Ice aggregate contacts at the nm-scale

Aggregation of ice particles is a fundamental process in the interstellar medium as well as in planet formation. Dedicated to study the contact physics of nm-ice particles, we developed a thermal gradient force microscope. This allows us and was used to measure pull-off forces with a resolution on the nN-scale and to measure rolling. Furthermore, based on a free probe, it also allows us to study twisting torques for the first time. The experiments show that torques required to twist are significantly larger than that macroscopic models scaled down to the nm-size range would predict. This implies that (ice) aggregates in astrophysical settings with small constituents are more robust against restructuring than previously thought. They likely grow as fractal aggregates to larger size before they restructure, and during later compact growth they likely retain a higher porosity during further evolution towards planetesimals.