R&D has responsibility for developing test methods that qualify materials for acceptability in manufacturing. The physical property that addresses flow behavior is viscosity, which is a measurement of resistance to flow. For liquids this seems reasonably obvious, but for semi-solid materials, like margarine, butter, jams, etc. the word “spreadability” may seem more appropriate. Observing and quantifying the way in which materials change shape during deformation is all part of the viscosity measurement world. The question is what type instrument to use when evaluating these “thicker” materials that do not flow readily.
Standard bench top viscometers and rheometers use a rotating spindle immersed in the fluid to make the viscosity measurement. Resistance to rotation at different speeds is measured as discreet torque values and converted mathematically into scientific units called “centipoise”. The resulting graph from this test looks like the display on the instrument in Figure 1. Viscosity values are recorded on the y-axis vs. rotational speeds on the x-axis. Non-Newtonian materials exhibit a decrease in viscosity as rotational speed increases. This type of flow behavior is referred to as “pseudoplastic” or “shear thinning”.
Non-flowing materials present more of a challenge when measuring viscosity because the spindle digs a hole when rotating in place. Once the material moves away from the spindle, it does not recover and leaves a void in the space adjacent to the spindle. Viscosity readings decrease quickly and there is no practical way to record data that is meaningful. Choosing a different spindle geometry is the solution.
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