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We studied clustering and gelation in low-volume-fraction (phi less than or similar to 0.2) hard-sphere colloids, some with charge, with added non-adsorbing polymers. The range of the effective 'depletion' attraction induced by the polymers between the particles is greater than or similar to2% of the particles' diameter. The effects of density mismatch between the particles and the solvent and unscreened charges on the particles were investigated. The onset of aggregation and gelation in a neutral, density-matched system is discussed in terms of mode-coupling theory (MCT) and a recent 'renormalized' version of MCT respectively. Gravity causes sedimentation of growing clusters and shifts the gelation boundary. In the charged and density-matched system, a 'cluster phase' in which finite-size aggregates 'coexisted' with monomers occurred before gelation. Its origins remain unclear.
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