RHEOLOGY OF LUBRICANTS PT1 #Oil Chat 97

Rheology is the science of flow and deformation of matter and describes the interrelation between force, deformation and time. Rheology is applicable to all materials, from gases to solids. Fluid rheology is used to describe the consistency of liquid matter by referring to viscosity and elasticity. Viscosity is the resistance to flow or thickness and elasticity relates to stickiness or structure.

Newtonian behaviour is a phenomenon that is firmly embedded in the science of Rheology and is also a key property that affects the performance of lubricants.

NEWTONIAN substances are matter of which the viscosity does not change with shear/stress or rate of flow. Typical examples of Newtonian fluids are water, alcohol and oil.

NON-NEWTONIAN materials are liquids or gels of which the viscosity does change when subjected to stress. Non-Newtonian fluids may thin down or thicken up when sheared or stressed as discussed below:

Thixotropy is the property of certain fluids and gels to become thinner when a constant force is applied, and after removal of the force the viscosity recovers fully to the initial state in a finite period of time. The higher the force that is applied, the lower the viscosity becomes. Thixotropy is a time-dependent phenomenon, as the viscosity of the substance must recover within a certain timeframe when the applied force is removed. Tomato sauce is a typical example. It is usually quite thick, leaving you frustrated waiting for it to run out of the bottle. In response you shake it, giving you that red shower that drowns the fries on your plate. Reason is that tomato sauce is thixotropic. Its thickness and viscosity decrease depending on for how long and how fast you shake it. When you allow the tomato sauce to settle for some time it “gels” again.

In some materials the structural strength/viscosity decreases while shearing but the viscosity does not fully recover after an appropriate rest period. It remains thinner than the initial state which indicates that the structure does not recover completely. A typical example of this behaviour is yogurt. After stirring, the viscosity of yogurt remains thinner than it was initially. Such substances may be classified as non-thixotropic.

Rheopexy is the rare property of some Non-Newtonian fluids to show a time-dependent increase in viscosity; the longer the fluid undergoes shearing forces, the higher its viscosity becomes. Rheopectic fluids, such as cream, thicken or solidify when stressed. Fresh cream usually flows readily, but when you shake or whip it long enough, its consistency changes, it stops to flow and eventually becomes solid. In summary, Rheopectic fluids thicken when subjected to shear forces.

In the next issue of OilChat we will discuss the significance of rheology in lubrication. If you have any questions about rheology (or any other lubricant related issues) in the interim, you are welcome to email us at info@bcl.co.za.