This article explores viscosity terms and definitions, unraveling the intricacies of fluid behavior.
Absolute Viscosity - The force required for a liquid to overcome its internal friction and commence flowing, also known as dynamic viscosity.
Centipoise - A unit of dynamic viscosity measurement equal to 1/100 of a poise, abbreviated as cP, cps, cp, and cPs.
Dilatant - Also termed shear thickening fluids, these exhibit increased viscosity with a rising shear rate. In simpler terms, the more you mix or stir these fluids, the thicker they get. Typical dilatant fluids include those with suspended solids, such as certain candies and sand/water mixtures.
Dynamic Viscosity - Also referred to as shear viscosity, it signifies the resistance of one layer of a fluid to move over another layer. It indicates the force needed to make a fluid flow at a specific rate.
dyne-cm - A traditional unit of measurement for surface tension and may also denote torque in viscosity measurements.
Fluid - A substance that continuously deforms under shear stress, which can be either a liquid or gas.
Kinematic Viscosity - A measure of a fluid’s internal resistance to flow under gravitational forces. It is determined by the time required for a fixed fluid volume to flow a known distance under gravity through a calibrated viscometer's orifice at a controlled temperature. Common instruments include Zahn cups, Ford Cups, and capillary viscometers of various types.
KREBS Unit (KU) - One Krebs unit (KU) equals the weight in grams that turns a paddle-type rotor submerged in the sample, 100 revolutions in 30 seconds. Typically measured with a Krebs Stormer type viscometer, using a paddle spindle rotating at 200 RPM, especially in the paint and coatings industry. Krebs units can be converted to centipoise using ASTM D 562, measuring from 40-141 KU equivalent to 27-5274 centipoise.
milliPascal Seconds - A unit of dynamic viscosity, abbreviated as mPa-s, where 1 pascal second equals 1000 milliPascal-seconds (mPa-s).
Newtonian - Sir Isaac Newton assumed that all fluids at a given temperature exhibit the same viscosity, irrespective of shear rate. Despite this assumption, many fluids change viscosity based on shear rate, but certain fluids like water maintain a constant viscosity regardless of shear rate. Such fluids are Newtonian, making their viscosity measurement straightforward, as it remains the same regardless of spindle, speed, or viscometer used.
Non-Newtonian - These fluids experience viscosity changes as the shear rate varies. Their shear stress does not vary proportionally, leading to changes in viscosity, either higher or lower. Applying more force can cause these fluids to thin or thicken, flowing slower or faster. This behavior is sometimes referred to as shear thinning or shear thickening. Various types of non-Newtonian behavior include Pseudoplastic, Dilatant, Plastic, Thixotropic, and Rheopectic.
Plastic - Under static conditions, this fluid type exhibits a solid-like behavior. Stress must be applied for the fluid to initiate flow, known as yield stress. An example is ketchup, which typically requires shaking or a firm hit to the bottle for pouring. The energy required to initiate this flow is termed Static Yield. These fluids may also demonstrate Newtonian, pseudoplastic, or dilatant flow characteristics.
Poise - A unit of dynamic viscosity in the centimeter-gram-second system, with 10 Poise (10 P) equaling 1 Pascal Second (Pa-s). Abbreviated as P.
Pseudoplastic - These fluids reduce in viscosity with applied force, meaning the more you stir them, the thinner they become. Examples include paint, nail polish, whipped cream, blood, milk, and quicksand. Also referred to as shear thinning fluids, they are the most common non-Newtonian fluids.
Relative Viscosity - The viscosity value of a non-Newtonian material at a specified shear rate.
Rheology - The study of the deformation and flow of materials, particularly non-Newtonian fluids.
Rheometer - A type of viscometer, rheometers measure how liquids flow in response to varying applied forces. Typically used for fluids with complex viscosity characteristics that a single viscosity value cannot define.
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. Examples include gypsum paste and some lubricants, with rheopectic fluids thickening or solidifying when shaken.
Reciprocal Seconds - A unit measuring shear rate, also denoted as seconds-1.
Shear (liquid) - The relative motion between adjacent layers of a moving liquid, where shear forces cause deformation tangentially to a surface.
Shear Rate - The rate at which a fluid is sheared during flow, also defined as the rate of velocity change as fluid layers move past each other. The shear rate is usually expressed in reciprocal seconds (1/s) or seconds-1. Calculated using a viscometer considering the spindle shape and rotational speed as it rotates in a sample fluid container.
Shear Stress - Primarily induced by the friction among fluid particles due to fluid viscosity. It is the force per unit area required to move a material. Shear stress represents tangential stress, acting along the surface and parallel to it. The primary source of shear stresses in a fluid is friction due to fluid viscosity. When applied to a fluid at rest, shear stress causes the fluid to move as it cannot remain stationary under the influence of the shear stress.
Static Yield - The force or torque necessary to initiate the flow of a material at rest. For instance, the force required on a ketchup bottle to start the flow from the bottle.
Stormer Viscometer - ASTM D562 defines a Stormer-type viscometer that utilizes a paddle-type spindle rotating at 200 RPMs. It is the most widely used viscometer type for testing the viscosity of paints and coatings.
- Stoke - A kinematic unit of measure expressible in terms of centistokes (cS or cSt); 1 stoke equals 100 centistokes, equivalent to 1 cm2s-1 or 0.0001 m2s-1. One stoke is equal to one poise divided by the density of the fluid in g/cm3.
Thixotropy - These fluids reduce viscosity under constant shear. For instance, certain gels become fluid when shaken or stirred but return to a gel state when the agitation stops. This non-Newtonian shear-thinning behavior is highly time-dependent for both the onset of shear thinning and the return to the initial state. Thixotropic behavior, which is rare, can be observed in gelatins, shortening, greases, heavy printing inks, colloidal solutions, etc.
Torque - The rotational equivalent of linear force. In a viscometer, it is measured as the energy required for the spindle to rotate a certain distance while immersed in a sample. Torque is calculated as Force (F) times Distance (r), where distance is measured from the pivot point to the point where the force acts. The SI unit of torque is Newton-meter (N-m).
Viscometer or Viscosimeter - The term "viscometer" is believed to have originated in 1883, defining an instrument used to measure the viscosity of liquids. Essentially, it gauges a fluid’s resistance to deformation under shear stress. In a Rotational Viscometer, a spindle moves through the sample fluid to assess viscosity. Viscometers like Zahn cups and Ford cups allow the sample to flow through an orifice due to gravity, measuring viscosity by calculating the time for a fixed amount of liquid to flow through a defined orifice size. Another type, the bubble tube viscometer, measures viscosity by timing the passage of an air bubble through a liquid in a tube.
Viscosity - Simply put, viscosity denotes a substance's thickness; for instance, grease is thicker than water and therefore possesses higher viscosity. In scientific terms, it quantifies a fluid's resistance to deformation at a specific rate or the material's resistance to flow. Viscosity is calculated as shear stress divided by shear rate.
Yield Stress - This represents the force a substance needs to initiate its flow. For instance, consider a tube of toothpaste—yield stress indicates the energy needed to start the toothpaste's flow from the tube. Similarly, in squeeze bottles of mustard, mayonnaise, or ketchup, yield stress signifies the force required for them to flow from the container. This factor is important in developing new food and consumer goods, especially those packaged in squeeze bottles. If the yield stress is excessively high, consumers may struggle to dispense the products; conversely, if it is too low, the material might flow too rapidly from the packaging.
This information has been sourced, reviewed and adapted from materials provided by Paul N. Gardner Company, Inc.
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