Our PhD students have been selected for their project on Leidenfrost droplets !

See the ESA web pages.

Please also note the beautiful logo of this new project !

We investigated the dynamics of a vertical column of 12 beads submitted to a series of taps. Impulses are strong enough to eject beads. The motion of the beads are recorded using a high-speed camera. Depending on the frequency of taps, complex trajectories are obtained as illustrated in the front picture. The trajectories are analyzed in our last article [1].

For low tap frequencies, the pulses travel through the pile and expel a few beads from the surface. After a few bounces, the system relaxes to the chain of contacting grains. When the tap frequency increases, fluidization is obtained. In the fluidized part of the pile, adjacent beads are bouncing in opposition of phase (see top picture). This phase locking of the top beads is observed even when the bottom beads experience chaotic motions. While the mechanical energy increases monotically with the bead vertical position, heterogeneous patterns in the kinetic energy distribution are found when the system becomes fluidized.

[1] G. Lumay, S. Dorbolo, O. Gerasymov and N. Vandewalle, Experimental study of a vertical column of grains submitted to a series of impulses, Eur. Phys. J. E 36 , 16 (2013) – PDF

First, identical soft ferromagnetic particles are placed on a liquid-air interface. Capillary attraction is prevented by applying a vertical magnetic field which provides a repulsive interaction between the particles. The balance of capillary attraction and magnetic repulsion creates a self-assembly [4] as shown in the front picture. These structures are then perturbed by applying an oscillating horizontal field. The resulting cooperative dance of the particles provides a net propulsion of the particles along the liquid surface (see trajectories on the picture below). The self-assembly is swimming ! See the movies below :

• Movie 1 - An efficient swimmer of N=3 beads. 
• Movie 2 - The horizontal field is switched off. The swimmer is seen to stop its motion. Then the field is switched on but in another direction. The motion resumes but in a different pulsating mode (with a different swimming speed).
• Movie 3 - The complex motion of a swimming (pentagonal) assembly made of N=6 beads.

This work opens new perspectives in mesoscopic physics. The most important feature of our system is that both self-assembling and periodic deformations can be rescaled to smaller sizes.

References

[1] E.Lauga, Soft Matter 7, 3060 (2011)
[2] P.Tierno, R.Golestanian, I.Pagonabarraga, and F.Sagués, Phys. Rev. Lett. 101, 218304 (2008)
[3] G.Lumay, N.Obara, F.Weyer, N.Vandewalle, Soft Matter 9, 2420 (2013) - PDF
[4] N.Vandewalle et al., Phys. Rev. E 85, 041402 (2012) - PDF - Post

In a recent paper [2], the dynamics of this foam/liquid interface has been investigated. The idea of this research is to see this interface as a separation between two fluids, characterized by a given (effective) surface tension. By vertically shaking the system, the forcing acceleration faces this effective surface tension and, above a critical threshold, causes the emergence of Faraday waves. Different threshold values have been observed as a function of the foam properties. A phenomenological model has been proposed.

[1] N.Vandewalle, H.Caps, G.Delon, A.Saint-Jalmes, E.Rio, L.Saulnier, M.Adler, A.L.Biance, O.Pitois, S.Cohen-Addad, R.Hohler, D.Weaire, S.Hutzler, D.Langevin, Foam Stability in Microgravity, J. Phys. Conf. Series 327, 012024 (2011) – PDF
[2] A.Bronfort and H.Caps, Phys. Rev. E 86, 066313 (2012) – PDF

In a letter [3], a dynamical model of the air drainage is proposed  that accounts for the particular geometry of the air flow and for the physical properties of the wall. Indeed, we show that according to the used surfactant, the lifetime is modified as predicted by the model. It is demonstrated that the antibubble constitutes a smart tool in order to get fine surface properties of a surfactant mixture as for example the surface shear viscosity.

