Ferro-hydrodynamics of droplet necking filaments

TL;DR

This study investigates how magnetic fields influence the filament thinning and breakup of ferrofluid droplets, revealing non-Newtonian behaviors and microstructure effects on stability and flow dynamics.

Contribution

It introduces a simple experimental method to study ferrofluid filament dynamics in a regime with Oh and De around 1, highlighting magnetic microstructure effects.

Findings

Magnetic fields induce extensional thickening and beads-on-a-string structures.

Magnetically assembled nanoparticle chains alter extensional viscosity and relaxation times.

Ferrofluids exhibit non-Newtonian behavior under magnetic forcing, affecting filament stability.

Abstract

We explore the necking, filament thinning, and pinchoff dynamics of ferrofluid droplets within a magnetic field, via a simple and low-cost experimental method. In our studies, both the Ohnesorge number Oh and the Deborah number De are O1, a typically inaccessible regime with conventional extensional rheometers. Under magnetic forcing, the nanoparticles assemble into field aligned, chainlike structures, that generate a tunable magnetoelastic response, and markedly alter the extensional flow. Although behaving as Newtonian liquids in the absence of a magnetic field, the field induces extensional thickening, and the emergence of beads on a string BOAS structures in the ferrofluid filaments, a non-Newtonian signature. By combining controlled elongation with high speed imaging, we directly quantify the magnetic field-dependent extensional viscosity and relaxation time. Our findings underscore how magnetically induced microstructures govern filament stability and extensional dynamics in ferrofluids.