Particles in Interfaces and the Stabilization of Colloidal Multi-Phase Systems

Colloidal multi-phase systems, such as foams and emulsions are ubiquitous in our daily life, existing from beer and milk to bubble baths; they also play important roles in industrial processes like enhanced oil recovery, cosmetics and food industries. Long-term stability is desirable for these systems, but often difficult to achieve. Colloidal multi-phase systems are thermodynamically unstable due to liquid interfacial tension and will separate into distinct phases given enough time. Therefore stabilizers such as surfactants, which lower interfacial tension and raise interfacial shear forces, are commonly added to stabilize them kinetically.

Colloidal particles with appropriate wettability can strongly adsorb to liquid interfaces and stabilize the large internal interface of emulsions, foams, and bijels. Estimates of the reduction in interfacial free energy due to particle adsorption allow for qualitative predictions of both the morphology and the stability of the particle-decorated interface. These predictions depend crucially on the contact angle of adsorbed particles with the interface. We discuss an experimental method of determining this contact angle that avoids commonly encountered artifacts, and we demonstrate how data obtained this way can be used for accurate predictions of the interfacial morphology and stability in a variety of different multi-phase systems.