[1] S.Dorbolo, H.Caps and N.Vandewalle, New J. Phys. 5, 66.1-66.9 (2003) – PDF
[2] S.Dorbolo, E.Reyssat, N.Vandewalle and D.Quéré, Europhys. Lett. 69, 966-970 (2005) – PDF
[3] B.Scheid, S.Dorbolo, L.R.Arriaga, and E.Rio, Phys. Rev. Lett. 109, 264502 (2012) – PDF

When a droplet is gently deposited onto a vibrated liquid bath, the droplet could bounce repeatedly, avoiding coalescence. This peculiar behavior discovered in 2005 [1] is the starting point of an active research on pilot wave dynamics (see for example our project QUANDROPS) and droplet resonance effects [2]. In our last article on the subject [3], we demonstrated that the bouncing behavior of polymeric droplets onto highly viscous bath allows us to study/measure the elongational viscosity of the polymeric samples. It is found that large elongational viscosity of the polymer solution droplets suppressed large droplet deformations.

[1] Y. Couder, E. Fort, C.-H. Gautier, and A. Boudaoud, Phys. Rev. Lett. 94, 177801 (2005). [2] S.Dorbolo, D.Terwagne, N.Vandewalle, and T.Gilet, New J. Phys. 10, 113021 (2008) - PDF
[3] S. Gier, S. Dorbolo, D. Terwagne, N. Vandewalle, and C. Wagner, Bouncing of polymeric droplets on liquid interfaces, Phys. Rev. E 86, 066314 (2012) – PDF

Recent articles [1-3] emphasized the interest of 3d printing for scientists. In addition to pedagogical activities, during which students experience 3d models, scientific research can benefit a lot from such a technique. Scientists may build complex structures and may dream about new possibilities for testing their ideas.

A 3d printer (Mojo, STRATASYS) is now used in our lab. Examples of 3d applications are :

• Granular matter : to design grains (see pictures above) with specific non-convex shapes for interlocking, providing unusual granular properties. The morphology of grains can also be determined by measurements of real particle systems.
• Self-assembly : to produce synthetic floating bodies for testing capillary attraction between small objects suspended to a liquid interface. The shape of objects will induce capillary multipoles for specific interactions.
• Microfluidics : to build complex microfluidic devices for creating encapsulated droplets or producing monodisperse bubbles.
• Foam : to create 3d complex rigid frames for trapping improbable (stable or metastable) soap film structures.
• Statistical Physics : to design specific lattices for playing with the granular counterpart of the Maxwell’s demon.
• Percolation : to print 3d fractal structures for testing fluid invasion and other physical processes such as diffusion.
• Complex fiber networks : to create frames for connecting complex digital microfluidics fiber-based networks.

Many more ideas are currently tested.

[1] J.N.A.Matthews, 3D printing breaks out of its mold, Physics Today 64, 25 (2011)
[2] M.D.Symes et al, Integrated 3D-printed reactionware for chemical synthesis and analysis, Nature Chemistry 4, 349 (2012)
[3] N.Jones, Three-dimensional printers are opening up new worlds to research, Nature 487, 22 (2012)

From sugar poured into a bowl to tons of grains discharge processes in industry, a peculiar feature of granular materials emerges : the formation of a pile. When the grains are not cohesive, only the friction and the grain geometry determine the shape of the assembly which resembles a conical pyramid. As known by any sandcastle architect, the addition of some liquid induces cohesion between the grains due to the surface tension and capillary effects. During the last years, different groups studied the mechanical properties of wet granular piles, finding a remarkably insensitivity on the liquid content. This unexpected result is explained by a particular organization of the liquid between the grains. Although a significant progress has been achieved, all these experiments only consider wet granular piles with a low percentage of liquid. In our last paper, we report what occurs when dry sand is poured on a highly humid granular bed. Above a threshold humidity, an interesting phenomenon appears: instead of a wet pile, stable sand towers emerge. The impacting grains have a non-zero probability to stick on the wet grains, due to instantaneous liquid bridges created during their impact. The growth velocity of this self-assembled structures is determined by the flux of grains and the liquid content of the bed. We found that the higher the humidity the greater the probability of the grains to stick, but the smaller the final height of the tower, which falls when the cohesive stress at its base is surpassed. Beyond an artistic technique to sculpt sandcastles, this experimental method represents a new alternative to study the mechanical properties of wet granular materials.

F.Pacheco-Vazquez, F.Moreau, N.Vandewalle and S.Dorbolo, Sculpting sandcastles grain by grain: Self-assembled sand towers, Phys. Rev. E 86, 051303 (2012) – PDF