Measuring a Critical Physical Property
Viscometers measure viscosity, the resistance of fluids to flow or stress. In common terms, viscosity is related to a fluid’s “thickness”—a physical property of great interest to manufacturers of liquids, slurries, and pastes. Viscosity is a critical characteristic of foods (e.g., dairy, honey, syrup, soft drinks), paints, cleaners, adhesives, polymers, fuel oils, and pharmaceuticals. Many industries use viscosity as an endpoint in the manufacture of liquid-formulated products.
Dozens of viscometer types are in use in academia, industry, and basic research, and these range in cost from about $100 for simple mechanical viscometers to $15,000 automated instruments. Rheometers, which measure viscosity and related properties, may cost as much as $200,000.
At the low end of the price range are U-tube and Cannon-Fenske tube viscometers found in schools, colleges, and some industries. Other simple designs include falling sphere, falling piston, rolling ball, and oscillating piston viscometers. These are not instruments in the truest sense; some require pourable fluids.
More sophisticated are vibrational viscometers used in process industries, and rotational viscometers. The latter operate on the principle that energy must be applied to a spindle or disk rotating within a liquid to overcome the resistance of that liquid. That energy is proportional to the fluid’s viscosity. Rotational viscometers do not require that the testing sample be pourable. For example, toothpaste cannot be tested in a tube viscometer because it does not “run,” but meaningful results can be obtained using paddle- or cylinder-type viscometers.
Higher-end viscometers may be connected to and operated through a computer, or readouts may be taken directly off the front panel display. Some industries favor one viscometer type. Paint and pigment industries prefer the Stormer viscometer, which uses paddles on a rotor submerged into the product. Similarly, resin labs use bubble viscometers, which measure the time it takes a bubble to emerge from varnishes, and petrochemical companies prefer the Stabinger viscometer, which employs a rotating cylindrical tube.
Viscometry is an old technique, as illustrated by the persistence of mechanical viscometers. But while manufacturers continue to finetune more-sophisticated electromechanical rotational viscometers, the underlying technology hasn’t changed much.
What is different, says Steven Colo, president at ATS RheoSystems (Bordentown, NJ), is user expertise. “Twenty years ago most users had an academic specialization in viscometry or rheology.” Today the specialists are gone and users tend to be generalists.
Mr. Colo views changing user demographics as an opportunity to provide support and expertise in instrument operation, sample testing, and data interpretation.
What viscosity tells you
In biotech and pharmaceuticals, the viscosity of protein and buffer solutions constitutes one of numerous quality checks; in beverages, viscosity is an indicator of sugar content and overall quality. Polymer scientists use viscosity to determine the concentration of plastics in acid solutions.
Viscosity measurements are usually conducted on dilute solutions and at varying concentrations. Measurement of the viscosity of polymer solutions at different strengths, for example, provides estimates of secondary properties such as intrinsic viscosity, molecular weight, and chain length.
Viscosity is related to solute concentration, which is what makes it one of the most useful physical measurements in research and product development. The viscosity of a liquid tends to rise with the concentration of solid solutes, as with sugar in water. But the viscosity of a blend of fully miscible and nonreacting liquids is usually somewhere between the viscosities of the components.
Rheometers are closely related to viscometers in that they measure viscosity and yield stress. Where viscometers determine a fluid’s “thickness” under native conditions, rheometers measure it as a function of applied shear or stress. In addition to viscosity, rheometers provide measures of modulus, shear modulus, “tan delta,” gel point, and curing profiles.
Rheology tends to study the mechanics of complex fluids that cannot be fully characterized solely by viscosity. Darren Wilson, product specialist at Anton Paar (Ashland, VA), refers to these materials as “non-Newtonian,” meaning their viscosity changes at different shear rates.
“Most samples are Newtonian, which means it doesn’t matter if you use a viscometer or a rheometer. But viscosity doesn’t tell the whole story with non-Newtonian fluids such as ketchup, mayonnaise, shampoos, and many types of paints and coatings.”
Useful viscometer/rheometer features that buyers should be aware of are temperature control, spindle rotational speed control, a range of sample holders, and ease of use. Temperature is critical, notes Mr. Colo, since viscosity generally rises as a fluid cools. Spindle rotation may also affect viscosity.
Mr. Wilson notes that sample size may be an issue when analyzing very expensive materials such as drugs or proteins, as is cost of ownership for highvolume applications. Another consideration is the instrument’s measurement range. “If you’re analyzing petroleum, from crude oil to gasoline, do you want to change out the capillary for each measurement, or use something that works all the way through?”
Angelo DePalma holds a Ph.D. in organic chemistry and has worked in the pharmaceutical industry. You can reach him at angelo@ adepalma.com.
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