When fluid interfaces become crowded or develop structures due to lateral interactions between the molecules at the interface, they gain unique mechanical properties. In this talk, I will first examine how the structure of fluid-fluid interfaces impacts their properties, driving us to rethink their rheological behavior. These interfaces can range from 2D fluid-like to viscoelastic, and even display auxetic (negative Poisson's ratio) solid characteristics. Next, I will discuss how this understanding is key to engineering multiphase materials. I’ll introduce new methods for creating ultra-stable Pickering-Ramsden foams with micrometer-sized bubbles through pressure-induced particle densification. These foams, stabilized by nanosilica particles, are formed at sub-atmospheric pressures, allowing particles to adsorb onto large bubbles. Returning to atmospheric pressure shrinks the bubbles, compressing the particle layers and forming a strong elasto-plastic network that resists disproportionation. Finally, I will explore the relevance of such interfaces in biomedical applications, focusing on pulmonary surfactants. These surfactants play a crucial role in lung functionality by reducing the energy required for breathing, improving compliance, and preventing lung collapse. Yet, questions about surfactant mechanisms remain. For example, exogenous surfactant therapy is not effective in restoring lung compliance against acute respiratory distress syndrome. Also, even in healthy lungs, compliance decreases over time but can be restored through deep breaths, leading to the incorporation of such lung recruitment maneuvers in mechanical ventilation protocols.