Star-shaped patterns caused by colloidal aggregation during the spreading process of a droplet

In this study, we discovered that, when an acidic solution with a low surface tension spreads on the surface of a glycerol solution mixed with milk, a star-shaped pattern is spontaneously formed on the surface in the horizontal plane during the spreading process. We experimentally investigated the emergence of the star-shaped pattern caused by an interfacial instability by using glycerol and aqueous 2-methoxyethanol solutions, which are acidic solutions; we chose the viscosity of the glycerol solution and concentration of both solutions as free parameters. The result demonstrated that the star-shaped pattern emerged when the concentration of 2-methoxyethanol was high. We proposed a phenomenological model based on our experimental results, which explains the following three points: the spreading of the aqueous 2-methoxyethanol solution on the surface of the glycerol solution; colloidal aggregation of the milk protein colloids caused by the denaturation that occurs when mixed with 2-methoxyethanol; accumulation of the aggregates toward the dent regions of the moving interface by a sweeping effect. The model reproduced the star-shaped pattern, which was similar to the experimental one. Furthermore, the concentration of the 2-methoxyethanol solution and the viscosity of the glycerol solution were taken as control parameters in our experiments and were varied, and a phase diagram was obtained. The phase diagram was similar to that obtained from our experiments. The results suggest that the above three points are important for the formation of the star-shaped pattern.