What happens when you drop a steel bearing ball into a fine sand? Turns out you get a granular post-impact “splash”, similar to what you would expect in a liquid, followed by a narrow upward-directed granular jet emanating from the sand. What is a bit counter-intuitive is that the air plays an important coupling mechanism in formation of such granular jet. For example, when you pump out air and repeat experiment in vacuum, the jet ceases to exist (see picture on the left).
In 2004, in what I believe was one of the first issues of then newly-formed Nature Physics, Heinrich Jaeger’s group at U Chicago has created fast x-ray movies of the granular jet [R. Royer et al., Nature Phys. 1, 164 (2005)].
This month two PRL papers expand on this issue – the first one is a follow-up by Jaeger’s group [R. Royer et al., Phys. Rev. Lett. 99, 038003 (2007)], and another one is by Detlef Lohse group in Netherlands, which has done a lot of wonderful work in this area [G. Caballero et al., Phys. Rev. Lett. 99, 018001 (2007)].
For more details (and fascinating movies of granular jets) go to Jaeger’s group page.
The use of x-rays in these experiments is quite fascinating – because physics happens at macroscopic lengthscales and timescales – millimeters and milliseconds, as opposed to some of the state-of-the-art synchrotron-based measurements in solid state, atomic physics and chemistry that take place at sub-angstrom lengthscales and sub-picosecond timescales – about 8 to 10 orders of magnitude decrease in both spatial and temporal dimensions. The use of radiography, which relies on x-ray adsorption as contrast mechanism, is perhaps one of the most basic application of x-rays dating back to Roentgen and commonly found in dental offices and hospitals around the world. Compared to other x-ray based techniques that take advantage of atomic-lengthscale resolution provided by short wavelength of x-ray photons, energy tunability to access adsorption edges of elements, sub-micron scale microscopy, magnetic scattering and coherent content of modern synchrotron sources, the use of third-generation synchrotrons to perform radiology on sand particles may seem almost as exotic as the use of hospital NMR imaging machine to study how many m&m candy one can fit in the bowl [A. Donev et al., Science, 303, 990 (2004)].
It is quite amazing how many hidden discoveries lurk in the macro-world of granular materials, and perhaps in other disciplines as well!