Ferrofluid drop impacts and Rosensweig peak formation in a non-uniform magnetic field

TL;DR

This study investigates ferrofluid drop impacts under non-uniform magnetic fields, revealing how magnetic forces influence drop spreading, peak formation, and stability without splashing, using high-speed imaging.

Contribution

It provides new insights into ferrofluid impact dynamics and Rosensweig peak formation under variable magnetic fields, with detailed experimental observations.

Findings

Peak formation occurs at the drop edge similar to crown instabilities.

High magnetic fields create stationary ferrofluid rings above the magnet.

Drop impacts produce Rosensweig instabilities without splashing.

Abstract

Vertical drop impacts of ferrofluids onto glass slides in a non-uniform magnetic field have been studied using high-speed photography. Outcomes have been classified based on the motion of the fluid-surface contact lines, and formation of peaks (Rosensweig instabilities) which affect the height of the spreading drop. The largest peaks are nucleated at the edge of a spreading drop, similarly to crown-rim instabilities in drop impacts with conventional fluids, and remain there for an extended time. Impact Weber numbers ranged from 18.0 to 489, and the vertical component of the B-field was varied between 0 and 0.37 T at the surface by changing the vertical position of a simple disc magnet placed below the surface. The falling drop was aligned with the vertical cylindrical axis of the 25 mm diameter magnet, and the impacts produced Rosensweig instabilities without splashing. At high magnetic field strengths a stationary ring of ferrofluid forms approximately above the outer edge of the magnet.