57 research outputs found
Droplets climbing a rotating helical fiber
A liquid droplet is placed on a rotating helical fiber. We find that the
droplet may slide down, attach or climb up the fiber. We inspect experimentally
the domain of existence of these three behaviors as a function of the
geometrical characteristics of the fiber, its angle relatively to the
horizontal, the wetting properties of the fluid and the rotating speed of the
helix. A theoretical model is proposed in order to capture the boundaries of
the experimental phase diagram
Are leaves optimally designed for self-support? An investigation on giant monocots
Leaves are the organs that intercept light and create photosynthesis.
Efficient light interception is provided by leaves oriented orthogonal to most
of the sun rays. Except in the polar regions, this means orthogonal to the
direction of acceleration due to gravity, or simply horizontal. The leaves of
almost all terrestrial plants grow in a gravity field that tends to bend them
downward and therefore may counteract light interception. Plants thus allocate
biomass for self-support in order to maintain their leaves horizontal. To
compete with other species (inter-species competition), as well as other
individuals within the same species (intra-species competition), self-support
must be achieved with the least biomass produced. This study examines to what
extent leaves are designed to self-support. We show here that a basic
mechanical model provides the optimal dimensions of a leaf for light
interception and self-support. These results are compared to measurements made
on leaves of various giant monocot species,especially palm trees and banana
trees. The comparison between experiments and model predictions shows that the
longer palms are optimally designed for self-support whereas shorter leaves are
shaped predominantly by other parameters of selection
Wicking through a confined micropillar array
This study considers the spreading of a Newtonian and perfectly wetting
liquid in a square array of cylindric micropillars confined between two plates.
We show experimentally that the dynamics of the contact line follows a
Washburn-like law which depends on the characteristics of the micropillar array
(height, diameter and pitch). The presence of pillars can either enhanced or
slow down the motion of the contact line. A theoretical model based on
capillary and viscous forces has been developed in order to rationalize our
observations. Finally, the impact of pillars on the volumic flow rate of liquid
which is pumped in the microchannel is inspected
The physics of badminton
International audienceThe conical shape of a shuttlecock allows it to flip on impact. As a light and extended particle, it flies with a pure drag trajectory. We first study the flip phenomenon and the dynamics of the flight and then discuss the implications on the game. Lastly, a possible classification of different shots is proposed
Anisotropic frictional model for an object sliding in a granular media
Several locomotion strategies are based on the anisotropic nature of the
forces experienced by the moving body with its environment. We report
experiments on the anisotropy of the frictional force experienced by a cylinder
moving in a granular medium as a function of the orientation between
the cylinder and its velocity. The component of the force in the direction
parallel to the velocity of the cylinder is always higher than the
perpendicular component and the force is therefore anisotropic. While the
parallel component increases continuously with the cylinder angle , we
observe that the perpendicular component reaches a maximum value for an
orientation of . In order to rationalise these
observations, we have developed a theoretical model which assumes that the
mechanical energy required to move the cylinder is dissipated by friction and
establishes a relationship between the parallel and perpendicular force
components that is consistent with experiments
Leidenfrost drops on a heated liquid pool
We show that a volatile liquid drop placed at the surface of a non-volatile
liquid pool warmer than the boiling point of the drop can experience a
Leidenfrost effect even for vanishingly small superheats. Such an observation
points to the importance of the substrate roughness, negligible in the case
considered here, in determining the threshold Leidenfrost temperature. A
theoretical model based on the one proposed by Sobac et al. [Phys. Rev. E 90,
053011 (2014)] is developed in order to rationalize the experimental data. The
shapes of the drop and of the substrate are analyzed. The model notably
provides scalings for the vapor film thickness. For small drops, these scalings
appear to be identical to the case of a Leidenfrost drop on a solid substrate.
For large drops, in contrast, they are different and no evidence of chimney
formation has been observed either experimentally or theoretically in the range
of drop sizes considered in this study. Concerning the evaporation dynamics,
the radius is shown to decrease linearly with time whatever the drop size,
which differs from the case of a Leidenfrost drop on a solid substrate. For
high superheats, the characteristic lifetime of the drops versus the superheat
follows a scaling law that is derived from the model but, at low superheats, it
deviates from this scaling by rather saturating
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
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