An important insight from our study is the different scales of interaction that give rise to the macroscopic optical response. We see clear evidence for aggregation or self-assembly of NRs in solution, manifest as increased optical anisotropy compared to isolated NRs. Further, we observe that these self-assembled aggregates are more readily aligned by an external electrostatic field, giving rise to greater overall orientational order as well as stronger field-driven modulation of absorbance. Our approach highlights the contrasting electro-kinetic behavior of individual vs aggregated NRs and allows for quantitative determination of the degree of order using an optical analysis that accounts for these mesoscale self-assembly effects as well as the macroscopic field-driven behavior of the entire ensemble.