# Rivalry of diffusion, external field and gravity in micro-convection of magnetic colloids

## TL;DR

This paper investigates how gravity-induced convection influences microfluidic mixing of magnetic colloids, revealing that it mimics diffusion and can be modeled with an effective diffusion coefficient, supported by simulations and experiments.

## Contribution

It introduces a theoretical model explaining enhanced diffusion due to gravity-driven convection in magnetic colloids within microchannels, validated by simulations and experiments.

## Key findings

- Gravity-induced convection causes interface smearing resembling diffusion.
- Effective diffusion coefficient can describe this convective mixing.
- Model applicability varies with microchannel thickness.

## Abstract

Magnetic fields and magnetic materials have promising microfluidic applications. For example, magnetic micro-convection can enhance mixing considerably. However, previous studies have not explained increased effective diffusion during this phenomenon. Here we show that enhanced interface smearing comes from a gravity induced convective motion within a thin microfluidic channel, caused by a small density difference between miscible magnetic and non-magnetic fluids. This motion resembles diffusive behavior and can be described with an effective diffusion coefficient. We explain this with a theoretical model, based on a dimensionless gravitational Rayleigh number, and verify it by numerical simulations and experiments with different cell thicknesses. Results indicate the applicability and limitations for microfluidic applications of other colloidal systems. Residual magnetic micro-convection follows earlier predictions.