Retarding the growth of the {Rosensweig} instability unveils a new scaling regime

TL;DR

This study slows down the Rosensweig instability in a viscous magnetic fluid, revealing a new scaling regime and highlighting discrepancies between theory and experiment due to shear thinning effects.

Contribution

It demonstrates a method to extend the observable growth regime of the Rosensweig instability and introduces a viscosity model to explain experimental deviations.

Findings

Growth and decay rates differ, contrary to predictions

Shear thinning affects the viscosity during instability dynamics

A new scaling regime for the instability is observed

Abstract

Using a highly viscous magnetic fluid, the dynamics in the aftermath of the Rosensweig instability can be slowed down by more than 2000 times. In this way we expand the regime where the growth rate is predicted to scale linearly with the bifurcation parameter by six orders of magnitude, while this regime is tiny for standard ferrofluids and can not be resolved experimentally there. We measure the growth of the pattern by means of a two-dimensional imaging technique, and find that the slopes of the growth and decay rates are not the same - a qualitative discrepancy to the theoretical predictions. We solve this discrepancy by taking into account a viscosity which is assumed to be different for the growth and decay. This may be a consequence of the measured shear thinning of the ferrofluid.