Interfacial rheology is a special branch of rheology that involves studying the unique two-dimensional systems formed at interfaces. Just as rheology is the study of flow in bulk fluids, interfacial rheology is the study of the flow properties of liquid interfaces. These flow properties are important in determining, for example, the behavior and stability of suspensions, emulsions, froth and foams.
Emulsion and foam stability, bubble and micelle formation, breakage and fusion and interfacial reactions are largely affected by the rheological properties of the interface. For industries utilizing emulsions and dispersions, such as coatings, food, oil and chemical industries, interfacial rheology can be key when developing and improving processes and products. Most biochemical reactions in nature occur at or in interfaces, such as cell walls and other membranes, and understanding the rheology is one factor in understanding and mimicking the biological system.
Interfacial rheology enables the study of surfactant kinetics, and the viscoelastic properties of the adsorbed interfacial layer have been shown to correlate with emulsion and foam stability. Surfactants and surface-active polymers are commonly used to stabilize emulsions and foams in, for example, the food and cosmetics industries. Surfactants have an amphiphilic character, which means that they consist of hydrophilic and hydrophobic parts. In the adsorption process, the surfactant molecules orientate themselves so that the hydrophilic head is in water and the hydrophobic head is in oil or vapor (see figure). Also, many polymers, like proteins, are surface active. They tend to adsorb at the interface, where they may change the conformation and influence on interfacial properties.
There two main methods for measuring interfacial rheology: dilatational and shear methods. These methods cannot be directly compared, but they complement each other. Interfacial rheology measures the response of the adsorbed interfacial layer to the deformation. Deformation of the interfacial layer can be caused either by changing the layer size or shape, and this corresponds to elasticity and viscosity of dilation and shear, respectively.
In dilational interfacial rheology, deformation is typically caused by dilation and compression of a hanging droplet.
Using oscillating barriers in a KSV NIMA Langmuir trough is also one way of measuring dilatational rheology. The benefit of the Langmuir trough is that surface packing before the oscillation can be controlled.
In interfacial shear rheology, a probe is dragged at the monolayer causing changes in the shape. This can be performed by a rotating ring/cone lowered to the interface and attached to a drive shaft, or by moving a light needle-like probe in a capillary on the interface by a magnetic field as with KSV NIMA ISR (see figure). The mass of the probe affects the sensitivity of the measurement tremendously, and the rotating ring has been shown to work with proteins with mass of several tens of kDa. The needle probe has been shown to be much more sensitive and capable of measuring even surfactants of less than 0.3 kDa. When the rheometer is integrated to a Langmuir trough it is also possible to control the surface packing of the monolayer, and, for example, experiment with conditions that are analogous to biological interfaces. The needle-like probe is suitable for long measurements as the probe moves naturally along the interface despite of evaporation, and does not require height adjustment of the probe.
KSV NIMA ISR involves using a magnetic probe to create shear deformation on the monolayer.
Prediction of emulsion, froth and foam stability
Viscoelasticity of an interface can predict the stability of a complex fluid. Micelle/droplet fusion and fission are largely dependent on the interface viscoelasticity.
Determination of thin film structure
The presence of networking, hydrogen bonding and other interactions can be detected from the viscoelastic behavior of films.
Examination of phase transitions
Phase transitions in a monolayer and thin film sometimes result in a change in the rheological properties of the layer.
Real time monitoring of surface reactions
Surface gelation, network formation and protein denaturation at interfaces can be detected from the changes in the viscoelastic properties in the interface.