Sedimentation Equilibrium of Mixtures of Charged Colloids

0.02210.83E x(mV/cm)0.60.40.0140.256078η i0 20 40 60 80 100x (cm)91000 20 40 60 80 100x (cm)Figure 4.9: Packing fractions of a suspension with 10 different types of colloids.The charge number Z n = n · 250, while the gravitational lengthL n = 0.1. The other parameters of the system can be found in tab. 4.1,except for the height H = 100cm, instead of 20cm. The inset shows theelectric field strength.The origin lies in the presence of Coulomb forces. Systems of charge neutralgases only need to find a balance between entropy and gravitational energy.Systems of charged particles need to find a balance with electrostatic energyas well, before reaching equilibrium. We will explain this balance in a slightlydifferent way.Non-barometric profiles The colloids prefer a barometric distribution.The salt prefers a homogeneous distribution. Charge neutrality prefers thecolloids to be mixed with sufficient counter-ions. A compromise is made bylifting the colloids, and shifting co-ions up and counter-ions down, leaving stilla (small) net electric field in the fluid. Depending on the salt concentration,the colloids can seriously be lifted in a situation with very few micro-ions.This lifting effect is caused by an electric field, generated by a thin layer ofcharge at the bottom and top of the distribution. Salty suspensions, likeseawater, do not show these effects, because only a small fraction of thecounter-ions can screen all colloids, resulting in (almost) barometric profiles28