text: A particle may diffuse to a surface in quiescent conditions, but this process is inefficient as a thick depletion layer develops, which leads to a progressive slowing down of the deposition. When particle deposition is efficient, it proceeds almost exclusively in a system under flow. In such conditions, the hydrodynamic flow will transport the particles close to the surface. Once a particle is situated close to the surface, it will attach spontaneously, when the particle-surface interactions are attractive. In this situation, one refers to favorable deposition conditions. When the interaction is repulsive at larger distances, but attractive at shorter distances, deposition will still occur but it will be slowed down. One refers to unfavorable deposition conditions here. The initial stages of the deposition process can be described with the rate equation 
  where ; is the number density of deposited particles,  is the time,  the particle number concentration, and  the deposition rate coefficient. The rate coefficient depends on the flow velocity, flow geometry, and the interaction potential of the depositing particle with the substrate. In many situations, this potential can be approximated by a superposition of attractive van der Waals forces and repulsive electrical double layer forces and can be described by DLVO theory. When the charge of the particles is of the same sign as the substrate, deposition will be favorable at high salt levels, while it will be unfavorable at lower salt levels. When the charge of the particles is of the opposite sign as the substrate, deposition is favorable for all salt levels, and one observes a small enhancement of the deposition rate with decreasing salt level due to attractive electrostatic double layer forces. Initial stages of the deposition process are relatively similar to the early stages of particle heteroaggregation, whereby one of the particles is much larger than the other.