Interfacial Rheology and The Controlled Fabrication and Disruption of Stabilized Emulsions

2019-06-10T18:36:19Z (GMT) by Jerome J Nash
Fluid interfaces containing surface-active species (e.g., surfactants, polymers, and particles) have rheological properties that are vital to the kinetic stability of emulsions. Many practical applications of emulsions necessitate superb stability during storage, such as in emulsion-based therapeutic delivery systems. While in other cases, stabilized systems are entirely unwanted (e.g., separating oil and aqueous phases in enhanced oil recovery and bilge water applications).

Techniques for modulating emulsion stability are highly desired and are largely determined by the mechanics of the interfacially-trapped species. However, the utility of these techniques is often limited by difficulties in measuring and interpreting the rheological characteristics of complex fluid interfaces. Lack of control over interface formation during emulsification magnifies this problem, further obscuring relationships between interfacial rheology and bulk emulsion stability. Thus, the objectives of this research were to (1) elucidate these fundamental relationships through emulsion stability and interfacial rheological measurements, and (2) present innovative methodologies for modulating the kinetic stability of model oil-in-water emulsions using physical chemistry principles.

Objective 1 was addressed by studying correlations between the dilatational rheology of single- and multi-component oil-water interfaces and the susceptibility to coalescence of the bulk systems they comprised. Oscillating pendant drop tensiometry was used to probe interfacial viscoelastic behavior, while dynamic light scattering and optical microscopy were used to characterize coalescence susceptibility in bulk oil-in-water emulsions. The magnitude of the low-frequency dilatational elastic modulus was shown to positively correlate with oil droplet coalescence resistance over time. Objective 2 was addressed by analyzing how physical chemistry principles can be applied to control various emulsion droplet destabilization phenomena and produce desirable bulk behavior. To this aim, two emulsion destabilization studies were performed; one related to the nanoparticle-induced flocculation of oil droplets in a dilute, electrostatically-stabilized emulsion and one related to the convective flows generated by the asymmetric dilatational rheology of coalescing droplets.

The knowledge garnered from this body of work is highly relevant to academic and industrial emulsion formulators who seek inexpensive, yet robust methods for predicting, characterizing and tailoring the kinetic stability of oil-in-water emulsions.