362 research outputs found
Basis for Calculating Cross Sections for Nuclear Magnetic Resonance Spin-Modulated Polarized Neutron Scattering
In this work we study the potential for utilizing the scattering of polarized neutrons from nuclei whose spin has been modulated using nuclear magnetic resonance (NMR). From first principles, we present an in-depth development of the differential scattering cross sections that would arise in such measurements from a hypothetical target system containing nuclei with non-zero spins. In particular, we investigate the modulation of the polarized scattering cross sections following the application of radio frequency pulses that impart initial transverse rotations to selected sets of spin-1/2 nuclei. The long-term aim is to provide a foundational treatment of the scattering cross section associated with enhancing scattering signals from selected nuclei using NMR techniques, thus employing minimal chemical or isotopic alterations, so as to advance the knowledge of macromolecular or liquid structure
Charge frustration in complex fluids and in electronic systems
The idea of charge frustration is applied to describe the properties of such
diverse physical systems as oil-water-surfactant mixtures and metal-ammonia
solutions. The minimalist charge-frustrated model possesses one energy scale
and two length scales. For oil-water-surfactant mixtures, these parameters have
been determined starting from the microscopic properties of the physical
systems under study. Thus microscopic properties are successfully related to
the observed mesoscopic structure.Comment: latex type, 13 page
Everybody Counts or Nobody Counts
Like most campuses, we constantly deal with change at RIT, ranging from policy to climate. We believe everyone needs to feel valued - “Everybody Counts or Nobody Counts, and are attempting to build a supportive culture that includes individual and group mentoring, funding opportunities, and recognition
Paracrystal model of the high-temperature lamellar phase of a ternary microemulsion system
The effect of solvent and counterion variation on inverse micelle CMCs in hydrocarbon solvents
Critical micelle concentrations (CMCs) for the formation of inverse micelles have been determined for anionic surfactants in nonpolar, hydrocarbon solvents. Sodium dioctylsulfosuccinate (Aerosol OT or AOT) was chosen as the model surfactant, with systematic variations in both the solvent (benzene, cyclohexane, and dodecane) and the surfactant counterion (sodium and tetrapropylammonium). Recent work (Langmuir 29 (2013) 3352–3258) has shown that high-resolution small-angle neutron scattering (SANS) measurements can be used to directly determine the presence or absence of aggregates in solution. No variation in the value of the CMC was found within the resolution of the measurements for changing either solvent or counterion; some effects on the structure of inverse micelles were observed. This lack of a significant difference in the onset of inverse micellization with changes to the molecular species is surprising, and the implications on the solvophobic effect in nonpolar solvents are discussed
A corresponding states approach to Small-Angle-Scattering for polydisperse ionic colloidal fluids
Approximate scattering functions for polydisperse ionic colloidal fluids are
obtained by a corresponding states approach. This assumes that all pair
correlation functions of a polydisperse fluid are
conformal to those of an appropriate monodisperse binary fluid (reference
system) and can be generated from them by scaling transformations. The
correspondence law extends to ionic fluids a {\it scaling approximation} (SA)
successfully proposed for nonionic colloids in a recent paper. For the
primitive model of charged hard spheres in a continuum solvent, the partial
structure factors of the monodisperse binary reference system are evaluated by
solving the Orstein-Zernike (OZ) integral equations coupled with an approximate
closure. The SA is first tested within the mean spherical approximation (MSA)
closure, which allows analytical solutions. The results are found in good
overall agreement with exact MSA predictions up to relevant polidispersity. The
SA is shown to be an improvement over the ``decoupling approximation'' extended
to the ionic case. The simplicity of the SA scheme allows its application also
when the OZ equations can be solved only numerically. An example is then given
by using the hypernetted chain (HNC) closure. Shortcomings of the SA approach,
its possible use in the analysis of experimental scattering data and other
related points are also briefly addressed.Comment: 29 pages, 7 postscript figures (included), Latex 3.0, uses aps.sty,
to appear in Phys. Rev. E (1999
Viscosity and Diffusion: Crowding and Salt Effects in Protein Solutions
