Scattering Wave Vector Research Articles

Modulus, Confinement, and Temperature Effects on Surface Capillary Wave Dynamics in Bilayer Polymer Films Near the Glass Transition

We report relaxation times (τ) for surface capillary waves on 27-127 nm polystyrene (PS) top layers in bilayer films using x-ray photon correlation spectroscopy. At ∼10 °C above the PS glass transition temperature (T(g)), τ tracks with underlayer modulus, being significantly smaller on softer substrates at low in-plane scattering wave vector. Relative to capillary wave theory, we also report stiffening behavior upon nanoconfinement of the PS layers. At PS T(g)+40 °C, both effects become negligible. We demonstrate how neighboring polymer domains impact dynamics over substantial length scales.

Salt-induced fast aggregation of nano-emulsions: structural and kinetic scaling

Salt-induced fast aggregation of oil-in-water nano-emulsions stabilized by the anionic surfactant sodium dodecyl sulfate was analyzed by employing dynamic and static light scattering techniques. Nano-emulsions with different ratios of dodecane/hexadecane were studied. The time evolution of the average size of all nano-emulsions collapsed in the same curve despite differences in the composition of the oil phase. A power law growing and a value of the homogeneity parameter similar to that typically found in model solid particle systems in diffusive aggregation regime were found. A value of 2.4 was estimated for the effective fractal dimension both from the scaling of scattered intensity with scattering wave vector modulus and from the kinetic scaling. This result indicates that the aggregates are more compact structures compared with model solid systems. It can be explained as a result of the effect of polydispersity in the primary droplet size and the coalescence events that occur inside the aggregates.

A calculation model for X-ray diffraction by curved-graphene nanoparticles

An approximation of the positions of carbon atoms in a curved graphene sheet is suggested for calculation of X-ray diffraction (XRD) patterns of curved-graphene nanoparticles. The model is tested for carbon nanotubes and newly calculated carbon nanotoroids consisting of several hundreds of atoms. It is shown that the random distribution of carbon atoms with graphene surface-averaged density and the local graphene-like rearrangement of atoms in a curved lattice are sufficient for describing the XRD patterns of an ensemble of respective exact carbon nanoparticles of random isotropic orientation in the range of scattering wave vector's modulus q from several units to several tens of inverse nanometers. The model is of interest to a fast-routine identification of curved-graphene nanoparticles in carbonaceous materials.

Power-law decay of standing waves on the surface of topological insulators

We propose a general theory on the standing waves (quasiparticle interference pattern) caused by the scattering of surface states off step edges in topological insulators, in which the extremal points on the constant energy contour of surface band play the dominant role. Experimentally we image the interference patterns on both Bi$_2$Te$_3$ and Bi$_2$Se$_3$ films by measuring the local density of states using a scanning tunneling microscope. The observed decay indices of the standing waves agree excellently with the theoretical prediction: In Bi$_2$Se$_3$, only a single decay index of -3/2 exists; while in Bi$_2$Te$_3$ with strongly warped surface band, it varies from -3/2 to -1/2 and finally to -1 as the energy increases. The -1/2 decay indicates that the suppression of backscattering due to time-reversal symmetry does not necessarily lead to a spatial decay rate faster than that in the conventional two-dimensional electron system. Our formalism can also explain the characteristic scattering wave vectors of the standing wave caused by non-magnetic impurities on Bi$_2$Te$_3$.

Emergent Rotational Symmetries in Disordered Magnetic Domain Patterns

Uniaxial systems often form labyrinthine domains that exhibit short-range order but are macroscopically isotropic and would not be expected to exhibit precise symmetries. However, their underlying frustration results in a multitude of metastable configurations of comparable energy, and driving such a system externally might lead to pattern formation. We find that soft x-ray speckle diffraction patterns of the labyrinthine domains in CoPd/IrMn heterostructures reveal a diverse array of hidden rotational symmetries about the magnetization axis, thereby suggesting an unusual form of emergent order in an otherwise disordered system. These symmetries depend on applied magnetic field, magnetization history, and scattering wave vector. Maps of rotational symmetry exhibit intriguing structures that can be controlled by manipulating the applied magnetic field in concert with the exchange bias condition.

