Friction, force chains, and falling fruit - Why is it that stacked apples seem so stable, but removing the "wrong" apple can cause the whole pile to tumble down?

I am on sabbatical leave working with Bob Berhinger, who is an expert on granular and fluid flows. We have recently applied our knowledge of the behavior of granular materials in motion to an age-old fruit market conundrum — how to pick a piece of fruit out of a pile without triggering collapse.

We studied and filmed piles of apples, oranges, and onions as one or more pieces of fruit were removed. Among other things, we discovered that increasing the friction of the onions (by peeling them) vastly decreased the likelihood of collapse. Theorists always thought that friction impacts collapse, and we devised an experimentally controlled way to prove it, accessible to all who dare try. [1] I will describe this and other activities I am engaged in this sabbatical year at Duke.

[1] Physics Today (Sept 2009), see also Youtube.com "unpeeled onions" or www.dukefruit.info.