Shaping drops with magnetic fields

Abstract

The control of small volumes of fluids (or drops) is important for a wide range of applications, including lab-on-chip devices, where drops are transported and merged for sensing and chemical mixing; liquid lenses, where drops are shaped to set optical properties; and printing, where drops are generated by nozzles. Electric techniques are widely used to generate, transport, split and merge drops. Equivalent magnetic techniques are less well-known. Similarly to electric dipoles in electric fields, magnetic dipoles experience a force in magnetic fields. This effect, called magnetophoresis, is used to shape ferrofluids in magnetic valves and seals. Interest in shaping drops with magnetic fields for microfluidics has recently increased, and ferrofluids and paramagnetic salt solutions have been studied. The rich phenomenology of the interaction of magnetic fields and fluids offers ample opportunities for exploration. Diamagnetic fluids for example have no natural electric equivalent and are rarely studied as a tool for microfluidics. In this thesis, I study the shaping of drops with magnetic fields. My research focus is on para- and diamagnetic salt solutions. Deformation of drops using external fields and induced magnetism has not been fully explored in the literature. I study here how induced magnetism can shape the liquid-vapour interface of drops and control solids that float on them. This thesis includes (i) an introduction to the background of the interaction of electromagnetic fields and fluids; (ii) a derivation of an expression for the shape of drops in electromagnetic fields; (iii) experimental validation of this expression through the measurement of the shape of para- and diamagnetic axisymmetric sessile drops in homogeneous magnetic fields; (iv) demonstration of the transport of para- and diamagnetic drops in magnetic field gradients; (v) explorations of the use of shaping drops with magnetic fields for rheological measurements, and for the controlled driving of objects floating on drops. In summary, I explore how drops can be shaped in homogeneous magnetic fields, and how the drops can be transported by magnetic field gradients. These fundamental investigations may help stimulate novel applications of the controlled shaping of drops with magnetic fields. In particular, I explore how this technique can be used in rheology for food or medical research.