In most studies devoted to dynamic wetting, the viscosity of the ambient air is neglected compared with that of the liquid, which equivalently means that the surrounding air is considered to be static. However, the presence of air flows underneath the sheet leads to an increase of the pressure in the wedge formed by the liquid sheet and the solid substrate. The pressure deforms the interface and tends to separate the liquid sheet from the solid substrate. This is a coupled problem, the two unknown functions being the meniscus shape and the pressure generated in the air wedge. A simple model is proposed here to investigate the possible influence of slight pressure fluctuations in the air on the displacement of the contact line. This model incorporates the liquid wetting properties as well as parameters characteristic of the process and of the air pressure oscillations. The main result is that small amplitude pressure oscillations in the air close to the contact line may generate large lateral displacements of the contact line. At the same time, the air gap is reduced as compared to its value in the static case. [S0021-8936(00)01904-8]

1.
Burley
,
R.
, and
Jolly
,
R. P. S.
,
1984
, “
Entrainment of Air Into Liquids by a High Speed Continuous Solid Surface
,”
Chem. Eng. Sci.
,
39
, No.
89
, pp.
1357
1372
.
2.
Burley
,
R.
, and
Kennedy
,
B. S.
,
1976
, “
A Study of the Dynamic Wetting Behavior of Polyester Tapes
,”
Br. Polym. J.
,
8
, pp.
140
143
.
3.
Burley
,
R.
,
1992
, “
Air Entrainment and the Limits of Coatability
,”
JOCCA
,
75
, No.
85
, pp.
192
202
.
4.
Gutoff
,
E. B.
, and
Kendrick
,
C. E.
,
1982
, “
Dynamic contact angles
,”
AIChE J.
,
28
, pp.
459
466
.
5.
Miyamoto, K., and Scriven, L. E., 1982, “Breakdown of Air Film Entrained by Liquid Coated on a Web,” AIChE Meeting, Los Angeles, CA, paper 101g.
6.
Mues
,
W.
,
Hens
,
J.
, and
Boiy
,
L.
,
1989
, “
Observation of a Dynamic Wetting Process Using Laser-Doppler Velocimetry
,”
AIChE J.
,
35
, No.
89
, pp.
1521
1526
.
7.
Veverka
,
P. J.
, and
Aidun
,
C. K.
,
1991
, “
Using High Speed Cinematography to Obtain Visual Evidence on Air Entrainment in Low-Speed Roll Coating
,”
Tappi J.
,
75
, pp.
203
210
.
8.
Veverka, P. J., and Aidun, C. K., 1996, “An Investigation of Interfacial Instability During Catastrophic Air Entrainment,” AIChE Spring National Meeting, New Orleans, LA, Paper 7d.
9.
Miyamoto
,
K.
,
1991
, “
On the Mechanism of Air Entrainment
,”
Ind. Coating Res.
,
1
, pp.
71
88
.
10.
Huh
,
C.
, and
Scriven
,
L. E.
,
1971
, “
Hydrodynamic Model of Steady Movement of a Solid/Liquid/Fluid Contact Line
,”
J. Colloid Interface Sci.
,
35
, No.
1
, pp.
85
101
.
11.
Emonot, J., 1994, “Contribution a l’E´tude du Contact Dynamique Entre une Nappe de Liquide et une Surface Solide,” Ph.D. thesis, Universite´ Claude Bernard de Lyon.
12.
Emonot, J., Bourgin, P., and Burley, R., 1995, “Microfluid Mechanics of Air Entrainment in High Speed Coating Processes,” Int. Symp. on Two-Phase Flow Modelling and Experimentation, Roma, Italy.
13.
Bourgin, P., and Tahiri, N., 1996, “Triple Line : Myth or Reality?,” AIChE Spring National Meeting, New Orleans, LA, Paper 7b.
14.
Tahiri, N., 1997, “Etude de l’E´coulement d’un Fluide Tres Visqueux sur un Substrat Solide se De´plac¸ant a Grande Vitesse : Entraı^nement d’Air,” Ph.D. thesis, Universite´ Louis Pasteur de Strasbourg.
15.
Tahiri, N., Bourgin, P., and Saintlos, S., 1997, “High Velocity Dynamic Contact Between a Viscous Liquid and a Solid Substrate : Analytical and Numerical Study,” Proc. Euromech Symposium 367, Strasbourg, France.
16.
Bourgin, P., and Saintlos, S., 1998, “Dynamic Effects of Air Entrainment in High Velocity Coating Flows,” Rheology and Fluid Mechanics of Nonlinear Materials, ASME FED-Vol. 246, ASME, New York, pp. 95–100.
17.
Levarlet, C., and Finnicum, D. S., 1996, “Unsteady State Bead Visualization,” AIChE Meeting, New Orleans, LA.
18.
Woehl, P., 1999, “Mouillage Dynamique a Grande Vitesse: Entraı^nement d’Air et Stabilite´,” Ph.D. thesis, Universite´ Louis Pasteur de Strasbourg.
19.
Bourgin, P., and Tahiri, N., 1995, “Generalized Jeffery-Hamel Flow : Application to High Velocity Coating,” Proc. First European Coating Symposium, Leeds, UK.