906 research outputs found
Screened Coulomb interactions of general macroions with nonzero particle volume
A semianalytical approach is developed to calculate the effective pair
potential of rigid arbitrarily shaped macroions with a nonvanishing particle
volume, valid within linear screening theory and the mean-field approximation.
The essential ingredient for this framework is a mapping of the particle to a
singular charge distribution with adjustable effective charge and shape
parameters determined by the particle surface electrostatic potential. For
charged spheres this method reproduces the well-known
Derjaguin-Landau-Verwey-Overbeek (DLVO) potential. Further exemplary benchmarks
of the method for more complicated cases, like tori, triaxial ellipsoids, and
additive torus-sphere mixtures, leads to accurate closed-form integral
expressions for all particle separations and orientations. The findings are
relevant for determining the phase behaviour of macroions with experiments and
simulations for various particle shapes.Comment: V2: 9 pages, 5 figures; Appendix: 6 pages, 5 figure
Topological phase transitions driven by next-nearest-neighbor hopping in two-dimensional lattices
For two-dimensional lattices in a tight-binding description, the intrinsic
spin-orbit coupling, acting as a complex next-nearest-neighbor hopping, opens
gaps that exhibit the quantum spin Hall effect. In this paper, we study the
effect of a real next-nearest-neighbor hopping term on the band structure of
several Dirac systems. In our model, the spin is conserved, which allows us to
analyze the spin Chern numbers. We show that in the Lieb, kagome, and T_3
lattices, variation of the amplitude of the real next-nearest-neighbor hopping
term drives interesting topological phase transitions. These transitions may be
experimentally realized in optical lattices under shaking, when the ratio
between the nearest- and next-nearest-neighbor hopping parameters can be tuned
to any possible value. Finally, we show that in the honeycomb lattice,
next-nearest-neighbor hopping only drives topological phase transitions in the
presence of a magnetic field, leading to the conjecture that these transitions
can only occur in multigap systems.Comment: 10 pages, 9 figures [erratum: corrected colors in Fig. 7(a)
Anisotropic electrostatic screening of charged colloids in nematic solvents
The physical behaviour of anisotropic charged colloids is determined by their
material dielectric anisotropy, affecting colloidal self-assembly, biological
function and even out-of-equilibrium behaviour. However, little is known about
anisotropic electrostatic screening, which underlies all electrostatic
effective interactions in such soft or biological materials. In this work, we
demonstrate anisotropic electrostatic screening for charged colloidal particles
in a nematic electrolyte. We show that material anisotropy behaves markedly
different from particle anisotropy: The electrostatic potential and pair
interactions decay with an anisotropic Debye screening length, contrasting the
constant screening length for isotropic electrolytes. Charged dumpling-shaped
near-spherical colloidal particles in a nematic medium are used as an
experimental model system to explore the effects of anisotropic screening,
demonstrating competing anisotropic elastic and electrostatic effective pair
interactions for colloidal surface charges tunable from neutral to high,
yielding particle-separated metastable states. Generally, our work contributes
to the understanding of electrostatic screening in nematic anisotropic media.Comment: 15 pages, 5 figures, SM under ancillary file
Topological-defect-induced surface charge heterogeneities in nematic electrolytes
We show that topological defects in an ion-doped nematic liquid crystal can
be used to manipulate the surface charge distribution on chemically
homogeneous, charge-regulating external surfaces, using a minimal theoretical
model. In particular, the location and type of the defect encodes the precise
distribution of surface charges and the effect is enhanced when the liquid
crystal is flexoelectric. We demonstrate the principle for patterned surfaces
and charged colloidal spheres. More generally, our results indicate an
interesting approach to control surface charges on external surfaces without
changing the surface chemistry.Comment: 6 pages, 4 figures, Supplemental Information can be found under
ancillary files. V2: Extra discussion, added additional images to the S
Exogenous application of platelet-leukocyte gel during open subacromial decompression contributes to improved patient outcome
Background: Platelet-leukocyte gel (PLG) is being used during various surgical procedures in an attempt to enhance the healing process. We studied the effects of PLG on postoperative recovery of patients undergoing open subacromial decompression (OSD). Methods: PLG was produced from platelet-leukocyte-rich plasma (P-LRP), prepared from a unit of whole blood. Forty patients were included in the study. Self-assessed evaluations, using the American Shoulder and Elbow Surgeons scoring system of activities of daily living (ADL), joint instability, pain levels, pain medications, and clinical evaluations for range of motion were conducted. Results: Platelet and leukocyte counts were significantly increased in the P-LRP compared to baseline counts. Treated patients demonstrated decreased visual analog scales for pain and used significantly less pain medication, had an improved range of motion during passive forward elevation, external rotation, external rotation with arm at 90 degrees abduction, internal rotation, and cross body adduction compared to control patients (p < 0.001). No differences in the instability score were observed between the groups. Furthermore, treated patients performed more ADL (p < 0.05). Conclusion: In the PLG-treated group, recovery was faster and patients returned earlier to daily activities and also took less pain medication than control subjects
Finite-temperature ordering in a two-dimensional highly frustrated spin model
We investigate the classical counterpart of an effective Hamiltonian for a
strongly trimerized kagome lattice. Although the Hamiltonian only has a
discrete symmetry, the classical groundstate manifold has a continuous global
rotational symmetry. Two cases should be distinguished for the sign of the
exchange constant. In one case, the groundstate has a 120^\circ spin structure.
