Mechanical properties of dense granular assemblies in the presence of wetting liquids.

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Mixtures of solid particles of sub-millimetric size and liquids play a pivotal role in many branches of process engineering and industrial production. The production of concrete, for instance, demands for a homogeneous mixture of the granular materials cement and sand on the one hand and water on the other hand. Pharmaceutical substances present in the form of powders are mixed with liquid binders before they are mold into pills. The cohesion between particles in a wet granular assembly at low liquid saturations can be well described by the effect of isolated capillary bridges. In the large range of liquid saturations above ca. 2% of the wetting liquid (with respect to the total sample volume) one finds large liquid clusters which dominate the mechanical properties of the particle assembly and the transport of liquid inside the assembly. In the framework of the present project we are aiming at the development of a unified model for cohesive forces in the presence of capillary bridges and liquid clusters which can be implemented to contact dynamics simulations of the particles. To this end we want to amend already existing models to account for the individual volume of capillary bridges, exchange of liquid between the bridges and clusters, and capillary many-body interaction. A further aim of this project is the quantification of different dissipative processes observed during approach, contact, and detachment of particles in the presence of a wetting liquid. The resulting collective behavior of the particles will be investigated in a variety of container geometries and for different boundary conditions. The long-term goal of this project is to combine the particle dynamics with models of liquid transport, that is, first of all to reproduce and then to realistically predict the collective dynamics of particles over large time and length scales.