Abstract
Anisotropic particles are known to exhibit a richer and more complex phase behavior in comparison to their spherical counterpart. While the majority of the existing studies address structural properties, the dynamic behavior of anisotropic particles is a relatively lesser explored avenue. Using multispeckle ultra-small-angle x-ray photon correlation spectroscopy (USA-XPCS), we have carried out a systematic investigation of the structural and dynamic properties of colloidal ellipsoids at the nearest-neighbor length scale. The USA-XPCS measurements have allowed us to probe, as a function of the volume fraction, the q-dependent effective structure factor, Seff(q), along with the effective long time diffusion coefficient, Deff(q), for this anisotropic system. Our results indicate a scaling behavior of Deff(q) with 1/Seff(q) from which we have estimated the effective amplitude function Aeff(q), which can be directly related to the effective hydrodynamic function Heff(q). Aeff(q) shows a similar q dependence to that of S(q). Our investigation also allows for the precise determination of the volume fraction corresponding to the arrest transition.
Highlights
Unlike their spherical counterparts, anisotropic particles exhibit a richer phase behavior with enhanced complexity due to their orientational degrees of freedom
We report the dynamics of colloidal ellipsoids at the nearest-neighbor length scale over a broad concentration range by employing ultra-small-angle x-ray photon correlation spectroscopy (XPCS)
The literature is replete with simulations investigating the glass transition for ellipsoidal particles, there is a scarcity of experimental reports on these systems
Summary
Anisotropic particles exhibit a richer phase behavior with enhanced complexity due to their orientational degrees of freedom. Three types of glass transitions have been predicted: the first is the conventional cage-driven one, which is typical for spherical particles, while the origins of the other two are related to the orientational degrees of freedom. Our results indicate that at all the concentrations investigated, the density fluctuations relax via a single decay process, which can be related to the long time effective diffusion coefficient, Deff. Using the aforementioned long time scaling, we have calculated Aeff(q) for colloidal ellipsoids with ρ = 2.9, which shows a similar q dependence to that of Seff(q). We found that both the structural correlations and the diffusion coefficients are isotropic except at the highest concentration studied, φmax = 0.42, where a nematic phase is formed.
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