Rheology studies the deformation and flow of matter. Considering different common liquids like water, shampoo, and honey, they all have different flow behaviors. All the liquids are composed of molecules. In addition, many formulations, like paints, pharmaceuticals and mineral slurries contain particles. Molecules or particles can be from below a nanometer (e.g. solvent molecules) to several micrometers (mineral particles) in size. As the rheology of the material depends on the forces that arise from molecules and particles sliding along to each other, the particle size and shape affect the (bulk) rheological properties of the fluid.
In addition to bulk properties, many of the above-mentioned formulations are emulsions, typically between oil and water. The stability of the emulsions is determined by the stability of the interfacial layer between the two phases. This branch of rheology is called interfacial rheology.
Viscosity is a rheological property
Typically, when the term rheology is used, one immediately thinks of viscosity. Viscosity is a property of fluids that indicates resistance to flow. Different liquids have different viscosities. If we compare water and honey with each other, it is easy to say that honey is more viscous as it doesn’t flow as easily as water.
Some of the materials can react differently to different stresses. For example, we can think of a toothpaste that stays on a tube even if the lid is off but is still very easy to squeeze out when wanted. Because of the way stress can affect the material property, it’s important to test liquid flow in a way that represents the industrial process. Knowledge of bulk rheology behavior is important in processes where pastes and slurries are involved. These include for example shape-forming processes like extrusion and injection and coating processes like dipping and spraying.
Interfacial rheology studies the rheological properties of interfacial layers
Interfacial rheology is a branch of rheology that studies the flow of matter at the interface between gas and liquid or two immiscible liquids. Unlike in bulk rheology, the flow of the bulk of the matter is not of interest, in fact, it should be minimized in interfacial rheology studies. The purpose is to study the rheological properties of the interfacial layer between the two phases. The interfacial layer can be composed of any surface-active material such as surfactants, polymers or even nanoparticles.
If you are interested in examples of how interfacial rheology studies are utilized in different industrial sectors, please download the overview below.