Microrheology of complex fluid-fluid interfaces.

When fluids such as air, water and oil come in contact with each another, they often remain separated while interacting through an interface. Examples of this are a simple air-over-water system and various oil and water mixtures. More complex examples include foams, emulsions and biological membranes. The behavior of these systems is dependent on the properties of the interfaces. For example, adding soap to water changes the properties of an air-water interface, allowing for the formation of stable foams, or soap suds. Interfacial rheology is a technique that allows us to quantify these properties. The result of this technology has been an increased understanding of the importance of interfacial properties in natural and industrial systems. One area where interfacial rheology will advance scientific understanding is in the study of cellular membranes. Methods have been developed that allow interfacial rheology to be applied to microscopic systems, creating the possibility of studying biological membranes in living cells. Unfortunately, measurements from these microrheological techniques do not agree with measurements from more established methods for studying large systems. The disagreement in results from micro- and macrorheological techniques represents a significant gap in our understanding. There are a few potential explanations for this discrepancy and testing those will require a combination of experimental equipment and the development of new experimental techniques.