Homogeneous percolation versus arrested phase separation in attractively-driven nanoemulsion colloidal gels

We elucidate mechanisms for colloidal gelation of attractive nanoemulsions depending on the volume fraction (ϕ) of the colloid. Combining detailed neutron scattering, cryo-transmission electron microscopy and rheological measurements, we demonstrate that gelation proceeds by either of two distinct pathways. For ϕ sufficiently lower than 0.23, gels exhibit homogeneous fractal microstructure, with a broad gel transition resulting from the formation and subsequent percolation of droplet–droplet clusters. In these cases, the gel point measured by rheology corresponds precisely to arrest of the fractal microstructure, and the nonlinear rheology of the gel is characterized by a single yielding process. By contrast, gelation for ϕ sufficiently higher than 0.23 is characterized by an abrupt transition from dispersed droplets to dense clusters with significant long-range correlations well-described by a model for phase separation. The latter phenomenon manifests itself as micron-scale “pores” within the droplet network, and the nonlinear rheology is characterized by a broad yielding transition. Our studies reinforce the similarity of nanoemulsions to solid particulates, and identify important qualitative differences between the microstructure and viscoelastic properties of colloidal gels formed by homogeneous percolation and those formed by phase separation.United States. Army Research Office (Institute for Collaborative Biotechnologies. Grant W911NF- 09-0001)National Science Foundation (U.S.) (Grants CMMI-1120724 and DMR-1006147

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

An insight into the growth of Alcanivorax borkumensis under different inoculation conditions

Alcanivorax borkumensis is a hydrocarbon degrading bacterium found to dominate bacterial communities in marine regions containing high levels of hydrocarbons. It has been linked to oil degradation around oil spill sites; thus, it has potential to be used actively in oil spill remediation. Here, we investigate the effect of solution and interfacial conditions on the growth of A. borkumensis. We show that providing A. borkumensis with dissolved organic nitrogen as an additional nutrient in solution leads to shorter lag times prior to hydrocarbon utilization at the oil-water interface. Hence, A. borkumensis can be encouraged to utilize n-alkanes present at the surface of the system quicker by supplementing the system with dissolved organic nitrogen. For fixed oil-water interfacial areas, the growth rates of bacteria show weak dependence on the initial bacteria concentrations; however, increasing bulk interfacial area leads to higher bacterial growth rates due to an increased amount of available surface area for degradation. To our knowledge, this is the first study to offer quantitative insight into how A. borkumensis can be actively supported in their utilization and degradation of oil for the bioremediation of marine oil spills

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

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. © 2013 AIP Publishing LLC

Phase and Steady Shear Behavior of Dilute Carbon Black Suspensions and Carbon Black Stabilized Emulsions

We use <i>para</i>-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 ϕ_CB = 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

Particle templated graphene-based composites with tailored electro-mechanical properties

A capillary-driven particle level templating technique was utilized to disperse graphite nanoplatelets (GNPs) within a polystyrene matrix to form composites that possess tailored electro-mechanical properties. Utilizing capillary interactions, highly segregated composites were formed via a melt processing procedure. Since the graphene particles only resided at the boundary between the polymer matrix particles, the composites possess tremendous electrical conductivity but poor mechanical strength. To improve the mechanical properties of the composite, the graphene networks in the specimen were deformed by shear. An experimental investigation was conducted to understand the effect of graphene content as well as shearing on the mechanical strength and electrical conductivity of the composites. The experimental results show that both the mechanical and electrical properties of the composites can be altered using this very simple technique and therefore easily be tailored for desired applications

Fixed-angle rotary shear as a new method for tailoring electro-mechanical properties of templated graphene-polymer composites

A capillary-driven particle-level templating technique was utilized to distribute graphite nanoplatelets (GNPs) into specially constructed architectures throughout a polystyrene matrix to form multi-functional composites with tailored electro-mechanical properties. By precisely controlling the temperature and pressure during a melt compression process, highly conductive segregated composites were formed using very low loadings of graphene particles. Since the graphene flakes form a honeycomb percolating network along the boundaries between the polymer matrix particles, the composites show very high electrical conductivity but poor mechanical strength. To improve the mechanical properties, a new processing technique was developed that uses rotary shear through pre-set fixed angles to gradually evolve the honeycomb graphene network into a concentric band structure over the dimensions of the sample. An experimental investigation was conducted to understand the effect of GNP loading as well as rotary shear angle on the mechanical strength and electrical conductivity of the composites. The experimental results show that both the electrical and mechanical properties of the composites are significantly altered using this very simple technique, which allows rational co-optimization of competing mechanical and electrical performance as appropriate for a given target application. © 2014 Elsevier Ltd

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. © 2013 American Chemical Society