Fluid interfaces acquire unique mechanical properties when they become crowded or develop structures due to lateral molecular interactions. In this talk, Prof. Jan Vermant (ETHZ) will first examine how the structure of fluid-fluid interfaces influences their properties, prompting a re-evaluation of their rheological behavior. These interfaces exhibit a spectrum of behaviors, from 2D fluid-like to viscoelastic, and can even display auxetic properties, characterized by a negative Poisson’s ratio. Next,he will discuss how this understanding is key to engineering multiphase materials. Prof. Vermant will introduce new methods for creating ultra-stable Pickering-Ramsden foams with micrometer-sized bubbles through pressure-induced particle densification. These nanosilica-stabilized foams form under sub-atmospheric pressure, enabling particle adsorption onto large bubbles. When pressure returns to atmospheric levels, the bubbles shrink, compressing the particle layers into a robust elasto-plastic network that resists disproportionation. Finally, he will explore the relevance of such interfaces in biomedical applications, focusing on pulmonary surfactants. These surfactants are essential for lung function, reducing breathing effort, enhancing compliance, and preventing lung collapse. However, challenges remain in understanding their mechanisms. For instance, exogenous surfactant therapy is ineffective in restoring lung compliance against acute respiratory distress syndrome. Moreover, even in healthy lungs, compliance gradually decreases but can be restored through deep breaths, a principle incorporated into mechanical ventilation protocols.