Viscosity is the quantification of how thick a fluid is, which is measured by applying friction. The principal of operation of a rotational viscometer is to drive a spindle immersed in sample through a calibrated spring. The drag of the fluid against the spindle is measured by the spring deflection, which is measured with a rotary transducer. The measurement range of a rotational viscometer (in centipoise or milliPascal seconds) is determined by the rotational speed of the spindle, the size and shape of the spindle, the container the spindle is rotating in, the temperature of the fluid (25°C default), and the full-scale torque of the calibrated spring. This testing plays a crucial role in 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 F/A=η dv/dx, 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 with respect 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 η=τ/γ=(shear stress)/(shear rate)×100.
To determine viscosity, a spindle is immersed in the sample container, which is kept at a specific temperature (25°C default). As the spindle rotates at a defined speed, the viscous drag of the fluid against the spindle causes spring deflection. The viscometer measures this resistance and produces results reported in units of centipoise or “cP”.
Many industries use viscosity 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. Viscosity is also used to determine how a liquid will move through production, such as moving viscous liquid through pipes or other equipment during the manufacturing process.
The pharmaceutical industry uses viscosity to predict how products will behave during consumer use, such as how a medicine will flow, pour or spread. For example, how fast an ointment will cover the skin, or how a cough syrup or other suspension will pour from a bottle.
Other industries that use viscosity measurements include the automotive industry, which 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 make-up 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.
PTL offers dynamic viscosity testing (Newtonian & Non-Newtonian) of liquids and semi-solids from 1 to 2 million cP. Two options are offered. One option tests for Newtonian material; three ascending torque measurements are included. The other option tests for Non-Newtonian material; a viscosity loop is performed which includes six ascending then descending torque measurements.