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Invited speakers

speaker institution current subject of lecture* as of
Jean-Pierre Delville CPMOH Bordeaux, France
"Optically forced fluid interfaces"
We discuss the pinning, the bending and the breakup of a liquid interface driven by an optical excitation. We investigate first the effect of the optical radiation pressure and survey results obtained in recent years such as interface bending in the linear regime, asymmetric radiation pressure effects under intense illumination, nonlinear deformations and laser-induced liquid jets, and laser-sustained liquid columns. Then, we analyze a dissipative coupling consisting in heating locally an interface between two immiscible fluids to produce thermocapillary stresses along this interface. This effect, also known as the optical Marangoni effect, is implemented in an adequate microchannel geometry to devise fundamental building blocks for two-phase flows in microfluidic devices. This allows the creation of contactless optical actuators such as mixers, valves, droplet sorters and switch, droplet dividers or droplet mergers. Finally, optical waves can as well couple with the bulk phases surrounding the interface. We illustrates this case by considering the bending of near-critical interfaces driven by scattering forces on bulk density fluctuations near a liquid/liquid critical point.
Haim Diamant Chemistry, Tel Aviv U., Israel
"Folds and jerks in compressed surfactant monolayers"
When we push laterally on an thin elastic sheet of paper or plastic, it will eventually either buckle or break. What about a one-molecule-thick layer of surfactant adsorbed at a fluid interface? (Such monolayers, for example, cover our lungs and are laterally compressed whenever we exhale.) Some surfactant monolayers do buckle or break under compression, yet others fail via a completely different mechanism which does not have a known macroscopic counterpart. They choose to relieve the stress through straight folded strips of micron width. The folding is accompanied by abrupt, macroscopic jerks of the monolayer. This type of failure is striking for its temporal sharpness, large spatial extent, and reversibility. Image and statistical analysis suggests that the folds occur in chain-reaction-like cooperative cascades.
Alfonso Ganan-Calvo Escula Superior Ingn., U. Seville Spain
"Ultimate limits of fine particles via electrospray and flow focusing"
I will give an overview of these two somehow analogous phenomena, their interest, their combination, their limits and what physically determines their limits (at least within a sufficiently ample parametrical realm)
Sascha Hilgenfeldt Mech. Engr., Northwestern U., Evanston USA
"Vesicle membranes under localized hydrodynamic stress"
Oscillating microbubbles can be used to drive fast, steady microfluidic flows with not just appealing propetries for fluid and particle transport (such as large flow speeds and throughput volumes), but with the unique feature of large hydrodynamic stresses in the flow. These stresses are concentrated in the vicinity of the bubbles and controllable via the ultrasonic pressure amplitude driving the oscillations. When soft objects, such as lipid membranes, are exposed to these localized hydrodynamic forces, they are deformed and - at sufficiently strong driving - destroyed. Often, however, the membrane is not ruptured, but undergoes a pinching-off of smaller vesicles. In other instances, the vesicle membrane successfully recloses after a hole was opened.
Eugenio Hamm Physics, U. Santiago, Chile Dynamics of developable cones under shear.
We study experimentally the response of a pair of crescent singularities (d-cones) in a cylindrical shell subjected to in plane shear. Imposed shear-field drives d-cones away from each other according to the same symmetries that rule the drift of dislocatios in crystals. We propose a mechanism in which the driving force is counterbalanced by the increasing bending energy of the system. The amplitude of displacement of the d-cones for a given shear angle is amplified by decreasing the thickness of the sheet, therefore concluding that the equilibrium state involves bending and stretching deformations
Alex Lips Unilever Corp. Trumbull CT USA Wrapup session leader
Detlef Lohse Applied Physics, Twente U. Netherlands "Giant bubble-pinchoff: Self-similarity and scaling "
Self-similarity has been the paradigmatic picture for the pinch-off of a drop. Here we will show through high-speed imaging and boundary integral simulations that the inverse problem, the pinch-off of an air bubble in water, does not obey self-similarity (of the first kind): A disk is quickly pulled through a water surface, leading to a giant, cylindrical void, which at collapse creates an upward and a downward jet. The neck radius h(tau) of the void does NOT scale with the inertial power law exponent 1/2 (i.e., does not obey ``Rayleigh-scaling''). This is due to a second length-scale, the inverse curvature of the void,which follows a power-law scaling with a different exponent. Only for infinite Froude numbers the scaling exponent 1/2 is recovered. In all cases we find the void-profile to be symmetric around the minimal void radius up to the time the airflow in the neck deforms the interface. This is work done by Raymond Bergmann, Devaraj van der Meer, Mark Stijnman, Marijn Sandtke, Andrea Prosperetti, and Detlef Lohse
Gareth McKinley Mech. Engr., Massachusetts Inst. of Tech., USA "Iterated Stretching and The Beads on a String Structure of Polymeric Jets"
I will discuss the dynamics of 'elasto-capillary' necking of fluid jets containing dissolved polymer. A self-similar balance of fluid elasticity and capillarity results in exponential thinning of the jet radius. Iterated instabilities on the jet (arising from Rayleigh disturbances) lead to a hierarchical beads-on-a-string structure and, ultimately, the formation of cusped interconnections between the beads and the main body of the jet.
Narayanan Menon Physics, U. Massachusetts, USA "Crumpling and uncrumpling of thin polymer films"
With a view to study the forced crumpling of thin elastic sheets, we have developed a procedure for creating thin copolymer films of thickness, h ~ 50nm and lateral dimensions X ~ 2 cm x 2 cm. This geometry yields an aspect ratio that is closer to the theoretical idealization of infinitely thin sheets than can be obtained with macroscopic sheets. Even though the films are exceedingly thin, they are robust enough that they can be mechanically manipulated without tearing. I will show experiments on crumpling and uncrumpling of these films, and our first attempts to characterize the geometry of the crumpled film.
