Pattern formation during healing of fluid-filled cracks: an analog experiment

The formation and subsequent healing of cracks and crack networks may control such diverse phenomena as the strengthening of fault zones between earthquakes, fluid migrations in the Earth's crust, or the transport of radioactive materials in nuclear waste disposal. An intriguing pattern-forming process can develop during healing of fluid-filled cracks, where pockets of fluid remain permanently trapped in the solid as the crack tip is displaced driven by surface energy. Here, we present the results of analog experiments in which a liquid was injected into a colloidal inorganic gel to obtain penny-shaped cracks that were subsequently allowed to close and heal under the driving effect of interfacial tension. Depending on the properties of the gel and the injected liquid, two modes of healing were obtained. In the first mode, the crack healed completely through a continuous process. The second mode of healing was discontinuous and was characterized by a 'zipper - like' closure of a front that moved along the crack perimeter, trapping fluid that may eventually form inclusions trapped in the solid. This instability occurred only when the velocity of the crack tip decreased to zero. Our experiments provide a cheap and simple analog to reveal how aligned arrays of fluid inclusions may be captured along preexisting fracture planes and how small amounts of fluids can be permanently trapped in solids, modifying irreversibly their material properties.     

RAVENS AND CROWS IN IRON AGE AND ROMAN BRITAIN

The raven and crow skeletons from Danebury are re‐examined, taking into account their taphonomy, their context and the associated finds. Raven and crow burials from other Iron Age and Roman sites are surveyed, again with a discussion of their context and associated finds where these could be ascertained. Taken together, the evidence makes it clear that most if not all were deliberate burials, often at the base of pits. We demonstrate how interpretations of such burials have changed, with zooarchaeologists initially proposing functional explanations and archaeologists readier to accept that the burials were deliberate. We go on to argue that the unique character of ravens and crows, including their tolerance of humans, their scavenging habits, and their voice, led to their playing an important role in Iron Age and Roman rites and beliefs.