(369d) Modulation of Dilatational Rheology with Interfacial Curvature and Phase Morphology

Adsorption of complex molecular or particulate species on curved interfaces of bubbles, foams or drops can render the interface into a complex two-dimensional (2D) viscoelastic fluid. Here, we show that the dilatational viscoelastic response of an insoluble 2-D fluid film of Survanta surfactant is influenced by the curvature of interface, phase-morphology of the film and its ageing time. Survanta is a clinically approved multicomponent natural lung surfactant, which is used for treating fatal neonatal respiratory distress syndrome in preterm babies. We investigated the interfacial rheological behavior of Survanta surfactant adsorbed and coated on curved air-aqueous interfaces created in our in-house built micro-bubble dilatational rheometer, where we apply pure dilatational, small amplitude, oscillatory surface area deformations on the interface at frequencies 0.25 to 0.5 Hz near equilibrium regime and calculate complex dilatational modulus ‘ε’ of the interfacial film. We find that the dilatational modulus of 2-D fluid film of Survanta connected with the bulk possessing surfactant vesicles goes up surprisingly as we increase the curvature of (bubble) interface from ~0.003 µm^-1 to 0.01 µm^-1. Our unique approach of combining 3-D imaging capabilities of confocal fluorescence microscopy with the dilatational rheometer setup reveals that the Survanta film which shows coexisting ‘solid-like’ ordered, circular domains flowing easily in the disordered, ‘liquid-like’ phase on a low curvature interface (~0.003 µm^-1) shifts its phase morphology to ordered phase organizing into a rigid, immobile framework of stripes with liquid phase organizing into isolated discontinuous patches at relatively higher curvature bubble interface (~0.01 µm^-1). Modulation of dynamics with the background curvature and phase morphology is found reversible up to certain extent. Further, ageing of the film at equilibrium yields increasing values of complex dilatational modulus and elastic modulus, and is found associated with the area fraction of solid phase being altered at equilibrium.

These findings on this complex 2D fluid film can be extrapolated to understand rheology of complex 3D fluids as well as how lung surfactants might be involved in avoiding Laplace instability among alveoli of different radii. Moreover, it offers insights for creating smart interfacial films of technological and commercial importance.