235 research outputs found
Time Resolved Correlation measurements of temporally heterogeneous dynamics
Time Resolved Correlation (TRC) is a recently introduced light scattering
technique that allows to detect and quantify dynamic heterogeneities. The
technique is based on the analysis of the temporal evolution of the speckle
pattern generated by the light scattered by a sample, which is quantified by
, the degree of correlation between speckle images recorded at
time and . Heterogeneous dynamics results in significant
fluctuations of with time . We describe how to optimize TRC
measurements and how to detect and avoid possible artifacts. The statistical
properties of the fluctuations of are analyzed by studying their
variance, probability distribution function, and time autocorrelation function.
We show that these quantities are affected by a noise contribution due to the
finite number of detected speckles. We propose and demonstrate a method to
correct for the noise contribution, based on a extrapolation
scheme. Examples from both homogeneous and heterogeneous dynamics are provided.
Connections with recent numerical and analytical works on heterogeneous glassy
dynamics are briefly discussed.Comment: 19 pages, 15 figures. Submitted to PR
Aging in Dense Colloids as Diffusion in the Logarithm of Time
The far-from-equilibrium dynamics of glassy systems share important
phenomenological traits. A transition is generally observed from a
time-homogeneous dynamical regime to an aging regime where physical changes
occur intermittently and, on average, at a decreasing rate. It has been
suggested that a global change of the independent time variable to its
logarithm may render the aging dynamics homogeneous: for colloids, this entails
diffusion but on a logarithmic time scale. Our novel analysis of experimental
colloid data confirms that the mean square displacement grows linearly in time
at low densities and shows that it grows linearly in the logarithm of time at
high densities. Correspondingly, pairs of particles initially in close contact
survive as pairs with a probability which decays exponentially in either time
or its logarithm. The form of the Probability Density Function of the
displacements shows that long-ranged spatial correlations are very long-lived
in dense colloids. A phenomenological stochastic model is then introduced which
relies on the growth and collapse of strongly correlated clusters ("dynamic
heterogeneity"), and which reproduces the full spectrum of observed colloidal
behaviors depending on the form assumed for the probability that a cluster
collapses during a Monte Carlo update. In the limit where large clusters
dominate, the collapse rate is ~1/t, implying a homogeneous, log-Poissonian
process that qualitatively reproduces the experimental results for dense
colloids. Finally an analytical toy-model is discussed to elucidate the strong
dependence of the simulation results on the integrability (or lack thereof) of
the cluster collapse probability function.Comment: 6 pages, extensively revised, final version; for related work, see
http://www.physics.emory.edu/faculty/boettcher/ or
http://www.fysik.sdu.dk/staff/staff-vip/pas-personal.htm
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Analytical simulation of the microbubble collapsing in a welding fusion pool
Data Availability Statement: Not applicable.Copyright © 2023 by the authors. This paper explains the use of remote ultrasound vibration at the optimum position and frequencies to vibrate plates under welding, with the aim of initiating cavitation in the molten pool area. It has been shown in the literature that ultrasound cavitation changes microstructure morphology and refines the grain of the weld. In practice, the plates are excited through narrow-band high-power ultrasound transducers (HPUTs). Therefore, a theoretical investigation is carried out to identify the plate-mode shapes due to the ultrasound vibration aligned with the frequency bandwidth of HPUTs available in the marketplace. The effect of exciting the plate at different locations and frequencies is studied to find the optimum position and frequencies to achieve the maximum pressure at the area of the fusion zone. It was shown that applying the excitation from the side of the plate produces an order of 103
higher vibration displacement amplitude, compared with excitation from the corner. The forced vibration of cavitation and bursting time are studied to identify vibration amplitude and the time required to generate and implode cavities, hence specifying the vibration-assisted welding time. Thus, the proposed computational platform enables efficient multiparametric analysis of cavitation, initiated by remote ultrasound excitation, in the molten pool under welding.Innovate UK, grant number 10018077. The APC was funded by Brunel University London
The Physics of the Colloidal Glass Transition
As one increases the concentration of a colloidal suspension, the system
exhibits a dramatic increase in viscosity. Structurally, the system resembles a
liquid, yet motions within the suspension are slow enough that it can be
considered essentially frozen. This kinetic arrest is the colloidal glass
transition. For several decades, colloids have served as a valuable model
