Path-Dependent Self-Assembly of Magnetic Anisotropic Colloidal Peanuts

Abstract

Here we present the field induced self-assembly of anisotropic colloidal particles whose shape resembles peanuts. Being made up of hematite core and silica shell, these particles align in a direction perpendicular to the applied external magnetic field. Using small-angle X-ray scattering with microradian resolution (μrad-SAXS) in sedimented samples, we have found that one can tune the self-assembled structures by changing the time of application of the external field. If the field is applied after the sedimentation, the self-assembled structure is a nematic one, while dipolar chains are formed if the field is applied during the sedimentation process. Interestingly, within each chain particles form a smectic phase with defects. Further, these aforementioned nematic and smectic phases are of oblate type in spite of the prolate shape of the individual particles. For dipolar chains, an unusual diffraction peak shape has been observed with highly anisotropic tails in the transverse direction (perpendicular to the external field). The peak shape can be rationalized by considering the fact that the dipolar chains can act as a building block aligned along the field direction to form a para-nematic phase.

Probing roto-translational diffusion of small anisotropic colloidal particles with a bright-field microscope

“…Before measurement, the sample is loaded in a glass capillary of thickness 100 µm, which is then sealed with vacuum grease. After about one hour at room temperature ((T = 22 ± 1) o C), the sample appears to be sedimented close to the bottom of the container as the sedimentation length l g ∼ 175 nm of these particles is smaller than their diameter [30]. The final number density is about 5•10 3 mm −2 , as obtained from particle counting (see Fig.…”

Probing roto-translational diffusion of small anisotropic colloidal particles with a bright-field microscope

“…Before measurement, the sample is loaded in a glass capillary of thickness 100 µm, which is then sealed with vacuum grease. After about one hour at room temperature ((T = 22 ± 1) • C), the sample appears to be sedimented close to the bottom of the container as the sedimentation length l g ∼ 175 nm of these particles is smaller than their diameter [30]. The final number density is about 5 • 10 3 mm −2 , as obtained from particle counting (see Fig.…”

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

“…We believe that the variation of the structure factor in this particular form is a quite general diffraction feature as it has been observed for other anisotropic colloids where one out of three rotational degrees of freedom is frozen. 24 …”