Large scale aggregation in magnetic colloids induced by high frequency magnetic fields

•250 nm superparamagnetic magnetic microgels in APTAC matrix were synthesized.•A 0.4 w% aqueous dispersion of magnetic microgels was investigated.•Large scale aggregation was discovered in weak 100 kHz AC magnetic fields.•The aggregates are microns thick and hundred microns long.•The aggregates consist of up 80% of the total amount of magnetic microgels. The paper presents evidence of large scale aggregation in an aqueous dispersion of magnetic microgels, induced by 100 kHz AC magnetic fields with amplitude ranging from 40 Oe to 120 Oe. The microgels with 250 nm z-average hydrodynamic diameter and 44 emu/g saturation magnetization are composed by 8 nm magnetite nanoparticles imbedded in (3-acrylamidopropyl)-trimethylammonium chloride (APTAC) matrix. The magnetically induced aggregation was investigated by means of light extinction (LE) and small angle light scattering (SALS) at room temperature. The SALS experiments show the formation and growth of field direction elongated aggregates with thickness in the range of a few microns and length in the range of tens to hundreds microns. The extinction experiments show the increasing of aggregate total volume with increasing magnetic field amplitude. It was estimated that, at the saturation of the aggregation process in 120 Oe amplitude field, the aggregates consist of about 80% of the total amount of magnetic microgels within the sample. These findings have potential interest for magnetic composite applications in magnetic hyperthermia and heat transfer applications.