Particles however have a tendency to aggregate, creating larger particles which settle more rapidly.
To aggregate, a collision must occur between the particles and when that collision occurs the particles must adhere to one another. All particles have a van der Waals attraction brought about by a variety of forces, principal among these the interaction of dipoles of neighboring atoms and molecules. Q ranges from a few kT for strongly polar molecules to of order 0.1 kT for crystalline silicates and oxides. There is an additional important term, the electrostatic interaction of particles. Particles in water generally have a surface charge, acquired by ionization of atoms at the surface of the particle.