Key Definitions:
Term | Definition |
|---|---|
Rheology | Study of how materials flow and deform during mixing, pouring or moulding |
Viscosity (η) | Resistance to flow, calculate by shear stress/shear rate |
Shear stress | Parallel force per unit area applied to a fluid |
Shear rate | A measure of how fast fluid layers move past each other. Calculated by dividing the velocity of one fluid layer by the distance to an adjacent fluid layer |
Newtonian fluid | Constant viscosity independent of shear rate |
Plastic (Bingham) behaviour | Requires a yield (starter) stress before flowing, then flows Newtonian |
Shear thickening (dilatant) | Viscosity increases as shear rate increases |
Shear thinning (pseudoplastic) | Viscosity decreases as shear rate increases |
Thixotropy | Becoming less viscous when subjected to an applied stress |
Viscoelasticity | Time-dependent combination of elastic (spring-like) and viscous responses; depends on load magnitude and duration |
Thermal conductivity | Rate of heat flow through a material per temperature gradient |
Specific heat | Heat needed to raise the temperature of 1g of material by 1 °C |
Thermal diffusivity | Speed at which temperature changes propagate |
Linear coefficient of thermal expansion | Fractional length change per °C |
Rheology
Dental materials are almost always patient-specific and are processed chairside or in the lab (mixed, syringed, packed, or moulded) before they harden. Predictable handling and placement therefore depend on rheology: how materials flow under shear. Many liquids are Newtonian (single viscosity value), but dental materials often show non-Newtonian behaviour.
Non-Newtonian flow behaviours
Some systems need a yield stress before moving (plastic), others thicken as shear increases (dilatant), while many useful dental pastes thin under shear (pseudoplastic), which aids syringeability and adaptation (e.g., silicone impressions). Certain fluids are thixotropic: their viscosity drops with continued mixing and recovers at rest which is important for consistent placement.
Viscoelasticity and Impression handling
Set or semi-set materials frequently combine elastic (spring-like) and viscous (dashpot-like) responses, so strain and recovery depend on load magnitude and duration. For elastomeric impressions, brief loading and a swift “snap” removal minimise permanent deformation and improve accuracy.
Thermal Considerations in the mouth
Oral temperatures can swing from ~5–60 °C. Thermal conductivity, specific heat, and especially thermal diffusivity govern how fast heat reaches the pulp; liners/bases and low-diffusivity materials help buffer extremes. Mismatch in the coefficient of thermal expansion (α) between restorations and tooth tissues can open margins or stress interfaces; alloy–investment and metal–ceramic pairs must be α-compatible to avoid defects.