Michael P. Godfrin

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.

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.

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.

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.