Strategies of carbon and nitrogen acquisition by saprotrophic and ectomycorrhizal fungi in Finnish boreal Picea abies-dominated forests.

We compared the δ13C and δ15N of forest material with an extensive sporocarp collection to elucidate the role of litter, wood and soil as fungal carbon and nitrogen sources in Finnish boreal Picea abies-dominated forests. Ectomycorrhizal Hydnum and Cortinarius had higher δ15N than other ectomycorrhizal fungi, suggesting use of 15N-enriched, deeper nitrogen. Russula had lower δ15N than other ectomycorrhizal fungi and resembled some litter decay genera, suggesting use of litter-derived nitrogen. There was little variation in δ15N among other genera of ectomycorrhizal fungi, indicating limited functional diversity in nitrogen use. Saprotrophic Leotia, Gymnopus, Hypholoma, Pholiota, Rhodocollybia and Calocera had δ15N values similar to ectomycorrhizal fungi, indicating overlap in use of older nitrogen from soil or roots or use of newly fixed nitrogen. Genera of litter and wood decay fungi varied up to 6‰ in δ13C and 10‰ in δ15N, suggesting large differences in carbon and nitrogen sources and processing. Similar δ13C between white and brown rot wood decay fungi also suggest that white rot fungi do not use lignin-derived carbon. Together, these δ13C and δ15N patterns of fungi from Finnish boreal forests enhance our knowledge of fungal functional diversity and indicate broad use of litter, wood and soil resources.

Large anisotropy in the magnetic susceptibility of metallic carbon nanotubes.

Through magnetic linear dichroism spectroscopy, the magnetic susceptibility anisotropy of metallic single-walled carbon nanotubes has been extracted and found to be 2-4 times greater than values for semiconducting nanotubes. This large anisotropy can be understood in terms of large orbital paramagnetism of metallic nanotubes arising from the Aharonov-Bohm-phase-induced gap opening in a parallel field, and our calculations quantitatively reproduce these results. We also compare our values with previous work for semiconducting nanotubes, which confirm that the magnetic susceptibility anisotropy does not increase linearly with the diameter for small-diameter nanotubes.

Wrinkling and strain softening in single-wall carbon nanotube membranes.

The nonlinear elasticity of thin supported membranes assembled from length purified single-wall carbon nanotubes is analyzed through the wrinkling instability that develops under uniaxial compression. In contrast with thin polymer films, pristine nanotube membranes exhibit strong softening under finite strain associated with bond slip and network fracture. We model the response as a shift in percolation threshold generated by strain-induced nanotube alignment in accordance with theoretical predictions.

Non-brownian microrheology of a fluid-gel interface.

We use stroboscopic video microscopy to study the motion of a sheared fluid-gel interface. Mechanical noise plays a role analogous to temperature, but with a low-frequency breakdown of linear response consistent with an underlying instability. We relate the fast motion of the interface to the rheological properties of the gel, laying the foundation for a non-Brownian optical microrheology.

Dielectric response of aligned semiconducting single-wall nanotubes.

We report measurements of the full intrinsic optical anisotropy of isolated single-wall carbon nanotubes (SWNTs). By combining absorption spectroscopy with transmission ellipsometry and polarization-dependent resonant Raman scattering, we obtain the real and imaginary parts of the SWNT permittivity from aligned semiconducting SWNTs dispersed in stretched polymer films. Our results are in agreement with theoretical predictions, highlighting the limited polarizability of excitons in a quasi-1D system.

Rheology of concentrated carbon nanotube suspensions.

The rheological properties of non-Brownian carbon nanotube suspensions are measured over a range of nanotube volume fractions spanning the transition from semidilute to concentrated. The polymer-stabilized nanotubes are "sticky" and form a quiescent elastic network with a well-defined shear modulus and yield stress that both depend strongly on nanotube volume fraction with different but related critical exponents. We compare controlled-strain-rate and controlled-stress measurements of yielding in shear flow, and we study the effect of slow periodic stress reversal on yielding and the arrest of flow. Our measurements support a universal scaling of both the linear viscoelastic and steady-shear viscometric response. The former allows us to extract the elastic shear modulus of semidilute nanotube networks for values that are near or below the resolution limit of the rheometers used, while the latter provides a similar extrapolation of the yield stress. A simple scaling argument is used to model the dependence of yield stress and elastic modulus on concentration.

