What the heck are colloids, anyway?

Colloids are Nature's and people's chief means of dispersing solid matter in a liquid. Immiscible liquids like oil and water can be dispersed in one another via the colloidal state. Important colloids in everyday life are milk, blood, ink, paint, and motor oil.

There are two requirements that must be met if you want to make a "stable" colloid that remains dispersed for a long time. First, the particles must be sufficiently small. (The fat droplets milk are prevented from rising to the top by the homogenizing process, which makes the droplets small.) Second, the particles must be made to repel each other. The main way of creating this repulsion in water is to incorporate ionizing chemical species in making the particles. In water, these ions disperse, leaving the colloidal particles with an electric charge. The typical charge on a particle is thought to be hundreds of elementary charges.

Under favorable conditions, the charged colloids repel each other so strongly that they stay as far as possible from each other by forming a crystal lattice. [P. Pieranski, Contemp. Phys. 24, 25 (1983)] But a number of recent reports have suggested that the particles may have mutual attraction under some conditions -- so that instead of dispersing, the particles clump together. This mysterious attraction is the focus of our study.

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