Yield stress and effusion rates in model cryovolcanic slurries

A recent publication on the low temperature viscometry of methanol-water slurries [Zhong et al, Icarus 202, 607-619 (2009)] has provided the first experimental evidence for the likely presence of a ‘yield stress’ in putative cryomagmas - a precursor for levee formation and steep sided volcanic structures on icy planets. This paper serves to build on this approach by applying a range of rheometrical techniques routinely employed in materials science. The system water-sucrose was used as a model ‘cryomagma’. This has the practical advantage of allowing experiments to be undertaken at 253 - 273K, prior to the planned construction of a low temperature measuring system capable of operating down to 120K. The ‘peritectic’ for the model system has a composition close to 66.3:33.7 sucrose:water and a viscosity at 255 K of ~ 15 Pa s - not dissimilar from regions of the ammonia-water-methanol phase diagram. The ice crystal volume fraction was varied in the range 0.05 - 0.35, by variation in the water:sucrose ratio prior to freezing - giving consistencies ranging from a ‘fluid slurry’ to a ‘soft solid’. A vane and serrated cup measuring geometry was employed to avoid artefacts from wall slip. Data from a range of experiments are presented - including flow curves (controlled stress and controlled rate), creep studies (‘static’ yield stress) and stress ramps (‘dynamic’ yield stress), linear viscoelastic measurements (varying strain, frequency, temperature) and assessments of time-dependent structural recovery (thixotropy) - and their geophysical significance highlighted. Rheometrical experiments were also performed using a novel extrusion flow cell that allows observation of slurry flow through a cylindrical channel. Combination of dynamic yield stress data from rotational experiments and pressure drop measurements from extrusion studies allows the prediction of the slurry effusion rate, following the treatments commonly employed in industrial extrusion situations. Initial results suggest that the slurry under shear deviates from published models used to predict the relative viscosity of suspensions (see Figure). Novel numerical approaches are being developed in parallel that have potential to explore the multi-phase rheological aspects of these systems in more detail.