The air/water interface in a lung is rheologically complex as the alveolar interfaces must be coated with a low tension surfactant film to ensure uniform lung inflation with minimal effort. The dependence of the Laplace pressure, ÎP = 2Î³/R, on the alveolar radius, R, means that interconnected bubbles or alveoli are at best metastable if Î³ is constant. The pressure difference between small and large alveoli can induce the Laplace Instability if the surface tension is constant during breathing, in which the smaller alveoli collapse while the larger alveoli overinflate. For (2Îµ-Î³) > 0 (dilatational modulus, Îµ=AâÎ³/âA), the surface tension, and hence the Laplace pressure decreases with decreasing radius and increases with increasing radius, which reverses the Laplace instability, thereby stabilizing the alveoli against collapse. Lysolipid, a byproduct produced by the innate immune system when the lungs become inflamed due to disease or injury, competes for the alveolar interface and pushes the lung towards instability with increasing lysolipid concentration. Our previous work has shown that the dilatational modulus increases with increasing oscillation frequency, which suggests that increasing breathing frequency during mechanical ventilation may restore lung functionality by elevating the dilational modulus of the lung interface, thereby reversing the Laplace Instability.
Our bubble tensiometer allows us to perform nearly isotropic deformations on small alveolar-sized bubbles, with the dilatational modulus dominating the surface rheology. As opposed to constant area rheometry, dilational rheometry depends more on the transport of surfactant to and from the interface, and as such, depends on the oscillation rate and the bubble size. In this work, we are going to examine the diffusion process of multiple inhibitors both above and below their critical micelle concentrations (CMC). We will show that the magnitude of the dilatational modulus peaks at the CMC. We will also discuss how Îµ can be decomposed into elastic and viscous components and how the ratio of the elastic to viscous components changes as insoluble lung surfactant is replaced by soluble lysolipids. We find that the dilatational modulus is extremely sensitive to the detailed composition of an interfacial film.