WET SOLIDS FLOW ENHANCEMENT Page: 3 of 11
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SUMMARY OF TECHNICAL PROGRESS:
The strain-stress behavior of a wet granular media was measured using a split Parfitt tensile tester.
In all cases the stress increases linearly with distance until the maximum uniaxial tensile stress is
reached. The stress then decreases exponentially with distance after this maximum is reached. The
linear region indicates that wet solids behave elastically for stresses below the tensile stresses and
can store significant elastic energy. The elastic deformation cannot be explained by analyzing the
behavior of individual capillary bridges and requires accounting for the deformation of the solids
particles. The elastic modulus of the wet granular material remains unexplained.
Figure 1 shows the tensile stress measured in a Parfitt tester against the displacement for
93-micron glass beads with 4% wt. moisture content. As can be seen the force required to pull
apart the sample increases with distance to reach a maximum at a separation of about 200 microns
or two particle diameters. Figures 2 and 3 show the distance/diameter to a maximum stress for a
variety of materials, particle sizes and moisture contents. In all cases a similar pattern is observed,
when stretching, with a linear region to the maximum stress followed by an exponential decay of
the stress. The objective of this part of the research was to explain and analyze the elastic
behavior of the wet granular material on a fundamental basis as it has been done with other
mechanical properties. This elastic behavior is unexpected, and difficult to explain quantitatively.
The basis of the cohesive behavior of wet solids is the attractive forces due to the presence
of capillary bridges, of a liquid with surface tension a and contact angle S, as the one shown in
Figure 4. The force exerted on the particles is, following the notation of Figure 4 [Reference 4],
F=ad sin(O) sin(O +S)+ (1.1)
where the starred variables are given by:
h*= sin(6 )+r*(sin(6 +S )-1)
2cos(6 +S )
Based on the moisture content it is possible (as discussed in reference ) to determine the
bridge volume and the filling angle 0. It is also easy to show that the filling angle is not sensitive
to the separation a between particles as is shown in Figure 5. For a constant filling angle equation
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Caram, Hugo S. & Foster, Natalie. WET SOLIDS FLOW ENHANCEMENT, report, July 1, 1999; Morgantown, West Virginia. (https://digital.library.unt.edu/ark:/67531/metadc722847/m1/3/: accessed April 19, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.