A Ritsumeikan University physicist has proposed a practical upper limit to viscosity, using Earth’s deep rock layers to estimate when matter effectively stops flowing.
The University of Queensland’s pitch drop experiment, famous for taking about a decade for a single drop to fall, has a viscosity of roughly 2.3 times 10 to the 8th power Pascal seconds. Professor Masaki Yoshida’s proposed ceiling sits at 10 to the 30th power, give or take two orders of magnitude, meaning pitch, the slowest flowing substance most people have heard of, is still trillions of times short of the limit.
Viscosity describes how easily a material flows, and it governs everything from water moving through a pipe to molten rock shifting deep inside the planet. Scientists have studied low and moderate viscosities extensively, but few have asked whether an upper bound exists, mainly because the highest viscosity materials tend to be minerals that make up rock rather than substances fluid dynamics typically studies.
Yoshida combined three independent lines of evidence. Geodetic observations tracked how stable regions of tectonic plates deform over years to decades. Laboratory experiments measured how minerals including olivine, clinopyroxene, diopside, anorthite and quartz deform under realistic heat and pressure over hours to years. Numerical simulations modeled how lithospheric plates bend and subduct over millions of years. All three approaches converged on a similar range, indicating tectonic plates reach effective viscosities of at least 10 to the 24th power Pascal seconds, and pointing toward a maximum of 10 to the 30th power, plus or minus two orders of magnitude.
“The proposed upper viscosity range represents a timescale dependent criterion,” Yoshida said.
Beyond that threshold, a material accumulates almost no strain over the age of the Earth and behaves as an effectively rigid body rather than a slow flowing liquid, according to the study. Yoshida argues this offers a more accurate way to think about extreme viscosity than treating it as infinite, since the finding shows materials stop behaving like fluids well before reaching any theoretical infinite value. The paper suggests the same framework could apply beyond geophysics, to thick non Newtonian fluids, glass like materials and other soft matter.
The study was published in the journal Physics of Fluids on June 29, 2026.
