An object falling in a fluid reaches a terminal velocity when the drag force and its weight are balanced. Contrastingly, an object impacting into a granular medium rapidly dissipates all its energy and comes to rest always at a shallow depth. Here we study, experimentally and theoretically, the penetration dynamics of a projectile in a very long silo filled with expanded polystyrene particles. We discovered that, above a critical mass, the projectile reaches a terminal velocity and, therefore, an endless penetration.

Gas bubbles dissolve slower and expand faster in a soft solid elastic medium, relative to a simple (inviscid) liquid medium.

Blood flow through intrapulmonary arteriovenous anastomoses (Q_IPAVA) occurs in healthy humans at rest and during exercise when breathing hypoxic gas mixtures at sea level and may be a source of right-to-left shunt. However, at high altitudes, Q_IPAVA is reduced compared with sea level, as detected using transthoracic saline contrast echocardiography (TTSCE). It remains unknown whether the reduction in Q_IPAVA (i.e., lower bubble scores) at high altitude is due to a reduction in bubble stability resulting from the lower barometric pressure (P_B) or represents an actual reduction in Q_IPAVA. To this end, Q_IPAVA, pulmonary artery systolic pressure (PASP), cardiac output (Q_T), and the alveolar-to-arterial oxygen difference (AaDO₂) were assessed at rest and during exercise (70–190 W) in the field (5,260 m) and in the laboratory (1,668 m) during four conditions: normobaric normoxia (NN; [ ] = 121 mmHg, P_B = 625 mmHg; n = 8), normobaric hypoxia (NH; [ ] = 76 mmHg, P_B = 625 mmHg; n = 7), hypobaric normoxia (HN; [ ] = 121 mmHg, P_B = 410 mmHg; n = 8), and hypobaric hypoxia (HH; [ ] = 75 mmHg, P_B = 410 mmHg; n = 7). We hypothesized Q_IPAVA would be reduced during exercise in isooxic hypobaria compared with normobaria and that the AaDO₂ would be reduced in isooxic hypobaria compared with normobaria. Bubble scores were greater in normobaric conditions, but the AaDO₂ was similar in both isooxic hypobaria and normobaria. Total pulmonary resistance (PASP/Q_T) was elevated in HN and HH. Using mathematical modeling, we found no effect of hypobaria on bubble dissolution time within the pulmonary transit times under consideration (<5 s). Consequently, our data suggest an effect of hypobaria alone on pulmonary blood flow. NEW & NOTEWORTHY Blood flow through intrapulmonary arteriovenous anastomoses, detected by transthoracic saline contrast echocardiography, was reduced during exercise in acute hypobaria compared with normobaria, independent of oxygen tension, whereas pulmonary gas exchange efficiency was unaffected. Modeling the effect(s) of reduced air density on contrast bubble lifetime did not result in a significantly reduced contrast stability. Interestingly, total pulmonary resistance was increased by hypobaria, independent of oxygen tension, suggesting that pulmonary blood flow may be changed by hypobaria.