Surface tension driven defects

Defects can arise during (hying of a coated film by building non-uniform surface tension gradient over the coating. They include convection or Benard cells, fat edges or picture framing, etc. 

The convection cells, which come from the surface tension gradient, can arise when the Marangoni number (Ma) exceeds 80.The Marangoni number is 

Convection cells can arise at a lower Marangoni number when the coating thickness is above 2 mm. Convection or Benard cells can be reduced or eliminated by adjusting operating conditions and formulation of coating solution. The possibility of having convection cells is reduced at the following conditions  

When the thickness of wet layer of coating is less than 1 mm, as is often the case with almost all coatings, convection cells are almost always due to the surface tension gradients.  

Concentration increases faster at edge than the bulk 

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The maximum line speed of drying and operating conditions of dryer are often restricted by the onset of defects. The drying related defects could be classified according to the cause of defects - stress induced defects, surface tension driven defects, defects caused by air motion. 

Figure 7.3.30. Surface tension driven defect Convection cells.

Single crystal silicon is one of the important fundamental materials for the modern photovoltaic industry. The Czochralski method of growing single crystal silicon is affected by the thermocapillary convection. Temperature and concentration gradients at the free surface of the melt give rise to surface tension-driven Marangoni flow, which can lead to crystal defects, if it is sufficiently large. 

An additional benefit of the enhanced coverage and minimal defect density can be seen in Figure 6. Alkaline conditions allow dissolution of Si02, such that even at neutral pH 5% of bare MCM-41 is dissolved after a period of 4 hours. Presumably this is driven by the elevated free energy inherent to high surface area materials. (By analogy, surface tension drives coalescence of small droplets in, say, an aerosol into larger droplets.)... 

A number of the defects described previously involve flow driven by surface tension, but there also are flow defects where surface tension has little or no involvement. When a paint is applied, it is expected to flow out and level to produce a smooth film. Unfortunately, this does not always happen. Sometimes the viscosity is so high when the paint arrives on the part or increases so quickly after application that there is little flow and the result is a rough, bumpy surface... 

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