Wetting, Adsorption, and Cleaning Processes - Surface and Colloid Chemistry: Principles and Applications - page 137

This is a very general statement but serves as a useful guideline for investigations.
For water to penetrate fabrics, the magnitude of θ has been found to be close to zero.
This can be achieved by using detergents. However, to achieve water repellency,
the magnitude of θ has to be as large (i.e., >90 o ) as possible. If θ < 90°, then Cos(θ)
is positive, and the liquid will penetrate a fabric. On the other hand, if θ >90°, the
sign of Cos(θ) is negative, and the liquid will not penetrate the material. This should
be compared to the capillary rise (or fall) of different liquids in a glass tubing (see
Chapter 2).
In Young's equation, the quantity (γ S - γ SL ) can be made negative. This is achieved
by coating the solid material with some suitable material (such as Teflon). That means
γ c should be reduced to less than 30 mN/m.

6.4 evaPoratIon rateS oF lIquId droPS

In many natural (raindrops, fogs, river water fall) and industrial systems (sprays, oil
combustion engines, cleaning processes), one encounters liquid drops. The rate of
evaporation of liquid from such drops can be of importance in the function of these
systems.
Extensive investigations on evaporation of liquid drops (free-hanging drops, drops
placed on solid surfaces) have been reported in the current literature (Birdi, 2002).
These drops have been analyzed as a function of

Liquid (water or organic)
Solids (plastics, glass)
Contact angle (θ)
Height and diameter and volume
Weight

In these analyses, some assumptions have been made as regards the shape of the
drops. The most accurate data obtained is when using the weight method. Different
analyses showed that the rate of evaporation was linearly dependent on the radius of
the drop. Further, the contact angle of the water drop on Teflon (i.e., a nonwetting
surface) remained constant under evaporation. On the other hand, the contact angle
decreased as the water drop evaporated on glass (i.e., a wetting surface).

6.5 adheSIon (GlueS) (SolId1-SolId2)
There are many kinds of materials that need to be joined together by using glues or
adhesives, both synthetic and organic, in manufacturing, the field of arts and crafts,
and everyday life, such as

Plastic on glass
Metal to glass
Wood to wood (furniture, housing, boats)
Airplane and windmill wings