Shape of the isotropic-(para)nematic coexistence curve in sheared nanotube suspensions.

A geometrical argument is used to account for the exponent of 1/3 describing the isotropic-(para)nematic phase boundary of sheared carbon nanotube dispersions [E. K. Hobbie and D. J. Fry, Phys. Rev. Lett. 97, 036101 (2006)]. Through simplifying assumptions motivated by the effects of concentration and attractive interactions, the behavior is explained by relating the mean strain-induced alignment to the average distance between nanotubes for the scenario of limited anisotropy in non-Brownian suspensions.

Comparative measures of single-wall carbon nanotube dispersion.

Model composites of DNA-wrapped single-wall carbon nanotubes in poly(acrylic acid) are used to evaluate metrics of nanotube dispersion. By varying the pH of the precursor solutions, we introduce a controlled deviation from ideal behavior. On the basis of small-angle neutron scattering, changes in near-infrared fluorescence intensity are strongly correlated with dispersion, while optical absorption spectroscopy and resonant Raman scattering are less definitive. Our results represent the first systematic comparison of currently accepted measures of nanotube dispersion.

Charge transport in melt-dispersed carbon nanotubes.

We investigate the effect of interfacial stabilizer on charge transport in polymer-dispersed carbon nanotubes. Despite mechanical contact, samples with dispersant show poor conductivity, which we attribute to a robust interfacial layer between contacted nanotubes. In comparison, results obtained when nanotubes are mechanically mixed into polymer melts without dispersant show much better conductivity. The difference is striking; at comparable loading, neat melt composites have resistivities five orders of magnitude smaller than those containing interfacial stabilizer. Our results highlight a fundamental issue for the engineering of conducting carbon nanotube composites; dispersion stability will typically be achieved at the expense of conductivity.

Nonequilibrium phase diagram of sticky nanotube suspensions.

We report a universal phase diagram describing the evolution from solidlike networks to flowing nematics for "sticky" nanotube suspensions under an applied shear stress. Although the nanotubes are strongly non-Brownian, we find features characteristic of first-order phase transitions, including a discontinuity in the nematic order parameter at the isotropic-(para)nematic phase boundary. Using simple physical arguments, we account for the shape of the coexistence curves, as well as the dependence of the order parameter on concentration and stress.

Rheo-optical studies of carbon nanotube suspensions.

We use a polarization-modulation technique to investigate the optical anisotropy of multi- and single-wall carbon nanotubes suspended in a variety of solvents under simple shear flow. Measurements of birefringence and dichroism are performed as a function of shear rate, tube concentration, and solvent viscosity. At fixed volume fraction, the anisotropy increases with increasing shear stress due to enhanced flow alignment. At fixed shear stress, the anisotropy increases with volume fraction due to rotational excluded-volume interactions. By considering the rotational diffusivity as a function of nanotube length, diameter, concentration, and solvent viscosity, we demonstrate a leading-order scaling relation for the optical anisotropy in terms of rotary Peclet number Pe. At low Pe, our results are in qualitative agreement with the theoretical predictions of Doi and Edwards. At high Pe, our data suggest that the degree of nanotube alignment scales as Pe16.

Colloidal particles coated and stabilized by DNA-wrapped carbon nanotubes.

Single-walled carbon nanotubes (SWNTs) are dispersed in water via wrapping with short segments of single-stranded DNA (ssDNA). Small angle neutron scattering suggests a power-law exponent that is consistent with clustered nanotubes and hence marginal stability. The SWNT-ssDNA complex is used to stabilize dispersions of hydrophilic colloidal particles with the nanotubes adhered to the surface of the colloids. Near-infrared fluorescence microscopy demonstrates the interfacial band-gap fluorescence of these SWNT-coated particles, suggesting potential routes to novel platforms and applications.

