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Séminaire Exceptionnel

                      
Vendredi 8 décembre 2006 à 11h30
Tour 33, couloir 33-43, 2ème étage, salle de réunion

Jacco Snoeijer
(University of Bristol, UK)

Dynamical wetting and collapsing bubbles

We have investigated several problems of interface dynamics: (1) relaxation behavior, singularity and shock formation, at receding contact lines (sliding drops or solid pulled out of a bath). (2) collapse of an axisymmetric cavity or bubble inside a fluid of small viscosity, like  water.   In this  second case, using a  slender-body description, we show that the minimum radius of the cavity scales like h ∝ t′^α , where t′ is the time from collapse. The exponent α very slowly approaches a universal value according  to α = 1/2 + 1/(4(− ln t′ )^(1/2)). Thus, as observed in a number of recent experiments, the scaling can easily be interpreted as evidence of a single non-trivial scaling exponent. Our predictions are  confirmed by numerical simulations.  In the case of receding contact line, one of our result leads to reconsider the classical result from Landau and Levich, at least for hydrophobic surfaces, about the thickness of the layer versus speed of withdrawal of a plate pulled out of a bath. We found that the forced wetting transition occurs through the formation of a remarkable structure: a thick liquid ridge is dragged upwards along with the plate and leads to a sharp shock formation. We have found experimentally that the existence of this ridge determines the critical speed of the wetting transition and affects the transient value of the layer thickness. Theoretically, however, the transition should be at a higher velocity: surprisingly, we show that stable meniscus solutions exist up to 20% above the ridge velocity. Finally, 3D aspects are considered with the possible formation of a point like singularity at the contact line with possible cusp formation and droplet deposition.