# Spin-coating of moderately concentrated superparamagnetic colloids in different magnetic field configurations

## TL;DR

This study investigates how different magnetic field configurations influence the spin-coating process of superparamagnetic colloids, revealing the significant role of magnetophoretic effects on deposit formation and fluid viscosity.

## Contribution

It introduces an analysis of magnetic field effects on spin-coating of superparamagnetic colloids, highlighting the impact of magnetophoretic forces on viscosity and deposit uniformity.

## Key findings

- Magnetic field configuration affects deposit thickness and uniformity.
- Non-uniform magnetic fields increase effective magnetoviscosity.
- Magnetophoretic effects significantly influence flow and deposit formation.

## Abstract

Spin-coating technique is very fast, cheap, reproducible, simple and needs less material to fabricate films of particulate systems/colloids. Their thickness and uniformity may be controlled by means of external fields. We apply magnetic fields during the spin-coating of a moderately concentrated superparamagnetic colloid (made of silica coated magnetite particles). We study the influence of different magnetic field configurations (homogeneous and inhomogeneous) on the resulting spin-coated deposits and compare experimental results under various conditions. Superparamagnetic colloids behave as, non-Newtonian, magnetorheological fluids. Their viscosity vary significantly under applied magnetic fields. We measure and compare the effect of uniform and non-uniform magnetic fields on their relative effective viscosity, using the spin-coated deposits and a previously existing model for simple colloids. The mechanisms involved in the deposits formation under different experimental conditions are also discussed. In particular, we show that the magnetophoretic effect plays an important role in the spin-coating of magnetic colloids subjected to non-uniform magnetic fields. We characterize an effective magnetoviscosity in non-uniform magnetic fields that is largely influenced by the magnetophoretic effect that enhances the flow of the magnetic fluid.