915 research outputs found
Constraints on global oceanic emissions of N2O from observations and models
We estimate the global ocean N2O flux to the atmosphere and its confidence interval using a statistical method based on model perturbation simulations and their fit to a database of ÎpN2O (nâ=ââŻ6136). We evaluate two submodels of N2O production. The first submodel splits N2O production into oxic and hypoxic pathways following previous publications. The second submodel explicitly represents the redox transformations of N that lead to N2O production (nitrification and hypoxic denitrification) and N2O consumption (suboxic denitrification), and is presented here for the first time. We perturb both submodels by modifying the key parameters of the N2O cycling pathways (nitrification rates; NH4+ uptake; N2O yields under oxic, hypoxic and suboxic conditions) and determine a set of optimal model parameters by minimisation of a cost function against four databases of N cycle observations. Our estimate of the global oceanic N2O flux resulting from this cost function minimisation derived from observed and model ÎpN2O concentrations is 2.4âŻÂ±âŻ0.8 and 2.5âŻÂ±âŻ0.8âŻTgâŻNâŻyrâ1 for the two N2O submodels. These estimates suggest that the currently available observational data of surface ÎpN2O constrain the global N2O flux to a narrower range relative to the large range of results presented in the latest IPCC report
Reshaping and Capturing Leidenfrost drops with a magnet
Liquid oxygen, which is paramagnetic, also undergoes Leidenfrost effect at
room temperature. In this article, we first study the deformation of oxygen
drops in a magnetic field and show that it can be described via an effective
capillary length, which includes the magnetic force. In a second part, we
describe how these ultra-mobile drops passing above a magnet significantly slow
down and can even be trapped. The critical velocity below which a drop is
captured is determined from the deformation induced by the field.Comment: Published in Physics of Fluids (vol. 25, 032108, 2013)
http://pof.aip.org/resource/1/phfle6/v25/i3/p032108_s1?isAuthorized=n
Wetting on a spherical wall: influence of liquid-gas interfacial properties
We study the equilibrium of a liquid film on an attractive spherical
substrate for an intermolecular interaction model exhibiting both fluid-fluid
and fluid-wall long-range forces. We first reexamine the wetting properties of
the model in the zero-curvature limit, i.e., for a planar wall, using an
effective interfacial Hamiltonian approach in the framework of the well known
sharp-kink approximation (SKA). We obtain very good agreement with a mean-field
density functional theory (DFT), fully justifying the use of SKA in this limit.
We then turn our attention to substrates of finite curvature and appropriately
modify the so-called soft-interface approximation (SIA) originally formulated
by Napi\'orkowski and Dietrich [Phys. Rev. B 34, 6469 (1986)] for critical
wetting on a planar wall. A detailed asymptotic analysis of SIA confirms the
SKA functional form for the film growth. However, it turns out that the
agreement between SKA and our DFT is only qualitative. We then show that the
quantitative discrepancy between the two is due to the overestimation of the
liquid-gas surface tension within SKA. On the other hand, by relaxing the
assumption of a sharp interface, with, e.g., a simple smoothing of the density
profile there, markedly improves the predictive capability of the theory,
making it quantitative and showing that the liquid-gas surface tension plays a
crucial role when describing wetting on a curved substrate. In addition, we
show that in contrast to SKA, SIA predicts the expected mean-field critical
exponent of the liquid-gas surface tension
Critical wind speed at which trees break
International audienceData from storms suggest that the critical wind speed at which trees break is constant (â42m/s), regardless of tree characteristics. We question the physical origin of this observation both experimentally and theoretically. By combining Hooke's law, Griffith's criterion, and tree allometry, we show that the critical wind speed indeed hardly depends on the height, diameter, and elastic properties of trees
Drop impact upon micro- and nanostructured superhydrophobic surfaces
We experimentally investigate drop impact dynamics onto different
superhydrophobic surfaces, consisting of regular polymeric micropatterns and
rough carbon nanofibers, with similar static contact angles. The main control
parameters are the Weber number \We and the roughness of the surface. At small
\We, i.e. small impact velocity, the impact evolutions are similar for both
types of substrates, exhibiting Fakir state, complete bouncing, partial
rebouncing, trapping of an air bubble, jetting, and sticky vibrating water
balls. At large \We, splashing impacts emerge forming several satellite
droplets, which are more pronounced for the multiscale rough carbon nanofiber
jungles. The results imply that the multiscale surface roughness at nanoscale
plays a minor role in the impact events for small \We \apprle 120 but an
important one for large \We \apprge 120. Finally, we find the effect of
ambient air pressure to be negligible in the explored parameter regime \We
\apprle 150Comment: 8 pages, 7 figure
Topography driven spreading
Roughening a hydrophobic surface enhances its nonwetting properties into superhydrophobicity. For liquids other than water, roughness can induce a complete rollup of a droplet. However, topographic effects can also enhance partial wetting by a given liquid into complete wetting to create superwetting. In this work, a model system of spreading droplets of a nonvolatile liquid on surfaces having lithographically produced pillars is used to show that superwetting also modifies the dynamics of spreading. The edge speed-dynamic contact angle relation is shown to obey a simple power law, and such power laws are shown to apply to naturally occurring surfaces
Mechanical tuning of the evaporation rate of liquid on crossed fibers
We investigate experimentally the drying of a small volume of perfectly
wetting liquid on two crossed fibers. We characterize the drying dynamics for
the three liquid morphologies that are encountered in this geometry: drop,
column and a mixed morphology, in which a drop and a column coexist. For each
morphology, we rationalize our findings with theoretical models that capture
the drying kinetics. We find that the evaporation rate depends significantly on
the liquid morphology and that the drying of liquid column is faster than the
evaporation of the drop and the mixed morphology for a given liquid volume.
Finally, we illustrate that shearing a network of fibers reduces the angle
between them, changes the morphology towards the column state, and so enhances
the drying rate of a volatile liquid deposited on it
Shapes, contact angles, and line tensions of droplets on cylinders
Using an interface displacement model we calculate the shapes of
nanometer-size liquid droplets on homogeneous cylindrical surfaces. We
determine effective contact angles and line tensions, the latter defined as
excess free energies per unit length associated with the two contact lines at
the ends of the droplet. The dependences of these quantities on the cylinder
radius and on the volume of the droplets are analyzed.Comment: 26 pages, RevTeX, 10 Figure
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