Alphonsus V. Pocius
3M Company, Corporate Research Materials Laboratory, St. Paul USA
"Soft Interfaces in Adhesion and other Industrially Important Phenomena" Detergency, emulsification, release and adhesion are all industrially important phenomena that include a soft interface as part of their commercial relevance. These phenomena play a role in such diverse applications as tissue scaffolding, oil recovery, cleaning products, anti-staining products, paints and pressure sensitive adhesives. Pressure sensitive adhesive tapes make use of soft interfaces in their manufacture and, in use, soft interfaces play a role in how the product performs. We have used the technique of contact mechanics to study the energy dissipation processes in the soft interfaces that are important to pressure sensitive adhesive technology. Our work makes a connection between the microscopic energy dissipation processes that occur at a soft interface and the commercially important strength of an adhesive bond.
Benoit Roman ESPCI, Paris, France "Tearing: instabilities and peel-off"
Cracks in thin sheets seem to propagate along a surprisingly robust path. I will present two examples: -- an oscillatory instability of the crack path when cutting with a blunt object, -- and the frustrating tearing experienced when trying to peel off the initial part of adhesive tape.
David Quere College de France, Paris
"A few pearl drops."
We first discuss the making of super-hydrophobic states, and then comment on the stability and fragility of these states. We describe in particular how pinning transitions can be induced, owing either to size effects or to impacts. It is finally shown that for dynamical pearls, stresses can be focussed on these deformable interfaces, leading to surprising patterns.
Eran Sharon Physics, Hebrew U., Israel
"Shaping Thin Sheets and the Geometry of Wavy Leaves"
Gauss's famous theorem (theorema egregium) establishes the connection between intrinsic metric properties of a surface and its possible shapes in space. This link provides a powerful mechanism, for the generation of complex three dimensional shapes from thin elastic sheets, by prescribing curved metrics on them. For example the edge of a torn plastic sheet is composed of an organized cascade of waves. The waves are similar in shape but differ greatly in scale, leading to the formation of a fractal edge as an equilibrium configuration. We show that the tearing process prescribes a hyperbolic equilibrium metric near the edge of the sheet. This metric should be satisfied in order to reduce the stretching energy, but the limitations on the embedding of such a metric in Euclidean space "force" the sheet to wrinkle. We use environmentally responsive gels to form "engineered sheets"---flat sheets that adopt prescribed equilibrium metrics upon induction by environmental conditions. With this system we can study the shaping mechanism in a large variety of metrics. We suggest that some complex shapes of leaves and flowers might result from this spontaneous wrinkling instability that links between simple growth and complex configuration. The complexity, in this case, results from elasticity and not from complex growth processes, as commonly accepted.
Kathleen J. Stebe Chemical Engr. Johns Hopkins U. Baltimore MD USA
"Surfactants on Detaching Drops"
When a bouyant viscous drop is injected into a viscous fluid, it evolves to form a distended shape that detaches via the rapid formation and pinching of a neck. THe effects of surfactants in altering this process are studied numerically. In the absence of surfactants, surface contractions are most pronounced in the vicinity of the neck. Thus, when surfactants are present, they are drawn to this location, where they strongly alter the ensuing dynamics by reducing the surface tension that drives the contraction. The surfactant is described by a non-linear surface equation of state that accounts for maximum packing in a monolayer. The surfactant is soluble. Depending upon surfactant mass transfer rates and the amount of surfactant adsorbed, the drop can form a variety of neck shapes, or fail to neck at all. A map of neck/ no-neck thresholds is constructed as a function of surfactant coverage and mass transfer rates that lays the basis fo r using drop shapes far from equilibrium to characterize surfactant dynamics. or to select surfactants for a desired drop detachment mode.
Sigurdur Thoroddsen Mech. Engr., National U. Singapore "On the coalescence of miscible drops and the pinch-off of a bubble"
We will present high-speed imaging of three free-surface flows First, we look at the coalescence of two liquid masses which have different liquid properties. The focus is on a difference in the surface tension, for example during the coalescence of a water drop with a layer of ethanol. The resulting Marangoni stress produces waves which travel along the drop with the higher surface tension. Secondly, the pinch-off of a bubble will be imaged at frame-rates as high as 1 million fps. We study the power-law for the decrease in neck radius, as the radius approaches zero. For gas in water the exponent is found to be slightly steeper than the 1/2 predicted by inviscid dimensional analysis. Finally, some videos will be shown of the 'puncturing' of a drop. This occurs when a soap layer on the drop is compressed and 'buckles', sending out a jet of fluid. more...
Shankar Venkataramani Math. U. Arizona, USA
"Ground states for nonconvex free energies"
The equilibria of many physical systems are determined by minimization of an appropriate free energy. If the free energy is non-convex, the ground states can display various multiple-scale behaviors including singularities, defects and micro-structure. I will illustrate these issues using two physically relevant examples -- Thin elastic sheets and the convection roll patterns. I will review recent results from the rigorous analysis of these problems. I will also discuss the implications of these results to the question of what (if any) general principles govern multiple scale behaviors in non-convex variational problems.more...
Stephane Zaleski LMM, U. Paris, France "Simulation of three dimensional focusing in splashes and atomizing liquid-gas mixing layers"
Large deformation of liquid masses , such as in jet atomisation or droplet splash on impact, lead to the formation of droplets through a sequence of two-dimensional and three-dimensional instabilities. This process and its analogies to focusing is analyzed using direct simulations and simple models.

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