The precise description of the motion of anisotropic particles in a flow rests on the understanding of the force and torque acting on them. Here, we study experimentally small, very elongated particles settling in a fluid at small Reynolds number. In our experiments, we can, to a very good approximation, relate the rate of rotation of cylindrical tungsten rods, of aspect ratios $\beta =8$ and $\beta =16$, settling in pure glycerol, to the torque they are experiencing. This allows us to compare the measured torque with expressions obtained either in the slender-rod limit or in the case of spheroids. Both theories predict a simple angle dependence for the torque, which is found to capture very well the experimental results. The slender-rod theory overestimates the results for the two aspect ratios considered, while the expression obtained for a spheroid provides a better approximation for $\beta =16$. Comparing our results with those of previous experiments provides further insight on the conditions of validity of the slender-rod theory. The translational dynamics is shown to be in qualitative agreement with the slender-rod and spheroid models, the former one being found to represent better the experimental data.

• Received 15 July 2021
• Accepted 24 January 2022

DOI:https://doi.org/10.1103/PhysRevFluids.7.024301