Drag forces on porous aggregates in protoplanetary disks

Abstract

Context. In protoplanetary disks, particle–gas interactions are a key part of the early stages of pre-planetary evolution. As dust particles grow into porous aggregates, treating drag forces of aggregates in the same way as those of monolithic compact spheres has always been an approximation. Aims. The substructures and building blocks of aggregates may respond differently to different drag regimes than the overall size of the porous body would suggest. The influence of porosity and substructure size on the drag on porous bodies is studied. Methods. We measured centimeter-sized porous aggregates with volume filling factors as low as ~10−4 for the first time in low-pressure wind tunnel experiments. Various substructures of different sizes down to micrometer (μm) resolution are tested. Knudsen numbers for the centimeter-sized superstructure are between 0.005 and 0.1 and Reynolds numbers are between 5 and 130. Results. We find that bodies are subject to increasingly large drag forces with increasing porosity, significantly larger than previously thought. In the parameter range measured, drag can increase by a factor of 23, and extrapolation suggests even larger values. We give an empirically determined model for an adjusted drag force. Conclusions. Our findings imply that the coupling of highly porous bodies in protoplanetary disks is significantly stronger than assumed in previous works. This decreases collision velocities and radial drift speeds and might allow porous bodies to grow larger under certain conditions before they become compacted.