(86d) Dilatational Modulus Beyond Gibbs

The dilational response of complex fluid-fluid interfaces is critically important in the flow behavior of emulsions and foams. The dilatational modulus promotes stability against interfacial relaxation and coalescence, and controls migration velocities of droplets and bubbles.  Despite its importance, measuring and understanding this interfacial property is elusive, since interfaces are strongly coupled with adjacent bulk material. Using a microtensiometer device that preserves a spherical cap shaped interface, we show that it is possible to measure the dilatational modulus of a complex fluid interface and isolate the effects of several different mechanisms via simultaneous measurement of surface pressure and total dilatational modulus as a function of surface coverage. Factors contributing to the overall dilatational modulus include the Gibbs modulus arising solely from surface concentration changes, transport from bulk to interface, Marangoni stresses, extra stresses arising from structural interactions within the interface, and bending stresses.  Among irreversibly adsorbed layers, phospholipids and fatty acids exhibit predominantly Gibbs effects, while surface active nanoparticles exhibit strong extra stresses that can be an order of magnitude or more greater than the Gibbs modulus.  Large extra stresses arise for densely packed interfaces containing surfactant-nanoparticle complexes or hydrophobic globular proteins that form nearly incompressible monolayers and multilayers.  The dilatational response of these particle-laden interfaces as a function of interface and particle composition is then related to the formation of stable nonspherical capsules via microfluidic control of transport timescales.