Magnetic-field-induced nematic-nematic phase separation and droplet formation in colloidal goethite

Analyzing spinodal decomposition of an anisotropic fluid mixture

Spinodal decomposition leads to spontaneous fluctuations of the local concentration. In the early stage, the resulting pattern provides explicit information about the material properties of the mixture. In the case of two isotropic fluids, the static structure factor shows the characteristic ring shape. If one component is a liquid crystal, the pattern is typically anisotropic and the structure factor is more complex. Using numerical methods, we investigate how structure factors can be used to extract information about material properties like the diffusion constant or the isotropic and the anisotropic contributions to the interfacial tension. The method is based on momenta taken from structure factors in the early stage of the spinodal demixing. We perform phase field calculations for an isotropic and an anisotropic spinodal decomposition. A comparison of the extracted results with analytic values is made. The calculations show that linear modes dominate in the beginning of the growth process, while non-linear modes grow monotonously in the region of the k-space for which damping is predicted by the linearized theory. As long as non-linear modes are small enough, linearized theory can be applied to extract material properties from the structure factor.

Colloidal aggregation and structural assembly of aspect ratio variant goethite (α-FeOOH) with nC60 fullerene in environmental media

Environmental mobility of C₆₀ fullerene can be significantly affected in the presence of naturally abundant α-FeOOH. However, α-FeOOH vary significantly in sizes, shapes and associated properties that can greatly influence the fate and transport of C₆₀ fullerene in environmental media. Therefore, colloidal hetero-association between well crystallized low aspect (L_Asp) α-FeOOH and nC₆₀ fullerene may differ substantially to weakly crystallized high-aspect (H_Asp) counterpart. In contrast to L_Asp α-FeOOH, inherent crystal defects and surface charge generation in H_Asp α-FeOOH facilitated strong Coulombic attraction and aggregation with fullerene in acidic pH. However, L_Asp α-FeOOH demonstrated subtle entropic depletion mediated interaction with fullerene prevalent in hard rods. Humic acid (HA) encapsulation of H_Asp α-FeOOH substantially blocked fullerene attachment. Minute enhancement in colloidal stability was detected for HA-coated H_Asp α-FeOOH and fullerene mixture to HA-coated H_Asp α-FeOOH alone. To investigate the interfacial assembly of α-FeOOH with fullerene "in situ" differential interference contrast (DIC) microscopic investigations were employed. This study showed significantly different interface behavior of the binary mixtures of fullerene and H_Asp α-FeOOH NPs, and L_Asp particles. On air-water interface, bare H_Asp α-FeOOH displayed liquid crystalline packing. However, addition of fullerene to H_Asp α-FeOOH suspension at pH5 produced closed-loop polygonal and circular ring structures. Head-to-tail alignment of magnetic dipoles as well as fullerene hydrophobicity facilitated such assembly formation. "Ex situ" AFM investigation revealed further the presence of magnetically derived ring structure which asserts that the formed "in situ" ensembles were not transient, hence, may abate fullerene transport through environmental interfaces. Barring hydrophobicity assisted attachment of fullerene to L_Asp α-FeOOHs, the absence of any close-packed structures may unlikely abate fullerene transport as envisaged in case of H_Asp α-FeOOH. Thus, aspect ratio variation and associated material properties of naturally abundant α-FeOOH may significantly impact fullerene transport through environmental media.