1,133 research outputs found
The Electrostatic Screening Length in Concentrated Electrolytes Increases with Concentration
According to classical electrolyte theories interactions in dilute (low ion
density) electrolytes decay exponentially with distance, with the Debye
screening length the characteristic length-scale. This decay length decreases
monotonically with increasing ion concentration, due to effective screening of
charges over short distances. Thus within the Debye model no long-range forces
are expected in concentrated electrolytes. Here we reveal, using experimental
detection of the interaction between two planar charged surfaces across a wide
range of electrolytes, that beyond the dilute (Debye-Huuckel) regime the
screening length increases with increasing concentration. The screening lengths
for all electrolytes studied - including aqueous NaCl solutions, ionic liquids
diluted with propylene carbonate, and pure ionic liquids - collapse onto a
single curve when scaled by the dielectric constant. This non-monotonic
variation of the screening length with concentration, and its generality across
ionic liquids and aqueous salt solutions, demonstrates an important
characteristic of concentrated electrolytes of substantial relevance from
biology to energy storage.Comment: This document is the unedited authors' version of a Submitted Work
that was subsequently accepted for publication in the Journal of Physical
Chemistry Letters, copyright American Chemical Society, after peer review. To
access the final edited and published work see
http://pubsdc3.acs.org/articlesonrequest/AOR-EW6FuIC6wIh6D9qqEeH
Scaling analysis of the screening length in concentrated electrolytes
The interaction between charged objects in an electrolyte solution is a
fundamental question in soft matter physics. It is well-known that the
electrostatic contribution to the interaction energy decays exponentially with
object separation. Recent measurements reveal that, contrary to the
conventional wisdom given by classic Poisson-Boltzmann theory, the decay length
increases with ion concentration for concentrated electrolytes and can be an
order of magnitude larger than the ion diameter in ionic liquids. We derive a
simple scaling theory that explains this anomalous dependence of the decay
length on ion concentration. Our theory successfully collapses the decay
lengths of a wide class of salts onto a single curve. A novel prediction of our
theory is that the decay length increases linearly with the Bjerrum length,
which we experimentally verify by surface force measurements. Moreover, we
quantitatively relate the measured decay length to classic measurements of the
activity coefficient in concentrated electrolytes, thus showing that the
measured decay length is indeed a bulk property of the concentrated electrolyte
as well as contributing a mechanistic insight into empirical activity
coefficients.Comment: To appear in Physical Review Letter
Response to Kuhse
In this short paper, we respond to critics of our original paper, The agony of agonal respiration: is the
last gasp necessary?. A common thread in both Hawryluckâ s and Kuhseâ s responses is the difficulties
encountered when using the agentâ s intentions to make moral distinctions between using neuromuscular
blocking drugs to palliate versus using neuromuscular blocking drugs to kill. Although this difficulty does
exist we maintain that the intentions of the physician must matter when providing end-of-life care. Originally published Journal of Medical Ethics, Vol. 28, No. 4, Aug. 200
Quantized Friction across Ionic Liquid Thin Films
Ionic liquids, salts in the liquid state under ambient conditions, are of
great interest as precision lubricants. Ionic liquids form layered structures
at surfaces, yet it is not clear how this nano-structure relates to their
lubrication properties. We measured the friction force between atomically
smooth solid surfaces across ionic liquid films of controlled thickness in
terms of the number of ion layers. Multiple friction-load regimes emerge, each
corresponding to a different number of ion layers in the film. In contrast to
molecular liquids, the friction coefficients differ for each layer due to their
varying composition
Calculation of static longitudinal aerodynamic characteristics of STOL aircraft with upper surface blown flaps
An existing prediction method developed for EBF aircraft configurations was applied to USB configurations to determine its potential utility in predicting USB aerodynamic characteristics. An existing wing-flap vortex-lattice computer program was modified to handle multiple spanwise flap segments at different flap angles. A potential flow turbofan wake model developed for circular cross-section jets was used to model a rectangular cross-section jet wake by placing a number of circular jets side by side. The calculation procedure was evaluated by comparison of measured and predicted aerodynamic characteristics on a variety of USB configurations. The method is limited to the case where the flow and geometry of the configuration are symmetric about a vertical plane containing the wing root chord. Comparison of predicted and measured lift and pitching moment coefficients were made on swept wings with one and two engines per wing panel, various flap deflection angles, and a range of thrust coefficients. The results indicate satisfactory prediction of lift for flap deflections up to 55 and thrust coefficients less than 2. The applicability of the prediction procedure to USB configurations is evaluated, and specific recommendations for improvements are discussed
Fruit and Vegetable Consumption, Body Mass Index, and Dieting Behaviors among Florida Middle and High School Youth
