Particle deposition onto micropatterned charge heterogeneous substrates: trajectory analysis

A trajectory analysis of particles near a micropatterned charged substrate under radial impinging jet flow conditions is presented to investigate the effect of surface charge heterogeneity on particle trajectory and deposition efficiency. The surface charge heterogeneity is modeled as concentric bands of specified width and pitch having positive and negative surface potentials. The flow distribution is obtained using finite element analysis of the governing Navier-Stokes equations. The particle trajectory analysis takes into consideration the hydrodynamic interactions, gravity, van der Waals and electrostatic double layer interactions. The presence of surface charge heterogeneity on the substrate gives rise to an oscillating particle trajectory near the collector surface due to repulsive and attractive forces. As a result of the coupled effects of hydrodynamic and colloidal forces, the particle trajectories and deposition efficiencies are increasingly affected by surface charge heterogeneity as one moves radially away from the stagnation point. The results indicate that it is possible to render collectors with up to 50% favorable surface fraction completely unfavorable by modifying the ratio of the radial to normal fluid velocity. Utilizing the real favorable area fraction of the collector, the patch model expression for calculating the deposition efficiency is modified for impinging jet flow geometry.     

Reversible association of thermoresponsive gold nanoparticles: polyelectrolyte effect on the lower critical solution temperature of poly(vinyl methyl ether)

Thermoresponsive gold nanoparticles (GNPs) have been prepared by the borohydride reduction of gold salt in the presence of water-soluble polymer, poly(vinyl methyl ether) (PVME). The PVME-coated GNPs (PVME-GNPs) have been assembled into large aggregates in the presence of polyelectrolytes, viz., poly(sodium-4 styrene sulfonate) and sodium salt of carboxymethylcellulose at low pH by raising the solution temperature from 20 to 40 degrees C. Increase of temperature triggers the interparticle association due to hydrophobic interaction of pendent methyl group of the surface adsorbed PVME. This assembly process is reversible with respect to temperature and pH of the medium and was studied by monitoring the change in surface plasmon resonance band of PVME-GNPs. Three-dimensional assemblies of various architectures, depending on the concentration of polyelectrolytes, were observed through transmission electron microscopy. A mechanistic model has been suggested for the reversible assembly formation that suits well with the experimental observations. The changes in optical properties of the PVME-GNPs due to their aggregation/disaggregation enabled us to use it as an effective tool to monitor the change in lower critical solution temperature (LCST) of PVME in the presence of polyelectrolytes due to interpolymer complexation at low pH. This result agrees well with the variation of LCST of pure aqueous PVME solution in the presence of polyelectrolytes measured by conventional turbidimetric technique.     

Reaction diffusion equation in the ultra cold coexisting atomic and molecular condensates and the importance of Feshbach interaction

We consider a gas of bosonic atoms near a Feshbach resonance. The dynamics of the atomic and molecular condensates can be described in the limit of small fluctuations by a set of coupled nonlinear Gross Piteaviskii equations. In the case of a strong atom-molecule conversion, the system has an integral motion for the spatially uniform solutions which exhibits temporal oscillation. The possible consequence of this oscillation may be the existence of Josephson like current in the condensates which has been investigated.     

Variation of multiplicities of the secondary charged particles on the geometry of collision

The total charge of the projectile spectator fragments, Q_pf is taken as a measure of the degree of centrality of collision thus defining the collision geometry. In this paper the mean multiplicities of the different charged secondaries emitted in the interaction of ²⁴Mg-Em at 4.5A GeV have been investigated as a function of the total charge Q_pf of the projectile spectator fragments. It has been observed that the average number of the produced particles, 〈N^s〉 and the heavily ionizing particles, 〈N_h> decreases exponentially with the increase of Q_pf showing strong correlation with the geometry of the collision. An attempt has also been made to compare these results with ⁸⁴Kr-Em interaction at 0.95A GeV.     