We report on a joint experimental-theoretical study of collective diffusion
in, and static shear viscosity of solutions of bovine serum albumin (BSA)
proteins, focusing on the dependence on protein and salt concentration. Data
obtained from dynamic light scattering and rheometric measurements are compared
to theoretical calculations based on an analytically treatable spheroid model
of BSA with isotropic screened Coulomb plus hard-sphere interactions. The only
input to the dynamics calculations is the static structure factor obtained from
a consistent theoretical fit to a concentration series of small-angle X-ray
scattering (SAXS) data. This fit is based on an integral equation scheme that
combines high accuracy with low computational cost. All experimentally probed
dynamic and static properties are reproduced theoretically with an at least
semi-quantitative accuracy. For lower protein concentration and low salinity,
both theory and experiment show a maximum in the reduced viscosity, caused by
the electrostatic repulsion of proteins. The validity range of a generalized
Stokes-Einstein (GSE) relation connecting viscosity, collective diffusion
coefficient, and osmotic compressibility, proposed by Kholodenko and Douglas
[PRE 51, 1081 (1995)] is examined. Significant violation of the GSE relation is
found, both in experimental data and in theoretical models, in semi-dilute
systems at physiological salinity, and under low-salt conditions for arbitrary
protein concentrations
Extending vaterite microviscometry to ex vivo blood vessels by serial calibration
The endothelial glycocalyx layer is a ~2 µm thick glycosaminoglycan rich pericellular matrix expressed on the luminal surface of vascular endothelial cells, which has implications in vessel mechanics and mechanotransduction. Despite its role in vascular physiology, no direct measurement has of yet been made of vessel glycocalyx material properties. Vaterite microviscometry is a laser tweezers based microrheological method, which has been previously utilized to measure the viscosity of linear and complex fluids under flow. This form of microrheology has until now relied on complete recollection of the forward scattered light. Here we present a novel method to extend vaterite microviscometry to relatively thick samples. We validate our method and its assumptions and measure the apparent viscosity as a function of distance from the vascular endothelium. We observe a differential response in conditions designed to preserve the EGL in comparison to those designed to collapse it
Concentration Independent Modulation of Local Micromechanics in a Fibrin Gel
Methods for tuning extracellular matrix (ECM) mechanics in 3D cell culture that rely on increasing the concentration of either protein or cross-linking molecules fail to control important parameters such as pore size, ligand density, and molecular diffusivity. Alternatively, ECM stiffness can be modulated independently from protein concentration by mechanically loading the ECM. We have developed a novel device for generating stiffness gradients in naturally derived ECMs, where stiffness is tuned by inducing strain, while local mechanical properties are directly determined by laser tweezers based active microrheology (AMR). Hydrogel substrates polymerized within 35 mm diameter Petri dishes are strained non-uniformly by the precise rotation of an embedded cylindrical post, and exhibit a position-dependent stiffness with little to no modulation of local mesh geometry. Here we present the device in the context of fibrin hydrogels. First AMR is used to directly measure local micromechanics in unstrained hydrogels of increasing fibrin concentration. Changes in stiffness are then mapped within our device, where fibrin concentration is held constant. Fluorescence confocal imaging and orbital particle tracking are used to quantify structural changes in fibrin on the micro and nano levels respectively. The micromechanical strain stiffening measured by microrheology is not accompanied by ECM microstructural changes under our applied loads, as measured by confocal microscopy. However, super-resolution orbital tracking reveals nanostructural straightening, lengthening, and reduced movement of fibrin fibers. Furthermore, we show that aortic smooth muscle cells cultured within our device are morphologically sensitive to the induced mechanical gradient. Our results demonstrate a powerful cell culture tool that can be used in the study of mechanical effects on cellular physiology in naturally derived 3D ECM tissues
Simultaneous SAXS and SANS Analysis for the Detection of Toroidal Supramolecular Polymers Composed of Noncovalent Supermacrocycles in Solution
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