To determine the transition temperature, we perform Monte-Carlo simulations and
measure specific heat, the order parameter as well as the associated Binder
cumulant. In the other case, the classical groundstates are macroscopically
degenerate. A thermal order-by-disorder mechanism is predicted to select
another 120^\circ spin-structure. A finite but very small transition
temperature is detected by Monte-Carlo simulations using the exchange method.Comment: 11 pages including 9 figures, uses IOP style files; to appear in J.
Phys.: Condensed Matter (proceedings of HFM2006
Comparison of Saturated Hydraulic Conductivity Measurement Methods for a Glacial-Till Soil
Hydraulic conductivity is the single most important hydraulic parameter for flow and transport-related phenomena in soil, but the results from different measuring methods vary under different field conditions. To evaluate the performance of four in situ saturated hydraulic conductivity (Ks) measuring methods, Ks measurements were made at four depths (15, 30, 60, and 90 cm) and five locations on a glacial-till soil of Nicollet (fine-loamy, mixed, mesic Aquic Hapludoll)-Clarion (fine-loamy, mixed, mesic Typic Hapludoll) association. The four in situ methods were: (i) Guelph permeameter, (ii) velocity permeameter, (iii) disk permeameter, and (iv) double-tube method. The Ks was also determined in the laboratory on undisturbed soil cores collected from all the five sites and four depths. The Guelph permeameter method gave the lowest Ks values, possibly because of small sample size, whereas the disk permeameter and double-tube methods gave maximum values for Ks with minimum variability, possibly because of large sample size. Maximum variability in Ks values for soil cores at shallow depths may have occurred because of the presence or absence of open-ended macropores. Estimates of Ks, however, are most comparable for the velocity permeameter and the laboratory method using a constant-head permeameter
Colloid-oil-water-interface interactions in the presence of multiple salts: charge regulation and dynamics
We theoretically and experimentally investigate colloid-oil-water-interface
interactions of charged, sterically stabilized, poly(methyl-methacrylate)
colloidal particles dispersed in a low-polar oil (dielectric constant
) that is in contact with an adjacent water phase. In this model
system, the colloidal particles cannot penetrate the oil-water interface due to
repulsive van der Waals forces with the interface whereas the multiple salts
that are dissolved in the oil are free to partition into the water phase. The
sign and magnitude of the Donnan potential and/or the particle charge is
affected by these salt concentrations such that the effective interaction
potential can be highly tuned. Both the equilibrium effective colloid-interface
interactions and the ion dynamics are explored within a Poisson-Nernst-Planck
theory, and compared to experimental observations.Comment: 13+2 pages, 5+3 figures; V2: small clarifications in the tex
Nematronics: Reciprocal coupling between ionic currents and nematic dynamics
Adopting a spintronics-inspired approach, we study the reciprocal coupling
between ionic charge currents and nematic texture dynamics in a uniaxial
nematic electrolyte. Assuming quenched fluid dynamics, we develop equations of
motion analogously to spin torque and spin pumping. Based on the principle of
least dissipation of energy, we derive the adiabatic "nematic torque" exerted
by ionic currents on the nematic director field as well as the reciprocal
motive force on ions due to the orientational dynamics of the director. We
discuss several simple examples that illustrate the potential functionality of
this coupling. Furthermore, using our phenomenological framework, we propose a
practical means to extract the coupling strength through impedance measurements
on a nematic cell. Exploring further applications based on this physics could
foster the development of nematronics -- nematic iontronics.Comment: 13 pages including the supplemental material, 3 figures, Physical
Review Letter
- …