17,944 research outputs found
Scaling in the crossover from random to correlated growth
In systems where deposition rates are high compared to diffusion, desorption
and other mechanisms that generate correlations, a crossover from random to
correlated growth of surface roughness is expected at a characteristic time
t_0. This crossover is analyzed in lattice models via scaling arguments, with
support from simulation results presented here and in other authors works. We
argue that the amplitudes of the saturation roughness and of the saturation
time scale as {t_0}^{1/2} and t_0, respectively. For models with lateral
aggregation, which typically are in the Kardar-Parisi-Zhang (KPZ) class, we
show that t_0 ~ 1/p, where p is the probability of the correlated aggregation
mechanism to take place. However, t_0 ~ 1/p^2 is obtained in solid-on-solid
models with single particle deposition attempts. This group includes models in
various universality classes, with numerical examples being provided in the
Edwards-Wilkinson (EW), KPZ and Villain-Lai-Das Sarma (nonlinear molecular-beam
epitaxy) classes. Most applications are for two-component models in which
random deposition, with probability 1-p, competes with a correlated aggregation
process with probability p. However, our approach can be extended to other
systems with the same crossover, such as the generalized restricted
solid-on-solid model with maximum height difference S, for large S. Moreover,
the scaling approach applies to all dimensions. In the particular case of
one-dimensional KPZ processes with this crossover, we show that t_0 ~ nu^{-1}
and nu ~ lambda^{2/3}, where nu and lambda are the coefficients of the linear
and nonlinear terms of the associated KPZ equations. The applicability of
previous results on models in the EW and KPZ classes is discussed.Comment: 14 pages + 5 figures, minor changes, version accepted in Phys. Rev.
The effect of water uptake on the behaviour of hydrophilic cements in confined environments
Physiological fluids will be in contact with the implant components from the first moments after a surgery. Therefore, the study of the
effect of water on the properties of the bone cements that are part of the arthroplasty procedure is of critical importance to predict the
long-term performance of the whole system. In our research group, we have developed a novel concept, the hydrophilic, partially
degradable and bioactive cements which uptake considerably more water than standard bone cements. In this paper, we aimed to study the
effect of water uptake (WU) by these cements on their behaviour. The tests were carried out in confined cavities, which represent more
accurately the in vivo situation the cement will face (constrained by the bone and prosthesis surfaces). We observed that the equilibrium
WU decreased up to 60% (as compared to non-confined situations), depending of the formulation. This decrease resulted in a latent
tendency of the cements to swell, and the hindering of such swelling generated a swelling pressure against the constraining walls. The
pressure, and consequent press-fitting effect, could be controlled by a number of mechanisms, and resulted in higher stability of the
hydrophilic cements, expressed as an increase in the push-out force, required to extract the specimens from such constrained cavities. This
effect was only observed in hydrophilic cements, not in commercial, hydrophobic ones used as controls. We conclude that such cements
will provide an additional and very useful source of immediate adhesion in the short-term after surgery: water induced press fitting
A review on the polymer properties of hydrophilic, partially degradable and bioactive acrylic cements (HDBC)
Acrylic bone cements were developed around 50 years ago for the fixation of hip prostheses during arthroplasty. Over
the intervening years, a series of drawbacks have been disclosed that have fostered intensive research on the development
of novel or alternative formulations to the standard acrylic cements. Here, we will review the development and
characterization of a novel class of cements, the Hydrophilic, partially Degradable and Bioactive Cements (HDBCs), an
example of multifunctional cements. They were developed to have improved biocompatibility and initial fixation to the
prosthesis and to induce the growth of bone on the surface of the cement and within pores generated by the degradation of
the solid component. HDBCs have higher water uptake than typical acrylic cements, leading to press-fitting inside
constrained cavities. They are tougher, albeit less stiff and strong than hydrophobic cements, and their mechanical
properties may be easily adjusted by small changes in composition. Last, the simultaneous bioactive and degradable
character of HDBCs have been shown to allow in vitro growth of calcium phosphates into pores within the bulk of the
cement
Finite-size effects in roughness distribution scaling
We study numerically finite-size corrections in scaling relations for
roughness distributions of various interface growth models. The most common
relation, which considers the average roughness . This illustrates how
finite-size corrections can be obtained from roughness distributions scaling.
However, we discard the usual interpretation that the intrinsic width is a
consequence of high surface steps by analyzing data of restricted
solid-on-solid models with various maximal height differences between
neighboring columns. We also observe that large finite-size corrections in the
roughness distributions are usually accompanied by huge corrections in height
distributions and average local slopes, as well as in estimates of scaling
exponents. The molecular-beam epitaxy model of Das Sarma and Tamborenea in 1+1
dimensions is a case example in which none of the proposed scaling relations
works properly, while the other measured quantities do not converge to the
expected asymptotic values. Thus, although roughness distributions are clearly
better than other quantities to determine the universality class of a growing
system, it is not the final solution for this task.Comment: 25 pages, including 9 figures and 1 tabl
Viscoelastic monitoring of starch-based biomaterials in simulated physiological conditions
Dynamic mechanical analysis (DMA) was used to investigate the solid-state rheological behaviour in a starch-based thermoplastic aimed to be used in different biomedical applications. The tested samples were processed by different injection moulding procedures. The dry samples were immersed in a simulated physiological solution and the relevant viscoelastic parameters were monitored against time. The decrease
of stiffness due to swelling can be followed in real time, being less pronounced for the composite sample with hydroxyapatite (HA). The temperature control of the liquid bath was found to be very good. Frequency scans were also performed in wet conditions in samples previously immersed during different times, indicating that DMA is a suitable method to control in-vitro the changes on the viscoelastic properties of biomaterials during degradation
Observational Constraints on Silent Quartessence
We derive new constraints set by SNIa experiments (`gold' data sample of
Riess et al.), X-ray galaxy cluster data (Allen et al. Chandra measurements of
the X-ray gas mass fraction in 26 clusters), large scale structure (Sloan
Digital Sky Survey spectrum) and cosmic microwave background (WMAP) on the
quartessence Chaplygin model. We consider both adiabatic perturbations and
intrinsic non-adiabatic perturbations such that the effective sound speed
vanishes (Silent Chaplygin). We show that for the adiabatic case, only models
with equation of state parameter are allowed: this
means that the allowed models are very close to \LambdaCDM. In the Silent case,
however, the results are consistent with observations in a much broader range,
-0.3<\alpha<0.7.Comment: 7 pages, 12 figures, to be submitted to JCA
Caging dynamics in a granular fluid
We report an experimental investigation of the caging motion in a uniformly
heated granular fluid, for a wide range of filling fractions, . At low
the classic diffusive behavior of a fluid is observed. However, as
is increased, temporary cages develop and particles become increasingly
trapped by their neighbors. We statistically analyze particle trajectories and
observe a number of robust features typically associated with dense molecular
liquids and colloids. Even though our monodisperse and quasi-2D system is known
to not exhibit a glass transition, we still observe many of the precursors
usually associated with glassy dynamics. We speculate that this is due to a
process of structural arrest provided, in our case, by the presence of
crystallization.Comment: 4 pages, 5 figures, submitted to Phys. Rev. Let
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