Abstracts - KTH Mechanics

30
Dynamic wetting transition: avoided critical behavior
G. Delon ∗ , J.H. Snoeijer ∗ ,B. Andreotti ∗ and M. Fermigier ∗
A solid surface can be coated by a thin film when the solid is withdrawn at
constant speed out of a bath of liquid. This dip coating process was analyzed initially
by Landau-Levich.
In a partial wetting situation, there is a threshold velocity below which the meniscusissteadyandthesolidremainsdry.
We investigate the dynamic wetting transition
between the two states : at low velocity, a stable meniscus, at high velocity, an entrained
film.
It has been shown that a receding contact line becomes unstable when the capillary
number exceeds a critical value Cac 1 . In our experiments, liquid entrainment occurs
at a capillary number Ca ∗ which is lower than Cac. The critical behavior expected at
Cac is thus avoided. The threshold velocity coincides precisely with the contact line
velocity above the transition. We explain the occurence of an early transition by the
nucleation of a a capillary ridge (see fig.) which moves ahead of the thin film. The
characteristics of this ridge determine the threshold velocity for liquid entrainment.
We discuss also the influence of the curvature of the solid surface on the wetting
transition.
∗ PMMH,ESPCI, 10 rue Vauquelin, 75005 Paris, France.
1 J. Eggers Phys. Fluids 17, 082106 (2005).
8
z (cm)
7
6
5
U p
100 µm
Figure 1: (Left) Liquid film (brighter part) entrained by a vertical plate pulled out
of bath of silicone oil. (Right) Time evolution of the ridge profile (vertical cross
sections in the middle of the liquid film. The three oblique lines in this space-time
diagram represent the velocity of the solid plate, the velocity of the contact line and
the velocity of the front between ridge and thin film.
0
100
200
U cl
300
U j
400
t (s)