Bulk-surface interactions occur in a wide range of applications ranging from emulsions, foams stabilized by surfactants to cell dynamics governed by proteins. Central to the modelling of these phenomena is the idea that the dynamics are not solely restricted to the bulk material, but that an active surface, i.e. a surface with its own dynamics, also governs the material behaviour. In this contribution, we present a continuum theory to study bulk-surface materials undergoing deformation and phase segregation. More specifically, we consider an immiscible binary bulk fluid enclosed by a thin immiscible binary fluid film that both deform in an incompressible manner. We treat this thin film as a material surface with finite thickness. In this talk, we detail the kinematics of the bulk-surface material, as well as the implications of the isochoric motion in terms of the mass balance. Additionally, we explain how a bulk-surface principle of virtual powers that is postulated on an arbitrary part where the boundary may lose smoothness, gives rise to additional geometric contributions in the mathematical formulation. Furthermore, we propose a set of boundary conditions that allow for slip between the surface and the bulk, and employ these to derive the Lyapunov decay relation, which characterizes the dissipative nature of our bulk-surface system and its interaction with the environment [1]. Applications of this framework can be found in mechanobiology, where biological cells consisting of a bulk volume (cytoplasm) enclosed by a surface (cell membrane) are shaped by various complex processes involving the cell membrane [2]. REFERENCES [1] Anne Boschman, Luis Espath, Kris van der Zee. A bulk-surface continuum theory for fluid flows and phase segregation with finite surface thickness. arXiv: 2308.01647, 2023. [2] B Ladoux and R Mège. Mechanobiology of collective cell behaviours. Nature Reviews, Molecular cell biology: 18(12):743-757, 2017.