Controlling the sands of time with ambient gas

Granular flows have kept time for centuries in hourglasses and sand timers. Although clogging in granular flows through hourglasses and hoppers has been extensively studied, the effect of air or fluid flow has been largely ignored. We show that the counterflow of gas in hourglasses, generated as grains trickle downward, dominates the timing until the gas becomes rarefied at low pressures. Our experiments employ enclosed glass timers and fine, sifted glass ballotini to fix the timer neck and particle diameters above a critical clogging ratio (R ≈ 5), ensuring smooth flow as we flip them repeatedly in a vacuum chamber. As pressure steadily decreases and both ends of the timer remain open, the pouring duration stays short and nearly constant down to vacuum since no counterflow forms. When only one side is open, the viscous gas counterflow slows the timer, and the timing remains constant until pressure falls below ~5% of atmosphere, where the pouring duration decreases by 40–70%. We used gases of varied viscosities, such as hydrogen and neon, and collapsed the timing data with a corrected time and relative pressure, P/P_c. The crossover is governed by the viscosity’s dependence on the Knudsen number, which matters when the gas mean free path approaches the interparticle spacing. We expect our results are especially relevant for granular flows in low pressure planetary environments, such as on Mars.