Particle Deposition Mechanisms: Impaction

Particle-Surface Interactions

Here a is the radius of the disc, h is the distance between the surfaces and m is the viscosity of the fluid. Based on Equation (1), the resistance force becomes infinitely large as . That implies that the surfaces could not come in contact. However, the surfaces will indeed come in contact due to the presence of London - van der Waals force. As noted before, the van der Waals force per unit area between two plane surfaces is given by

(2)

Equating (1) and (2), one finds

(3)

and the two disks will come in contact in a finite time. The Stokes drag on a particle approaching a surface increases rapidly as the distance from the wall decreases. For ,

(4)

Equation (4) is valid for . For smaller h, Equation (4) is not applicable and the van der Waals force may lead to adherence.