Figure 2.

Oxide skin that forms on alloys of gallium has a yield stress. (a) It can be measured by placing a thin sample (∼1 mm thick) of the metal between a parallel plate rheometer. The oxide spans the periphery between the two plates. (b) Beyond a critical surface yield stress (∼0.5–0.6 N/m), the oxide breaks and the metal flows readily. Below the surface yield stress, the oxide skin is elastic and mechanically stabilizing. Adapted with permission from ref (23). Copyright 2008 Wiley.