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.
Highlights
The self-assembly of peanut shaped colloidal particles was studied in the presence of external magnetic fields under two different conditions
The magnetic field was applied after the sedimentation process was over, whereas in the second case, the field was applied while the particles were still sedimenting
We have shown that one can tune the selfassembled structures formed by anisotropic magnetic colloidal peanuts in the presence of external magnetic field by changing the time of application of the external field
Summary
The ability to manipulate the self-assembly in colloidal systems has opened up an alternative route toward the development of smart materials with designable properties.[1,2] As a result, a considerable amount of research has been carried out over the last couple of decades to understand as well as to manipulate the self-assembled structures formed by colloidal particles possessing basic shapes like spheres,[3,4] rods,[5−7] disks,[8] and ellipsoids.[9,10] Recent progress in the synthetic techniques of colloidal particles has resulted in the fabrication of particles with complex shapes resembling peanuts,[11−13] dumbbells,[14−16] polyhedras,[17,18] and octapods,[19] to name but a few. Controlling the self-assembly of these particles requires a thorough control over the interparticle interactions. There are some other major challenges that need to be overcome control over particle orientation, their localization, and their registry into self-assembled structures. The possibility to explore the different self-assembled structures formed from diverse complex building blocks has contributed significantly toward reinvigorating colloidal science in recent times
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