616 research outputs found
Ethical issues in urban planning and development
Thesis (Ph.D.)--Boston UniversityProblem: The problem of this dissertation is to relate ethical issues in modern city planning in the United States and the emerging context of choice in urban society to a systematic ethical framework which can serve as a normative foundation for responsible planning.
Method: The dissertation is developed in three stages: first, an historical study of the development of ethical issues in city planning in the United States; secondly, an analysis of the context of choice in contemporary urban society; finally, the development of a systematic ethical framework for urban planning which explores the emerging coherence between Talcott Parsons' theory of action, Edgar S. Brightman's formulation of the moral laws, the idea of the responsible society, and contemporary discussions of planning theory [TRUNCATED
Automated Analysis of Spoken Language: A Gender Comparison
We studied the number of words spoken by adult males versus females throughout a six-hour day and during three structured monologues. The six-hour samples were captured and analyzed using an automated speech monitoring and assessment system. The three monologues required different language tasks, and analyses of syntactic and semantic complexity were performed for each. There were no significant gender differences except during a reminiscent monologue when males spoke significantly more words and sentences than females. These results conflict with past research and popular (mis)conceptions
Experimental Evidence for a Structural-Dynamical Transition in Trajectory Space
Among the key insights into the glass transition has been the identification
of a non-equilibrium phase transition in trajectory space which reveals phase
coexistence between the normal supercooled liquid (active phase) and a glassy
state (inactive phase). Here we present evidence that such a transition occurs
in experiment. In colloidal hard spheres we find a non-Gaussian distribution of
trajectories leaning towards those rich in locally favoured structures (LFS),
associated with the emergence of slow dynamics. This we interpret as evidence
for an non-equilibrium transition to an inactive LFS-rich phase. Reweighting
trajectories reveals a first-order phase transition in trajectory space between
a normal liquid and a LFS-rich phase. We further find evidence of a purely
dynamical transition in trajectory space.Comment: 5 page
Effects of vertical confinement on gelation and sedimentation of colloids
We consider the sedimentation of a colloidal gel under confinement in the
direction of gravity. The confinement allows us to compare directly experiments
and computer simulations, for the same system size in the vertical direction.
The confinement also leads to qualitatively different behaviour compared to
bulk systems: in large systems gelation suppresses sedimentation, but for small
systems sedimentation is enhanced relative to non-gelling suspensions, although
the rate of sedimentation is reduced when the strength of the attraction
between the colloids is strong. We map interaction parameters between a model
experimental system (observed in real space) and computer simulations.
Remarkably, we find that when simulating the system using Brownian dynamics in
which hydrodynamic interactions between the particles are neglected, we find
that sedimentation occurs on the same timescale as the experiments, however the
thickness of the "arms" of the gel is rather larger in the experiments,
compared with the simulations. An analysis of local structure in the
simulations showed similar behaviour to gelation in the absence of gravity
Characterization of Aircraft Produced Soot and Contrails Near the Tropopause
Participation in the SUCCESS project primarily involved development and deployment of specific instruments for characterizing jet aircraft exhaust emissions as particulates and their subsequent evolution as contrail particles, either liquid or solid, as cirrus. Observations can be conveniently considered in two categories - close or distant from the aircraft. Thus close to the aircraft the exhaust is mixing through the engine turbulence with a much drier and colder environment and developing water/ ice supersaturation along the trail depending on circumstances (near field), whereas distant from the aircraft (far field) the exhaust has cooled essentially to ambient temperature, the turbulence has decayed and any particle growth or evaporation is controlled by the prevailing ambient conditions. Intermediate between these two regions the main aircraft vortices form (one on each side of the aircraft) which tend to inhibit mixing under some conditions, a region extending from a few aircraft lengths to sometimes a hundred times this distance. Our approach to the problem lay in experience gained in characterizing the smoke from hydrocarbon combustion in terms of its cloud forming properties and its potential influence on the radiation properties of the smoke and subsequent cloud from the viewpoint of reduction (absorbtion and scattering ) of solar radiation flux leading to significant global cooling (Hudson et al 1991; Hallett and Hudson 1991). Engine exhaust contains a much smaller proportion of the fuel carbon than is sometimes present in ordinary combustion (less than 0.01% compared with 10%) and influences condensation in quite different ways, to be characterized by the Cloud Condensation Nucleus, CCN - supersaturation spectrum. The transition to ice is to be related to the dilution of solution droplets to freeze by homogeneous nucleation at temperatures somewhat below -40C (Pueschel et al 1998). The subsequent growth of ice particles depends critically on temperature, supersaturation and to some extent pressure, as is demonstrated in an NSF funded project being carried out in parallel with the work reported here. As will be discussed below, nucleation processes themselves and also exhaust impurities also influence the growth of ice particles and may control some aspects of growth of ice in contrails. Instrumentation was designed to give insight into these questions and to be flown on the NASA DC- 8 as a platform. In addition a modest program was undertaken to investigate the properties of laboratory produced smoke produced under controlled conditions from the viewpoint of forming both CCN and CN. The composition of the smoke could inferred from a thermal characterization technique; larger particles were captured by formvar replicator for detailed analysis; ice particles were captured and evaporated in flight on a new instrument, the cloudscope, to give their mass, density and impurity content
