Ten years ago, GRASP members reported experimental results about the unusual crack dynamics of clay and mud during desiccation . In 2d, typical fracture networks appear during the drying process, dividing the material into cells (see left picture). In fact, the drying produces shrinkage of the clay onto a stable substrate, therefore inducing tension leading to rupture events. Mechanical stresses are relaxed along single directions, leading to secondary ruptures perpendicular to primary cracks.
Our original idea was to study such a process in 1d : clay lines were created. A typical space-time picture of the clay line is illustrated below (time goes down). Different grey levels are seen due to a change of the liquid content during drying. Cracks are seen to appear at different successive times, releasing the tension due to retraction. The total opening of the cracks increases linearly with time and is strongly correlated to retraction phenomenon. A careful analysis [1,2] shows that the crack events are self-avoiding, producing a complex hierarchy of cracks.
It is remarkable that such dynamics, evidenced 10 years ago in this experiment, is today a central idea of studies on elastic sheets and thin objects , and in particular the hierarchy of wrinkles.
 N.Lecocq and N.Vandewalle, Experimental study of cracking induced by desiccation in 1-dimensional systems, Eur. Phys. J. E 8, 445 (2002) – PDF
 N.Lecocq and N.Vandewalle, Dynamics of crack opening in a one-dimensional desiccation experiment, Physica A 321, 431 (2003) – PDF
 H.Vandeparre et al., Phys. Rev. Lett. 106, 224301 (2011) – “A hierarchy of wrinkles“