Time-resolved coherent X-ray scattering analysis of nanorod colloids: dynamics and photon-statistical effects in angular X-ray cross-correlation analysis

Understanding the dynamic fluctuations of anisotropic particles is essential for elucidating the mechanisms of order formation through self-organization. Because anisotropic particles possess both translational and rotational degrees of freedom, their dynamics are intrinsically more complex than those of spherical particles. Time-resolved coherent X-ray scattering has the potential to probe such complex dynamics. Translational motion can be evaluated by X-ray photon correlation spectroscopy (XPCS), whereas orientational information can be extracted using the angular X-ray cross-correlation function (AXCCF). In this study, we investigated dynamic fluctuations in a colloidal nanorod system at concentrations near the coexistence region of isotropic and liquid-crystalline phases using time-resolved coherent X-ray scattering at 17.4 kHz. XPCS analysis revealed a pronounced slowing down of translational motion as the isotropic phase approached the liquid-crystalline phase. To examine possible temporal fluctuations in orientational order near the coexistence region, we applied a statistical approach based on AXCCF and principal component analysis. Temporal variations were indeed observed in the AXCCF. However, quantitative analysis showed that the magnitude and distribution of these variations are largely explained by photon-counting statistics expected for coherent scattering under low-count conditions. Although the present experiment did not provide definitive evidence for intrinsic orientational fluctuations beyond statistical noise, the analysis establishes an important framework for evaluating the detectability of orientational dynamics in time-resolved coherent X-ray scattering experiments and provides practical guidance for future measurements and validation.