What’s a Pseudoplastic?
March 9, 2024 - If you want to travel safely in remote areas, you must prepare. This round of preparation was automotive liquids. This means a small supply of windshield washer fluid, motor oil, and transmission fluid along with the items we already had packed some of which are not automotive related but need leak-proof storage. This includes wasp spray, soap, slime tire repair (green label), rags, a funnel and similar stuff.
We were headed into town, so we planned a stop at the local automotive shop. But I had questions. What does 0W-20 actually mean?
The “0” and “20” refer to the viscosity of the oil. The “W” means winter. If you’re not familiar with viscosity, the simplified definition is, a measure of a fluid’s resistance to movement. The larger the number, the greater the resistance. In our case, the “0” indicates the oil has a very low viscosity under winter starting conditions. As the oil moves around the engine the oil becomes more viscous until it reaches “20” under normal running conditions.
That’s interesting because most fluids (Newtonian fluids) become less viscous when heated. Motor oil is designed to become more viscous. But that isn’t the whole story. It isn’t actually the heat that determines the viscosity of the motor oil but the mixing, or more technically, the shear stress within the fluid. This type of fluid is called a pseudoplastic fluid. There are two types, shear-thinning and shear-thickening. Motor oil is shear-thickening.
So why go to the trouble of manufacturing a fluid designed to get thicker with use? When your engine starts, it’s cold and not well lubricated. All the oil is down in the oil pan with the oil molecules randomly oriented. Ideally on startup the oil would immediately be disbursed throughout the engine to lubricate it. A thin oil will help to lubricate all the engine parts quickly and seep into every groove. The problem is if it is too thin, the oil will quickly drain back to the oil pan. Therefore, the oil is designed to resist flow, to a point, as it continues to be pumped around the engine. That way it will stay where it’s needed, up in the engine.
You will notice above, I said the oil molecules are randomly oriented when at rest in the oil pan. With pumping or mixing, the oil molecules are forced to move around each other (increasing shear) but this also causes the chain molecules to become align in planes which can maximize the tension between molecules (shear-thickening) as in the case of motor oil. The other type of pseudoplastic exhibits a decrease in viscosity or (shear-thinning) under the same conditions. That type would not be good for motor oil.
As your engine runs, the oil molecules (chains) begin to break down and dirt interrupts the interaction between molecules. Extreme hot, cold, or dusty environments will make the oil break down faster require more frequent oil changes. Typically, the recommended mileage between synthetic oil changes is 7,500 to 10,000 miles even though the label says “20,000 miles guaranteed.”
We’re summer travelers to cool locations on dusty roads. For a long trip, we should plan to get an oil change before we leave and maybe even while we are still on the road. For good maintenance on the road we will need to routinely check our oil level and color. The oil we are buying today is only to top off if needed. If we need an oil change, we’ll take it to a shop. Where on the road do you responsibly dispose of used oil?
Different engines and driving conditions require different oil. Check your owner’s manual, but now you know what those number mean and why someone would choose one oil over another.
If you want to experiment with pseudoplastics, you probably have some around your house including cornstarch, ketchup, whipping cream, paint, nail polish, and plant oils like coconut or palm.
Place equal parts of cornstarch and water in a small cup and mix. It will get thicker as you mix. Then try mixing ketchup or mayonnaise. At first it will cling to your spoon almost like a solid and not want to flow. Once you start mixing it will seem thinner. Compare those to mixing water. Water’s viscosity won’t change.
Fun with science!
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