Colloids and Fine Particles
5.4 RESULT OF SURFACE FORCES ON BEHAVIOUR IN AIR AND WATER
From the equations for van der Waals forces [Equation (5.8)] and EDL repulsion [Equation (5.11)] one can see that the magnitude of surface forces increases linearly with particle size. Body forces which depend on the mass of the particle, however, increase with the cube of the particle size (since the mass is related to the volume multiplied by the density and volume depends upon the cube of the particle size). It is therelative values of body forces and interparticle surface forces that are important. Although, small particles have very small interparticle surface forces compared with big particles, relative to the body forces, the interparticle surface forces are large for small particles. This effect occurs because of the much stronger dependence of body forces on size than the surface forces.
When particles’ surfaces interact across air, such as in dry fine powders, the dominant interaction is attraction due either to van der Waals interactions or capillary bridges (see Chapter 13). In air, other gases and vacuum the only mechanism which could generate repulsion is electrostatic charging (for example due to friction). If the charge on particles is the same sign, repulsion will result due to Coulomb’s law while attraction would result between oppositely charged particles. Electrostatic interaction, although possible is usually not of significance if the relative humidity is greater than about 45 % because the charge rapidly dissipates at room temperature in humid air.
The result is that fine powders in air are cohesive due to van der Waals and capillary attraction. Attraction between particles results in cohesive behaviour of
-2.0 -1.5 -1.0 -0.5 0.0 0.5 25 20 15 10 5 0 Separation distance (nm) Fo rce (nN)
van der Waals only for comparison steric repulsion on approach bridging attraction on retraction (b) Figure 5.9 ðContinuedÞ
the powder. The strong cohesion of the particles is the reason why fine particles are difficult to fluidize (Geldart’s Group C powders described in Chapter 7). The strong cohesion is also the cause of the high unconfined yield stresses of powders described in Chapter 10. The high unconfined yield stress of these powders means that the powders are not free flowing and will require a larger dimension hopper opening relative to free flowing powders of the same bulk density. Either larger primary particles or granules of the fine powder will have greater mass so that body forces (rather than adhesive surface forces) will dominate behaviour of these bigger particles and free flowing powders will result.
The influence of attractive forces between fine dry powders is observed as the effect of particle size on bulk density. As the particle size decreases the loose packed and tapped bulk densities tend to decrease. This is because as the particle size becomes smaller, the influence of the attractive surface forces becomes stronger than the body forces. Consolidation is aided by body forces (such as gravity) which allow the particles to rearrange into denser packing structures. The attractive surface forces between fine particles hinder their rearrangement into dense packing structures. Note this may seem counterintuitive to some readers (who may think that attractive forces would increase packing density). This is not the case because attractive forces create strong bonds that hinder rearrangement of particles into more dense packing structures.
One difference between dry fine powders and colloids in liquids is that the low viscosity of air (and other gases) make hydrodynamic drag forces minimal for dry powders in many instances, except when the particles have very low density (such as dust and smoke) or the gas velocity is very high. However, fine particles in liquids are strongly influenced by hydrodynamic drag forces as described in Chapter 2 because the viscosity of liquids is much greater than that of gases.
When fine particles are suspended or dispersed in liquids, such as water, we are able to control the interaction forces by prudent choice of the solution chemistry. This control of interaction forces is of significant technological importance because we can thus control the suspension behaviour such as stability, sedimentation rate, viscosity, and sediment density. Additives such as acids, bases, polymers and surfactants can easily be used in formulations to develop the range and magnitude of either repulsion or attraction as demon- strated in Figure 5.9. When fine particles are suspended or dispersed in liquids, such as water, there are several mechanisms that can produce repulsive forces between particles that can overwhelm the attractive van der Waals interaction between like particles if we want to keep the particles dispersed. One example of a situation where dispersed particles are desirable is in ceramic processing. In this application, the low viscosity of dispersed particles is desirable as well as the uniform and dense packing of particles in the shaped ceramic component afforded by the repulsion. However, suspended particles with high magnitude zeta potential with strong repulsion between them may be made to aggregate by development of an attractive interaction so that they settle rapidly to increase efficiency of solid/liquid separation. This application of bridging polymers is discussed in Section 5.5.
In general, as shown in Figure 5.10, suspension behaviour depends upon the interparticle forces which in turn depend upon the solution conditions. The
suspension behaviour of interest such as stability, sedimentation, sediment density, particle packing and rheological (flow) behaviour are discussed in the following sections.