The study of matter flow characteristics is the basis for the science of rheology. In dentistry rheology is important, because during insertion and placement of many dental materials the dentist uses the rheological properties of a material. For example, a restorative material should be fluid to adapt to a cavity wall but also firm enough to retain its shape after the dentist has shaped the material.
Most liquids, wwhen placed in motion, resist imposed forces that cause them to move. This resistance to motion is called viscosity, and is controlled by internal forces within the liquid. Thus, viscosity is a measure of the consistency of a fluid and its ability to flow. For example, syrup has a higher viscosity than water at room temperature.
The viscosity of a material can be determined by measuring the shear force needed to move a surface located at a certain distance from another surface when the liquid material is located between the two surfaces. By relating the surface of the area over which the shear force acts to the shear force, we can meassure the shear stress in the liquid. By increasing the speed with which we move the surface we increase the shear rate. If the shear stress increases linearly with an increase in shear rate and the linear relationship intercepts the shear stress axis at zero when the shear rate is zero, we say that the liquid is Newtonian.
The rheological behavior can be described as being either plastic, dilatant, Newtonian and pseudoplastic. Plastic behavior means that there is a certain shear stress level that is needed to initiate very small shear rate. A dilatant medium increases the shear stress more than a Newtonian liquid as the shear rate increases, while a pseudoplastic liquid do not increase the shear stress as fast as a Newtonian liquid does as the shear rate increases. Many dental materials are pseudoplastic in their liquid stage.
The behaviors of the different fluids are shown below.
