2,140 research outputs found
Correlation factor for diffusion in cubic crystals with solute-vacancy interactions of arbitrary range
A formalism using a double Laplace Fourier transform of the transport
equation yields the return probabilities of the vacancy in the vicinity of the
tracer atom in the presence of solute-vacancy interactions of arbitrary
extension. Studying model cases, it is shown that taking into account the full
range of the interaction may change noticeably the correlation factor. The
latter depends tightly on the pattern of migration barriers which is chosen to
describe the vacancy jumps around the tracer atom. A thorough ab initio
evaluation of all barriers is rarely available in the literature. It is shown
that approximations often used to overcome this lack of information can be
misleading. The examination of dilute systems recently studied shows that the
interactions within the first three neighbour shells dictate the final value
with a good precision. The main improvement of the modelling comes from
dropping the restrictive assumption which impose an equal value to the jump
frequencies leading to a dissociation of the solute-vacancy pair.Comment: 45 pages; 8 figure
Hydrodynamics of fluid-solid coexistence in dense shear granular flow
We consider dense rapid shear flow of inelastically colliding hard disks.
Navier-Stokes granular hydrodynamics is applied accounting for the recent
finding \cite{Luding,Khain} that shear viscosity diverges at a lower density
than the rest of constitutive relations. New interpolation formulas for
constitutive relations between dilute and dense cases are proposed and
justified in molecular dynamics (MD) simulations. A linear stability analysis
of the uniform shear flow is performed and the full phase diagram is presented.
It is shown that when the inelasticity of particle collision becomes large
enough, the uniform sheared flow gives way to a two-phase flow, where a dense
"solid-like" striped cluster is surrounded by two fluid layers. The results of
the analysis are verified in event-driven MD simulations, and a good agreement
is observed
Probing the nanohydrodynamics at liquid-solid interfaces using thermal motion
We report on a new method to characterize nano-hydrodynamic properties at the
liquid/solid interface relying solely on the measurement of the thermal motion
of confined colloids. Using Fluorescence Correlation Spectroscopy (FCS) to
probe the diffusion of the colloidal tracers, this optical technique
--equivalent in spirit to the microrheology technique used for bulk
properties-- is able to achieve nanometric resolution on the slip length
measurement. It confirms the no-slip boundary condition on wetting surfaces and
shows a partial slip b=18 +/- 5 nm on non-wetting ones. Moreover, in the
absence of external forcing, we do not find any evidence for large nano-bubble
promoted slippage on moderately rough non-wetting surfaces.Comment: 4 pages, 3 figure
Flow boundary conditions from nano- to micro-scales
The development of microfluidic devices has recently revived the interest in
"old" problems associated with transport at, or across, interfaces. As the
characteristic sizes are decreased, the use of pressure gradients to transport
fluids becomes problematic, and new, interface driven, methods must be
considered. This has lead to new investigations of flow near interfaces, and to
the conception of interfaces engineered at various scales to reduce flow
friction. In this review, we discuss the present theoretical understanding of
flow past solid interfaces at different length scales. We also briefly discuss
the corresponding phenomenon of heat transport, and the influence of surface
slip on interface driven (e.g. electro-osmotic) flows.Comment: submitted to "Soft Matter
Where does a cohesive granular heap break?
In this paper, we consider the effect of cohesion on the stability of a
granular heap. We first briefly review literature results on the cohesion force
between two rough granular beads and specifically consider the dependence of
the adhesion force on the normal load. We then compute the dependence of the
maximum angle of stability of the heap as a function of the cohesion. We point
out that the dependence of the cohesive forces on the external normal load
between grains is a key point in determining the localization of the failure
plane. While for a constant cohesive force, slip occurs deep inside the heap,
surface failure is obtained for a linear dependence of the cohesion on the
normal stress.Comment: 6 pages, 6 figures. Submitted to Phys. Rev.
Shear localization in a model glass
Using molecular dynamics simulations, we show that a simple model of a glassy
material exhibits the shear localization phenomenon observed in many complex
fluids. At low shear rates, the system separates into a fluidized shear-band
and an unsheared part. The two bands are characterized by a very different
dynamics probed by a local intermediate scattering function. Furthermore, a
stick-slip motion is observed at very small shear rates. Our results, which
open the possibility of exploring complex rheological behavior using
simulations, are compared to recent experiments on various soft glasses.Comment: 4 pages, 4 figures (5 figure files
Nucleation in hydrophobic cylindrical pores : a lattice model
We consider the nucleation process associated with capillary condensation of
a vapor in a hydrophobic cylindrical pore (capillary evaporation). The
liquid-vapor transition is described within the framework of a simple lattice
model. The phase properties are characterized both at the mean-field level and
using Monte-Carlo simulations. The nucleation process for the liquid to vapor
transition is then specifically considered. Using umbrella sampling techniques,
we show that nucleation occurs through the condensation of an asymmetric vapor
bubble at the pore surface. Even for highly confined systems, good agreement is
found with macroscopic considerations based on classical nucleation theory. The
results are discussed in the context of recent experimental work on the
extrusion of water in hydrophobic pores
The Doppler Spectra of Medium Grazing Angle Sea Clutter; Part 1: Characterisation
This paper is concerned with the characterisation of Doppler spectra from high range resolution X-band radar sea clutter observed from an airborne platform over the range of grazing angles, 15° to 45°. It is observed that when looking up or down wind there is a strong correlation between mean Doppler shift and local spectrum intensity. When combined with random fluctuations of spectrum width, these characteristics give the spectra a temporal and spatial variability. This behaviour has previously been observed in low grazing angle data and these results confirm the wider applicability of the models developed using that data. The modelling method is also extended here to capture the bimodal behaviour observed with high intensity returns from breaking waves looking up or down-wind
- …