Density fluctuations of hard-sphere fluids in narrow confinement

Spatial confinement induces microscopic ordering of fluids, which in turn alters many of their dynamic and thermodynamic properties. However, the isothermal compressibility has hitherto been largely overlooked in the literature, despite its obvious connection to the underlying microscopic structure and density fluctuations in confined geometries. Here, we address this issue by probing density profiles and structure factors of hard- sphere fluids in various narrow slits, using x-ray scattering from colloid-filled nanofluidic containers and integral-equation-based statistical mechanics at the level of pair distributions for inhomogeneous fluids. Most importantly, we demonstrate that density fluctuations and isothermal compressibilities in confined fluids can be obtained experimentally from the long-wavelength limit of the structure factor, providing a formally exact and experimentally accessible connection between microscopic structure and macroscopic, thermodynamic properties. Our approach will thus, for example, allow direct experimental verification of theoretically predicted enhanced density fluctuations in liquids near solvophobic interfaces

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 alone to critical Casimir forces. 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.Comment: 5 pages, 4 Figure

Nanostructure-specific X-ray tomography reveals myelin levels, integrity and axon orientations in mouse and human nervous tissue

Myelin insulates neuronal axons and enables fast signal transmission, constituting a key component of brain development, aging and disease. Yet, myelin-specific imaging of macroscopic samples remains a challenge. Here, we exploit myelin’s nanostructural periodicity, and use small-angle X-ray scattering tensor tomography (SAXS-TT) to simultaneously quantify myelin levels, nanostructural integrity and axon orientations in nervous tissue. Proof-of-principle is demonstrated in whole mouse brain, mouse spinal cord and human white and gray matter samples. Outcomes are validated by 2D/3D histology and compared to MRI measurements sensitive to myelin and axon orientations. Specificity to nanostructure is exemplified by concomitantly imaging different myelin types with distinct periodicities. Finally, we illustrate the method’s sensitivity towards myelin-related diseases by quantifying myelin alterations in dysmyelinated mouse brain. This non-destructive, stain-free molecular imaging approach enables quantitative studies of myelination within and across samples during development, aging, disease and treatment, and is applicable to other ordered biomolecules or nanostructures

Orientation Distributions of Cellulose Nanofibrils and Nanocrystals in Confined Flow

The pursuit of sustainable and environmentally friendly materials has been driving a tremendous interest in biobased alternatives in the last decade. Nanocellulose has been widely seen as a prime contender due to its impressive properties as well as being abundant and biodegradable. Recently, it has been demonstrated how nanocellulosic materials can be hydrodynamically aligned in flows and assembled continuously into materials with tunable macroscopic properties. However, the aligning mechanisms of the highly entangled system of elongated nanoparticles in different flow situations still remain largely unknown. Here, we investigate the orientation distributions of cellulose nanofibrils and nanocrystals (CNF and CNC) in a straight quadratic channel at various flow rates using small-angle X-ray scattering (SAXS), where CNF and CNC are aligned by strong shear flow close to the walls. In dilute systems, CNC behave as Brownian ellipsoids, while at semi-dilute concentrations there seems to be a limit to how high alignment of CNF and CNC can be achieved in a shear dominated flow even though particle interactions clearly aid in aligning the system at low flow rates. Furthermore, we show how some essential parameters in the orientational distribution can be obtained with polarized optical microscopy

Study on the Subgel-Phase Formation Using an Asymmetric Phospholipid Bilayer Membrane by High-Pressure Fluorometry

The myristoylpalmitoylphosphatidylcholine (MPPC) bilayer membrane shows a complicated temperature–pressure phase diagram. The large portion of the lamellar gel (L_β′), ripple gel (P_β′), and pressure-induced gel (L_βI) phases exist as metastable phases due to the extremely stable subgel (L_c) phase. The stable L_c phase enables us to examine the properties of the L_c phase. The phases of the MPPC bilayers under atmospheric and high pressures were studied by small-angle neutron scattering (SANS) and fluorescence spectroscopy using a polarity-sensitive fluorescent probe Prodan. The SANS measurements clearly demonstrated the existence of the metastable L_βI phase with the smallest lamellar repeat distance. From a second-derivative analysis of the fluorescence data, the line shape for the L_c phase under high pressure was characterized by a broad peak with a minimum of ca. 460 nm. The line shapes and the minimum intensity wavelength (λ″_min) values changed with pressure, indicating that the L_c phase has highly pressure-sensible structure. The λ″_min values of the L_c phase spectra were split into ca. 430 and 500 nm in the L_βI phase region, which corresponds to the formation of a interdigitated subgel L_c (L_cI) phase. Moreover, the phase transitions related to the L_c phase were reversible transitions under high pressure. Taking into account the fluorescence behavior of Prodan for the L_c phase, we concluded that the structure of the L_c phase is highly probably a staggered structure, which can transform into the L_cI phase easily