Coupling neutron imaging and thermochromic liquid crystals to investigate the properties of a laboratory-made subducting slab.

Ludox colloidal dispersions exhibit viscous, elastic, plastic and brittle rheological properties depending on their water content. This makes these dispersions a relevant model system to study a wide variety of phenomena, from drying paint to columnar joints. As for now, they are the only system that enables to generate one-sided subduction from convection in the laboratory. Rayleigh numbers, constraining the intensity of convection,  have a similar order of magnitude in the laboratory experiments and in the mantle. Prandtl numbers are much greater than 100, insuring negligible inertial effects. Ludox is thus a relevant analog system to study convection in planetary mantles, the water content playing the role of temperature in determining its rheological properties. 

We investigate here convective patterns  in a Ludox suspension (TM50) heated from below and dried and cooled from above, coupling neutron imaging (NeXT, ILL) and thermochromic liquid crystals (TLCs). Both imaging methods are complementary. Neutron imagery is used to estimate the local volume fraction of silica in the solution, which can be linked to the local rheological properties. TLCs  give us access to the temperature field. We therefore can follow in situ the development  of hot thermal plumes, and of a skin at the surface, that will eventually subduct spontaneously. 

In addition to the imagery, the evaporation rate, the surface, ambient and heating temperatures, and the ambient humidity rate are recorded. They are  used to estimate the heat and mass transfer at the surface and how the formation of a skin affects them compared to a case with an homogeneous newtonian solution.