Particle deposition
Deposition processes may be triggered by appropriate hydrodynamic flow conditions and favorable particle-surface interactions.
The latter process is known as particle release and is often triggered by the addition of appropriate chemicals or a modification in flow conditions.
When macromolecules, such as proteins, polymers or polyelectrolytes attach to surfaces, one rather calls this process adsorption.
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.
Once a particle is situated close to the surface, it will attach spontaneously, when the particle-surface interactions are attractive.
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.
At one point, such a surface layer will repel any particles that may still make attempts to deposit.
With increasing number density of deposited particles, the blocking function decreases.
The blocking process has been studied in detail in terms of the random sequential adsorption (RSA) model.
[4] The simplest RSA model related to deposition of spherical particles considers irreversible adsorption of circular disks.
Since the repulsion between particles in aqueous suspensions originates from electric double layer forces, the presence of salt has an important effect on surface blocking.
Therefore, the surface will be blocked at a much lower coverage than what would be expected based on the RSA model.
[5] At higher salt and for larger particles, this effect is less important, and the deposition can be well described by the RSA model.
This situation will result in a porous layer made of particle aggregates at the surface, and is referred to as ripening.
The porosity of this layer will depend whether the particle aggregation process is fast or slow.
Optical microscopy has the advantage that the deposition of particles can be followed in real time by video techniques and the sequence of images can be analyzed quantitatively.
[6] On the other hand, the resolution of optical microscopy requires that the particle size investigated exceeds at least 100 nm.
Another approach to study particle deposition is to investigate their transport in a chromatographic column.