Ubiquity of domain patterns in sheared viscoelastic fluids.

A ubiquitous domain pattern is observed in two-phase viscoelastic fluids falling within the simple paradigm of soft viscoelastic domains suspended in a less viscoelastic fluid under shear flow. Three strikingly different complex fluids exhibit the same shear-induced domain structure, which we relate to the elasticity of the dispersed phase via an approximate Weissenberg number. We suggest a physical mechanism for the formation of this pervasive pattern, independent of the dynamic origin of the elasticity of the suspended phase.

Shear-induced structure in polymer-clay nanocomposite solutions.

The equilibrium structure and shear response of model polymer-clay nanocomposite gels are measured using X-ray scattering, light scattering, optical microscopy, and rheometry. The suspensions form physical gels via the "bridging" of neighboring colloidal clay platelets by the polymer, with reversible adsorption of polymer segments onto the clay surface providing a short-range attractive force. As the flow disrupts this transient network, coupling between composition and stress leads to the formation of a macroscopic domain pattern, while the clay platelets orient with their surface normal parallel to the direction of vorticity. We discuss the shear-induced structure, steady-shear rheology, and oscillatory-shear response of these dynamic networks, and we offer a physical explanation for the mesoscale shear response. In contrast to flow-induced "banding" transitions, no stress plateau is observed in the region where macroscopic phase separation occurs. The observed platelet orientation is different from that reported for polymer-melt clay nanocomposites, which we attribute to effects associated with macroscopic phase separation under shear flow.

Elastic flow instability in nanotube suspensions.

We report an elastic instability associated with flow-induced clustering in semidilute non-Brownian colloidal nanotubes. Rheo-optical measurements are compared with simulations of mechanical flocculation in sheared fiber suspensions, and the evolving structure is characterized as a function of confinement and shear stress. The transient rheology is correlated with the evolution of highly elastic vorticity-aligned aggregates, with the underlying instability being somewhat ubiquitous in complex fluids.

Kinetic pathway of the bilayered-micelle to perforated-lamellae transition.

Using time-resolved small-angle neutron scattering, we have studied the kinetics of the recently observed bilayered-micelle (or so-called "bicelle") to perforated-lamellar transition in phospholipid mixtures. The data suggest that phase-ordering occurs via the early-time coalescence of bicelles into stacks of lamellae that then swell. Our measurements on this biomimetic system highlight the ubiquitous role of transient metastable states in the phase ordering of complex fluids.

Anisotropic ordering in sheared binary fluids with viscous asymmetry: experiment and computer simulation.

Optical measurements of the structure and morphology of phase-separating polymer blend under simple shear flow have been performed and the results are compared with computer simulations of sheared phase-separating binary mixtures with viscous asymmetry in the fluid components. Information about the structure is obtained from the two-point composition correlation function. Both experiment and simulation suggest subtle differences in the shear response depending on whether the more viscous phase is dispersed or continuous. Measurements of the string width along the neutral direction suggest power-law decay in the shear rate with an exponent of 1/3 when the more viscous phase is dispersed. The simulations suggest that the mean string width, measured along the velocity-gradient direction in the two-dimensional model calculation, exhibits power-law decay in the shear rate with an exponent of 1/3 independent of which phase is dispersed.

Foliar nitrogen concentrations and natural abundance of (15)N suggest nitrogen allocation patterns of Douglas-fir and mycorrhizal fungi during development in elevated carbon dioxide concentration and temperature.

