Particles on Surfaces 1 pp 129-142
A Theoretical Review of Particle Adhesion
This paper constitutes a theoretical review of the forces of adhesion of small particles to surfaces. The primary force of adhesion of small, less than 50 micron diameter, particles on a dry surface are van der Waals forces. These van der Waals forces can be increased due to particle and/or surface deformations which increase the particle contact area. Particles less than 1 micron diameter can be held to surfaces by forces exceeding 100 dynes, which corresponds to forces per unit area of 109 dyn/cm2 or more. Total forces of adhesion for 1 micron diameter particles can exceed the gravitational force acting on that particle by factors greater than 106. Electrostatic forces, although they only become important and predominate for particles larger than 50 microns diameter, may play a significant role in bringing particles to surfaces for adhesion. The presence of a liquid between the particle and surface, due to either immersion and subsequent removal from a liquid or due to high humidity conditions, can add a very large capillary force to the total force of adhesion. This capillary force is known to remain, in some cases, even after baking at above the liquid boiling point for many hours. Although some possibilities exist for particle removal, removal of small particles has been in practice extremely difficult due to these large forces. Each of these forces will be discussed theoretically with some attempts at quantification versus particle diameter. Some data will also be presented on attempted particle removal as a function of removal method. Clearly, emphasis should be placed on prevention of particle deposition on surfaces rather than relying on achieving subsequent removal.
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