![]() ![]() Engine oil is truly an engineered fluid since you want it to be thin when you start your engine but not get too thin as it heats up. Motor oil is not thixotropic but its viscosity does change with temperature. One of the most common references to viscosity is when we go to change our motor oil. Degree Engler goes back further than Saybolt Seconds and is basically a ratio of the time it takes for 200 ml of a fluid to flow through a tube relative to water. Since 60 ml is only a couple of ounces and the viscosity of water is 31 SSU, that calibrated tube must be pretty small in diameter. Voila, the time it takes for the fluid to pass through the tube in seconds is your viscosity in SSU. Basically, 60 milliliters of fluid is placed in the device and allowed to flow through a calibrated tube. The test method utilizes a viscometer developed by George Saybolt and utilized by Standard Oil starting in the 1880s. In classic imperial fashion, the test method is unique unto itself. For reference, the SSU (Saybolt Second Universal) viscosity of water is about 31 SSU. ASTM D88 is the empirical method for determining the Saybolt Universal viscosity, specifically for petroleum products. Stepping outside the realm of metric units we have a couple of units that involve test procedures developed by ASTM (American Society for Testing and Materials). The list includes units like Poise, Stokes, Saybolt Seconds Universal and Degree Engler. ![]() There is a whole plethora of units used to describe viscosity, or how thick a fluid is. ![]() It is measured in mass per unit area at a certain rate, known as the rate of shear deformation. Viscosity is the measure of a fluid’s resistance to flowing at a given rate. Just like materials change their physical state based on temperature, going from liquid to solid to gas fluids like ketchup and pudding change their physical viscosity (thickness or thinness) based on how much they are agitated. Thixotropy arises because the particles in the fluid require time to re-organize. One of the most fascinating things about a thixotropic fluid is that it can return back to its more viscous state. Put simply, after a thixotropic fluid starts to move it gets thinner, its viscosity decreases and it starts to flow more freely. Thixotropy is defined as a time-dependent shear thinning property. As you sling it around you notice that it thickens up again, maybe even caking up on the wall or your parent’s clothes.Ī thixotropic fluid is one that exhibits characteristics of thixotropy. As you play with your food, trying to coordinate the use of a spoon something may have put in your hand, you may have mixed it around or slung a bit here and there. Your caregivers may have put a bowl of pudding-like food in front of you. Think back to your own formative years, sitting in a high chair with a bib. I’d like to meet the person who came up with that idea and kindly ask, “What were you thinking?” How many of us would sit in diners hitting the bottom of the bottle with the heel of our hand or shake it aggressively, finally giving up and jabbing a butter knife into the neck and making a big ketchupy mess? Whoever decided to put the ketchup in the bottle and play that joke on us may or may not have realized that ketchup is a special type of fluid known as thixotropic, meaning it gets thinner as it is shaken.Īnother thixotropic fluid, although less common table fare than ketchup, is pudding. Some of us may remember a day when ketchup was placed in thick glass bottles with skinny necks. ![]()
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