Confined gravity currents of non-Newtonian fluids with surface tension effects: experiments and comparison with theory

This study reports mainly experimental findings on the evolution of confined axisymmetric gravity currents of non-Newtonian fluids within a gap of fixed height, focusing on the combined effects of rheology and surface tension. The inflow rate follows a power-law dependence on time, with the exponent related to the fluid behaviour index. Prior theoretical work, validated against axisymmetric experiments with Newtonian fluids, has shown that (i) surface tension acting primarily at the grounding line can significantly modify the structure of classical self-similar solutions and (ii) the presence of a meniscus at the grounding line can be incorporated into the similarity formulation through a parameter relating its height to the gap thickness, thereby preserving self-similarity. Here, this framework is extended to non-Newtonian fluids, adopting a power-law rheology for analytical convenience, which is appropriate since the tested shear-thinning and shear-thickening fluids behave as power-law fluids over the relevant shear-rate range. The influence of surface tension is confirmed through twenty-one axisymmetric experiments using Newtonian and non-Newtonian fluids. Good agreement between theory and experiments is obtained only when the grounding-line meniscus is included in the analysis. In addition, surface tension is found to play a central role in the hysteresis observed during the initial stages of current propagation, determining whether the current remains confined or transitions to an unconfined state. Finally, we show that the self-similar solution is robust, remaining valid even when the inflow rate deviates from the ideal power-law form, for example, in cases involving a delay in filling the experimental cell.