Oil viscosity is crucial for maintaining your engine's health. A scientist has recently discovered its "absolute cosmic limit."

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      Vehicles depend on various lubricants to prevent their moving components from turning into costly debris, and this is primarily related to viscosity, which is the capacity of a material to flow without changing shape (referred to as shear). Lubricants must flow at different temperatures without getting too thin to effectively reduce friction between surfaces. Interestingly, there is a maximum viscosity limit that far exceeds the weights indicated on each motor oil bottle.

      In practical terms, this upper viscosity limit is virtually limitless. This is because when discussing higher viscosity values, the conversation shifts from familiar liquids to the rock-forming minerals that flow beneath the Earth at incredibly high temperatures. Masaki Yoshida, a professor at Japan's Ritsumeikan University, chose to determine the physical upper limit of viscosity and published his findings on this "absolute cosmic limit" in the Physics of Fluids journal last month.

      Ritsumeikan University

      Professor Yoshida derived a figure of 10^30±2 pascal-seconds (Pa s), a metric of dynamic viscosity that indicates a material's capacity to flow a specific quantity per second under a specified pressure. To give some context to this immense figure, water has a viscosity of approximately 10^-3 Pa s, while honey registers around 10^1 Pa s. A substance with such high viscosity flows so slowly that any measurable movement would exceed the Earth's age. Essentially, a material of this viscosity behaves rigidly, which is why it is typically not included in fluid-dynamics studies.

      The thought of an engine generating internal temperatures high enough to necessitate motor oil of such viscosity is quite frightening. The dynamic viscosity of motor oils is generally measured in millipascal-seconds (mPa s), which is one-thousandth of a Pa s. For non-winter oils, this value is divided by the oil's density (known as kinematic viscosity). In both instances, the weight number displayed on the bottle is established by measuring viscosity at various temperatures and compiling those measurements into an index.

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      To determine this "cosmic limit," the study drew on decades of geodetic data (related to the Earth's shape), laboratory experiments on rock deformation, and simulations based on what we know about geological processes that occur beneath the Earth's surface over millions of years. Since the materials with the highest viscosities are not lubricants but rather the foundational elements of rocks that flow during tectonic plate movements, they were the focus.

      What was the purpose behind this research? For scientists, it removes the commonly used placeholder of "infinite viscosity" in fluid-dynamics studies, establishing a definitive upper limit. However, it’s so high that it won’t impact more conventional research, which is why viscosity was often referred to as "infinite" previously. In a press release, Ritsumeikan University also stated that these findings might aid in understanding geological activity both past and present, as well as studying high-viscosity non-Newtonian fluids, which change viscosity when a force is applied (like ketchup, which thins when the bottle is shaken). It also serves as an intriguing reminder that the same principles that maintain engine functionality also sustain the Earth.