast tracking GISAXS technique, we were able to observe the immediate response to the compression of the self-assembled nanoparticle layer at the air/water interface and to identify all relevant intermediate stages including those far from the equilibrium. In particular, a new nonequilibrium phase before the monolayer collapse via the 2D to 3D transition was found that is inaccessible by the competing direct space imaging techniques such as the scanning and transmission electron microscopies due to the high water vapor pressure and surface tension.

In situ characterization of block copolymer ordering on chemically nanopatterned surfaces by time-resolved small angle x-ray scattering

The assembly of lamella-forming block copolymer blend thin films on chemically nanopatterned striped surfaces was monitored in real time with small angle x-ray scattering (SAXS) in transmission mode. The strongest diffraction from the assembled grating structure was detected after 4.5min of annealing as the temperature was ramped from 100to240°C at a rate of about 20°C∕min. Real-space images were also obtained from samples annealed for specific times using top-down scanning electron microscopy (SEM) and this identified structures formed during annealing that are unique to the block copolymer blends. The data are compared to previously reported SEM and molecular simulation studies with pure block copolymers. Because it can be used in real time and probes the entire film thickness, transmission SAXS proved to be a useful tool for better understanding the block copolymer annealing process.

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

Mechanism and dynamics of block copolymer directed assembly with density multiplication on chemically patterned surfaces

In this work, we used scanning electron microscopy (SEM), in situ coherent small angle x-ray scattering (SAXS), and Monte Carlo molecular simulation to gain insights into the dynamics of block copolymer directed assembly with density multiplication on chemically patterned surfaces. During directed assembly, it was observed with SEM that poly(styrene-block-methyl methacrylate) initially formed discrete polystyrene domains that lacked long-range order at the free surface. After further annealing, the polystyrene domains gradually coalesced into linear domains that were not registered fully with the underlying chemical pattern. The linear domains could be trapped in metastable morphologies. Finally, the linear polystyrene domains formed perpendicular lamellae in full registration with the underlying chemical pattern. It was revealed with SAXS that scattering peaks characteristic of the period of the chemical pattern appeared and disappeared at the early stages of assembly. Finally, the morphological evolutio...

Local order variations in confined hard-sphere fluids

Pair distributions of fluids confined between two surfaces at close distance are of fundamental importance for a variety of physical, chemical, and biological phenomena, such as interactions between macromolecules in solution, surface forces, and diffusion in narrow pores. However, in contrast to bulk fluids, properties of inhomogeneous fluids are seldom studied at the pair-distribution level. Motivated by recent experimental advances in determining anisotropic structure factors of confined fluids, we analyze theoretically the underlying anisotropic pair distributions of the archetypical hard-sphere fluid confined between two parallel hard surfaces using first-principles statistical mechanics of inhomogeneous fluids. For this purpose, we introduce an experimentally accessible ensemble-averaged local density correlation function and study its behavior as a function of confining slit width. Upon increasing the distance between the confining surfaces, we observe an alternating sequence of strongly anisotropic versus more isotropic local order. The latter is due to packing frustration of the spherical particles. This observation highlights the importance of studying inhomogeneous fluids at the pair-distribution level.

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.