(549f) Probing Dilational Interfacial Stresses of Complex Interfaces Using a Microscale Spherical Bubble

The dilational properties of complex fluid-fluid interfaces are often correlated with the stability and bulk rheology of emulsions and foams. Probing the dilational response of an interface is challenging since it is often difficult to isolate dilational, shear, and other deformation modes. In this talk, we generate purely dilational deformation of a spherical bubble pinned at the tip of a capillary tens of micrometers in diameter. We oscillate the pressure jump across the interface at small amplitude while optically measuring the time-dependent radius of the spherical interface. The resulting dilational modulus can contain components arising from both interfacial rheological properties and transport to the interface from the bulk solution. Each of these phenomena can depend on radius of curvature of the bubble and applied frequency. Through careful analysis of the force balance on the interface in conjunction with several existing constitutive models, we show that radius of curvature and frequency can be used together to  separate thermodynamic and dynamic effects. Various interfacial phenomena can be separated by examining the real and imaginary parts of the complex dilational modulus, as well as the radius-dependent crossover frequency at which the two moduli are equivalent.  We validate these findings experimentally using two common nonionic surfactant systems: C₁₂E₈ and Tween 80 at air-water interfaces. In the case of C₁₂E₈, which adsorbs reversibly, the dilational modulus is solely a result of diffusion-limited transport to the interface. Tween 80, which is known to form an irreversibly adsorbed monolayer, exhibits a Kelvin-Voigt-type viscoelastic response. Finally, we apply this analysis to an interface stabilized by a mixture of colloidal silica and the cationic surfactant CTAB that is used in the generation of particle-stabilized emulsions and foams.