An ephemeral crinkle

kw: observations, photographs, natural phenomena

Nearly every time I open a 2-liter bottle of soda, I notice a regular, but brief, pattern appear as the bottle shrinks beneath the label. Once air leaks between, the pattern vanishes. If you look carefully, you'll see that the pattern covers the whole visible label, though the lighting does not show it up equally everywhere. This regular crenelation is similar to the pattern of folds used to make a sheet that can respond to compressive stress by changing size.

I am familiar with the usual pattern of collapse when air is drawn from a thin-walled cylinder. It will usually form a three-lobed figure. Try sucking the air out of a plastic soda bottle; it will usually form first a triangle (in cross section) then upon further air withdrawal, a figure with a cross section more like a clover leaf.

But this pattern appears when the high pressure in a carbonated bottle is released. I believe the labels are attached to already-filled bottles, so when the pressure is released the bottle will shrink a little but the label is being drawn with it by external air pressure (external to the gap between the label and the bottle). Thus the label experiences biaxial (two-directional) compression and this pattern is the result.

This image, from the Center for Nonlinear Mechanics at the University of Bath, shows the theoretical analysis of a single row of alternate crenelations in a cylinder under uniaxial compression. Though it is not the same system, the pattern is suggestive of a real self-organizing compression pattern that develops before permanent deformation sets in.

This photo of a cylinder that crumpled in the way shown just above is from the article Buckling at the University of Western Australia, School of Mechanical Engineering. So far, I haven't found any reference to buckling in the style I photographed in articles on biaxial compression.