Shape Matters in Magnetic-Field-Assisted Assembly of Prolate Colloids

Abstract

An anisotropic colloidal shape in combination with an externally tunable interaction potential results in a plethora of self-assembled structures with potential applications toward the fabrication of smart materials. Here we present our investigation on the influence of an external magnetic field on the self-assembly of hematite-silica core–shell prolate colloids for two aspect ratios ρ = 2.9 and 3.69. Our study shows a rather counterintuitive but interesting phenomenon, where prolate colloids self-assemble into oblate liquid crystalline (LC) phases. With increasing concentration, particles with smaller ρ reveal a sequence of LC phases involving para-nematic, nematic, smectic, and oriented glass phases. The occurrence of a smectic phase for colloidal ellipsoids has been neither predicted nor reported before. Quantitative shape analysis of the particles together with extensive computer simulations indicate that in addition to ρ, a subtle deviation from the ideal ellipsoidal shape dictates the formation of this unusual sequence of field-induced structures. Particles with ρ = 2.9 exhibit a hybrid shape containing features from both spherocylinders and ellipsoids, which make their self-assembly behavior richer than that observed for either of the “pure” shapes. The shape of the particles with higher ρ matches closely with the ideal ellipsoids, as a result their phase behavior follows the one expected for a “pure” ellipsoidal shape. Using anisotropic building blocks and external fields, our study demonstrates the ramifications of the subtle changes in the particle shape on the field-directed self-assembled structures with externally tunable properties.

Extending depolarized DLS measurements to turbid samples

“…Both D T and D R are indeed independent of the concentration, which confirms that 3D-DLS using the mod3D technique is capable of reliably suppressing multiple scattering effects and allows us to correctly determine translational and rotational diffusion coefficients for highly turbid suspensions with transmission values as low as around 1%. Using the TEM data as input, D T and D R were calculated using the theoretical approach of Ortega et al for elongated particles, and the results are also shown as the green line in Fig 9 . Given the known shape imperfections of these particles [36], the agreement between the theoretical calculation and the measured values is surprisingly good. Note that two alternative expressions for different geometrical models, ellipsoids and spherocylinders, as described in Martchenko et al [13], resulted in significantly larger discrepancies FIG.…”

Stacking Interactions and Flexibility of Human Telomeric Multimers

“…For more details about this model, see the Methods section. Scattering intensities from MC simulations were obtained following a procedure already successfully used to describe reversible self-aggregation processes in other DNA-based systems and especially suitable for representing the hydrophobic (stacking) forces acting between G4 units. − …”

Connecting Structure, Conformation and Energetics of Human Telomere G-quadruplex Multimers

“…To obtain more quantitative information on the G4 ensembles, we developed a novel approach comparing SAXS experimental intensities with those computationally estimated through Monte Carlo (MC) simulations, by modeling the G4 unit as a hard cylinder (HC) with two attractive sites at the basis, as schematized in Figure 2 (for more details see the Methods section). Scattering intensities from simulations of HCs have been obtained following a procedure similar to that discussed in Reference [36]. Within this procedure each cylinder is replaced with a set of scattering points randomly placed inside its volume with a fixed number density, and by using these points the structure factor is then calculated.…”