  Aqueous and brine suspensions of corn starch show striking discontinuous
shear thickening. We have found that a suspension shear-thickened throughout
may remain in the jammed thickened state as the strain rate is reduced, but an
unjamming front may propagate from any unjammed regions. Transient shear
thickening is observed at strain rates below the thickening threshold, and
above it the stress fluctuates. The jammed shear-thickened state may persist to
low strain rates, with stresses resembling sliding friction and effective
viscosity inversely proportional to the strain rate. At the thickening
threshold fluid pressure depins the suspension's contact lines on solid
boundaries so that it slides, shears, dilates and jams. In oil suspensions
lubrication and complete wetting of confining surfaces eliminate contact line
forces and prevent jamming and shear thickening, as does addition of immiscible
liquid surfactant to brine suspensions. Starch suspensions in glycerin-water
solutions, viscous but incompletely wetting, have intermediate properties.
