Magnetically responsive manipulation of droplets and bubbles

Abstract

Droplets and bubbles have a wide range of applications in industry, agriculture, and daily life, and their controllable manipulation is of significant scientific and technological importance. Versatile magnetically responsive manipulation strategies have been developed to achieve precise control over droplets and bubbles. To manipulate nonmagnetic droplets or bubbles with magnetic fields, the presence of magnetic medium is indispensable. Magnetic additives can be added to the surface or interior of droplets and bubbles, allowing for on‐demand manipulation by direct magnetic actuation. Alternatively, magnetically responsive elastomer substrates can be used to actuate droplets and bubbles by controlling the deformation of microstructures on the substrates through magnetic stimulation. Another strategy is based on untethered magnetic devices, which enables free mobility, facilitating versatile manipulation of droplets and bubbles in a flexible manner. This paper reviews the advances in magnetically responsive manipulation strategies from the perspective of droplets and bubbles. An overview of the different classes of magnetic medium, along with their respective corresponding droplet/bubble manipulation methods and principles, is first introduced. Then, the applications of droplet/bubble manipulation in biomedicine, microchemistry, and other fields are presented. Finally, the remaining challenges and future opportunities related to regulating droplet/bubble behavior using magnetic fields are discussed.

Portable Triboelectric Electrostatic Tweezer for External Manipulation of Droplets within a Closed Femtosecond Laser-Treated Superhydrophobic System

“…13,14 However, droplet manipulation based on gradient structures is limited by short transport distances, single and fixed transport directions, and irreversible movement, arising from the foundation of these methods (i.e., the limited gradient range and the fixed gradient direction). 19,21 Another idea is to change the morphology or other physical and chemical properties of the substrate supporting the droplet through external stimuli (such as magnetism, 22 light, 23,24 and electricity 25 ) or to directly apply force to the droplets, 26−28 making the droplet follow the stimulus source to move forward. Although stimulus strategies allow droplets to move farther and in a more flexible direction than gradient structures, they often rely on essential surface pretreatment or droplet pretreatment.…”

“…In contrast to the magnetism and light stimuli, which drive droplets indirectly by changing the physical and chemical properties of the operating platform, − an electrostatic field can directly apply electrostatic force to droplets. , In addition, the electric field can easily penetrate the insulating material, so electrostatic forces show great potential for droplet manipulation in confined environments. However, electrostatic operating systems based on a high-voltage input are unsafe, not easy to carry, and not convenient enough.…”

“…Controllable droplet manipulation plays an indispensable role in various applications, such as microfluidics, , printing technology, , biological detection and analysis, , combinatorial chemistry, , water harvesting, , and heat management. , Due to the risk of contamination associated with contact operation, contactless droplet manipulation has attracted increasing attention in recent years. There are usually two strategies for achieving noncontact transport of droplets. − One is to design geometric, chemical, wetting, or even charge gradient structures on the surface of solid materials. − The droplets on the gradient structures have asymmetric three-phase contact lines or contact angles, which enable the droplets to move along the gradient direction spontaneously under the asymmetric forces (e.g., Laplace force). , However, droplet manipulation based on gradient structures is limited by short transport distances, single and fixed transport directions, and irreversible movement, arising from the foundation of these methods (i.e., the limited gradient range and the fixed gradient direction). , Another idea is to change the morphology or other physical and chemical properties of the substrate supporting the droplet through external stimuli (such as magnetism, light, , and electricity) or to directly apply force to the droplets, − making the droplet follow the stimulus source to move forward. Although stimulus strategies allow droplets to move farther and in a more flexible direction than gradient structures, they often rely on essential surface pretreatment or droplet pretreatment. ,,− Nonetheless, despite extensive progress, these contactless manipulations are usually carried out on open material surfaces, and few methods are capable of manipulating droplets in a confined space from the outside without surface or droplet pretreatment.…”