AFM Force Measurements to Explore Grain Contacts with Relevance for Planetary Materials

Abstract

Most small asteroids are defined as “rubble piles” or bodies with zero tensile strength and large bulk porosity. The cohesive forces that hold them together act at the grain scale, and their magnitude is often estimated from similar materials when used in simulations. Improving the accuracy of predictions of asteroid strengths requires suitable laboratory measurements of relevant materials, as well as increasing the availability of materials from sample return. Atomic force microscopy (AFM) is well suited for force measurements relative to particle–particle interactions. In this work, we use AFM force measurements to evaluate the cohesive forces that act between micron-sized grains. We investigate the effect of the sizes of the interacting grains of JSC-1 lunar simulant using three sample sizes (<45, 75–125, and 125–250 μm) and three spherical AFM tip diameters (2 μm, 15 μm, and 45 μm). In all cases, adhesion forces were larger at ambient relative humidity (RH), where the water layer on the surface of the grains is more prominent, creating a larger meniscus between the tip and the grain upon contact. We observed weaker adhesion with larger grain/tip size, which can be attributed to the changing contact area between the samples and the tips. We expect that our approach will pave the way to a better understanding of regolith surface properties such as adhesion and cohesion and provide suitable input for models that can be used to predict the evolution of asteroids and their particle behaviors.