The purpose of this study was to examine fruit and vegetable consumption reported by Florida middle school and high school students and to determine if consumption was associated with: (1) demographic characteristics; (2) body mass index (BMI); and/or (3) weight loss behaviors. Data from the 2009 Florida YRBS and the MSHBS were used. Both surveys used a two-stage cluster sample design, producing a representative sample of students. In addition to socio- demographic variables, independent variables included engaging in weight loss behaviors. Dependent variables were adequate fruit and adequate vegetable consumption. Descriptive, chi-square and logistic regression analyses were performed. Results indicated that less than 25% of youth reported consumption of adequate amounts of fruit and less than 10% reported consuming adequate amounts of vegetables. No significant relationships were found between BMI and either fruit or vegetable consumption. Exercising, both exercising and dieting, fasting, and using pills /powders to lose weight were significantly associated with adequate fruit and vegetable consumption. Dieting to lose weight was significantly associated with adequate vegetable but not adequate fruit consumption. Interventions promoting healthy weight for children and adolescents need to address positive and negative behaviors and encourage greater consumption of fruits and vegetables
SO(10) Cosmic Strings and Baryon Number Violation
SO(10) cosmic strings formed during the phase transition Spin(10)
SU(5) are studied. Two types of strings ---
one effectively Abelian and one non-Abelian --- are constructed and the string
solutions are calculated numerically. The non-Abelian string can catalyze
baryon number violation via the ``twisting'' of the scalar field which causes
mixing of leptons and quarks in the fermion multiplet. The non-Abelian string
is also found to have the lower energy possibly for the entire range of the
parameters in the theory. Scattering of fermions in the fields of the strings
is analyzed, and the baryon number violation cross section is calculated. The
role of the self-adjoint parameters is discussed and the values are computed.Comment: LaTex (RevTex), 36 pages, 6 figures (available upon request),
MIT-CTP#215
Counterion-mediated Electrostatic Interactions between Helical Molecules
We study the interaction of two cylinders with helical charge distribution
mediated by neutralizing counterions, by analyzing the separation as well as
the azimuthal angle dependence of the interaction force in the weak and strong
coupling limit. While the azimuthal dependence of the interaction in the weak
coupling limit is overall small and mostly negligible, the strong coupling
limit leads to qualitatively new features of the interaction, among others also
to an orientationally dependent optimal configuration that is driven by angular
dependence of the correlation attraction. We investigate the properties of this
azimuthal ordering in detail and compare it to existing results.Comment: 11 pages, 12 figure
Fragmentation and dewatering transform Great Plains stream fish communities
Citation: Perkin, J. S., Gido, K. B., Cooper, A. R., Turner, T. F., Osborne, M. J., Johnson, E. R., & Mayes, K. B. (2015). Fragmentation and dewatering transform Great Plains stream fish communities. Ecological Monographs, 85(1), 73-92. doi:10.1890/14-0121.1Biodiversity in stream networks is threatened globally by interactions between habitat fragmentation and altered hydrologic regimes. In the Great Plains of North America, stream networks are fragmented by >19000 anthropogenic barriers, and flow regimes are altered by surface water retention and groundwater extraction. We documented the distribution of anthropogenic barriers and dry stream segments in five basins covering the central Great Plains to assess effects of broad-scale environmental change on stream fish community structure and distribution of reproductive guilds. We used an information-theoretic approach to rank competing models in which fragmentation, discharge magnitude, and percentage of time streams had zero flow (a measure of desiccation) were included to predict effects of environmental alterations on the distribution of fishes belonging to different reproductive guilds. Fragmentation caused by anthropogenic barriers was most common in the eastern Great Plains, but stream desiccation became more common to the west, where rivers are underlain by the depleted (i.e., extraction > recharge) High Plains Aquifer. Longitudinal gradients in fragmentation and desiccation contributed to spatial shifts in community structure from taxonomically and functionally diverse communities dominated by pelagic reproductive guilds where fragmentation and desiccation were least, to homogenized communities dominated by benthic guilds where fragmentation and desiccation were common. Modeling results revealed these shifts were primarily associated with decline of pelagic reproductive guilds, notably small-bodied pelagophilic and lithopelagophilic fishes that declined in association with decreased fragment length and increased number of days with zero flow. Graph theory combined with a barrier prioritization approach revealed specific fragments that could be reconnected to allow fishes within these guilds to colonize currently unoccupied fragments with the mitigation or removal of small dams (<10 m height). These findings are useful for natural resource managers charged with halting or reversing the prevailing pattern of declining fish diversity in the Great Plains. Our study represents one of the most comprehensive assessments of fish diversity responses to broad-scale environmental change in the Great Plains and provides a conservation strategy for addressing the simultaneous contributions of fragmentation and flow alteration to the global freshwater biodiversity crisis
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