Magnetic-resonance-imaging rheometrical experiments show that concentrated suspensions or emulsions cannot flow steadily at a uniform rate smaller than a critical value ( ${\dot{\gamma }}_c$). As a result, a “liquid” region (sheared rapidly, i.e., at a rate larger than ${\dot{\gamma }}_c$) and a “solid” region (static) coexist. The behavior of the fluid in the liquid region follows a simple power-law model, while the extent of the solid region increases with the degree of jamming of the material.

• Received 4 December 2001

DOI:https://doi.org/10.1103/PhysRevLett.88.218301