Viscometers measure the viscosity—the thickness or stickiness—of a liquid resulting from internal friction. Viscometers typically measure the drag between a moving liquid across a surface, a property indicative of a host of industrially relevant properties like the lubricating capacity of oil, the protein concentration during a biomanufacturing process, or the wetting characteristics of a liquid.
Many types of viscometers are used in research and development settings. The most common varieties are orifice, falling/moving piston, rotational, falling ball, vibrational, and capillary, which operate by measuring vibration, fluid pressure, and Coriolis forces.
Capillary viscometers utilize a U-shaped glass tube with bulbs at either end—one higher than the other. The fluid’s viscometry is related to how long a defined quantity of fluid takes to pass a defined distance through the tube (of a defined diameter).
Capillary viscometry works only on Newtonian fluids—those that exhibit ideal flow behavior; viscosity, as a definable characteristic of a liquid, is a subset of the more-encompassing science of rheology, which is the measure of how materials flow. As such, the term viscosity is normally applied to pure liquids (e.g., ethanol, water), whereas rheology is a characteristic of complex mixtures (yogurt, paint).
For Newtonian fluids, the relationship between stress and shear rate is constant, or at least consistent, which holds direct implications for the real-world reproducibility of viscometry measurements.
Take lubricants, a significant component of the viscometry marketplace. The normal operation of machinery and large equipment relies on lubricants, but during normal operation, the composition of lubricants changes.
“Soot, carbon, glycol, particulates, refrigerant gases, water, and other contaminants entering lubricants during service reduce end users’ ability to obtain consistent or repeatable viscosity measurements,” says Dan Walsh, director of technical support at Spectro Scientific (Chelmsford, MA). In other words, lubricants exhibiting Newtonian viscosity off the shelf may not exhibit that property after a week or month of use. “Users need to be aware of this when acquiring a viscometer.”
Modern lubricants themselves, out of the bottle, may not exhibit Newtonian behavior either. “The base stock ingredient, a petroleum product, shows ideal viscosity, but manufacturers add components to enhance or inhibit some property of the main ingredient,” Walsh adds, “and these may contribute to non- Newtonian behavior.”
Unlike most analysis modes where the sample is isolated from the innards of the instrument, with capillary viscometers, the sample compartment is the instrument. Cleaning, therefore, becomes critical for achieving reproducible and accurate results. Joseph T. Mastropierro, deputy laboratory technical director at Cannon Instrument Company (State College, PA), stresses the need for thorough, careful cleaning. “Draining as much of the test material from the viscometer as possible before cleaning is critical to accelerating the cleaning process,” he says. “This can be tricky when the viscosity of the test material is high, as drainage can take time.” Raising the temperature can lower the test material’s viscosity and help it drain more efficiently.
“The second critical step is selecting a cleaning solvent that will dissolve the material being tested. After adding solvent to the viscometer, allow adequate soak time to assure complete dissolution and removal of the test material. A second volatile, low-viscosity solvent can help assure cleanliness. After solvent cleaning, allowing a suitable time for air drying helps assure that removal of residual solvent is complete.”
Inadequate cleaning can drastically affect the quality of subsequent viscosity tests. “Erroneously higher or lower results are possible, depending on the viscosity difference from the previous test sample to the current test sample, or the cleaning solvent utilized,” Mastropierro adds.
Capillary viscometers are reliable and time-tested, but users should select this instrument type only after considering their test fluid and the conditions under which its viscosity will be measured. “Don’t just consider the analysis,” cautions Walsh, “also think about sample preparation and any interferences that may be present or develop over time.”