Adding particles to liquids modifies the liquids’ optical and physical properties, e.g. color, density, and viscoelasticity. Solvents, such as water or acetone, are typically ideally viscous liquids, while particle-bearing liquids display a more complex rheological behavior. If the particles are soluble in the solvent, the final product is termed a solution. Particles are dissolved as molecules or ions (<1 nm). In case the particles are insoluble, the product is a two-phase (or multi-phase) mixture which is termed a suspension if particles are larger than 1 µm or termed a colloid if particles are between 1 nm and 1 µm in size.
Many liquid products, paints, inks, beverages, medicine, slurries, or shower gels contain various types of particles to achieve the desired final product or adjust processing properties. A low particle volume fraction (termed “solid fraction” hereafter) typically induces shear-thinning behavior, whereas high particle concentrations might result in shear thickening. However, the materials’ viscoelasticity not only depends on the concentration of particles but also on particle shape and particle size. In most suspensions, particles differ in size, which can be expressed as the particle size distribution. Furthermore, the particle surface’s electrical charge (zeta potential) influences the suspension’s rheological behavior. This article aims to separate the effects of particle concentration, particle shape, particle size distribution, and particle surface charge, and to name common methods to measure these properties.