Elasto-capillarity

We study the competition between capillarity and elasticity that can lead to surprising phenomena when the interfacial energy is comparable to the elastic one.

Capillary Origami

[\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 . If the sheet is stiffer, the corners start bending but the sheet quickly reopens.
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, 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.
The opening and closure of the structure can finally be actuated by an electric field applied between the droplet and the ground/]

Capillary buckling of a floating annulus

Can surfactant molecules induce wrinkles in a floating sheet?
Out-of-plane buckling of a flexible annulus floating on a bath of water is observed as surfactant molecules are added outside the annulus. The difference in surface tension induces compressive stresses, which result in regular orthoradial wrinkles beyond a critical difference in surface tension. The wrinkles first appear in the vicinity of the inner edge of the annulus and progressively grow as the concentration of surfactant is increased. Conversely, the wavenumber remains constant and relies on a simple balance between gravity and the bending stiffness of the membrane within the range of our experimental parameters.

Wrapping a sphere with a sheet

[\Can we wrap a sphere with an adhesive sheet?
Cartographers know from centuries that it is not possible to provide a planar representation of the earth without stretching the continents. This geometrical constraint results from the theorema egregium from the mathematician Gauss. As a consequence the contact zone between the adhesive sheet and the sphere adopts a complex morphology depending on the bending and stretching rigidities of the sheet. If the sheet is thin enough, we observe a ramified oscillatory shape. /]