Evaporation-induced patterns from droplets containing motile and nonmotile bacteria

In this letter, we report the observations of specific pattern formation from the evaporation of aqueous droplets containing motile and nonmotile bacteria. We found that when motile bacteria were present the droplet evaporated into disclike patterned deposits of bacteria. However, when the bacteria were made nonmotile by treatment with liquid nitrogen, the droplet evaporated into ringlike deposits. We also observed that bacteria with higher motility produced more uniformly deposited disclike patterns. Furthermore, we propose a model with numerical simulations to explain the mechanism of formation of these patterns. The model is based on the advective fluid flow from the center of the droplet toward the edge due to enhanced evaporation from the edge of the pinned droplet in comparison to that from the free surface. For the case of motile bacteria, we have added another velocity parameter toward the axis of the droplet and directed against the fluid flow in order to account for the disclike pattern formation. The numerical simulations match the experimental observations well. The present work, by qualitative and quantitative understanding of the evaporation of bacteria droplets, demonstrates that the inherent bacterial motility is primarily responsible for the formation of these differential patterns.     

Star–block polymers of multiple polystyrene-b-polyisobutylene arms radiating from a polydivinylbenzene core

The synthesis together with mechanical property and rheological characterization of novel star–block copolymers comprising multiple polystyrene (PSt)-b-polyisobutylene (PIB) arms emanating from polydivinylbenzene (PDVB) cores are described. The synthesis strategy involved the preparation of PSt-b-PIB-Cl^t (i.e., diblocks fitted with a tert-chlorine terminus at the PIB end) by sequential living block polymerization of St and IB, ionizing the -Cl^t terminus by TiCl₄ at room temperature, and linking the PSt-b-PIB^⊕ prearms by DVB. Molecular characterization was effected mainly by triple detector GPC including refractive index (RI)-, UV-, and laser light scattering (LLS)-GPC traces. Evidence for intra- and intermolecular reactions between individual star–blocks is presented and a comprehensive mechanism to the final product is proposed. The stress–strain behavior of star–blocks has been studied and is compared with those of linear triblocks (i.e., two-arm stars) of similar arm molecular weights and composition in the 25–70°C range. The mechanical properties of star–blocks are invariably superior to those of the triblocks over the entire temperature range. The rheological behavior of star–blocks and linear triblocks has been compared in terms of dynamic viscosity at various frequencies. Star–blocks exhibit significantly lower melt viscosities than their linear counterparts, which signals improved processing behavior. We have also compared select rheological properties of the commercially available PSt-b-(hydrogenated-1,4-polybutadiene)-b-PSt thermoplastic elastomer (Kraton G 1650) with those of PIB-based linear triblocks and multiarm star–blocks of similar glassy/rubbery compositions. The melt viscosities of PIB-based triblocks and star–blocks were significantly lower than that of Kraton G over the entire frequency range investigated. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 2235–2243, 1999     

Inelastic behavior of the single domain of metal-ceramic composites with lamellar microstructure

Metal-ceramic composites are widely applied in the different brunches of industry. The composites are produced by squeeze-casting of the ceramic preform by molten aluminum alloy. The lamellar microstructure is obtained during freezing of ceramic suspension. The internal structure of the domains can be controlled via freeze-casting parameters. The material has high anisotropy level and its effective properties depend on lamella orientation. The aim of this study is numerical simulation of the inelastic behavior of the material and its verification by experiment. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

The effect of the finite size of ions and Debye layer overspill on the screened Coulomb interactions between charged flat plates

Screened repulsion between uniformly charged plates with an intervening electrolyte is analyzed for strongly overlapped electrical double layers (EDL), accounting for the steric effect of ions and their expulsion from EDL edges into the surrounding solution. As a generalization of a study by Philipse et al. which does not account for these effects, an analytical expression is derived for the repulsion pressure in the limit of infinitely long plates with a zero-field assumption, which agrees closely with the corresponding numerical solution at low inter-plate separations. Our results show an augmented repulsive pressure for finite-sized ions at strong EDL overlaps. For plates with a finite lateral size, we demonstrate a further extended domain of low inter-plate gaps where the repulsion pressure increases with ion size due to a strong interplay between the steric interaction of ions and the EDL overspill phenomenon, considered earlier in a study by Ghosal & Sherwood limited to the linear Debye-Hückel regime (which cannot account for the steric effect of ions). This investigation on a simple model should enhance our understanding of the interaction between charged particles in electrophoresis, nanoscale self-assembly, active particles, and various other electrokinetic systems.     