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A Neutron Scattering Study of the Structure of Poly(dimethylsiloxane)-Stabilized Poly(methyl methacrylate) (PDMS–PMMA) Latexes in Dodecane
Hard-sphere particles in nonpolar solvents are an essential tool for colloid scientists. Sterically stabilized poly(methyl methacrylate) (PMMA) particles have long been used as the exemplary hard-sphere system. However, neither the particles themselves nor the poly(12-hydroxystearic acid) (PHSA) stabilizer necessary to prevent aggregation in nonpolar solvents are commercially available. To counter this, several alternatives have been proposed. In recent years, there has been an increased interest in poly(dimethylsiloxane) (PDMS) stabilizers as a commercially available alternative to PHSA, yet the structure of particles made in this way is not as well understood as those produced using PHSA. In this work, we employ small-angle neutron scattering to determine the internal structure of PDMS-stabilized PMMA particles, synthesized with and without an additional crosslinking agent. We report data consistent with a homogeneous PMMA core with a linearly decaying PDMS shell. The thickness of the shell was in excess of 50 nm, thicker than the PHSA layer typically used to stabilize PMMA but consistent with reports of the layer thickness for similar molecular weight PDMS at planar surfaces. We also show that the amount of the hydrogenous material in the particle core of the crosslinked particles notably exceeds the amount of added ethylene glycol dimethacrylate crosslinker, suggesting some entrapment of the PDMS stabilizer in the PMMA matrix
The Devil is in the details:Pentagonal bipyramids and dynamic arrest
Colloidal suspensions have long been studied as a model for atomic and
molecular systems, due to the ability to fluorescently label and individually
track each particle, yielding particle-resolved structural information. This
allows various local order parameters to be probed that are otherwise
inaccessible for a comparable molecular system. For phase transitions such as
crystallisation, appropriate order parameters which emphasise 6-fold symmetry
are a natural choice, but for vitrification the choice of order parameter is
less clear cut. Previous work has highlighted the importance of icosahedral
local structure as the glass transition is approached. However, counting
icosahedra or related motifs is not a continuous order parameter in the same
way as, for example, the bond-orientational order parameters and .
In this work we investigate the suitability of using pentagonal bipyramid
membership, a structure which can be assembled into larger, five-fold symmetric
structures, as a finer order parameter to investigate the glass transition. We
explore various structural and dynamic properties and show that this new
approach produces many of the same findings as simple icosahedral membership,
but we also find that large instantaneous displacements are often correlated
with significant changes in pentagonal bipyramid membership, and unlike the
population of defective icosahedra, the pentagonal bypyramid membership and
spindle number do not saturate for any measured volume fraction, but continues
to increase.Comment: accepted by JStat Mech: Theory and Experiment 201
Counterion Condensation on Spheres in the Salt-free Limit
A highly-charged spherical colloid in a salt-free environment exerts such a
powerful attraction on its counterions that a certain fraction condenses onto
the surface of a particle. The degree of condensation depends on the curvature
of the surface. So, for instance, condensation is triggered on a highly-charged
sphere only if the radius exceeds a certain critical radius \collrad^{*}.
\collrad^{*} is expected to be a simple function of the volume fraction of
particles. To test these predictions, we prepare spherical particles which
contain a covalently-bound ionic liquid, which is engineered to dissociate
efficiently in a low-dielectric medium. By varying the proportion of ionic
liquid to monomer we synthesise nonpolar dispersions of highly-charged spheres
which contain essentially no free co-ions. The only ions in the system are
counterions generated by the dissociation of surface-bound groups. We study the
electrophoretic mobility of this salt-free system as a function of the colloid
volume fraction, the particle radius, and the bare charge density and find
evidence for extensive counterion condensation. At low electric fields, we
observe excellent agreement with Poisson-Boltzmann predictions for counterion
condensation on spheres. At high electric fields however, where ion advection
is dominant, the electrophoretic mobility is enhanced significantly which we
attribute to hydrodynamic stripping of the condensed layer of counterions from
the surface of the particle.Comment: 13 pages, 9 figures and two table
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Molecular exchange in spherical diblock copolymer colloids synthesised by polymerisation-induced self-assembly
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