Alumnus Solves Quandary in Porous Media Experiments

Porous media—solids containing pores—are all around us: Skin, snow, concrete, and cartilage are all examples. Civil engineers are constantly working with a subset of porous media called granular media: Earthen structures, filter sands, and soil particles provide the foundation for our infrastructure, the water we drink, and the food we eat. In granular media, the solids are disconnected, and the pores are the spaces between grains where fluids can rest, flow, mix, or react. Because they are so important in so many fields of science and technology, there is a long tradition of porous media research. Porous media specialists even have their open professional society, the International Society for Porous Media.

However, until recently, there was a quandary in granular media experiments. To understand the trouble, consider this stack of spheres, photographed on the Greek island of Rhodes:

These spheres are arranged in the densest possible way, that is, the geometric orientation with the smallest possible porosity, called hexagonal close packing. Note how the second layer of spheres fits neatly into the spaces above the first layer of spheres. This interlocking pattern makes the stack uniformly dense. In this photograph, gravel fills the space under the first layer. But what happens in an idealized experiment, where all the granular media are the same size? It there any way to pack the media to make the density constant right up to the wall? The short answer is no. Moreover, even if one considers non-spherical media, the answer is still no. This has been a quandary for porous media research, because the less-dense space near the wall can—and does—carry a disproportionate share of the flow through the porous media. This is called the wall effect.

Enter CU Denver civil engineering alumnus Eric Roth, who earned his bachelor’s in 2011 and his master’s in 2013. During his doctoral research at the University of Colorado Boulder, Dr. Roth devised not just one but two ways to solve the wall effect for porous media experiments in an apparatus with flat walls (see “a” below). One way is to start with a layer of smaller spheres, one-third the diameter of the regular spheres, and fit them into the space between the first layer and the wall (see “b” below). Another way is to coat the wall with a layer of silicone, just thick enough that when the first layer of spheres is positioned, they will be exactly 50% saturated in silicone (see “c” below). Working with CU Boulder collaborators John Crimaldi, Roseanna Neupauer, and Lauren Sather and CU Denver collaborator David Mays, Dr. Roth has recently published experimental results showing that either way does the job, providing uniform fluid velocity right up to the wall. Quandary solved!

For details, please see Roth’s article in Transport in Porous Media, or e-mail David Mays to request a copy.