Friday, July 01, 2011

Quicksand! It's Everywhere!

I remember as a youngster being terrorized by black-and-white movie scenes where a jungle explorer walks into quicksand and disappears forever, leaving only his pith hat on top of the sand for others to find. We now know that quicksand is relatively uncommon and isn't that dangerous anyway since you would only sink until Archimedes' Law kicks and you end up floating in the mess.

However, a new report (subscription required - an independent review) in Physics Review Letters suggests that the rather unusual rheology of quicksand might be much more common than we think. Before I completely scare the bejeebers out of you, let me explain the report.

The dominant thought pattern regarding granular media, and in this case, dry granular media, is that objects falling into will eventually come to a stop because the energy of the falling body is absorbed by the inefficiencies of the media (friction). It was discovered however, that that is not always true. The researchers filled a ping-pong ball with steel, dropped it in a 20 foot tube of polystyrene beads and found that not only did the ball not stop falling, it eventually fell at a steady rate - a terminal velocity. Since the existence of a terminal velocity meant that the forces on the ball were balanced, the ball would never stop falling and should continue downward at that same speed forever.

Theoretical analysis supported this and suggests that even ordinary sand could fit the criteria - meaning that if you were walking on the beach, you could end up falling through the sand forever never to be seen again. Before you panic and decide to take your chances with the sharks and not the sand next time you are at the ocean, there is only one catch to this: for a ping-pong ball to fall into dry sand, it would need a density of 400 g/cm3, which is about 20 times higher than the densest metals on earth (osmium, 22.5 g/cm3.)

It is also interesting that this result is independent of the initial velocity, which sort of plays against intuition, but then again when you think about it, bullets are stopped by sand in very short order.

3 comments:

Eric F. Brown said...

This article, from Slate ( http://www.slate.com/id/2264312/ )talks about the rise and fall of quicksand in the movies.

John said...

What a wonderful article, even in blaming Ralph Nader.

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