Studies on Lactoferricin-derived Escherichia coli Membrane-active Peptides Reveal Differences in the Mechanism of N-Acylated Versus Nonacylated Peptides

To improve the low antimicrobial activity of LF11, an 11-mer peptide derived from human lactoferricin, mutant sequences were designed based on the defined structure of LF11 in the lipidic environment. Thus, deletion of noncharged polar residues and strengthening of the hydrophobic N-terminal part upon adding a bulky hydrophobic amino acid or N-acylation resulted in enhanced antimicrobial activity against Escherichia coli, which correlated with the peptides' degree of perturbation of bacterial membrane mimics. Nonacylated and N-acylated peptides exhibited different effects at a molecular level. Nonacylated peptides induced segregation of peptide-enriched and peptide-poor lipid domains in negatively charged bilayers, although N-acylated peptides formed small heterogeneous domains resulting in a higher degree of packing defects. Additionally, only N-acylated peptides perturbed the lateral packing of neutral lipids and exhibited increased permeability of E. coli lipid vesicles. The latter did not correlate with the extent of improvement of the antimicrobial activity, which could be explained by the fact that elevated binding of N-acylated peptides to lipopolysaccharides of the outer membrane of gram-negative bacteria seems to counteract the elevated membrane permeabilization, reflected in the respective minimal inhibitory concentration for E. coli. The antimicrobial activity of the peptides correlated with an increase of membrane curvature stress and hence bilayer instability. Transmission electron microscopy revealed that only the N-acylated peptides induced tubular protrusions from the outer membrane, whereas all peptides caused detachment of the outer and inner membrane of E. coli bacteria. Viability tests demonstrated that these bacteria were dead before onset of visible cell lysis.

Momentum transfer resolved memory in a magnetic system with perpendicular anisotropy

We have used resonant, coherent soft x-ray scattering to measure wave vector resolved magnetic domain memory in Co/Pd multilayers. The technique uses angular cross correlation functions and can be applied to any system with circular annuli of constant values of scattering wave vector q. In our Co/Pd film, the memory exhibits a maximum at q=0.0384 nm−1 near initial reversal that decreases in magnitude as the magnetization is further reversed. The peak is attributed to bubble domains that nucleate reproducibly near initial reversal and which grow into a labyrinth domain structure that is not reproduced from one magnetization cycle to the next.

Few-nanometer-wide carbon toroids in the hydrocarbon films deposited in tokamak T-10

Measurements of X-ray scattering by the hydrocarbon films, deposited in tokamak T-10, in the range of scattering wave vector’s modulus q ∼ 5 ÷ 70 nm - 1 are interpreted via solving an optimization problem within a broad class of carbon nanostructures (tubules, fullerenes and bigger spheres, ellipsoids, toroids, half-fragments of all these structures, flat flakes, etc.), with allowance for amorphous hydrocarbon component and heavy impurities. It is shown that (i) wide peaks at q ∼ 10 , 30 and 55 nm - 1 may be caused by the nanostructures formed by the curved single-layer graphene, (ii) respective most probable candidates are 2–3 nm wide, low-aspect-ratio toroids.

Investigating the microstructure of a yield-stress fluid by light scattering

We report the results of an experimental study of the microstructure of dispersions of Carbopol ETD 2050, a model yield-stress fluid. Using two different light scattering instruments, measurements were made over three decades in scattering wave vector, from 0.02 to 25 μm⁻¹. These measurements reveal microstructure characterized by two length scales: a longer length scale, 6 μm and larger, that depends on Carbopol concentration and the pH of the dispersion and a shorter length scale of approximately 400 nm that is independent of both sample concentration and pH. We relate these results to shear rheology measurement of the yield stress of these materials.

Quasiparticle Interference ofC2-Symmetric Surface States in a LaOFeAs Parent Compound

We present scanning tunneling microscopy studies of the LaOFeAs parent compound of iron pnictide superconductors. High resolution spectroscopic imaging reveals strong standing wave patterns induced by quasiparticle interference of two-dimensional surface states. Fourier analysis shows that the distribution of scattering wave vectors exhibits pronounced twofold (C(2)) symmetry, strongly reminiscent of the nematic electronic state found in CaFe(1.94)Co(0.06)As(2). The implications of these results to the electronic structure of the pnictide parent states will be discussed.

Resonance enhanced dynamic light scattering

We present a novel light scattering setup that enables probing of dynamics near solid surfaces. An evanescent wave generated by a surface plasmon resonance in a metal layer is the incident light field in the dynamic light scattering experiment. The combination of surface plasmon resonance spectroscopy and dynamic light scattering leads to a spatiotemporal resolution extending a few hundred nanometers from the surface and from microseconds to seconds. The comparison with evanescent wave dynamic light scattering identifies the advantages of the presented technique, e.g., surface monitoring, use of metal surfaces, and biorelevant systems. For both evanescent wave geometries, we define the scattering wave vector necessary for the analysis of the experimental relaxation functions.

