Charge regulation and ionic screening of patchy surfaces

The properties of surfaces with charge-regulated patches are studied using nonlinear Poisson-Boltzmann theory. Using a mode expansion to solve the nonlinear problem efficiently, we reveal the charging behavior of Debye-length sized patches. We find that the patches charge up to higher charge densities if their size is relatively small and if they are well separated. The numerical results are used to construct a basic analytical model which predicts the average surface charge density on surfaces with patchy chargeable groups.     

Phase diagrams of colloidal spheres with a constant zeta-potential

We study suspensions of colloidal spheres with a constant zeta-potential within Poisson-Boltzmann theory, quantifying the discharging of the spheres with increasing colloid density and decreasing salt concentration. We use the calculated renormalized charge of the colloids to determine their pairwise effective screened-Coulomb repulsions. Bulk phase diagrams in the colloid concentration-salt concentration representation follow, for various zeta-potentials, by a mapping onto published fits of phase boundaries of point-Yukawa systems. Although the resulting phase diagrams do feature face-centered cubic and body-centered cubic phases, they are dominated by the (re-entrant) fluid phase due to the colloidal discharging with increasing colloid concentration and decreasing salt concentration.     

Methodology for porting retention prediction data from old to new columns and from conventional-scale to miniaturised ion chromatography systems

Several procedures are available for simulating and optimising separations in ion chromatography (IC), based on the application of retention models to an extensive database of analyte retention times on a wide range of columns. These procedures are subject to errors arising from batch-to-batch variability in the synthesis of stationary phases, or when using a column having a different diameter to that used when the database was acquired originally. Approaches are described in which the retention database can be recalibrated to accommodate changes in the stationary phase (ion-exchange selectivity coefficient and ion-exchange capacity) or in the column diameter which lead to changes in phase ratio. The entire database can be recalibrated for all analytes on a particular column by performing three isocratic separations with two analyte ions. The retention data so obtained are then used to derive a "porting" equation which is employed to generate the required simulated separation. Accurate prediction of retention times is demonstrated for both anions and cations on 2mm and 0.4mm diameter columns under elution conditions which consist of up to five sequential isocratic or linear gradient elution steps. The proposed approach gives average errors in retention time prediction of less than 3% and the correlation coefficient was 0.9849 between predicted and observed retention times for 344 data points comprising 33 anionic or cationic analytes, 5 column internal diameters and 8 complex elution profiles.     

Open tubular capillary columns with basic templates made by the generalized preparation protocol in capillary electrochromatography chiral separation and template structural effects on chiral separation capability

Some open tubular (OT) molecule imprinted polymer (MIP) silica capillary columns have been prepared using atenolol, sulpiride, methyl benzylamine (MBA) and (1-naphthyl)-ethylamine (NEA) as templates by the pre-established generalized preparation protocol. The four MIP thin layers of different templates showed quite different morphologies. The racemic selectivity of each MIP column for the template enantiomers was optimized by changing eluent composition and pH. The template structural effects on chiral separation performance have been examined. This work verifies the versatility of the generalized preparation protocol for OT-MIP silica capillary columns by extending its boundary toward templates with basic functional group moieties. This study is the very first report to demonstrate a generalized MIP preparation protocol that is valid for both acidic and basic templates. The chiral separation performances of atenolol and sulpiride by the MIPs of this study were found better than or comparable to those of atenolol and sulpiride obtained by non-MIP separation techniques and those of some basic template enantiomers obtained by MIP based techniques.     

Derivatization-independent cholesterol analysis in crude lipid extracts by liquid chromatography/mass spectrometry: applications to a rabbit model for atherosclerosis

Direct measurement of various sterols in crude lipid extracts in a single experiment from limited biological samples is challenging. Current mass spectrometry (MS) based approaches usually require chemical derivatization before subjecting to MS analysis. Here, we present a derivatization-independent method for analyzing various sterols, including cholesterol and its congeners, using liquid chromatography and atmospheric pressure chemical ionization mass spectrometry. Based on the specific tandem mass spectrometry pattern of cholesterol, multiple reaction monitoring (MRM) transitions were used to quantify free cholesterol and its fatty acyl esters. Several cholesterol oxidation products could also be measured using the upfront liquid chromatography separation and specific MRM transitions. The method was validated alongside established enzymatic assays in measuring total cholesterol. As a proof of concept, we analyzed plasma sterols in rabbits administrated with a high cholesterol diet (HCD) which is a classical atherosclerotic model. Free cholesterol, cholesterol esters, 7-hydroxycholesterol, and 7-ketocholesterol were elevated in plasma of rabbits on HCD. This method could also serve as an excellent tool for quantitative analysis of other sterols such as ergosterol and sitosterol in other organisms beside mammalian. In Saccharomyces cerevisiae, our results indicated dramatic increases of the ratio of ergosterol esters to free ergosterol in both yeh2Δ and tgl1Δ cells, which are consistent with the function of the respective enzymes.     

