The Basics of Non-Evaporative Parts Drying

John B. Durkee, Ph.D., P.E.

For most managers of critical cleaning operations, drying may not be synonymous with evaporation. Drying of rinse water by evaporation can be the most costly, energy, and time-consuming stage in the cleaning of parts. Even worse, whatever is non-volatile in the rinse water (minerals, detergents, ions), is left behind as surface imperfections. In some critical applications that can be fatal; the point of cleaning work, after all, is to remove surface imperfections—not generate them.
MAY THE FORCE BE WITH YOU
The second general method for drying parts of liquid films is to apply force to the films so as to dislodge them from surfaces. At least three types of force may be used: centrifugal, mechanical, and surface tension.
CENTRIFUGAL DRYERS
This approach makes too much sense to have been ignored in industrial cleaning. One simply places the parts in a circular-shaped basket, rotates the basket, the water is pulled off by the centrifugal force, and the dry parts are removed.
Relative to evaporation, energy consumption is negligible. Some heat air and blow it onto the rotating parts which makes no sense as the point is to avoid the energy debit necessary for evaporation. Cycle time can be 30 seconds to ten minutes.
There is mistaken concern about the force causing part movement within the basket, and therefore possible damage. Actually the centrifugal force holds parts in place and keeps them from moving.
Yet, there are good reasons why centrifugal dryers aren’t commonly used in critical cleaning: (1) not all water is removed, perhaps only 95% (which could serve as a preliminary drying step), and (2) most cleaning machines are built to use square baskets, and not circular-shaped ones.
MECHANICAL IMPACT WITH AIR
Long used in industrial cleaning, blowoff with forced cold air is low-cost and can be useful as a preliminary drying step. The operative device is called an air knife. It can be fed with compressed air at ~75 psi or with low pressure air at a velocity of hundreds of feet per second. The airflow is often referred to as an air curtain; in a cleanroom without an enclosure, it would be referred to as a particle spreading disaster! See Figure 1.
Most common applications involve high-production such as strip or sheet or other items on a moving conveyor.

MARANGONI DRYING
Useful only for perfectly flat surfaces, of which semiconductor wafers are the best example, this approach removes films of water without leaving residues.¹
It’s based on an observation that there is a net surface force between locations (a gradient) where the surface tension is different. This force pulls thin fluid films from the location of low surface tension to the location of high surface tension.
Such a gradient can be produced by a difference in temperature between two positions. It can also be produced by a difference in composition. This is shown in Figure 2.
In a Marangoni dryer, silicon wafers are mounted in an open cassette box immersed in DI water. A tiny amount of isopropanol is added to the water surface at the top of the dryer.² In other words, the surface tension in the top layer of fluid is made less than that of the fluid into which the wafers are immersed.
As the open cassette of wafers is slowly (~1 mm/min) pulled upward through the top layer of low surface tension, the water film on the surface of the wafer is pulled downward, and off the wafer. The dry wafer has no water on either side. No water-soluble materials (“spots”) have been left behind.

THE LAST WORD
There is one good reason why non-evaporative drying is practiced in critical cleaning. It is to remove water without leaving residues insoluble in water. Energy conservation has yet to be a factor in the costs of critical cleaning so that the two other schemes for non-evaporative drying are not considered. Perhaps in the future they will be.
References

1. “Drying Without Evaporation – The Marangoni System”, Controlled Environments Magazine, June 2005.
2. An excellent multi-page view of Marangoni drying process can be seen at,
   www.feu.edu.tw/edu/mse/%E6%AA%94%E6%A1%88%E4%B8%8B%E8%BC%89/maragoni%20dryer.pdf.