Insights into Permanent Encodings of Macroscopic Spike Patterns by Magnetic-Field-Directed Evaporative Self-Assembly from Ferrofluids

Field-directedassembly has the potential to make large hierarchicallyordered structures from nanoscale objects. Shear forces and optical,electric, and magnetic fields have been used for this purpose. Ferrofluidsconsist of magnetic nanoparticles hosted in mobile liquids. Thoughthey exhibit rich structures and lattice patterns in response to anapplied magnetic field, the patterns collapse when the field is removed.Recently, we adapted evaporation-induced self-assembly to obtain permanentencodings of the complex field response of magnetite nanoparticlesin alkane media. The encodings are characterized by order that culminatesin macrostructures comprising kinetically trapped spike patterns.The present work examines a number of variables that control patternformation associated with this encoding. Control variables includeapplied magnetic field strength, magnetic field gradient, nanoparticleconcentration, solvent evaporation conditions, and alkane solventchain length. The pattern formation process is captured in six stagesof evolution until the solvent host has evaporated and the patternis permanently fixed. The macropatterns consist of hexagonal arraysthat coexist with different pentagonal and heptagonal defects. TheVoronoi entropy is calculated for different patterns that arise dueto changes in the control parameters. Insight into order in the latticepatterns is achieved by extracting measurables like peak-to-peak spikewavelength, spike population, spike height, and base diameter fromthe patterns. The pattern measurables depend nonlinearly on the magneticfield gradient, solvent evaporation rate, and solvent chain length.Nanoparticle concentration does not impact the measurables significantly.Nonetheless, the results agree qualitatively with a linear expressionfor the critical magnetization and wavelength that explicitly containsthe field gradient and surface tension.

Automated summary

Researchers have successfully obtained permanent encodings of the complex field response of magnetite nanoparticles in alkane media using evaporation-induced self-assembly. They examined several variables that control the pattern formation and found that the magnetic field gradient, solvent evaporation rate, and solvent chain length nonlinearly affect the pattern measurables. The nanoparticle concentration, however, has only a minor impact on the measurables.