Development of absolute intensity multi-angle light scattering for the determination of polydisperse soot aggregate properties

This work investigates the capability of multi-angle light scattering to determine polydisperse aggregate parameters including the size distribution and the fractal dimension. The investigation considers one laser wavelength in the visible at 527nm and a fixed angle range from 10° to 160°. The corresponding range of the scattering wave vector limits the measured section of the overall structure factor significantly and makes an unambiguous determination of the size distribution parameters using relative multi-angle scatter intensities difficult. An interplay of the distribution width parameter and the fractal dimension is significant and impairs the possibilities for determining both properties simultaneously. The ambiguity of solutions can be overcome by including the absolute scattering intensity which is obtained via comparison with a known Rayleigh scatterer. However, additional parameters like the primary particle diameter, the soot volume fraction, the fractal prefactor, and the scattering function of the refractive index F(m) must be known with sufficient accuracy. We show how the analysis can be applied to a variety of soot sources ranging from very small aggregates in premixed flames, over intermediate aggregates in a laminar diffusion flame to large cooled soot emitted from a diffusion flame in inverted geometry.

Aggregation Behavior of Triple Helical Polysaccharide with Low Molecular Weight in Diluted Aqueous Solution

It has been proven in our previous work that the lentinan triple helical chains with high weight-average molecular weight (M(w) = 1.71 x 10(6)) formed easily a self-entangle one, and no ordered aggregates were detected. In the present work, we used the ultrasonic method to degrade the lentinan triple helical chains to obtain a sample with a mean value of M(w) approximately 5.0 x 10(5) g/mol. Subsequently, its dilute aqueous solution properties were studied by dynamic light scattering (DLS). The relaxation time distributions exhibited two modes (fast and slow) with different relaxation time scales. The fast mode was attributed to the relaxation of an individual triple helical lentinan, whereas the slow mode indicated the formation of large aggregates. On the basis of the scattering wave vector dependencies for the scattering intensity and for the amplitudes and characteristic times associated with the relaxation modes, the molecular parameters were calculated by combing static LS with DLS. The values of the radius of gyration for individuals (R(g)(indi)) and aggregates (R(g)(agg)) were 48.2 and 75.4 nm, and those of the hydrodynamic radius for individuals (R(h)(indi)) and aggregates (R(h)(agg)) were 14.9 and 98.4 nm, respectively. Furthermore, the structure-sensitive dimensionless parameter of individuals (rho(indi) = 3.23) and aggregates (rho(agg) = 0.766) indicated that the individual triple helical chains were stiff, whereas the aggregates existed as compact clusters. The aggregates consisted of short triple helical chains by packing close to form "faggot-like" assembly, and the ordered aggregates (near 10%) coexisted with the predominant triple helical chain in the aqueous solution. Atomic force microscopy provided straightforward evidence on the shape of the triple helical chains and their aggregates in water.

Explanation of the patterns in Mie theory

The far-field scattered light intensity, or the related phase function, for a spherical particle is known to display an overall power-law structure when formulated in terms of the scattering wave vector. Empirically determined patterns in the intensity relating to the particle size and refractive index are known. The cause of the patterns, however, has not been satisfactorily explained. This work applies an exact microphysical model to explain most of the patterns, and specifically, to reveal the physical cause of crossovers from one power-law to another. A unique aspect of this microphysical approach is phasor analysis, which provides a visually based way to examine the angle-dependent wavelet superposition involved in the model. A simple color coding scheme connects the phasors to the interior of the particle, and it is this connection that reveals the meaning of the crossovers.

Scattering information obtained by optical microscopy: Differential dynamic microscopy and beyond

We describe the use of a bright-field microscope for dynamic light scattering experiments on weakly scattering samples. The method is based on collecting a time sequence of microscope images and analyzing them in the Fourier space to extract the characteristic time constants as a function of the scattering wave vector. We derive a theoretical model for microscope imaging that accounts for (a) the three-dimensional nature of the sample, (b) the arbitrary coherence properties of the light source, and (c) the effect of the finite numerical aperture of the microscope objective. The model is tested successfully against experiments performed on a colloidal dispersion of small spheres in water, by means of the recently introduced differential dynamic microscopy technique [R. Cerbino and V. Trappe, Phys. Rev. Lett. 100, 188102 (2008)]. Finally, we extend our model to the class of microscopy techniques that can be described by a linear space-invariant imaging of the density of the scattering centers, which includes, for example, dynamic fluorescence microscopy.

Dynamics of Concentrated Hard-Sphere Colloids Near a Wall

We investigate the Brownian motion of hard-sphere colloids near a solid wall by Evanescent Wave Dynamic Light Scattering (EWDLS). We carried out measurements for various volume fractions of sterically stabilized poly(methyl methacrylate) (PMMA) particles over a range of scattering wave vectors, q. While in the dilute regime, the near wall short-time diffusion is significantly slowed down due to particle-wall hydrodynamic interactions (HI); as volume fraction increases, the wall effect is progressively diminished at all q's. We present a new analysis for the EWDLS short-time self- and collective diffusivities applicable to all volume fractions and a simple model for the self-diffusion describing the interplay between particle-wall and particle-particle HI. Moreover, a weaker decay of the near-wall self-diffusion coefficient with volume fraction is predicted by Stokesian dynamics simulations.

