Problem database

There are two parallel disks in a vessel with a gas whose pressure can be changed. One disk hangs on an elastic thread, the other rotates at a constant angular velocity. Twist angle of the first disk at pressure $P1$ is equal to $\varphi_1$. As the gas pressure increases, the angle of twisting of the thread first increases, and then, having reached the value $\varphi_2$, it ceases to depend on the gas pressure. Explain this effect. How does the thread twist angle depend on the gas pressure at $\varphi \ll \varphi_2$?