Figure 20.

Mean molecular displacement ${\overline{s}}_x$ in helium flow at delays $\mathsf{\Delta }t$ = 20, 30, 40, and 50 µs: (O) data at 1.6 kPa; (◁) data at 920 Pa. The black solid line represents the linear interpolation of the MTV data, which provides an average velocity ${\overline{u}}_{MTV}$ = 42.2 m/s for the data at 1.6 kPa and ${\overline{u}}_{MTV}$ = 27.28 m/s for the data at 920 Pa.