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Effect of liquids on the stability and segregation of granular materials

We studied the effect of adding small amounts of liquids to granular matter. This is a topic in which little quantitative work has been done, even though humidity or liquids are almost always present in natural situations where granular matter occurs.

Two different experimental setups were used. First, our experiments were conducted with mixtures poured into a quasi-two dimensional silo that allows visualization through the transparent side walls. Our data for the increase in the angle of repose and subsequent saturation appears to be inconsistent with some of the models of wet granular matter.

Our experiments showed the importance of viscosity of the liquid in determining the angle of repose of the pile formed after pouring the granular mixture (Samadani 2001). We also reported one of the first systematic studies of segregation transition of bidisperse granular mixtures in the presence of liquids (Samadani 2000).

Then, the maximum angle of stability of a cohesive pile was investigated using a rotating drum apparatus to understand the discrepancies noted in previous studies. We first showed the effect of the side walls by varying the width of the drum. The maximum angle of stability was then measured in the limit where side walls are unimportant.

We developed a new liquid bridge model that takes into account the nature of the grain contacts and the cohesive force due to liquid bridges to show the grain size, system size, and surface tension behavior (Nowak 2005). In this model, the friction between particles is considered less important compared to geometric stability of the particles. The experimental data is in excellent agreement with the prediction of our model.