Behavior of Marine Bacteria in Clean Environment and Oil Spill Conditions.

Alcanivorax borkumensis is a bacterial community that dominates hydrocarbon-degrading communities around many oil spills. The physicochemical conditions that prompt bacterial binding to oil/water interfaces are not well understood. To provide key insights into this process, A. borkumensis cells were cultured either in a clean environment condition (dissolved organic carbon) or in an oil spill condition (hexadecane as the sole energy source). The ability of these bacteria to bind to the oil/water interface was monitored through interfacial tension measurements, bacterial cell hydrophobicity, and fluorescence microscopy. Our experiments show that A. borkumensis cells cultured in clean environment conditions remain hydrophilic and do not show significant transport or binding to the oil/water interface. In sharp contrast, bacteria cultured in oil spill conditions become partially hydrophobic and their amphiphilicity drives them to oil/water interfaces, where they reduce interfacial tension and form the early stages of a biofilm. We show that it is A. borkumensis cells that attach to the oil/water interface and not a synthesized biosurfactant that is released into solution that reduces interfacial tension. This study provides key insights into the physicochemical properties that allow A. borkumensis to adhere to oil/water interfaces.

Phase and steady shear behavior of dilute carbon black suspensions and carbon black stabilized emulsions.

We use para-amino benzoic acid terminated carbon black (CB) as a model particulate material to study the effect of salt-modulated attractive interactions on phase behavior and steady shear stresses in suspensions and particle-stabilized emulsions. Surprisingly, the suspension displayed a yield stress at a CB volume fraction of ϕCB = 0.008. The yield stress scaled with CB concentration with power law behavior; the power law exponent changed abruptly at a critical CB concentration, suggesting a substantial change in network structure. Cryogenic scanning electron microscopy revealed structural differences between the networks found in each scaling regime. Randomly oriented pores with thick CB boundaries were observed in the scaling region above the critical particle concentration, suggesting a strong gel network, and long, oriented pores were found in the scaling region below the critical particle concentration, suggesting a weak network influenced by an induced shear stress. These findings correlate with the existence of gels and transient networks. Transient networks break down under gravitational forces over time periods of 12-24 hours. The yield stresses of CB-gels containing oil emulsion droplets were found to scale with carbon black concentration similar to the CB-gels without oil. These results offer insight into salt-induced attractive colloidal networks and the difference in structure and yield-stress behavior between transient networks and gels. Furthermore, CB offers the ability to stabilize an oil phase in discrete droplets and contain them within a rigid network structure.

Massive electrical conductivity enhancement of multilayer graphene/polystyrene composites using a nonconductive filler.

We report a massive increase in the electrical conductivity of a multilayer graphene (MLG)/polystyrene composite following the addition of nonconducting silica nanoparticles. The nonconducting filler acts as a highly effective dispersion aid, preventing the sheetlike MLG from restacking or agglomerating during the solvent casting process used to fabricate the composite. The enhanced dispersion of the MLG leads to orders of magnitude enhancement in electrical conductivity compared to samples without this filler.

Shear-directed assembly of graphene oxide in aqueous dispersions into ordered arrays.

A wide variety of new carbon-based materials are being developed from graphene oxide (GO) precursor sheets, whose assembly in aqueous phases determines the form, structure, and properties of the resultant carbon. Here we show that graphene oxide forms ordered linear arrays of aggregates when aqueous suspensions are subjected to shear flow in the presence of soluble salts. These linear arrays align along the vorticity direction, normal to the direction of flow. We propose that salt addition screens electrostatic repulsion and allows formation of fractal-like GO sheet aggregates by hydrophobic forces. Fluid shear in a confined gap then guides the assembly of these primary aggregates into optically visible, ordered linear arrays or "superaggregates" whose characteristics are a function of GO concentration, salt valency, salt concentration, and gap confinement. This is the first reported observation of vorticity banding in graphene oxide suspensions and the first reported observation of such banding based on salt-induced interactions. We also demonstrate that simple isometric nanoparticles of carbon or gold do not form such linear superaggregate arrays but can be assembled into such arrays using graphene oxide as a two-dimensional colloidal template.

A platform for retaining native morphology at sub-second time scales in cryogenic transmission electron microscopy.

The advantage of cryogenic transmission electron microscopy for morphological analysis of complex fluids is the ability to capture native specimen morphology in solution. This is often limited by available sample preparation devices and procedures, which expose the sample to high shear rates leading to non-native artifacts, are unable to capture evolving samples at a time resolution shorter than a few seconds, and often non-specifically adsorb sample species from suspension resulting in a non-native sample concentration on the grid. In this paper we report the development of a new sample preparation device based on capillary action that overcomes all of these limitations. The use of a removal capillary placed parallel to the grid results in reduced shear and lower absorption of particulate material from the sample. A deposition capillary placed perpendicular to the grid allows for precise and sub-second resolution for time resolved studies. We demonstrate each of the features of this platform using model samples, and where appropriate, compare our results to those prepared using current vitrification platforms. Our results confirm that this new sample vitrification device opens up previously unattainable regimes for sample preparation and imaging and is a powerful new tool for cryogenic transmission electron microscopy.