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| Delamination blisters, with Pedro Reis, Dominic Vella, Arezki Boudaoud & Benoit Roman |
 Delamination
blisters are commonly observed when a thin layer deposited on a
substrate happens to be compressed. This is for instance the case for a
sticker on a window when sun heat tends to expend the sticker
material. Although
destructive in most practical applications, such blisters may be useful
to develop stretchable electronic devices which could be used for
medical applications or electronic paper for instance. Indeed,
blister-shaped electronic circuits experience less mechanical stress
when the substrate gets deformed.
Macroscopic experiments with well controlled and tunable physical parameters teach us what are the laws dictating the formation and the final size of such blisters. image credit: D. Coveney, MIT Click on the picture to watch the formation of blisters on a compressed substrate (1.7Mb)
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| Bibliography |
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 What happens when a water droplet is deposited on a flexible sheet? Does the sheet spontaneously wrap the droplet? Yes, if driving capillary forces overtake the elastic bending resistance of the sheet (linked movie1). If the sheet is stiffer, the corners start bending but the sheet quickly reopens (linked movie 2). What are geometrical shapes eventually obtained after the partial evaporation of the droplet? The final conformation is dictated by the initial cut of the sheet. Pyramids, cubes or quasi-spheres are obtained from triangles, crosses or flowers shapes, respectively. Beyond scientific curiosity (the problem is in strong connection with Gauss classical theorema egregium on topology), we believe this capillary origami mechanism to be relevant for building three dimensional micro-structures from two dimensional templates. At small scales capillary forces indeed dominate and minute droplets may serve as micro-pliers. |
| Wet hair, with Benoit Roman
,
Charlotte Py , Arezki
Boudaoud , Sébastien
Neukirch &
Charles Baroud |
The hair of a dog coming out of a pond assemble into clumps giving him a spiny appearance. What is the number of hairs in a clump? The answer relies on a balance betwen capillary forces and elasticity of the hairs. More generally, the stiking of flexible elements dramatically damages mechanical microsystems or lung airways, but also allows beetles to climb on walls. |
| Windswept droplets, with
Marc Fermigier , François Besselièvre,
Gabriel Amselem & A.Kevin Njifenju |
A small droplet impacting a glass window usually remains stuck on the pane. How can we expel it? One possible solution consists in depositing a hydrophobic coating the glass surface. Another solution is to blow it out. We propose to explore this last solution (partly combined with the first one). What is the running speed of the droplet as a function of the wind velocity? Is there any threshold to allow its motion? How does the shape of the droplet evolve? Does it leave any tail and satellite droplets? |
| Gobbling droplets,
with Christian Clasen , Gareth McKinley & Vladimir Entov |
A jet of liquid is unstable because of surface tension and usually breaks into small droplets. The addition of minute quantities of polymeric molecules provides an additive elastic stress which stabilizes the liquid column. In this situation the terminal droplet has the time to gobble many of its incoming neighbors before its detachment. Click on the picture to watch the video (5.9Mb) |
| Spreading flowers, with Angelina Aessopos , Vladimir Entov, Anette (Peko) Hosoi , Marc Fermigier & Gareth McKinley |
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The spreading of a
droplet of food dye on a layer of semi-dilute polymer solution leads to
a fingering pattern. Although Carlo Marangoni identified the motor of
these phenomena as the contrast between the surface trensions of the
diferent interfaces in the 1880's, the origin of these dendrites
remains an open
question.
Click on the picture to watch the video (6.5Mb) |
| Dripping of a jelly liquid,
with Christian Clasen , Gareth McKinley & Vladimir Entov |
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Concentrated surfactant
solutions may exhibit particular molecular structures
(“worm like micelles”) which lead to a jelly
liquid. The material behaves as a soft elastic solid when a light
stress is applied but flows as a liquid under higher stresses.
When a droplet of such a liquid drips from a pipette, a long thread connects the droplet to the pipette and progressively necks and breaks down when the thread reaches a critical diameter. Click on any picture to watch the video (2.2Mb).
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| Rolling Stones, |
  Once deposited on a
tilted planar surface coated with some viscous liquid, a sphere rolls
down and slides at the same time. Tire prints like patterns are formed
behind the ball. Click on the
pictures to watch "stones" racing inside a rotating drum
(2.7Mb)
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| Fingering instabilities in
viscoelastic liquids,
with Ryan Welsh & Gareth McKinley |
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The separation
of two plates bridged by a thin layer of a viscoelastic
liquid leads to complex instabilities. The experiment shown was done using a polystyrene Boger fluid characterized by a relaxation time of 3 min. The larger fingering structure corresponds to a Saffman Taylor: a fluid of low viscosity (air) penetrate into a viscous liquid which generates viscous fingering. Elastic stresses in the solution are also induced when the polymeric molecules get stretched enough. They are responsible for the secondary structure which develops at larger strains. The endplate diameter is 5mm, the initial gap between the endplate is 0.1 mm and the Deborah number (relaxation time/experiment time) is 43. Click on the picture to watch a video of this experiment (1.2Mb) .
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| Ganesha instability,
with Gareth McKinley &
L.Mahadevan |
 Multi-walled nanotubes
exhibits periodic ripples when they are bent. This instability
essentially depends on the tubes geometry but not on the material
properties. The same popliteal ripples are also observed with
macroscopic rubber sheet scrolls or even on elephant trunks!
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| Super Hydrophobic Surfaces,
initially with David
Quéré & Denis Richard, |
 The combination of an hydrophobic material and and rough surface can lead to super hydrophobic surfaces. A water droplet deposited on such a substrate remains at rest on the tops of the roughness which widely reduces the contact area of the liquid with the solid. The drop is mainly in contact with air and keeps the shape it would have in the air. Click on the picture to watch a video (2Mb) of the dynamics of water on such a surface. Click here to watch a water droplet boucing on a superhydrophobic surface (7Mb) |
| The shape of Tektites,
with Tim Kreider, John Bush , Linda Elkins Tanton , Pascale Aussillous and David Quéré |
 Tektites are believed to
be the "splashes" from the impact of big meteorites on the Earth. The
molten silica material (mainly from terrestrial origin) would be
ejected with some spinning motion which deforms the liquid drops. These
rotating shapes are often frozen by the cooling down of the
liquid during its flight. |
| Liquid trains in a tube,
with David Quéré |
If two adjacent liquids
are introduced in a capillary tube, a spontaneous motion
of this bislug is generally observed when the tube
is hold horizontally. This flow relies on the asymmetry of the system: capillary
forces on the three menisci do not necessarily balance and
their contrast drives the bislug. Click on the picture to watch a video of a bislug experiment (3.3Mb) . |
| Ex-vivo
rheology of spider silk, with Nikola Kojić, Christian Clasen & Gareth McKinley |
 Nephila
clavipes spider (female) and one of her major ampulate glands
Although spider silk has been revered during last decade for its unmatched mechanical properties, very little is known about the rheological properties of the spinning dope. Determining the shear and extensionnal rheology of the spinning dope should give some guidance to understand the complex spinning process occurring along the spider spinning canal . |
