Ribbing instability

Figure 10. Imaging ellipsometry of rib instability near the reservoir meniscus...

Figure 11. Wavelength of rib instability as function of withdrawal speed for several concentrations of methanol-water.

An interesting manifestation of surface flows in dip coating is a rib instability often observed near the reservoir in binary solvent systems and in some pure solvents (ethanol). Figure 50.10 shows a set of wavy interference fringes, indicative of a 20-nm thickness undulation that has a wave vector that runs perpendicular to the withdrawal direction. Typically, this undulation is a standing wave, but under some conditions (not well defined at... 

In reverse-roU coating, as in forward-roU coating, instabilities can form. However, it is possible to obtain defect-free coatings at high speed and sometimes increasing the speed can lead to a smooth coating when a ribbing condition is present. 

Many coating flows are subject to instabilities that lead to unacceptable coating defects. Three-dimensional flow instabilities lead to such problems as ribbing. Air entrainment is another common defect. 

During the coating process, unwanted instabilities can occur and are mainly related to the viscosity and surface tension of the solution, as well as the coating speed. The dimensionless capillary number is useful to characterize the various instabilities such as ribbing, cascading, mist, etc. 

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