Chiral Bora[1]ferrocenophanes: Syntheses,Mechanistic Insights,and Ring‐Opening Polymerizations

A series of new boron‐bridged [1]ferrocenophanes ([1]FCPs) was prepared by salt‐metathesis reactions between enantiomerically pure dilithioferrocenes and amino(dichloro)boranes (Et₂NBCl₂, iPr₂NBCl₂, or tBu(Me₃Si)NBCl₂). The dilithioferrocenes were prepared in situ by lithium–bromine exchange from the respective planar‐chiral dibromides (S_p,S_p)‐[1‐Br‐2‐(HR₂C)H₃C₅]₂Fe (R=Me or Et). In most of the cases, mixtures of the targeted [1]FCPs 4 and the unwanted 1,1′‐bis(boryl)ferrocenes 5 were formed. The product ratio depends on the bulkiness of the amino group, the speed of addition of the amino(dichloro)borane, the alkyl group on Cp rings, and in particular on the reaction temperature. The formation of strained [1]FCPs is strongly favored by increased reaction temperatures. Secondly, CHEt₂ groups at Cp rings favored the formation of the targeted [1]FCPs stronger than CHMe₂ groups. These discoveries open up new possibilities to further suppress the formation of unwanted byproducts by a careful choice of the reaction temperature and through tailoring the bulkiness of CHR₂ groups on ferrocene. Thermal ring‐opening polymerizations of selected boron‐bridged [1]FCPs gave metallopolymers with a M_w of 10 kDa (GPC).     

Preparation and characterization of an anionic dye-polycation molecular films by electrostatic layer-by-layer adsorption process

This communication reports the formation and characterization of self-assembled films of a low molecular weight anionic dye amaranth and polycation poly(allylamine hydrochloride) (PAH) by electrostatic alternating layer-by-layer (LBL) adsorption. It was observed that there was almost no material loss occurred during adsorption process. The UV-vis absorption and fluorescence spectra of amaranth solution reveal that with the increase in amaranth concentration in solution, the aggregated species starts to dominate over the monomeric species. New aggregated band at 600 nm was observed in amaranth-PAH mixture solution absorption spectrum. A new broad low intense band at the longer wavelength region, in the amaranth-PAH mixture solution fluorescence spectrum was observed due to the closer association of amaranth molecule while tagged into the polymer backbone of PAH and consequent formation of aggregates. The broad band system in the 650-750 nm region in the fluorescence spectra of different layered LBL films changes in intensity distribution among various bands within itself, with changing layer number and at 10 bilayer LBL films the longer wavelength band at 710 nm becomes prominent. Existence of dimeric or higher order n-meric species in the LBL films was confirmed by excitation spectroscopic studies. Almost 45 min was required to complete the interaction between amaranth and PAH molecules in the one-bilayer LBL film.     

Mössbauer spectroscopic study of the thermal spin crossover in [Fe(II)(isoxazole)6](ClO4)2

The ⁵⁷Fe Mössbauer spectroscopy of mononuclear [Fe(II)(isoxazole)₆](ClO₄)₂ has been studied to reveal the thermal spin crossover of Fe(II) between low-spin (S=0) and high-spin (S=2) states. Temperature-dependent spin transition curves have been constructed with the least-square fitted data obtained from the Mössbauer spectra measured at various temperatures between 84 and 270 K during a cooling and heating cycle. This compound exhibits an unusual temperature-dependent spin transition behaviour with T_C(↓)=223 and T_C(↑)=213 K occurring in the reverse order in comparison to those observed in SQUID observation and many other spin transition compounds. The compound has three high-spin Fe(II) sites at the highest temperature of study of which two undergo spin transitions. The compound seems to undergo a structural phase transition around the spin transition temperature, which plays a significant role in the spin crossover behaviour as well as the magnetic properties of the compound at temperatures below T_C. The present study reveals an increase in high-spin fraction upon heating in the temperature range below T_C, and an explanation is provided.     

Electrical excitation of surface plasmons

We exploit a plasmon mediated two-step momentum down-conversion scheme to convert low-energy tunneling electrons into propagating photons. Surface plasmon polaritons (SPPs) propagating along an extended gold nanowire are excited on one end by low-energy electron tunneling and are then converted to free-propagating photons at the other end. The separation of excitation and outcoupling proves that tunneling electrons excite gap plasmons that subsequently couple to propagating plasmons. Our work shows that electron tunneling provides a nonoptical, voltage-controlled, and low-energy pathway for launching SPPs in nanostructures, such as plasmonic waveguides.