Persistent Holes

Have been observed in a shaken non-Newtonian fluid. Normally, a fluid takes the shape of its container and any puncture of the surface quickly fills. However, in an experiment performed at the University of Texas at Austin, stable holes appeared in a cornstarch–water mixture that was vertically vibrated at frequencies above 120 Hz, with accelerations between 12 and about 20 times the gravitational acceleration. Holes did not form spontaneously; but if a puff of air poked a hole in the fluid, the hole could persist indefinitely. As seen from above in the top photograph, the holes had diameters comparable to the 5-mm depth of the fluid and extended to the bottom of the container, which was 9.4 cm across. The researchers attribute the holes’ stability to shear thickening—an increase of viscosity with shear rate. At higher accelerations, as shown in the side view (bottom) of a single hole, the rim grew fingerlike protrusions that rose 2 cm above the surface and undulated erratically. ( F. S. Merkt et al., Phys. Rev. Lett. 92, 184501, 2004.http://dx.doi.org/10.1103/PhysRevLett.92.184501 )