Suspensions or dispersions of particles or droplets in a liquid medium are encountered in a variety of industries and find use in a diverse range of applications. These include liquid abrasives, ceramics, medicines, foodstuffs and inks to name a few. One key criterion which is important across this range of applications is suspension stability. For a suspension to be functional it must be capable of suspending the dispersed phase for the lifetime of the product and/or be easily dispersed should sedimentation occur. A number of factors contribute to dispersed phase stability and these may be thermodynamic or kinetic in origin. Examples of the former include steric and to some extent electrostatic stabilization which induce stability through particle repulsion (the latter is really kinetic as electrostatic repulsion barrier is not infinite), while kinetic stability can be induced by increasing the viscosity of the suspending medium thus slowing down particle aggregation and sedimentation.
For sub-micron suspensions Brownian motion is usually significant to maintain the particles in a dispersed phase, however, for larger particles the effect of gravity becomes significant if there is a sizeable difference in density between dispersed and continuous phases. In this case the likelihood for sedimentation can be predicted from the ratio of gravitational to Brownian forces using Equation 1 .
where a is the particle radius, Δρ is the density difference between the dispersed and continuous phases, g is acceleration due to gravity, kB is the Boltzmann constant and T is the temperature. If this ratio is greater than unity some degree of sedimentation can be expected while a ratio less than unity is likely to indicate a stable system. This equation, however, does not take into account potential interactions between particles. Due to Brownian motion, particles will be continually colliding with each other and consequently particles may become aggregated due to Van der Waals attractive forces. This can lead to formation of secondary particles (flocs) of much larger size and therefore a larger gravitational contribution to Equation 1, with consequent settling.
To prevent particles becoming aggregated it is necessary to provide some barrier. This can be achieved through steric or electrostatic means by adsorbing polymers or introducing a charge onto the particle surface by, for example, modifying the pH. If the repulsive force exceeds the attractive force then a stable system should result. For an electrically charged suspension such a force balance can be described by DLVO theory where the combined/total energy (VT) is the sum of attractive (VA) and repulsive ((VR) contributions as shown in Figure 1a. This theory proposes that an energy barrier resulting from the repulsive force prevents two particles approaching one another and adhering together unless the particles have sufficient thermal energy to overcome this barrier. The size of this potential barrier can be indicated by the magnitude of the zeta potential which is the potential at the slipping plane between the particle and associated double layer with the surrounding solvent [4, 5]. If all the particles in suspension have a large negative or positive zeta potential then they will tend to repel each other and there will be no tendency for the particles to come together. However, if the particles have low zeta potential values then there will be insufficient repulsion to prevent the particles coming together and flocculating. The general dividing line between stable and unstable suspensions is generally taken as +30 or -30 mV with particles having zeta potentials outside of these limits normally considered stable [6, 7]. Such an assumption, however, is very much dependent on particle properties [1, 4]. In this paper we consider the importance of particle size, zeta potential and rheology on sedimentation behavior and demonstrate how these properties can be manipulated to induce stability.
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