6 research outputs found
Force-velocity correlations in a dense, collisional, granular flow
We report measurements in a 2-dimensional, gravity-driven, collisional,
granular flow of the normal force delivered to the wall and of particle
velocity at several points in the flow. The wall force and the flow velocity
are negatively correlated. This correlation falls off only slowly with distance
transverse to the flow, but dies away on the scale of a few particle diameters
upstream or downstream. The data support a picture of short-lived chains of
frequently colliding particles that extend transverse to the flow direction,
making transient load-bearing bridges that cause bulk fluctuations in the flow
velocity. The time-dependence of these spatial correlation functions indicate
that while the force-bearing structures are local in space, their influence
extends far upstream in the flow, albeit with a time-lag. This leads to
correlated velocity fluctuations, whose spatial range increases as the jamming
threshold is approached.Comment: to be submitted for publicatio
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Microbe-electrode interactions: The chemico-physical environment and electron transfer
This thesis presents studies that examine microbial extracellular electron transfer that an emphasis characterizing how environmental conditions influence electron flux between microbes and a solid-phase electron donor or acceptor. I used bioelectrochemical systems (BESs), fluorescence and electron microscopy, chemical measurements, 16S rRNA analysis, and qRT-PCR to study these relationships among chemical, physical and biological parameters and processes.Engineering and Applied Science
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Electron Uptake by Iron-Oxidizing Phototrophic Bacteria
Oxidation–reduction reactions underlie energy generation in nearly all life forms. Although most organisms use soluble oxidants and reductants, some microbes can access solid-phase materials as electron-acceptors or -donors via extracellular electron transfer. Many studies have focused on the reduction of solid-phase oxidants. Far less is known about electron uptake via microbial extracellular electron transfer, and almost nothing is known about the associated mechanisms. Here we show that the iron-oxidizing photoautotroph Rhodopseudomonas palustris TIE-1 accepts electrons from a poised electrode, with carbon dioxide as the sole carbon source/electron acceptor. Both electron uptake and ruBisCo form I expression are stimulated by light. Electron uptake also occurs in the dark, uncoupled from photosynthesis. Notably, the pioABC operon, which encodes a protein system essential for photoautotrophic growth by ferrous iron oxidation, influences electron uptake. These data reveal a previously unknown metabolic versatility of photoferrotrophs to use extracellular electron transfer for electron uptake.Engineering and Applied SciencesOrganismic and Evolutionary Biolog
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A Simple, Inexpensive Holographic Microscope
We have built a simple holographic microscope completely out of consumer components. We obtain at least 2.8 micrometer resolution and depth of field greater than 200 micrometers from an instrument costing less than $1000.Engineering and Applied SciencesPhysic
Dynamical Fluctuations in Dense Granular Flows
We have made measurements of force and velocity fluctuations in a variety of dense, gravity-driven granular flows under flow conditions close to the threshold of jamming. The measurements reveal a microscopic state that evolves rapidly from entirely collisional to largely frictional, as the system is taken close to jamming. On coarse-grained time scales, some descriptors of the dynamics—such as the probability distribution of force fluctuations, or the mean friction angle—do not reflect this profound change in the micromechanics of the flow. Other quantities, such as the frequency spectrum of force fluctuations, change significantly, developing low-frequency structure in the fluctuations as jamming is approached. We also show evidence of spatial structure, with force fluctuations being organized into local collision chains. These local structures propagate rapidly in the flow, with consequences far away from their origin, leading to long-range correlations in velocity fluctuations
Duty Cycling Influences Current Generation in Multi-Anode Environmental Microbial Fuel Cells
Improving microbial fuel cell (MFC) performance continues
to be
the subject of research, yet the role of operating conditions, specifically
duty cycling, on MFC performance has been modestly addressed. We present
a series of studies in which we use a 15-anode environmental MFC to
explore how duty cycling (variations in the time an anode is connected)
influences cumulative charge, current, and microbial composition.
The data reveal particular switching intervals that result in the
greatest time-normalized current. When disconnection times are sufficiently
short, there is a striking decrease in current due to an increase
in the overall electrode reaction resistance. This was observed over
a number of whole cell potentials. Based on these results, we posit
that replenishment of depleted electron donors within the biofilm
and surrounding diffusion layer is necessary for maximum charge transfer,
and that proton flux may be not limiting in the highly buffered aqueous
phases that are common among environmental MFCs. Surprisingly, microbial
diversity analyses found no discernible difference in gross community
composition among duty cycling treatments, suggesting that duty cycling
itself has little or no effect. Such duty cycling experiments are
valuable in determining which factors govern performance of bioelectrochemical
systems and might also be used to optimize field-deployed systems