Suspension Stability: Why particle size, zeta potential and rheology are important

This paper presents the combined technologies, rheology, zeta potential and particle size measurements, that are important to characterize particle suspensions. We will use a system that sediments at room temperature and explain why different techniques are needed to characterize the system to show its potential stability INTRODUCTION 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 electrostatic stabilization which induce stability through particle repulsion, 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 Eq. 1. T k g a B / 4   (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 Eq. 1, with consequent settling. Suspension Stability; Why Particle Size, Zeta Potential and Rheology are