3,276 research outputs found
Cirad - Highlights 2016 : Activities report. Spotlight Tropical supply chains and sustainable development
How geometry determines the coalescence of low-viscosity drops
The coalescence of water drops on a substrate is studied experimentally. We
focus on the rapid growth of the bridge connecting the two drops, which very
quickly after contact ensues from a balance of surface tension and liquid
inertia. For drops with contact angles below , we find that the
bridge grows with a self-similar dynamics that is characterized by a height
. By contrast, the geometry of coalescence changes dramatically
for contact angles at , for which we observe , just as
for freely suspended spherical drops in the inertial regime. We present a
geometric model that quantitatively captures the transition from 2/3 to 1/2
exponent, and unifies the inertial coalescence of sessile drops and freely
suspended drops.Comment: 5 pages, 3 figure
Short time dynamics of viscous drop spreading
Liquid drops start spreading directly after coming into contact with a solid
sub- strate. Although this phenomenon involves a three-phase contact line, the
spread- ing motion can be very fast. We experimentally study the initial
spreading dy- namics, characterized by the radius of the wetted area, for
viscous drops. Using high-speed imaging with synchronized bottom and side views
gives access to 6 decades of time resolution. We show that short time spreading
does not exhibit a pure power-law growth. Instead, we find a spreading velocity
that decreases logarithmically in time, with a dynamics identical to that of
coalescing viscous drops. Remarkably, the contact line dissipation and wetting
effects turn out to be unimportant during the initial stages of drop spreading
Marangoni spreading due to a localized alcohol supply on a thin water film
Bringing the interfaces of two miscible fluids into contact naturally
generates strong gradients in surface tension. Here we investigate such a
Marangoni-driven flow by continuously supplying isopropyl alcohol (IPA) on a
film of water, using micron-sized droplets of IPA-water mixtures. These
droplets create a localized depression in surface tension that leads to the
opening of a circular and thin region in the water film. At the edge of the
thin region, there is a rim growing and collecting the water of the film. We
find that the spreading radius scales as . This result can be
explained from a balance between Marangoni and viscous stresses, assuming that
the gradients in surface tension are smoothened out over the entire size of the
circular opening. We derive a scaling law that accurately predicts the
influence of the IPA flux as well as the thickness of the thin film at the
interior of the spreading front.Comment: 10 pages, 5 figure
Cirad - Les faits marquants 2016 : Rapport d'activité. Thema Filières tropicales et développement durable
Revisiting time reversal and holography with spacetime transformations
Wave control is usually performed by spatially engineering the properties of
a medium. Because time and space play similar roles in wave propagation,
manipulating time boundaries provides a complementary approach. Here, we
experimentally demonstrate the relevance of this concept by introducing
instantaneous time mirrors. We show with water waves that a sudden change of
the effective gravity generates time-reversed waves that refocus at the source.
We generalize this concept for all kinds of waves introducing a universal
framework which explains the effect of any time disruption on wave propagation.
We show that sudden changes of the medium properties generate instant wave
sources that emerge instantaneously from the entire space at the time
disruption. The time-reversed waves originate from these "Cauchy sources" which
are the counterpart of Huygens virtual sources on a time boundary. It allows us
to revisit the holographic method and introduce a new approach for wave
control.Comment: 5 figure
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