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Viscosity Terms and Definitions

Absolute Viscosity - The force needed for a liquid to overcome its internal friction and start to flow. Also known as dynamic viscosity.

Centipoise - A unit of measurement for dynamic viscosity equivalent to 1/100 of a poise. It is abbreviated cP, cps, cp and cPs.

Dilatant - Also described as shear thickening fluids, they are characterized by increased viscosity with increases in shear rate. In other words, the more you mix or stir these fluids, the thicker they become. Fluids containing suspended solids, such as some candies and sand/water mixtures are typical dilatant fluids.

Dynamic Viscosity - Also known as shear viscosity, defined as the resistance of one layer of a fluid to move over another layer. In other words, the amount of force needed to make a fluid flow at a certain rate.

dyne-cm - A unit of measurement traditionally used to measure surface tension. May also refer to torque in viscosity measurements.

Fluid - A fluid is a substance which deforms continuously under the application of a shear stress and can be either a liquid or gas.

Kinematic Viscosity - A measure of a fluid’s internal resistance to flow under gravitational forces. It is measured by determining the time in seconds required for a fixed volume of fluid to flow a known distance under gravity through an orifice of a calibrated viscometer at a controlled temperature. Typical instruments used are Zahn cups & Ford Cups of distinct types, as well as capillary viscometers.

KREBS Unit (KU) - One Krebs unit (KU) is the weight in grams that will turn a paddle type rotor, that is submerged in the sample, 100 revolutions in 30 seconds. It is typically measured using a Krebs Stormer type viscometer with a paddle spindle rotating at 200 RPM. It is commonly used in the paint and coatings industry. Krebs units can be converted to centipoise using ASTM D 562. These viscometers typically measure from 40-141 KU which is equivalent to 27-5274 centipoise.

milliPascal Seconds - A dynamic viscosity unit of measure for viscosity; abbreviated as mPa-s. 1 pascal second is equal to 1000 milliPascal-second (mPa-s).

Newtonian - Sir Isaac Newton assumed that all fluids at a given temperature exhibited the same viscosity, independent of the shear rate. In other words, twice the force would move a fluid twice as fast. We have since found this not to be the case - many fluids do change viscosity based on shear rate. But the viscosity of many fluids, such as water, remain constant regardless of shear rate. Hence, we refer to these fluids as Newtonian. Measuring Newtonian fluids is simple, as measured viscosity will be the same regardless of which spindle, speed, or viscometer is used.

Non-Newtonian - These fluids are those where viscosity changes as shear rate changes. When their shear rate is varied, their shear stress does not vary in the same proportion, and their viscosity changes, either higher or lower. In other words, when more force is applied to the fluid it will then thin or thicken, and flow slower or faster. This is sometimes referred to as shear thinning and shear thickening. There are many types of non-Newtonian behavior, including Pseudoplastic, Dilatant, Plastic, Thixotropic, and Rheopectic. See these terms for further explanations.

Plastic - Under static conditions this type of fluid behaves as a solid. Stress must be applied to the fluid for it to start flowing. This stress is the yield stress. An example of this type of fluid is ketchup, it will not normally pour from the bottle unless the bottle is shaken or hit with your palm. The amount of energy needed to start this flow is called the Static Yield. These fluids may also have Newtonian, pseudoplastic, or dilatant flow characteristics.

Poise - A dynamic viscosity unit of measure in the centimeter-gram-second system of units. 10 Poise (10 P) = 1 Pascal Second (Pa-s). Abbreviated as P.

Pseudoplastic - These fluids decrease in viscosity as force is applied. In other words, the more you stir these fluids the thinner they will become. Paint, nail polish, whipped cream, blood, milk, and quicksand are all examples of pseudoplastic fluids. Also known as shear thinning fluids. These are the most common non-Newtonian fluids.

Relative Viscosity - The viscosity value of a non-Newtonian material at a defined shear rate.

Rheology - The study of the deformation and flow of materials, especially non-Newtonian fluids.

Rheometer - A type of viscometer, rheometers measure the way in which liquids flow in response to varying applied forces. It is typically used with fluids that have complex viscosity characteristics that cannot be defined by a single viscosity value.

