Vendredi
8 décembre
2006
à 11h30

Tour 33, couloir 33-43, 2^{ème}
étage, salle de réunion

Jacco Snoeijer

(University of Bristol, UK)

Tour 33, couloir 33-43, 2

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.