Granular materials have long been the subject of scientific studies due to
their fascinating physical properties which differ significantly from materials in bulk liquid or
solid form. These studies have focused primarily on dry granular materials in which the intergrain
forces are purely repulsive, but the addition of a wetting fluid to the material adds an
attractive force to the system and therefore a new dimension to the underlying physics. Indeed the physics of
such a wet granular system is relatively unexplored, and there is no basic understanding of
how exactly the addition of a small quantity of wetting fluid affects physical properties of
granular media. This problem is of great scientific interest, but it is also of practical importance
since granular materials relevant to many industrial and commercial applications often contain
significant liquid between the grains. Recent work by our group has demonstrated that large changes
in the physical properties of granular materials can be induced by even a
nanometer-scale layer of liquid on the grains, offering a glimpse of the complex physical behavior which
wetting can induce (see references below). For example, in glass beads coated with oil, we
find three different regimes of correlated behavior among the grains with different
dynamics and stability (see figure 1 below).
A study of the effects of liquid on the properties of granular materials is of
particular interest in the context of microgravity. A reduction in the gravitational force
would change not only the distribution and strength of contact forces between the grains in a
granular sample, but also the capillary forces due to the liquid bridges between the grains and the
dynamics of the liquid draining from the grains. Furthermore, in the microgravity environment
where grains will not come to rest at the bottom of a container, liquid-induced adhesive forces
would lead to the agglomeration of grains which would otherwise move independently. Wetting will
thus significantly affect all physical properties of granular media in microgravity,
and there is a significant potential to observe new physical phenomena in wet granular
materials in reduced gravity which are otherwise hidden by the presence of gravity.
Since there is currently only a very limited fundamental understanding of the effects of interstitial liquid on granular materials, we are conducting a ground-based research program to establish a basic understanding of the phenomena on Earth. This program combines the efforts of two principal investigators in order to simultaneously investigate both the experimental and theoretical aspects of wet granular media. The goal of the research program is to understand how the addition of liquid changes both the static and dynamic macroscopic physical properties of granular materials, and in particular to understand how the liquid-induced cohesion leads to the development of correlations between the grains. We are probing the liquid-induced phenomena systematically by experimental investigations of various granular properties as a function of liquid content using a variety of liquids and grains. We plan three research thrusts: a) Investigations of surface phenomena and the development of correlations through clumps on the surface of a rotating granular sample. b) Studies of the impact of interstitial liquid on flow properties through measurements of the drag force in a granular material. c) Examination of liquid-induced effects on granular segregation dynamics through magnetic resonance imaging of vibrated granular materials and studies of segregation during flow. The experimental results will guide the theoretical efforts, while at the same time theoretical research will both provide the basis for interpretation of the experimental data and suggest new experiments to be performed.
Schiffer, P., Barabasi, A.L., Tegzes, P., Albert, A.I., Studies of Wet Granular Media, Proceedings of the Fifth Microgravity Fluid Physics and Transport Phenomena Conference, NASA Glenn Research Center, Cleveland, OH, CP-2000-210470, pp. 1390-1393, August 9, 2000.