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A theory of the small-q behaviour of the static structure factor S(q) for monodisperse colloidal dispersions is presented in which the long-range forces between the colloid particles are treated in the random phase approximation. By using a hard-sphere reference fluid, explicit results for the coefficients to 0(q4) in a small-q expansion of c ( q ) , the Fourier transform of the direct correlation function, are obtained.The theory gives an expression for the osmotic compressibility of a dispersion that is equivalent to an empirical relation used to interpret light-scattering experiments on microemulsions. When applied to charge-stabilised dispersions the theory predicts the existence of a phase diagram closely analagous to one normally observed in molecular systems.

Monte Carlo simulations have been used to investigate the dynamic phase behavior of a classical Heisenberg spin system with a bilinear exchange anisotropy Λ in a planar thin film geometry. Studies of the field amplitude, frequency and temperature dependence show dynamic phase transitions in films subject to a pulsed oscillatory external field. Thin films with competing surface fields show separate and distinct dynamic phase transitions for the bulk and surface layers of the film. Between the two transitions, a mixed state with coexisting dynamically ordered and dynamically disordered phases is observed in the film. In contrast, the free film with no surface fields shows a single dynamic phase transition as in a bulk system. PACS number(s) : 75.40.Mg,

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