Dynamics of Magnetic Droplets on Liquid-Repellent Surfaces

Ferrofluids are colloidal suspensions of superparamagnetic nanoparticles in a carrier liquid. The combination of liquid properties and a strong magnetic response makes ferrofluids versatile materials exhibiting fascinating phenomena. The main focus of this thesis is on the intersection of ferrofluids and highly water-repellent surfaces. The basic properties of ferrofluids and the limitations of contact angle goniometry are experimentally investigated, and novel field-induced instabilities and ferrofluid-based wetting characterisation methods are demonstrated. Publication I is an in-depth comparative analysis of two ferrofluids with electrostatic and electro-steric stabilisation. Structure, colloidal stability, magnetic and flow properties of the fluids are investigated with a range of advanced techniques. Publication II reviews recent literature on wetting of ferrofluids, which is important for microfluidics applications and magnetic actuation. The Publication discusses contact angle goniometry, the most common wetting characterisation method, and the challenges caused by the magnetic-field-induced deformation of ferrofluid droplets. Publications III and IV examine the challenges of contact angle goniometry on highly water-repellent surfaces. In the superhydrophobic wetting regime, an error of just a single pixel in locating the solid-liquid interface leads to large inaccuracies in the measured contact angles, rendering this technique poorly suited for these surfaces. Publications V and VI describe wetting characterisation methods for superhydrophobic surfaces based on oscillating magnetic droplets. A water droplet with a tiny amount of superparamagnetic nanoparticles is brought to an oscillatory motion in a parabolic magnetic field on the surface under investigation. The wetting properties can be measured from the rate at which the oscillations decay due to the dissipative forces. Publications VII and VIII investigate magnetic-field-induced ferrofluid droplet splitting on water-repellent surfaces. The field-induced instability is used to create self-assembled droplet populations, which can be magnetically actuated. Reversible switching between static and dynamic self-assembly is demonstrated using an oscillating field, and novel concepts for microfluidics and interfacial tensiometry are introduced. This thesis improves our understanding on the basic properties of ferrofluids and examines the limitations of contemporary wetting characterisation methods. Building on this foundation, it introduces novel phenomena and concepts, which can help the development of advanced ferrofluid-based applications.