  The compression of soft elastic matter and biological tissue can lead to
creasing, an instability where a surface folds sharply into periodic
self-contacts. Intriguingly, the unfolding of the surface upon releasing the
strain is usually not perfect: small scars remain that serve as nuclei for
creases during repeated compressions. Here we present creasing experiments with
sticky polymer surfaces, using confocal microscopy, which resolve the contact
line region where folding and unfolding occurs. It is found that surface
tension induces a second fold, at the edge of the self-contact, which leads to
a singular elastic stress and self-similar crease morphologies. However, these
profiles exhibit an intrinsic folding-unfolding asymmetry that is caused by
contact line pinning, in a way that resembles wetting of liquids on imperfect
solids. Contact line pinning is therefore a key element of creasing: it
inhibits complete unfolding and gives soft surfaces a folding memory.
