Chapter 4: Dynamical Deformation of Interfaces Induced by Aggregate Formation

Surfactant is one of typical component to realize self-motile droplet system. It is often used to modify surface tension and to produce Marangoni instability, resulting in droplet motion. However, surfactant is also known to form aggregate within a bulk phase; for example, bilayers, spherical and cylindrical micelles. Such aggregate formation leads to change in rheological properties in bulk phase. It is known that such aggregate formation in a limited region can produce motility of microorganisms. In this chapter, we show that chemical system composed of oil, water, surfactant, and co-surfactant system can show characteristic interfacial deformation, repetitive extension and retraction of spherical extrusions due to aggregate formation. In addition to interfacial deformation, we show that translation of a droplet can be realized as well.