Print this page 6 July 2007 2007/29

Atmospheric pressure determines vigour 'jet' after impact solid object

If you drop a cannonball on a bed of very loosely impacted sand, it will disappear completely. Shortly afterwards, a vigorous jet of sand will be shooting up from the bed. The height of this sandblast appears to be dependent on the atmospheric pressure above the sandbed. Experiments by researchers at the FOM Foundation and the University of Twente have demonstrated this phenomenon. Their findings will be published in the Physical Review Letters of 6 July 2007. The research proves that the prevailing pressure certainly has to be taken into account, when creating space wagons and robots that have to land or walk on sandy planets like Mars.

A steel ball that is being dropped on a bed of very loosely impacted grains of sand, causes a vigorously upwards shooting jet of sand. A team of researchers at the University of Twente and the FOM Foundation, led by Detlef Lohse discovered this in 2004. The cannonball pulls in its track a cavity that is being filled with air from the spaces between the grains of sand in the vicinity of the cannonball. The grains are very loosely impacted and so, there is relatively much air between them. The hydrostatic pressure in the sand compresses the cavity. This takes place at such a power that a jet of grains of sand shoots up. When the atmospheric pressure is being lowered above the sandbed then the jet becomes less intemperate, as one year later became clear from experiments by John Royer and fellow scientistst in the United States of America. Lohse and scientific fellow assistents Gabriel Caballero (FOM), Raymond Bergmann (FOM), Devaray van der Meer (University of Twente) and Andrea Prosperetti (University of Twente, John Hopkins) went more deeply into this subject. They have established that the atmospheric pressure above the sandbed have an effect on the velocity at which the cannonball penetrates the sand. The more the atmospheric pressure, the deeper the bullet is able to penetrate and, as a result, the higher the hydrostatic pressure that compresses the cavity behind the cannonball. The travelling cannonball produces a flow of air, which turns out to weaken the still loosely impact of the grains of sand any further. Observations during experiments at the University of Twente have been showing that the cannonball produces some quicksand on the spot, so to speak, which makes it easier for the cannonball to penetrate further into the sand. A higher atmospheric pressure above the sandbed reinforces the effect. The researchers have their findings published in the Physical Review Letters of 6 July 2007.

The observations at the University of Twente show that space robots are able to land safely at Mars. The atmospheric pressure there is just about one percent of the earth's and so, in dusty areas a landing wagon will not go down very quickly. In 2005, however, it proved that going out for a ride in the dunes of Mars may still be a tricky situation. Then, the mobile Martian robot 'Opportunity' was stuck in a sandy hill for nearly a month and a half.

For more information, please contact Devaraj van der Meer, phone (053) 489 23 87 or Professor Detlef Lohse, phone (053) 489 80 76.

Reference:
'The role of air in grammar jet formation' by Gabriel Caballero, Raymond Bergmann, Devaraj van der Meer, Andrea Prosperetti and Detlef Lohse, Physical Review Letters, vol. 99, 6 July 2007