Homology in Abelian Lattice Models

We study Abelian lattice gauge theory defined on a simplicial complex with arbitrary topology. The use of dual objects allows one to reformulate the theory in terms of different dynamical variables; however, we avoid entirely the use of the dual cell complex. Topological modes which are present in the transformation now appear as homology classes, in contrast to the cohomology modes found in the dual cell picture. Irregularities of dual cell complexes do not arise in this approach. We treat the two and three-dimensional cases in detail, and prove a general vanishing theorem for Wilson line correlators.     

Lifting of the degeneracy in semibullvalenes by remote and direct substituents: a quantitative study using variable-temperature carbon-13 NMR spectroscopy

A series of six 1,5-(ethylmethyl)semibullvalenes (1a <==> 1a', 2 <==> 2', 3 <==>3') and two 4(2)-substituted semibullvalenes (4 <==> 4'), each undergoing Cope equilibria between nondegenerate valence tautomers, was investigated by carbon-13 NMR spectroscopy at a range of temperatures in several different solvents. Gompper's treatment of substituent perturbation was extended, specifically accounting for the effects of the substituents on chemical shifts, to allow the determination of the thermodynamic parameters for these skewed equilibria. These new treatments were used to determine the population difference (f - f ') between the valence tautomers and the perturbation thermodynamic quantities DeltaH(P), DeltaS(P), and DeltaG(P). The slow-exchange limit was reached for the parent 1,5-(ethylmethyl)semibullvalenes 3a <==> 3a' from which it was established that the preferred valence tautomer is 3a with the ethyl group on the cyclopropane ring. Despite considerable effort, the slow-exchange limit could not be reached in any of our other remotely substituted semibullvalenes. Provided that the ethyl group always prefers the cyclopropyl position as in 3a, the 1-ethyl-5-methylsemibullvalenes 1a, 2, and 3 are more stable by DeltaH(P) = 0.7-1.7 kJ mol(-1) than their valence tautomers 1a', 2', and 3'. In the directly substituted semibullvalenes (4 left harpoon ovet right harpoon 4'), the preferred valence tautomers 4a and 4b have the bromine atom or the nitrile group on the vinyl position (C(4)) rather than on the cyclopropane ring (C(2)) and are more stable than 4a' and 4b' by DeltaH(P) = 4.8 and 7.0 kJ mol(-1), respectively.     

Solvent-Dependent Nanostructures Based on Active π-Aggregation Induced Emission Enhancement of New Carbazole Derivatives of Triphenylacrylonitrile

In the present study, the carbazole and 2,3,3-triphenylacrylonitrile (TPAN) nanostructures (2-CTPAN and 2,2′-CTPAN) have been designed and synthesized by Pd-catalyzed Sonogashira cross-coupling reaction. CTPAN exhibit aggregation-induced emission enhancement (AIEE) behavior in water with high fluorescence quantum yield. Both the compounds show tunable self-assembly in water as well as in N,N-dimethylformamide (DMF) by extended π–π stacking interactions. CTPAN can be self-assembled into spherical particles in water and the structures of these self-assemblies have been investigated using X-ray diffraction. Interestingly, 2-CTPAN and 2,2′-CTPAN form organogels with a critical gelation concentration (CGC) of 11 and 15 mg mL⁻¹, respectively, in DMF and exhibit acicular and rod shaped morphology, respectively. The single-crystal structure of 2-CTPAN shows that the intermolecular C−H⋅⋅⋅π interactions lock the molecular conformation into a staircase-shaped supramolecular assembly. These AIEE active compounds reveal high water dispersibility, strong yellow fluorescence with high quantum yield, promising photostability and excellent biocompatibility, which make them potential bioimaging agents.