Formation of nematic order in 3D systems of hardcolloidal ellipsoids

Jörg Roller, John David Geiger, Markus Voggenreiter, Janne-Mieke Meijer and Andreas Zumbusch Soft Matter, 2020, 16, 1021 DOI: 10.1039/c9sm01926j

Abstract: Suspensions of hard ellipsoidal particles exhibit complex phase behavior as shown by theoretical predictions and simulations of phase diagrams. Here, we report quantitative confocal microscopy experiments of hard prolate colloidal ellipsoids with different aspect ratio a/b. We studied different volume fractions phi of ellipsoids in density and refractive index matched suspensions. Large 3D sample volumes were investigated and the positions as well as the orientations of all ellipsoids were extracted by image analysis routines. By evaluating the translational and orientational order in the system we determined the presence of isotropic and nematic phases. For ellipsoids with a/b= 2.0 we found that isotropic phases form at all phi, while ellipsoids with a/b= 7.0 formed nematic phases at high phi,as expected from theory and simulations. For a/b= 3.5 and a/b= 4.1, however, we observed the absence of long-range orientational order even at phi where nematic phases are expected. We show that local orientational order formed with the emergence of nematic precursors for a/b= 3.5 and short-ranged nematic domains for a/b= 4.1. Our results provide novel insight into the phase behavior and orientational order of ellipsoids with different aspect ratios.