Viscosity is a measure of a fluid’s resistance to flow, effectively describing how thick or sticky a liquid is. It quantifies the internal friction within the fluid as its layers move past one another. A fluid with low viscosity, like water, flows easily due to minimal internal resistance, while a fluid with high viscosity, like honey, flows slowly because of greater internal friction and thickness. The ability to measure viscosity accurately is essential in various industries for quality control and process optimization.
Full Scale Range
Viscometers operate within a defined range, and it is critical to record readings within 10% to 100% of the instrument’s torque capacity. The torque generated by the fluid’s resistance causes spring deflection in the instrument, which is translated into a viscosity value. This deflection gives specific viscosity ranges with high accuracy. Ensuring the sample is conditioned to suit the viscometer’s range is crucial for reliable measurements.
Sample Conditioning
Viscometers can measure fluids with varying degrees of flow, categorized as:
- Free Flowing (Low viscosity, small value): Fluids like water flow easily and require less force to measure.
- Slow Flowing (Medium viscosity, medium value): Fluids such as motor oils exhibit moderate flow behavior, demanding intermediate torque.
- Very Slow to Non-Flowing (High viscosity, big value): Thick fluids like honey or gels flow very slowly, requiring greater force to measure.
Units for Fluid Rheology
Viscosity can be measured using different methods, each with its own unit system:
- Dynamic (Rotational) Viscosity: Measured using rotational viscometers like those from Brookfield, the SI unit is mPa•s, with centipoise (cP) being the common unit used. Conversion is straightforward: 1 cP = 1 mPa•s.
- Kinematic Viscosity: Measured with capillary tubes or flow cups, the SI unit is mm²/s, and the common unit is centistokes (cSt). Conversion is 1 mm²/s = 1 cSt. To convert kinematic to dynamic viscosity, multiply by the fluid's density:
cSt x Density (g/mL) = cP or mm²/s x Density (g/mL) = mPa•s.
(This conversion applies only to Newtonian fluids.)
Common Mathematical Models for Fluid Behavior
Newtonian Fluids
Newtonian fluids exhibit consistent viscosity regardless of the shear rate or shear stress applied. This means the viscosity remains constant regardless of how much force or stirring speed is applied during measurement. Examples of Newtonian fluids include low molecular weight oils and water-soluble polymers with small molecular structures. Since their viscosity is unaffected by external forces, viscometer readings for these fluids are highly reliable and unaffected by changes in spindle speed or the spring of the instrument.
Non-Newtonian Fluids
In contrast, non-Newtonian fluids show varying viscosity depending on shear rate and shear stress, meaning the viscosity value is only accurate when specific experimental conditions are carefully controlled. Non-Newtonian fluids can exhibit both time-dependent and time-independent behaviors.
- Time-Dependent Behaviors:
- Thixotropy: Viscosity decreases over time under shear (e.g., certain gels and creams).
- Rheopecty: Viscosity increases over time under shear (e.g., some slurries and greases).
- Time-Independent Behaviors:
- Pseudoplastic (Shear-Thinning): Viscosity decreases as the shear rate increases (e.g., ketchup, paint).
- Dilatant (Shear-Thickening): Viscosity increases with higher shear rates (e.g., cornstarch in water).
- Plastic: These fluids require a yield stress to initiate flow (e.g., toothpaste, chocolate).
Mathematical Models for Fluid Flow
- Bingham Plastic: This modification of the Newtonian model introduces a yield stress, which must be overcome before the fluid begins to flow. It is frequently used in oil-field applications, particularly for calculating yield point (YP) and plastic viscosity (PV).
- Casson Model: A variation of the Bingham model, the Casson equation is often used to extrapolate yield stress from low shear rate data and is commonly applied in testing chocolate.
- Power Law Model: The most widely used model for water-soluble polymers. Xanthan gum, for instance, is a classic power law fluid, where viscosity decreases predictably with increasing shear rate.
Understanding the principles of viscometer operation and the behavior of Newtonian and non-Newtonian fluids is critical for achieving accurate and reliable viscosity measurements. Whether dealing with simple fluids like water or complex, non-Newtonian materials like gels or oils, viscometers provide essential data for ensuring product quality, optimizing production processes, and maintaining consistency in a variety of industries.