Particle interactions - an overview

A strong repulsive force between particles arises from the repulsion between like charges on the surfaces of adjacent particles. The approximate operating distance of the electrostatic force resulting from charges adsorbed on particles in an aqueous environment is 1/k where k is known as the double-layer thickness. The thickness of this layer is strongly dependent on the electrolyte concentration and is given approximately (in nanometre units) by 

As well as producing repulsion by introducing a polymer onto the surface of the particles, one can combine both effects using absorbed polyelectrolytes, i.e. polymers with charge distributed along the chains. The particular nature of the forces arising is different in each case but the overall effect is always the same—they produce a repulsive force. We need say no more about normal polymers at this stage with respect to the details of these force, but it is useful to note that the 

In summary we can say that the overall effect of the attractive van-der-Waals and depletion forces with the various repulsive forces shown in figure 10, where two extremes are shown—very large attractions due to van-der-Waal plus depletion forces, and the large repulsion present if we have very large surface charge and low electrolyte in aqueous systems. 

However, most systems have a mixture of both effects, so that we have the appearance of a secondary minimum. If this secondary minimum is shallow (only a few kTs), then the particles remain dispersed, but if it is much larger than that, then the particles stick together, that is they flocculate. 

All these kinds of interactions lead to an increased viscosity. However, although these colloidal forces dictate the form of the dispersion at low shear rate, when the shear rate is greater than some critical value, the viscosity begins to decrease. Eventually, the viscosities approach similar values at high shear rates. 

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