15,458 research outputs found
Statistics of Bubble Rearrangements in a Slowly Sheared Two-dimensional Foam
Many physical systems exhibit plastic flow when subjected to slow steady
shear. A unified picture of plastic flow is still lacking; however, there is an
emerging theoretical understanding of such flows based on irreversible motions
of the constituent ``particles'' of the material. Depending on the specific
system, various irreversible events have been studied, such as T1 events in
foam and shear transformation zones (STZ's) in amorphous solids. This paper
presents an experimental study of the T1 events in a model, two-dimensional
foam: bubble rafts. In particular, I report on the connection between the
distribution of T1 events and the behavior of the average stress and average
velocity profiles during both the initial elastic response of the bubble raft
and the subsequent plastic flow at sufficiently high strains
Droplet evaporation in one-component fluids: Dynamic van der Waals theory
In a one-component fluid, we investigate evaporation of a small axysymmetric
liquid droplet in the partial wetting condition on a heated wall at . In the dynamic van der Waals theory (Phys. Rev. E {\bf 75}, 036304
(2007)), we take into account the latent heat transport from liquid to gas upon
evaporation. Along the gas-liquid interface, the temperature is nearly equal to
the equilibrium coexisting temperature away from the substrate, but it rises
sharply to the wall temperature close to the substrate. On an isothermal
substrate, evaporation takes place mostly on a narrow interface region near the
contact line in a late stage, which is a characteristic feature in
one-component fluids.Comment: 6 pages, 6 figure
Signature of elasticity in the Faraday instability
We investigate the onset of the Faraday instability in a vertically vibrated
wormlike micelle solution. In this strongly viscoelastic fluid, the critical
acceleration and wavenumber are shown to present oscillations as a function of
driving frequency and fluid height. This effect, unseen neither in simple
fluids nor in previous experiments on polymeric fluids, is interpreted in terms
of standing elastic waves between the disturbed surface and the container
bottom. It is shown that the model of S. Kumar [Phys. Rev. E, {\bf 65}, 026305
(2002)] for a viscoelastic fluid accounts qualitatively for our experimental
observations. Explanations for quantitative discrepancies are proposed, such as
the influence of the nonlinear rheological behaviour of this complex fluid.Comment: 4 pages, 4 figure
Assessment of oxygen plasma ashing as a pre-treatment for radiocarbon dating
This study investigates the potential of low-temperature oxygen plasma ashing as a technique for decontaminating charcoal and wood samples prior to radiocarbon dating. Plasma ashing is demonstrated to be rapid, controllable and surface-specific, and clear differences are identified in the rate of ashing in different organic materials. However, the ability of plasma ashing to selectively remove these different components is limited in heterogeneous sample matrices. This is because oxidation is confined to the immediate sample surface. Comparison of radiocarbon dates obtained from identical aliquots of contaminated ancient charcoal pre-treated by acid-base-acid (ABA), acid-base-oxidation-stepped combustion (ABOx-SC) and plasma ashing suggests that the technique performs as well as the ABA pre-treatment but does not remove as much contamination as the ABOx-SC technique. Plasma-ashing may be particularly useful in cases where sample size is limiting
Thermodiffusion in model nanofluids by molecular dynamics simulations
In this work, a new algorithm is proposed to compute single particle
(infinite dilution) thermodiffusion using Non-Equilibrium Molecular Dynamics
simulations through the estimation of the thermophoretic force that applies on
a solute particle. This scheme is shown to provide consistent results for
simple Lennard-Jones fluids and for model nanofluids (spherical non-metallic
nanoparticles + Lennard-Jones fluid) where it appears that thermodiffusion
amplitude, as well as thermal conductivity, decrease with nanoparticles
concentration. Then, in nanofluids in the liquid state, by changing the nature
of the nanoparticle (size, mass and internal stiffness) and of the solvent
(quality and viscosity) various trends are exhibited. In all cases the single
particle thermodiffusion is positive, i.e. the nanoparticle tends to migrate
toward the cold area. The single particle thermal diffusion 2 coefficient is
shown to be independent of the size of the nanoparticle (diameter of 0.8 to 4
nm), whereas it increases with the quality of the solvent and is inversely
proportional to the viscosity of the fluid. In addition, this coefficient is
shown to be independent of the mass of the nanoparticle and to increase with
the stiffness of the nanoparticle internal bonds. Besides, for these
configurations, the mass diffusion coefficient behavior appears to be
consistent with a Stokes-Einstein like law
Polymeric filament thinning and breakup in microchannels
The effects of elasticity on filament thinning and breakup are investigated
in microchannel cross flow. When a viscous solution is stretched by an external
immiscible fluid, a low 100 ppm polymer concentration strongly affects the
breakup process, compared to the Newtonian case. Qualitatively, polymeric
filaments show much slower evolution, and their morphology features multiple
connected drops. Measurements of filament thickness show two main temporal
regimes: flow- and capillary-driven. At early times both polymeric and
Newtonian fluids are flow-driven, and filament thinning is exponential. At
later times, Newtonian filament thinning crosses over to a capillary-driven
regime, in which the decay is algebraic. By contrast, the polymeric fluid first
crosses over to a second type of flow-driven behavior, in which viscoelastic
stresses inside the filament become important and the decay is again
exponential. Finally, the polymeric filament becomes capillary-driven at late
times with algebraic decay. We show that the exponential flow thinning behavior
allows a novel measurement of the extensional viscosities of both Newtonian and
polymeric fluids.Comment: 7 pages, 7 figure
- …