Droplet Manipulation under a Magnetic Field: A Review

Abstract

The magnetic manipulation of droplets is one of the emerging magnetofluidic technologies that integrate multiple disciplines, such as electromagnetics, fluid mechanics and so on. The directly driven droplets are mainly composed of ferrofluid or liquid metal. This kind of magnetically induced droplet manipulation provides a remote, wireless and programmable approach beneficial for research and engineering applications, such as drug synthesis, biochemistry, sample preparation in life sciences, biomedicine, tissue engineering, etc. Based on the significant growth in the study of magneto droplet handling achieved over the past decades, further and more profound explorations in this field gained impetus, raising concentrations on the construction of a comprehensive working mechanism and the commercialization of this technology. Current challenges faced are not limited to the design and fabrication of the magnetic field, the material, the acquisition of precise and stable droplet performance, other constraints in processing speed and so on. The rotational devices or systems could give rise to additional issues on bulky appearance, high cost, low reliability, etc. Various magnetically introduced droplet behaviors, such as deformation, displacement, rotation, levitation, splitting and fusion, are mainly introduced in this work, involving the basic theory, functions and working principles.

Keywords: droplet manipulation; liquid actuation; magnetic field; magnetization; microfluidics.

Figure 1

Ferrofluid droplet generation devices: (a) Straight channels with permanent magnetic field [80]; (b) Driven by a permanent magnet without a pump [81]; (c) In cross-shaped channel with a perpendicular uniform magnetic field [88]; (d) In T-shaped microfluidic chip with a permanent magnet [84]; (e) Horizontal magnetic field [90]; (f) Radial magnetic field [91].

Figure 2

Suspended and sessile ferrofluid droplet deformation in a magnetic field: (a) Suspended ferrofluid droplet undergoes tensile deformation [15]; (b) Sessile ferrofluid droplet with magnetic dots [95]; (c) Sessile ferrofluid droplet above permanent magnet [101]; (d) Sessile ferrofluid droplet beneath permanent magnet [102]; (e) Asymmetrical deformation with gravitational force [105]; (f) Ferrofluid peak with coupling of magnetic and electric fields [107].

Figure 3

Droplet transportation in magnetic actuation: (a) Ferrofluid plugs with rotating permanent magnets [115]; (b) Ferrofluid droplet with sliding permanent magnets [73]; (c) Ferrofluid droplets with electromagnetic coils [56]; (d) Ferrofluid droplets with permanent and electromagnetic fields [122]; (e) Water droplet moving on substrate deformation [128]; (f) Water droplet moving based on magnetic nano/ micropillar arrays [130].

Figure 4

Different magnetic sorting methods: (a) Based on the difference in magnetism [136]; (b) Droplets deflected to the indicated outlet [18]; (c) Magnetic rails [21]; (d) Based on the difference in flow speed and droplet size [19]; (e) Deflect of water droplets in ferrofluid medium [137].

Figure 6

Magnetic levitation of droplets: (a) Ferrofluid droplet levitation and splitting in an electromagnetic field [154]; (b) Molten silicon droplets levitation in a coupled field [161].

Figure 5

Droplet coalescence and splitting in a magnetic field: (a) Uniform magnetic field induced droplet merging in a cross-shaped channel [22]; (b) Permanent magnet induced droplet merging in a cross-shaped channel [23]; (c) Permanent magnet induced droplet merging in a Y-shaped channel [24]; (d) Droplet merging in coupling uniform and non-uniform magnetic fields [25]; (e) Droplet splitting in T-shaped channel [26]; (f) Droplet splitting in Y-shaped channel [27].