The spreading of surfactants on gel layers has been found to be accompanied by an intriguing instability which involves the formation of crack-like patterns on the surface of the gel. In an attempt to extend the findings on the spreading on agar gels presented in part 1 of this series, this paper examines the case of surfactant spreading on gelatin, which is a characteristic example of a protein-based gel. Aqueous solutions of Silwet L-77 of varying concentrations were spread on thick gelatin layers of varying concentrations. The resulting pattern formation was found to have many similarities to the corresponding phenomenon on agar. In terms of spreading dynamics, the values of the spreading exponent, n, of the power law L(t) ~ kt(n), which describes the temporal evolution of the cracks, are similar to those of the agar case, within the predicted limits for surface tension gradient-induced spreading on thick films, highlighting the dominant presence of Marangoni stresses. However, the values of the spreading coefficient, k, are much smaller compared to those measured during the spreading on agar. Further observations are linked with the rheological properties of gelatin, which are also measured in detail.