Rheopexy - A rare non-Newtonian liquid behavior where viscosity increases over time under a constant shear force. In other words, the longer a fluid is mixed or stirred the higher its viscosity becomes. Many rheopectic fluids will thicken or even solidify when shaken. Gypsum paste, as well as some lubricants, are examples of rheopectic fluids.

Reciprocal Seconds - Unit of measurement of shear rate. Also written as seconds-1.

Shear (liquid) - The relative motion between adjacent layers of a moving liquid. Shear forces act tangentially to a surface causing deformation.

Shear Rate - This is the rate at which a fluid is sheared during flow, also defined as the rate of change of velocity at which fluid layers move past each other. Shear rate is normally expressed in reciprocal seconds (1/s) or seconds -1. It is calculated with a viscometer by considering the spindle shape and rotational speed as it rotates in a sample container of fluid.

Shear Stress - Primarily caused by the friction between fluid particles, due to fluid viscosity. Defined as the force per unit area used to move a material. A shear stress is an example of a tangential stress, i.e., it acts along the surface, parallel to the surface. Friction due to fluid viscosity is the primary source of shear stresses in a fluid. When shear stress is applied to a fluid at rest fluid, the fluid cannot remain at rest but will move because of the shear stress.

Static Yield - The amount of force/torque needed to initiate flow of a material at rest. For example, the amount of force needed on a bottle of ketchup to start it flowing from the bottle.

Stormer Viscometer - Defined in ASTM D562, a Stormer type viscometer uses a paddle type spindle rotating at 200 rpms. They are the most widely used viscometer type for paints and coatings viscosity testing.

  • Stoke - A Kinematic unit of measure that can be expressed in terms of centistokes (cS or cSt); 1 stokes = 100 centistokes = 1 cm2 s-1 = 0.0001 m2 s-1. One stoke is equivalent to one poise divided by the density of the fluid in g/cm3.

Thixotropy - These fluids decrease in viscosity when subjected to constant shear. For example, some gels become fluid when shaken or stirred, but revert to a gel state when shaking or stirring is stopped. This is a non-Newtonian shear thinning behavior that is highly time dependent, both for the shear thinning and to begin, as well as for the liquid to return to its previous state. Thixotropic behavior is time dependent and can occur in conjunction with other flow behaviors. It can also be observed with only with certain shear forces. Thixotropy is rare, but this behavior can be found in gelatins, shortening, greases, heavy printing inks, colloidal solutions, etc.

Torque - The rotational equivalent of linear force. In a viscometer this is measured as the amount of energy that the spindle needs to rotate a certain distance while immersed in a sample. Force (F) times Distance (r) = Torque. Distance is measured from the pivot point to the point where ethe force will act. The SI unit of torque is a in Newton-meter (N-m).

Viscometer or Viscosimeter - The first use of the word viscometer is said to have been in 1883, and its definition is an instrument with which to measure viscosity, typically liquids. In other words, it measures a fluid’s resistance to deformation under shear stress. In a typical Rotational Viscometer, a spindle moves through the sample fluid to measure the viscosity. Zahn cups and Ford cups are examples of viscometers where the sample flows through an orifice under the force of gravity, and viscosity is measured by determining the time in seconds necessary for a fixed amount of liquid to flow through a defined orifice size. Another type of viscometer is a bubble tube viscometer, which measures viscosity by measuring the time it takes for an air bubble to pass through a liquid in a tube.

Viscosity - A simple definition is that it is a measure of thickness, for example grease is thicker than water, and therefore has a higher viscosity. In a scientific sense, the viscosity of a fluid is a measure of its resistance to deformation at a given rate, or the resistance of a material to flow. Viscosity equals shear stress divided by shear rate.

Yield Stress - This is defined as the amount of force required for a material to start to flow. A practical example is a tube of toothpaste - the yield stress is the amount of energy required to start the flow of the toothpaste from the tube. Another example is squeeze bottles of mustard, mayonnaise, or ketchup - the yield stress is the amount of force needed to get them to flow from the bottle. This is a key factor to control when developing new foods and other consumer goods such as personal care products that will be packaged in a squeeze bottle. If the yield stress is too high the consumer will find it difficult to dispense the products, and if too low the material will flow from the packaging too fast.

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