Studies on pore morphology of titanium and its oxide by small angle neutron scattering

Titanium metal bodies have been prepared from the sintered powder compacts of TiO2 by a novel molten salt electrochemical approach, known as FFC Cambridge process. The phase and compositional characterizations of both Ti and TiO2 have been carried out by X-ray diffraction. The pore morphologies of sintered TiO2 pellet and the metallic Ti pellet, obtained after electrochemical reduction have been studied by SANS over a scattering wave vector q range of 0.003–3.5 nm−1 using a double crystal diffractometer and a pin-hole collimated SANS instrument. In the case of reduced metal pellet, average pore size was found to be larger than that of the oxide pellet as the voids left behind after the oxygen atoms left the oxide matrix, could not coalesce.

Small angle neutron scattering study of pore microstructure in ceria compacts

Ceria powders were prepared by gel combustion process using cerium nitrate and hitherto unexplored amino acids such as aspartic acid, arginine and valine as fuels. The powders have been characterized by X-ray and laser diffraction. Cold pressed compacts of these powders have been sintered at 1250°C for 2 h. Internal pore microstructure of the sintered compacts has been investigated by small angle neutron scattering (SANS) over a scattering wave vector q range of 0.003–0.17 nm−1. The SANS profiles indicate surface fractal morphology of the pore space with fractal dimensionality lying between 2.70 and 2.76.

The Rouse–Mooney model for coherent quasielastic neutron scatterings of single chains well entangled in polymer melts

The dynamic structure factor (DSF) for single (labeled) chains well entangled in polymer melts has been developed based on the Rouse-Mooney picture; the DSF functions derived from the Langevin equations of the model in both discrete and continuous forms are given. It is shown that for all practical purposes, it is sufficient to use the continuous form to analyze experimental results in the "safe" q region (q being the magnitude of the scattering wave vector q) where the Rouse-segment-based theories are applicable. The DSF form reduces to the same limiting form as that of the free Rouse chain as q(2)a(2) or q(2)R(2)-->infinity (a and R being the entanglement distance and the root mean square end-to-end distance, respectively), confirming what has been expected physically. The natural reduction to the limiting form allows the full range of DSF curves to be displayed in terms of the reduced Rouse variable q(2)(Z(d)t)(0.5) in a unified way. The displayed full range represents a framework or "map," with respect to which effects occurring in different regions of the DSF may be located and studied in a consistent manner. One effect is the significant or noticeable deviations of the theoretical DSF curves from the limiting curve in the region approximately 4>q(2)(Z(d)t)(0.5)> approximately 0.1 (a time region where t<tau(1) (e)) to the faster side as qa is in the range 1-5. This is supported by the comparison of the experimental results of an entangled poly(vinylethylene) sample with the theoretical curves. The DSF functional forms predict plateaus with heights depending on the value of q-q-split plateaus-as can be experimentally observed in the time region greater than the relaxation time tau(1) (e) of the lowest Rouse-Mooney mode, when qa falls between approximately 1 and approximately 7. High sensitivity of the distribution of the q-split plateaus to a enables its value to be extracted from matching the calculated with the experimental results. The thus obtained a value for a well-entangled poly(ethylene-co-butene) polymer is in close agreement with the rheological result. It is shown that the fixed-end boundary conditions in the Rouse-Mooney model are responsible for the correct prediction of the distribution of the q-split plateaus.

Structural and Biochemical Properties of Lipid Particles from the Yeast Saccharomyces cerevisiae

The two most prominent neutral lipids of the yeast Saccharomyces cerevisiae, triacylglycerols (TAG) and steryl esters (SE), are synthesized by the two TAG synthases Dga1p and Lro1p and the two SE synthases Are1p and Are2p. In this study, we made use of a set of triple mutants with only one of these acyltransferases active to elucidate the contribution of each single enzyme to lipid particle (LP)/droplet formation. Depending on the remaining acyltransferases, LP from triple mutants contained only TAG or SE, respectively, with specific patterns of fatty acids and sterols. Biophysical investigations, however, revealed that individual neutral lipids strongly affected the internal structure of LP. SE form several ordered shells below the surface phospholipid monolayer of LP, whereas TAG are more or less randomly packed in the center of the LP. We propose that this structural arrangement of neutral lipids in LP may be important for their physiological role especially with respect to mobilization of TAG and SE reserves.