The image at left was taken of a droplet of food coloring spreading within self-generated fractures at the surface of gel agar in a Petri dish. This is quite different from the circular way which surfactant-laden droplets spread on either solid or liquid surfaces, and is related to the fact that the gel substrate is a viscoelastic material which can fracture. We are studying how this instability arises from competition between the surface tension of the droplet and the elastic properties of the substrate. Such instabilities are important to understand in order to reliably work with non-Newtonian fluids in industrial and biomedical settings.
The image at right shows the photoelastic stress field present during crack-crack interactions. We have investigated the origin of the curved shapes made by pairs of interacting cracks, which occur in situations as diverse as dental enamel, cleaved silicon, geological faults, and planetary ice crusts. Through experiments on diverse materials (not just gels) we have quantified this universal shape and formulated a geometric model to understand how it arises. The work identifies an easily-measured shape parameter which could serve as a diagnostic tool for identifying the stress state under which cracks were formed in natural systems where history and dynamics are inaccessible.