Tunable Superparamagnetic Ring (tSPRing) for Droplet Manipulation

Abstract

The manipulation of droplets via a magnetic field forms the basis of a fascinating technology that is currently in development. Often, the movement of droplets with magnets involves adding magnetic particles in or around the droplet; alternatively, magneto responsive surfaces may also be used. This work, presents and characterizes experimentally the formation and properties of a tunable superparamagnetic ring (tSPRing), which precisely adjusts itself around a water droplet, due to liquid–liquid interaction, and enables the physical manipulation of droplets. The ring is made of an oil‐based ferrofluid, a stable suspension of ferromagnetic particles in an oily phase. It appears spontaneously due to the oil–water interfacial interaction under the influence of a magnetic field. The ferrofluid–water interaction resembles a cupcake assembly, with the surrounding ring only at the base of the droplet. The ring is analogous to a soft matter ring magnet, showing dipole repulsive forces, which stabilizes the droplets on a surface. It enables robust, controllable, and programmable manipulation of enclosed water droplets. This work opens the door to new applications in open surface upside or upside‐down microfluidics and lays the groundwork for new studies on tunable interfaces between two immiscible liquids.

Horizontal and Vertical Coalescent Microrobotic Collectives Using Ferrofluid Droplets

“…[33][34][35][36] Nevertheless, previous research has mainly focused on the simple splitting and horizontal self-assembled structures of the ferrofluid droplets induced by magnetic-field-gradient forces, which typically exhibit a single spike-array mode. [37,38,41,45] The controlled splitting behavior of ferrofluid droplets under magnetic torque is also overlooked. Moreover, the horizontal and vertical dynamic self-assembly behavior of ferrofluid droplets still needs to be explored.…”

Bioinspired materials for droplet manipulation: Principles, methods and applications

“…As shown in Figure 10b, various types of droplet manipulation techniques have been realized on surfaces. Nasirimarekani et al 119 presented a tunable superparamagnetic ring (tSPRing), which was composed of an oil‐based ferrofluid. Under the action of a magnetic field, the tSPRing could manipulate the droplets in a powerful, controllable and programmable way (Figure 10c).…”

“…(c) Ferrofluid spikes deposited on the surface, holding the water droplets in place, when a magnetic field over the saturation magnetization of the ferrofluid was applied to the tSPRing. Reproduced with permission 119 . Copyright 2021, Wiley‐VCH.…”

Self‐Transportation of Superparamagnetic Droplets on a Magnetic Gradient Slippery Surface with On/Off Sliding Controllability

“…Because of so many superiorities such as remote operability, and rapid response, dynamic control of magnetic droplets under a magnetic field has attracted more and more attention. [41][42][43][44] Applying superhydrophobic surfaces with high water-repellent ability and slippery surfaces with low friction can avoid lots of challenges during the traditional manipulation process such as mass loss and cross contamination. [45][46][47][48] For example, Jiang et al reported no-loss transportation of a magnetic droplet by a high adhesive superhydrophobic polystyrene film; [49] Guo et al demonstrated regulation of the sliding state of a magnetic droplet on a superhydrophobic surface by adjusting the intensity of the magnetic field; [50] Rigoni et al demonstrated an unique stick-slip motion of a magnetic droplet on a slippery surfaces under the action of the patterned magnetic field; [51] we recently also reported the in-situ on/off sliding control of a superparamagnetic microdroplet on a superhydrophobic surface.…”