2,122 research outputs found
Frank's constant in the hexatic phase
Using video-microscopy data of a two-dimensional colloidal system the
bond-order correlation function G6 is calculated and used to determine the
temperature-dependence of both the orientational correlation length xi6 in the
isotropic liquid phase and the Frank constant F_A in the hexatic phase. F_A
takes the value 72/pi at the hexatic to isotropic liquid phase transition and
diverges at the hexatic to crystal transition as predicted by the KTHNY-theory.
This is a quantitative test of the mechanism of breaking the orientational
symmetry by disclination unbinding
Fluid-Induced Propulsion of Rigid Particles in Wormlike Micellar Solutions
In the absence of inertia, a reciprocal swimmer achieves no net motion in a
viscous Newtonian fluid. Here, we investigate the ability of a reciprocally
actuated particle to translate through a complex fluid that possesses a network
using tracking methods and birefringence imaging. A geometrically polar
particle, a rod with a bead on one end, is reciprocally rotated using magnetic
fields. The particle is immersed in a wormlike micellar (WLM) solution that is
known to be susceptible to the formation of shear bands and other localized
structures due to shear-induced remodeling of its microstructure. Results show
that the nonlinearities present in this WLM solution break time-reversal
symmetry under certain conditions, and enable propulsion of an artificial
"swimmer." We find three regimes dependent on the Deborah number (De): net
motion towards the bead-end of the particle at low De, net motion towards the
rod-end of the particle at intermediate De, and no appreciable propulsion at
high De. At low De, where the particle time-scale is longer then the fluid
relaxation time, we believe that propulsion is caused by an imbalance in the
fluid first normal stress differences between the two ends of the particle
(bead and rod). At De~1, however, we observe the emergence of a region of
network anisotropy near the rod using birefringence imaging. This anisotropy
suggests alignment of the micellar network, which is "locked in" due to the
shorter time-scale of the particle relative to the fluid
Harmonic lattice behavior of two-dimensional colloidal crystals
Using positional data from video-microscopy and applying the equipartition
theorem for harmonic Hamiltonians, we determine the wave-vector-dependent
normal mode spring constants of a two-dimensional colloidal model crystal and
compare the measured band-structure to predictions of the harmonic lattice
theory. We find good agreement for both the transversal and the longitudinal
mode. For , the measured spring constants are consistent with the
elastic moduli of the crystal.Comment: 4 pages, 3 figures, submitte
The experimental realization of a two-dimensional colloidal model system
We present the technical details of an experimental method to realize a model
system for 2D phase transitions and the glass transition. The system consists
of several hundred thousand colloidal super-paramagnetic particles confined by
gravity at a flat water-air interface of a pending water droplet where they are
subjected to Brownian motion. The dipolar pair potential and therefore the
system temperature is not only known precisely but also directly and
instantaneously controllable via an external magnetic field B. In case of a one
component system of monodisperse particles the system can crystallize upon
application of B whereas in a two component system it undergoes a glass
transition. Up to 10000 particles are observed by video microscopy and image
processing provides their trajectories on all relative length and time scales.
The position of the interface is actively regulated thereby reducing surface
fluctuations to less than one micron and the setup inclination is controlled to
an accuracy of 1 microrad. The sample quality being necessary to enable the
experimental investigation of the 2D melting scenario, 2D crystallization, and
the 2D glass transition, is discussed.Comment: 13 pages, 11 figure
Crystal nuclei and structural correlations in two-dimensional colloidal mixtures: experiment versus simulation
We examine binary mixtures of superparamagnetic colloidal particles confined
to a two-dimensional water-air interface both by real-space experiments and
Monte-Carlo computer simulations at high coupling strength. In the simulations,
the interaction is modelled as a pairwise dipole-dipole repulsion. While the
ratio of magnetic dipole moments is fixed, the interaction strength governed by
the external magnetic field and the relative composition is varied. Excellent
agreement between simulation and experiment is found for the partial pair
distribution functions including the fine structure of the neighbour shells at
high coupling. Furthermore local crystal nuclei in the melt are identified by
bond-orientational order parameters and their contribution to the pair
structure is discussed
Functional Brain Imaging with Multi-Objective Multi-Modal Evolutionary Optimization
Functional brain imaging is a source of spatio-temporal data mining problems.
