Sharpening surface of magnetic paranematic droplets

TL;DR

This paper investigates how magnetic fields induce cusp formation in colloidal droplets containing nickel nanorods and nanoparticles, revealing controlled assembly mechanisms with potential applications in nanomaterial fabrication.

Contribution

It introduces a model explaining cusp formation through magnetostatic and surface tension forces and demonstrates controlled nanorod alignment at droplet surfaces.

Findings

Nanorods form reversible cusps under magnetic fields.

Cusp formation explained by magnetostatic and surface tension forces.

Potential for aligning nanomaterials like carbon nanotubes before fiber spinning.

Abstract

In a non-uniform magnetic field, the droplets of colloids of nickel nanorods and nanoparticles aggregate to form a cusp at the droplet surface not deforming the entire droplet shape. When the field is removed, nanorods diffuse away and cusp disappears. Spherical particles can form cusps in a similar way, but they stay aggregated after release of the field; finally, the aggregates settle down to the bottom of the drop. X-ray phase contrast imaging reveals that nanorods in the cusps stay parallel to each other without visible spatial order of their centers of mass. Formation of cusps can be explained with a model that includes magnetostatic and surface tension forces. The discovered possibility of controlled assembly and quenching of nanorod orientation under the cusped liquid surface offers vast opportunities for alignment of carbon nanotubes, nanowires and nanoscrolls, prior to spinning them into superstrong and multifunctional fibers. Magneto and electrostatic analogy suggests that similar ideal alignment can be achieved with the rod-like dipoles subject to a strong electric filed.