Exploring dilatancy in soft granular materials using advanced radiography techniques

• Applicant: Dr.-Ing. Sascha Heitkam
• Subject Area: Statistical Physics, Nonlinear Dynamics, Complex Systems, Soft and Fluid Matter, Biological Physics
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 551239760

Dilatancy is a well-known effect in hard granular materials such as sand or soil. When exposed to shear, the inter-particle distance increases and liquid is entrained. This has strong implications for example in civil engineering. For soft granular materials, such as foam or emulsions, dilatancy has been predicted analytically. However, despite significant efforts there is no direct experimental proof of dilatancy, yet. In this project, we aim to measure the effect of shear on the liquid hold-up in soft granular materials. This will prove or disprove the predicted dilatancy in foam and emulsions. Assemblies of droplets or bubbles with well-defined diameters are generated in millifluidic devices. Monodispersed assemblies will exhibit crystalline packings while small levels of polydispersity result in random packings. 3D-printed templates will trigger mono-crystalline clusters. Neutrally-buoyant oil-in-water emulsions will yield steady-state packings with relatively high water content. Air-in-water foams undergo drainage, yielding unsteady and inhomogeneous liquid fraction distributions. By means of movable boundaries, the clusters will be exposed to defined shear angles. Radiographic methods will yield the phase fractions within the clusters. For foams, X-ray or synchrotron radiography is sufficient to yield high contrast between gas and liquid phase, allowing to measure liquid contents and cluster structures. Furthermore, optical observation allows to measure the stress distribution near the wall. For emulsions, neutron radiography is required, employing the attenuation contrast between heavy water and oil. This will yield the water fraction and the crystal structure of the cluster. Analyzing the change in liquid fraction between sheared and unsheared state allows to evaluate whether there is dilatancy in soft granular matter. At the same time, radiography yields the local shear angle for crystalline arrangements and optical measurements allow to estimate the shear angle in random arrangement. Combining both information yields a quantitative description for dilatancy in soft matter. Comparing neutrally-buoyant emulsions to draining foam will allow to cover a large range of liquid fractions and to rule out drainage effects on dilatancy.

• DFG Programme: Research Grants
• International Connection: South Korea, Switzerland
• Cooperation Partners: Dr. Joonwoo Jeong; Professor Dr. Pavel Trtik