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 in key position when developing and improving processes and products. Most of biochemical reactions in nature happen in interfaces, such as cell walls and other membranes, and understanding the rheology is one factor in understanding and mimicking the biological system.
There two main methods for measuring interfacial rheology, dilatational and interfacial methods. Dilatational and shear rheology cannot be directly compared, but they complement each other. In dilatational measurements the surface concentration is changing during the measurement, introducing a new factor of complexity to the measurement. So also relaxation, absorption/desorption and solubility affect the measurement, but in some cases this is also desirable.
Oscillating barriers in a Langmuir trough are one way of measuring dilatational rheology, another one being oscillating drop. 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. This can be performed by a rotating ring/cone lowered to the interface and attached to a drive shaft, or by moving a light needlelike probe in a capillary on the interface by a magnetic field. 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, and for example do experiments with conditions that are analogous to biological interfaces.
• 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 among other things.
• 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 some time 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.
• Continuous monitoring of molecule adsorption into interfaces
lysosyme application note, small molecule note