Cars rely on a variety of lubricants to keep their moving parts from expensive shine, and it all depends on viscosity, the ability of a material to flow without deforming (known as shear). To do its job of reducing friction between surfaces, the lubricant must be able to flow at different temperatures without becoming too thin. And as it turns out, there is an upper limit to viscosity that far exceeds the weight listed on each bottle of motor oil.
Practically, the upper limit of viscosity is unlimited. That’s because when you start looking at higher values for viscosity, the discussion moves away from recognizable fluids to the rock-forming minerals that flow beneath the planet’s surface under ridiculously high temperatures. Professor Masaaki Yoshida of Ritsumeikan University, Japan, nevertheless decided to find the physical upper limit of viscosity, and published findings on this “absolute cosmic limit”. physics of fluids magazine last month.
Professor Yoshida came up with the number 1030±2 Pascal-second (Pa s), a measure of dynamic viscosity that expresses the ability of a material to flow a certain amount of fluid per second under a certain amount of pressure. To put that very large number into perspective, the viscosity of water is about 10-3 near, while the viscosity of honey is about 101 Near. This viscous substance flows so slowly that the timescale of any measurable motion exceeds the age of the Earth. In practice, such materials are rigid, which is why it is not usually the subject of fluid-dynamics studies.
I would be a little intimidated by an engine that could generate such high internal temperatures that a sticky motor oil would be required. Dynamic viscosity for motor oils is usually measured in millipascal-seconds (MPa or equivalent), which is one thousandth of a millipascal. For non-winter weight, that value is divided by the density of the oil (known as kinematic viscosity). And in both cases, the weight number you see on the bottle is determined by measuring the viscosity at several temperatures and creating an index from those measurements.
To arrive at that “cosmic limit,” the study relied on simulations based on decades of geologic data (related to Earth’s shape), laboratory rock-deformation experiments, and knowledge of geological processes occurring beneath Earth’s surface over millions of years. Because the materials exhibiting the highest levels of viscosity are not lubricants, they are the building blocks of rocks, which flow when tectonic plates move.
What did all this mean? For researchers, this eliminates the “infinite viscosity” placeholder often used in fluid-dynamics studies, providing a solid upper bound. Granted, this is so high that it doesn’t impact much conventional research, which is why scientists first described viscosity as “infinite.” In a press release, Ritsumeikan University also claimed that the findings could be useful in studying past and present geologic activity, as well as high-viscosity non-Newtonian fluids, whose viscosity changes when subjected to force (like ketchup, which becomes thinner when the bottle is shaken). It’s also a good reminder that the same principles that keep your engine alive also keep the planet alive.
