At least 13 TEM images were acquired to represent each iron suspension, using a JEOL 1230 instrument. Suspensions were vortexed, and then a 50 µL aliquot from each was loaded onto a 3 mm copper TEM grid (Formvar/Carbon FCF200). Images were scaled and then processed using ImageJ 1.53k by thresholding to differentiate particles from the background. Two metrics-the Feret diameter (the maximum of all transects across each particle perimeter) and the ASTM roundness (the ratio of the observed area to the area of a circle with a diameter equal to the Feret diameter)-were estimated for each particle. We used a lower cutoff for particle identification of 335 nm 2 , which corresponds to the smallest spherical particle that the MALS detector can identify, based on manufacturer specifications (8 nm nominal radius of gyration).