Magneto-capillary dynamics of amphiphilic Janus particles at curved liquid interfaces

Abstract

A homogeneous magnetic field can exert no net force on a colloidal particle. However, by coupling the particle's orientation to its position on a curved interface, even static homogeneous fields can be used to drive rapid particle motions. Here, we demonstrate this effect using magnetic Janus particles with amphiphilic surface chemistry adsorbed at the spherical interface of a water drop in decane. Application of a static homogeneous field drives particle motion to the drop equator where the particle's magnetic moment can align parallel to the field. As explained quantitatively by a simple model, the effective magnetic force on the particle scales linearly with the curvature of the interface. For particles adsorbed on small droplets such as those found in emulsions, these magneto-capillary forces can far exceed those due to magnetic field gradients in both magnitude and range. This mechanism may be useful in creating highly responsive emulsions and foams stabilized by magnetic particles.

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

“…The assembled structures can be tuned by varying the directions of the magnetic fields (Movie S1, Supporting Information), which has potential applications in sensor or display technology. To estimate the time scale of the assembly and the structural transition, we assume that colloidal particles of radius a = 4 µm are adsorbed at a decane-water interface, with a surface tension γ = 53.2 mN m −1 , and the effective viscosity μ = 0.91 mPa s. [25,26] Based on our simulation results, the estimated time scale of structural formation and transition is about t ≈ 1 ms, which is sufficiently fast to satisfy the requirements of responsive materials for advanced sensor or display technologies. 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.…”

“…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.…”

Patterns that persist: Heritable information in stochastic dynamics

“…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. The circular track was fabricated in polydimethyl siloxane (PDMS) by soft lithography and treated with a surfactant solution (0.2 w/v% Pluronic F127 in water) to further prevent particle adhesion.…”

Collective transport of polar active particles on the surface of a corrugated tube

“…Ellis [16] studied the effect of self-propulsion and curvature on the degree of defect unbinding. On the other hand, curvature can induce spontaneous flow and flocking [22][23][24][25][26][27][28]. Sanchez and coworkers [22] experimentally studied selfpropelled microtubule bundles on curved surfaces and reported the spontaneous generation of a streaming flow.…”