832 research outputs found
Avalanche of particles in evaporating coffee drops
The pioneering work of Deegan et al. [Nature 389, (1997)] showed how a drying
sessile droplet suspension of particles presents a maximum evaporating flux at
its contact line which drags liquid and particles creating the well known
coffee stain ring. In this Fluid Dynamics Video, measurements using micro
Particle Image Velocimetry and Particle Tracking clearly show an avalanche of
particles being dragged in the last moments, for vanishing contact angles and
droplet height. This explains the different characteristic packing of the
particles in the layers of the ring: the outer one resembles a crystalline
array, while the inner one looks more like a jammed granular fluid. Using the
basic hydrodynamic model used by Deegan et al. [Phys. Rev. E 62, (2000)] it
will be shown how the liquid radial velocity diverges as the droplet life comes
to an end, yielding a good comparison with the experimental data.Comment: This entry contains a Fluid Dynamics Video candidate for the Gallery
of Fluid Motion 2011 and a brief article with informatio
Role of liquid driving on the clogging of constricted particle suspensions
Forcing dense suspensions of non-cohesive particles through constrictions
might either result in a continuous flow, an intermittent one, or indefinite
interruption of flow, i.e., a clog. While one of the most important (and
obvious) controlling parameters in such a system is the neck-to-particle size
ratio, the role of the liquid driving method is not so obvious. On the one
hand, wide-spread volume-controlled systems result in pressure and local liquid
velocity increases upon eventual clogs. On the other hand, pressure-controlled
systems result in a decrease of the flow through the constriction when a clog
is developed. The root of the question therefore lies on the role of
interparticle liquid flow and hydrodynamic forces on both the formation and
stability of an arch blocking the particle transport through a constriction. In
this work, we experimentally analyse a suspension of non-cohesive particles in
channels undergoing intermittent regimes, in which they are most sensitive to
parametric changes. By exploring the statistical distribution of arrest times
and of discharged particles, we surprisingly find that the transport of
non-cohesive suspensions through constrictions actually follows a "slower is
faster" principle under certain conditions.Comment: 9 pages, 5 figures, 2 table
Building water bridges in air: Electrohydrodynamics of the floating water bridge
The interaction of electrical fields and liquids can lead to phenomena that
defies intuition. Some famous examples can be found in Electrohydrodynamics as
Taylor cones, whipping jets or non-coalescing drops. A less famous example is
the Floating Water Bridge: a slender thread of water held between two glass
beakers in which a high voltage difference is applied. Surprisingly, the water
bridge defies gravity even when the beakers are separated at distances up to 2
cm. In the presentation, experimental measurements and simple models are
proposed and discussed for the stability of the bridge and the source of the
flow, revealing an important role of polarization forces on the stability of
the water bridge. On the other hand, the observed flow can only be explained
due to the non negligible free charge present in the surface. In this sense,
the Floating Water Bridge can be considered as an extreme case of a leaky
dielectric liquid (J. R. Melcher and G. I. Taylor, Annu. Rev. Fluid Mech.,
1:111, 1969).Comment: Paper submitted to Physics of Fluids journal, an illustrative video
is included with three experiment
Universality of Tip Singularity Formation in Freezing Water Drops
A drop of water deposited on a cold plate freezes into an ice drop with a
pointy tip. While this phenomenon clearly finds its origin in the expansion of
water upon freezing, a quantitative description of the tip singularity has
remained elusive. Here we demonstrate how the geometry of the freezing front,
determined by heat transfer considerations, is crucial for the tip formation.
We perform systematic measurements of the angles of the conical tip, and reveal
the dynamics of the solidification front in a Hele-Shaw geometry. It is found
that the cone angle is independent of substrate temperature and wetting angle,
suggesting a universal, self-similar mechanism that does not depend on the rate
of solidification. We propose a model for the freezing front and derive
resulting tip angles analytically, in good agreement with observations.Comment: Letter format, 5 pages, 3 figures. Note: authors AGM and ORE
contributed equally to the pape
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