A new framework hybridizing multi-objective and multi-modal optimization is
proposed to formalize these data mining problems, and addressed through
Evolutionary Computation (EC). The merits of EC for spatio-temporal data mining
are demonstrated as the approach facilitates the modelling of the experts'
requirements, and flexibly accommodates their changing goals
Performance and carcass characteristics of steers fed with two levels of metabolizable energy intake during summer and winter season
Climate change is producing an increase on extreme weather events around the world such as flooding, drought and extreme ambient temperatures impacting animal production and animal welfare. At present, there is a lack of studies addressing the effects of climatic conditions associated with energy intake in finishing cattle in South American feed yards. Therefore, two experiments were conducted to assess the effects of environmental variables and level of metabolizable energy intake above maintenance requirements (MEI) on performance and carcass quality of steers. In each experiment (winter and summer), steers were fed with 1.85 or 2.72 times of their requirements of metabolizable energy of maintenance. A total of 24 crossbred steers per experiment were used and located in four pens (26.25 m2/head) equipped with a Calan Broadbent Feeding System. Animals were fed with the same diet within each season, varying the amount offered to adjust the MEI treatments. Mud depth, mud scores, tympanic temperature (TT), environmental variables, average daily gain, respiration rates and carcass characteristics plus three thermal comfort indices were collected. Data analysis considered a factorial arrangement (Season and MEI). In addition, a repeated measures analysis was performed for TT and respiration rate. Mean values of ambient temperature, solar radiation and comfort thermal indices were greater in the summer experiment as expected (P\u3c 0.005). The mean values of TT were higher in steers fed with higher MEI and also in the summer season. The average daily gain was greater during summer v. winter (1.10 ± 0.11 v. 0.36 ± 0.06) kg/day, also when steers were fed 2.72 v. 1.85 MEI level (0.89 ± 0.12 v. 0.57 ± 0.10) kg/day. In summer, respiration rate increased in 41.2% in the afternoon. In winter, muddy conditions increased with time of feeding, whereas wind speed and rainfall had significant effects on TT and average daily gain. We conclude that MEI and environmental variables have direct effects on the physiology and performance of steers, including TT and average daily gain, particularly during the winter. In addition, carcass characteristics were affected by season but not by the level of MEI. Finally, due to the high variability of data as well as the small number of animals assessed in these experiments, more studies on carcass characteristics under similar conditions are required
Tracing melioidosis back to the source: using whole-genome sequencing to investigate an outbreak originating from a contaminated domestic water supply
Melioidosis, a disease of public health importance in Southeast Asia and northern Australia, is caused by the Gram-negative soil bacillus Burkholderia pseudomallei. Melioidosis is typically acquired through environmental exposure, and case clusters are rare, even in regions where the disease is endemic. B. pseudomallei is classed as a tier 1 select agent by the Centers for Disease Control and Prevention; from a biodefense perspective, source attribution is vital in an outbreak scenario to rule out a deliberate release. Two cases of melioidosis within a 3-month period at a residence in rural northern Australia prompted an investigation to determine the source of exposure. B. pseudomallei isolates from the property's groundwater supply matched the multilocus sequence type of the clinical isolates. Whole-genome sequencing confirmed the water supply as the probable source of infection in both cases, with the clinical isolates differing from the likely infecting environmental strain by just one single nucleotide polymorphism (SNP) each. For the first time, we report a phylogenetic analysis of genomewide insertion/deletion (indel) data, an approach conventionally viewed as problematic due to high mutation rates and homoplasy. Our whole-genome indel analysis was concordant with the SNP phylogeny, and these two combined data sets provided greater resolution and a better fit with our epidemiological chronology of events. Collectively, this investigation represents a highly accurate account of source attribution in a melioidosis outbreak and gives further insight into a frequently overlooked reservoir of B. pseudomallei. Our methods and findings have important implications for outbreak source tracing of this bacterium and other highly recombinogenic pathogens
Dynamics of particles and cages in an experimental 2D glass former
We investigate the dynamics of a glass forming 2D colloidal mixture and show
the existence of collective motions of the particles. We introduce a mean
square displacement MSD with respect to the nearest neighbors which shows
remarkable deviations from the usual MSD quantifying the individual motion of
our particles. Combined with the analysis of the self part of the Van Hove
function this indicates a coupled motion of particles with their cage as well
as intra cage hopping processes.Comment: Submitted to EP
Quantifying Stock Price Response to Demand Fluctuations
We address the question of how stock prices respond to changes in demand. We
quantify the relations between price change over a time interval
and two different measures of demand fluctuations: (a) , defined as the
difference between the number of buyer-initiated and seller-initiated trades,
and (b) , defined as the difference in number of shares traded in buyer
and seller initiated trades. We find that the conditional expectations and of price change for a given or
are both concave. We find that large price fluctuations occur when demand is
very small --- a fact which is reminiscent of large fluctuations that occur at
critical points in spin systems, where the divergent nature of the response
function leads to large fluctuations.Comment: 4 pages (multicol fomat, revtex
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