Magneto-Capillary Particle Dynamics at Curved Interfaces: Time-Varying Fields and Drop Mixing

Abstract

Spatially uniform magnetic fields induce nonzero forces on magnetic particles adsorbed at curved liquid interfaces thereby driving their motion. Such motions, prohibited in bulk fluids, arise due to interfacial constraints that couple magnetic torques to capillary forces at curved interfaces. Here, we show that time-varying (spatially uniform) magnetic fields can be used to drive a variety of steady particle motions on water drops in decane. Upon application of a precessing field, magnetic Janus particles with amphiphilic surface chemistry move either along circular orbits at the drop poles or along zigzag paths at the drop equator. The different magneto-capillary motions depend on the frequency and precession angle of the field as well as the initial position of the particle on the drop surface. Our experimental observations are reproduced and explained by a mathematical model that accounts for the relevant magnetic, capillary, and hydrodynamic forces and torques that contribute to particle motion. In addition to ferromagnetic Janus particles, we show that similar dynamics can be achieved using superparamagnetic carbonyl iron particles, which are manufactured on industrial scales and respond to even weaker magnetic fields. We demonstrate how the field-driven motion of such particles at the drop interface can induce fluid flows that effectively mix the drop interior. These results suggest that magneto-capillary particle motions could be used to enhance mass transfer within emulsions stabilized by magnetic particles.

Patterns that persist: Heritable information in stochastic dynamics

“…We prepared magnetic Janus particles (MJPs) by e-beam deposition of successive layers of metal-5 nm Ti adhesive layer, 25 nm Ni magnetic layer-onto monolayers of 4 µm fluorescent sulfonated polystyrene (PS) particles. 45 The nickel layer gave the particles a permanent magnetic moment m ≈ 3 × 10 −14 A m 2 directed parallel to the Janus equator. 46 To mitigate their agglomeration in water, the Janus particles were coated with a thin layer of silicon dioxide by sputter deposition.…”

Controllable Capillary Assembly of Magnetic Ellipsoidal Janus Particles into Tunable Rings, Chains and Hexagonal Lattices

“…For a possible experimental realization of our system, we note that magnetic spherical Janus particles have been experimentally fabricated [27][28][29][30] and investigated at a liquid-liquid interface. [25,31] Such Janus particles can have various amphiphilicities [32,33] and may be stretched mechanically to form ellipsoidal particles with various aspect ratios. [34][35][36] Assuming Janus particles with radius a = 100 nm, aspect ratio α = 3 and magnetic moment m ≈ 4 × 10 −12 A m 2 adsorbed at a water-decane interface with a surface tension 70 ws γ = mN m −1 , an external magnetic field B ≈ 0.1 T is able to introduce a magnetic torque larger than the capillary torque and to produce the various structures observed in our simulations.…”

Manipulating Dispersions of Magnetic Nanoparticles

“…FERROFLUIDS To prevent aggregation of the MNPs in FFs, functional polymer ligands are usually grafted onto the surface of the particles, for example, polyethylene glycol stabilized magnetite (Fe 3 O 4 −PEG) nanoparticles in water. 13 Iron oxide nanoparticles with a slight size-dependent magnetic moment are commonly used in FFs. Multiple interactions, including magnetic dipolar interactions and static electrical repulsions between the MNPs are dynamically balanced.…”

“…To prevent aggregation of the MNPs in FFs, functional polymer ligands are usually grafted onto the surface of the particles, for example, polyethylene glycol stabilized magnetite (Fe 3 O 4 –PEG) nanoparticles in water . Iron oxide nanoparticles with a slight size-dependent magnetic moment are commonly used in FFs.…”