Rheology is the study of the flow or deformation of matter. Viscosity, specifically, describes that of a free-flowing fluid. Often, materials may exhibit both viscous (fluid) and elastic (solid) behavior—these are known as viscoelastic materials, which require more than a standard viscosity analysis for complete quantification of material behavior.
In general, viscosity is the quantification of the internal friction of a fluid. This is measured using a standard viscometer (or rheometer) by placing the analysis area of a spindle in contact with a sample and applying some stress either by rotation or oscillation of the spindle—this is called shear. The drag of the fluid against the spindle—torque required to produce some amount of shear stress—as well as the spindle speed, are measured by the instrument.
The viscometer measures this resistance and produces results reported in units of centipoise (cP) or milliPascal seconds (mPa-s). The size and shape of the spindle, rotational speed, sample container, and temperature of the fluid (25°C by default) all greatly affect a viscosity measurement, so it is important to have consistent analysis conditions across analyses and instruments. This testing plays a crucial role in the research, development, and process control of liquid and semi-liquid products.
When friction is applied, one plane of fluid moves relative to another plane, and this force is called shear. Viscous materials require greater shear than those less viscous. The definition of viscosity is illustrated in the figure below. If two planes of fluid, separated by dx, are moving in the same direction, at two different velocities, the force required to maintain dv is proportional to the velocity gradient, or:
In which, η is a constant, known as viscosity. The velocity gradient, represented as dv/dx, is the change in the speed of multiple layers of fluid relative to each other, which is defined as shear rate (γ). The F/A is the force per area, also known as the shear stress (τ). Viscosity can then be calculated as:
Viscoelastic materials pose a bit more of a challenge when trying to gain a full picture of material behavior. Classic viscosity measurements may give information about the viscous fluid properties, but won’t be able to give information about the elastic properties of such a material. In this case, oscillatory tests are preferred. An oscillatory test will partially rotate back and forth in a sinusoidal manner. The sample will take time to respond to the rotation with some amount of lag—a phase shift. The angle of the phase shift can determine at what shear rates viscous properties dominate the sample’s behavior, and what shear rates the elastic properties dominate sample behavior. This is useful for materials that go through periods of low strain, then periods of high strain—think of lotion sitting in a bottle, then being pumped through a nozzle for application.
Many industries use rheology testing as a quick and reliable way to analyze important factors that can affect the performance, quality, and/or efficiency of a product, both during manufacturing and for quality control testing of final consumer goods. For instance, the food industry uses viscosity measurements to determine or troubleshoot the appearance, texture or consistency of food products which are factors in a consumer’s sensory experience. Rheology is also used to determine how a material will move through production, such as moving viscous liquid through pipes or other equipment during the manufacturing process.
The pharmaceutical industry uses rheology to predict how products will behave during consumer use, such as how a medicine will flow, pour or spread. For example, how well an ointment will cover the skin, or how a cough syrup or other suspension will pour from a bottle.
Other industries also use viscosity measurements. The automotive industry uses measurements for engine lubricants or hydraulic systems. The printing industry tests the properties of semi-liquid inks. The paint and coating industries test coatings and finishes to improve their application properties. The cosmetic industry tests the flowability and spreadability of liquid makeup and creams to enhance coverage performance. In short, any product or industry which uses liquid in manufacturing or their final product has a use for determining and controlling viscosity.
Sample amount is not an easy question to answer, since the volume is dependent on the target viscosity. Please share your expected viscosity and we can guide you to the appropriate volume of test material.
For additional questions on your sample needs, please contact us to share specifics about your sample and options for suitable sample quantities.
PTL is capable measuring dynamic viscosity for liquids and semi-solids from 1 to 2 million cP.
Dynamic Viscosity is the ratio of shear stress to shear rate for a test material which is reported in Centipoise, cP.
Particle Technology Labs uses the Brookfield DV-II+ Pro Viscometer and the Anton Paar Modular Compact Rheometer (MCR) 302e to meet your viscosity and rheology needs.
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