PerspectiveMaterials Science

Controlling the Flow of Suspensions

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Science  18 Feb 2011:
Vol. 331, Issue 6019, pp. 868-869
DOI: 10.1126/science.1201543

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If you've painted a wall, you know that you want the paint to flow smoothly onto the surface but be viscous enough that it doesn't drip. Paint is a suspension—small, solid particles of pigment and polymer dispersed in a liquid—and manufacturers devote much effort to controlling its flow behavior, or rheology. Suspension rheology is critical not only in coatings but also in many food and materials processing steps, and it depends on two factors. First, the content and shape of the dispersed particles are important, but these factors usually cannot be varied to optimize performance. The size and shape of the particles are usually predetermined, and a high volume ratio of particles to solvent is often required. The second factor is the interaction between the particles, which affects viscosity. To achieve low viscosity yet deliver a high volume ratio of particles, the particles should repel each other. To achieve a high viscosity and create an elastic material like a gel, the particles need to aggregate via attractive forces. Forces between the particles are determined by their surface properties and are traditionally adjusted by adding surfactants. On page 897 of this issue, Koos and Willenbacher (1) propose another option, the addition of a small amount of a second liquid, immiscible with the primary liquid.