Routes to colloidal gel formation

We discuss features of simple inter-particle potentials which are able to generate low-packing fraction arrested states, i.e. gels, in the absence of a macroscopic phase separation phenomenon. We suggest that the ratio between surface and bulk free energy is crucial in favoring ideal gel states. Two possible models for gels, one based on the competition of short range attraction and long range repulsions and the other on local constraints disfavoring packed local structures are discussed.