system for understanding the glass transition in molecular systems. The spatial
and temporal scales involved allow these systems to be studied by a wide
variety of experimental techniques. The focus of this review is the current
state of understanding of the colloidal glass transition. A brief introduction
is given to important experimental techniques used to study the glass
transition in colloids. We describe features of colloidal systems near and in
glassy states, including tremendous increases in viscosity and relaxation
times, dynamical heterogeneity, and ageing, among others. We also compare and
contrast the glass transition in colloids to that in molecular liquids. Other
glassy systems are briefly discussed, as well as recently developed synthesis
techniques that will keep these systems rich with interesting physics for years
to come.Comment: 56 pages, 18 figures, Revie
Tracing the Evolution of Temperature in Near Fermi Energy Heavy Ion Collisions
The kinetic energy variation of emitted light clusters has been employed as a
clock to explore the time evolution of the temperature for thermalizing
composite systems produced in the reactions of 26A, 35A and 47A MeV Zn
with Ni, Mo and Au. For each system investigated, the
double isotope ratio temperature curve exhibits a high maximum apparent
temperature, in the range of 10-25 MeV, at high ejectile velocity. These
maximum values increase with increasing projectile energy and decrease with
increasing target mass. The time at which the maximum in the temperature curve
is reached ranges from 80 to 130 fm/c after contact. For each different target,
the subsequent cooling curves for all three projectile energies are quite
similar. Temperatures comparable to those of limiting temperature systematics
are reached 30 to 40 fm/c after the times corresponding to the maxima, at a
time when AMD-V transport model calculations predict entry into the final
evaporative or fragmentation stage of de-excitation of the hot composite
systems. Evidence for the establishment of thermal and chemical equilibrium is
discussed.Comment: 9 pages, 5 figure
A Ghoshal-like Test of Equilibration in Near-Fermi-Energy Heavy Ion Collisions
Calorimetric and coalescence techniques have been employed to probe
equilibration for hot nuclei produced in heavy ion collisions of 35 to 55 MeV/u
projectiles with medium mass targets. Entrance channel mass asymmetries and
energies were selected in order that very hot composite nuclei of similar mass
and excitation would remain after early stage pre-equilibrium particle
emission. Inter-comparison of the properties and de-excitation patterns for
these different systems provides evidence for the production of hot nuclei with
decay patterns relatively independent of the specific entrance channel.Comment: 7 pages, 2 figure
Properties of the Initial Participant Matter Interaction Zone in Near Fermi-Energy Heavy Ion Collisions
The sizes, temperatures and free neutron to proton ratios of the initial
interaction zones produced in the collisions of 40 MeV/nucleon Ar +
Sn and 55 MeV/nucleonAl + Sn are derived using total
detected neutron plus charged particle multiplicity as a measure of the impact
parameter range and number of participant nucleons. The size of the initial
interaction zone, determined from a coalescence model analysis, increases
significantly with decreasing impact parameter. The temperatures and free
neutron to proton ratios in the interaction zones are relatively similar for
different impact parameter ranges and evolve in a similar fashion.Comment: 7 pages, 8 figure
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Physiologically-based pharmacokinetic and toxicokinetic models for estimating human exposure to five toxic elements through oral ingestion
Biological monitoring and physiologically-based pharmacokinetic (PBPK) modelling are useful complementary tools in quantifying human exposure to elements in the environment. In this work, we used PBPK models to determine the optimal time for collecting biological samples in a longitudinal study to determine if participants who consumed allotment produce had been exposed to arsenic, cadmium, chromium, nickel or lead. There are a number of PBPK models for these elements published in the literature, which vary in size, complexity and application, given the differences in physiochemical properties of the elements, organs involved in metabolism and exposure pathways affected. We selected PBPK models from the literature to simulate the oral ingestion pathway from consumption of allotment produce. Some models required modification by reducing or removing selected compartments whilst still maintaining their original predictability. The performance of the modified models was evaluated by comparing the predicted urinary and blood elemental levels with experimental data and other model simulations published in the literature. Overall, the model predictions were consistent with literature data (r > 0.7, p < 0.05), and were influential in predicting when samples should be collected. Our results demonstrate the use of mathematical modelling in informing and optimising the design of longitudinal studies
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