Size controls on the crossover from normal to self-inhibited sintering of ice spheres

Anomalies in the sintering of ice spheres were observed in previous studies as a growth of protrusions that evolve into porous bonds which eventually retreat. These anomalies were discussed in terms of Mullins-Sekerka theory which predicts the self-amplified growth of small perturbations. This interpretation would imply that the anomalous (mechanically weakening) sintering should crossover to normal (mechanically strengthening) sintering if the sphere size is reduced below a critical size. Here we show that this is indeed the case with sintering experiments in random sphere packs. Six samples, varying in particle diameter from 1 to 2.3 mm were sintered over ten days at −10 °C and then imaged by 3D micro-computed tomography. For two additional samples with small (1 mm) and large (2.1 mm) particles, we monitored the sintering evolution. Surface and bond properties were quantified by image analyses, which revealed solidly sintered bonds in small, and porous bonds in large spheres. Microstructure-based finite element simulations of the temperature field revealed higher temperature gradients between the particles in the bond region, as a driver for the protrusion growth. The temperature gradients, as well as the bond porosity, decreased after an initial increase up to five days and signal the retreat of the bonds. Our work details microstructural controls on anomalous sintering in ice as a self-inhibited bond growth process if only slight deviations from isothermal conditions occur. Besides the insight into fundamental sintering mechanisms, the results may have practical consequences for piste preparation, food processing, or research on icy planetary bodies.