Pseudotsuga menziesii (Mirb.) Franco (Douglas-fir) seedlings were grown in a 2 x 2 factorial design in enclosed mesocosms at ambient temperature or 3.5 degrees C above ambient, and at ambient CO2 concentration ([CO2]) or 179 ppm above ambient. Two additional mesocosms were maintained as open controls. We measured the extent of mycorrhizal infection, foliar nitrogen (N) concentrations on both a weight basis (%N) and area basis (Narea), and foliar delta15N signatures (15N/14N ratios) from summer 1993 through summer 1997. Mycorrhizal fungi had colonized nearly all root tips across all treatments by spring 1994. Elevated [CO2] lowered foliar %N but did not affect N(area), whereas elevated temperature increased both foliar %N and Narea. Foliar delta15N was initially -1 per thousand and dropped by the final harvest to between -4 and -5 per thousand in the enclosed mesocosms, probably because of transfer of isotopically depleted N from mycorrhizal fungi. Based on the similarity in foliar delta15N among treatments, we conclude that mycorrhizal fungi had similar N allocation patterns across CO2 and temperature treatments. We combined isotopic and Narea data for 1993-94 to calculate fluxes of N for second- and third-year needles. Yearly N influxes were higher in second-year needles than in third-year needles (about 160 and 50% of initial leaf N, respectively), indicating greater sink strength in the younger needles. Influxes of N in second-year needles increased in response to elevated temperature, suggesting increased N supply from soil relative to plant N demands. In the elevated temperature treatments, N effluxes from third-year needles were higher in seedlings in elevated [CO2] than in ambient [CO2], probably because of increased N allocation below ground. We conclude that N allocation patterns shifted in response to the elevated temperature and [CO2] treatments in the seedlings but not in their fungal symbionts.

Shear viscosity of phase-separating polymer blends with viscous asymmetry.

Rheo-optical measurements of phase separating polymer mixtures under simple shear flow have been used to investigate the influence of domain morphology on the viscosity of emulsionlike polymer blends, in which the morphology under weak shear is droplets of one coexisting phase dispersed in a matrix of the second. The structure and viscosity of low-molecular-weight polybutadiene and polyisoprene mixtures, phase separated by quenching to a temperature inside the coexistence region of the phase diagram, were measured as a function of shear rate and composition. In the weak shear regime, the data are in qualitative agreement with an effective medium model for non-dilute suspensions of slightly deformed interacting droplets. In the strong shear regime, where a stringlike pattern appears en route to a shear-homogenized state, the data are in qualitative agreement with a simple model that accounts for viscous asymmetry in the components.

Correlations between foliar δ15N and nitrogen concentrations may indicate plant-mycorrhizal interactions.

Nitrogen isotope measurements may provide insights into changing interactions among plants, mycorrhizal fungi, and soil processes across environmental gradients. Here, we report changes in δ15N signatures due to shifts in species composition and nitrogen (N) dynamics. These changes were assessed by measuring fine root biomass, net N mineralization, and N concentrations and δ15N of foliage, fine roots, soil, and mineral N across six sites representing different post-deglaciation ages at Glacier Bay, Alaska. Foliar δ15N varied widely, between 0 and -2‰ for nitrogen-fixing species, between 0 and -7‰ for deciduous non-fixing species, and between 0 and -11‰ for coniferous species. Relatively constant δ15N values for ammonium and generally low levels of soil nitrate suggested that differences in ammonium or nitrate use were not important influences on plant δ15N differences among species at individual sites. In fact, the largest variation among plant δ15N values were observed at the youngest and oldest sites, where soil nitrate concentrations were low. Low mineral N concentrations and low N mineralization at these sites indicated low N availability. The most plausible mechanism to explain low δ15N values in plant foliage was a large isotopic fractionation during transfer of nitrogen from mycorrhizal fungi to plants. Except for N-fixing plants, the foliar δ15N signatures of individual species were generally lower at sites of low N availability, suggesting either an increased fraction of N obtained from mycorrhizal uptake (f), or a reduced proportion of mycorrhizal N transferred to vegetation (T r). Foliar and fine root nitrogen concentrations were also lower at these sites. Foliar N concentrations were significantly correlated with δ15N in foliage of Populus, Salix, Picea, and Tsuga heterophylla, and also in fine roots. The correlation between δ15N and N concentration may reflect strong underlying relationships among N availability, the relative allocation of carbon to mycorrhizal fungi, and shifts in either f or T r.