Enhanced Raman spectrum of lawsone on Ag surface: vibrational analyses, frequency shifts, and molecular geometry

The surface-enhanced Raman spectrum of lawsone, a well known natural dye, has been investigated. Activation with KNO(3) or Na(2)SO(4) solution was necessary to enhance the Raman signal, whereas addition of NaCl solution depletes the effects. In the enhanced Raman spectrum, the strong double-bond stretching bands are most distinctive and show large red shifts from those in the infrared and FT-Raman spectra. The observed strong double-bond stretching bands reflect lawsone coordinated perpendicular to the Ag surface. DFT computations have been carried out for the plausible configurations of lawsone coordinated to an adatom on the Ag surface. Lawsone coordinated to an Ag(+) adatom with H(+) released best reproduces the observed vibrational characteristics.     

Lines Pinning Lines

A line ℓ is a transversal to a family F of convex polytopes in ℝ³ if it intersects every member of F. If, in addition, ℓ is an isolated point of the space of line transversals to F, we say that F is a pinning of ℓ. We show that any minimal pinning of a line by polytopes in ℝ³ such that no face of a polytope is coplanar with the line has size at most eight. If in addition the polytopes are pairwise disjoint, then it has size at most six.     

On open problems in polling systems

In the present paper we address two open problems concerning polling systems, viz., queueing systems consisting of multiple queues attended by a single server that visits the queues one at a time. The first open problem deals with a system consisting of two queues, one of which has gated service, while the other receives 1-limited service. The second open problem concerns polling systems with general (renewal) arrivals and deterministic switch-over times that become infinitely large. We discuss related, known results for both problems, and the difficulties encountered when trying to solve them.     

Sources and technology for an atomic gravitational wave interferometric sensor

We study the use of atom interferometers as detectors for gravitational waves in the mHz–Hz frequency band, which is complementary to planned optical interferometers, such as laser interferometer gravitational wave observatories (LIGOs) and the Laser Interferometer Space Antenna (LISA). We describe an optimized atomic gravitational wave interferometric sensor (AGIS), whose sensitivity is proportional to the baseline length to power of 5/2, as opposed to the linear scaling of a more conservative design. Technical challenges are briefly discussed, as is a table-top demonstrator AGIS that is presently under construction at Berkeley. We study a range of potential sources of gravitational waves visible to AGIS, including galactic and extra-galactic binaries. Based on the predicted shot noise limited performance, AGIS should be capable of detecting type Ia supernovae precursors within 500 pc, up to 200 years beforehand. An optimized detector may be capable of detecting waves from RX J0806.3+1527.     

An improved penalty immersed boundary method for fluid–flexible body interaction

An improved penalty immersed boundary (pIB) method has been proposed for simulation of fluid–flexible body interaction problems. In the proposed method, the fluid motion is defined on the Eulerian domain, while the solid motion is described by the Lagrangian variables. To account for the interaction, the flexible body is assumed to be composed of two parts: massive material points and massless material points, which are assumed to be linked closely by a stiff spring with damping. The massive material points are subjected to the elastic force of solid deformation but do not interact with the fluid directly, while the massless material points interact with the fluid by moving with the local fluid velocity. The flow solver and the solid solver are coupled in this framework and are developed separately by different methods. The fractional step method is adopted to solve the incompressible fluid motion on a staggered Cartesian grid, while the finite element method is developed to simulate the solid motion using an unstructured triangular mesh. The interaction force is just the restoring force of the stiff spring with damping, and is spread from the Lagrangian coordinates to the Eulerian grids by a smoothed approximation of the Dirac delta function. In the numerical simulations, we first validate the solid solver by using a vibrating circular ring in vacuum, and a second-order spatial accuracy is observed. Then both two- and three-dimensional simulations of fluid–flexible body interaction are carried out, including a circular disk in a linear shear flow, an elastic circular disk moving through a constricted channel, a spherical capsule in a linear shear flow, and a windsock in a uniform flow. The spatial accuracy is shown to be between first-order and second-order for both the fluid velocities and the solid positions. Comparisons between the numerical results and the theoretical solutions are also presented.