Programmable droplet manipulation by a magnetic-actuated robot

Abstract

Droplet manipulations are fundamental to numerous applications, such as water collection, medical diagnostics, and drug delivery. Structure-based liquid operations have been widely used both in nature and in artificial materials. However, current strategies depend mainly on fixed structures to realize unidirectional water movement, while multiple manipulation of droplets is still challenging. Here, we propose a magnetic-actuated robot with adjustable structures to achieve programmable multiple manipulations of droplets. The adjustable structure redistributes the resisting forces from the front and rear ends of the droplets, which determine the droplet behaviors. We can transport, split, release, and rotate the droplets using the robot. This robot is universally applicable for manipulation of various fluids in rough environments. These findings offer an efficient strategy for automated manipulation of droplets.

Reconfigurable multifunctional ferrofluid droplet robots

“…The magnetization of ferrofluids is given by M = χ(H ext + H ferro (M)) due to the paramagnetic property, where χ is the volume susceptibility of the ferrofluid (31). When smartly programming H ext (B ext is equivalently used in later sections) by using either electromagnets (30) or permanent magnets (29) as illustrated in Fig. 1B, unique capabilities in terms of both morphology and mobility can be realized, such as splitting and merging behaviors, elongating to navigate through confined spaces, morphing into complex shapes, and coordinating group motions (Fig.…”

“…However, when B a =B b ≥ 0.4, the volume ratio V a =V b increases linearly as B a =B b increases (the "unequally splitting" phase), which enables an FDR to split into small droplets with various splitting volume ratios. This mechanism could allow an FDR to split into multiple subdroplets in different sizes on demand, which is useful for creating a team of FDRs for cooperative tasks or controlling microreactions in chemical engineering applications (29).…”

“…Fundamental studies have provided striking insights into the physics of ferrofluid droplets at the micro and macro scales (23)(24)(25). Recent works have also demonstrated utilizing these magnetic droplets for engineering applications (26)(27)(28)(29)(30). For example, magnetically actuated droplets have been controlled to navigate on various terrains by forming self-assembled structures and disassemble into multiple smaller droplets to pass through a narrow gap (26), while the functionality of such collective droplets is simple and their ability to complete cooperative tasks is also restricted by the lack of addressability of individual droplets.…”

“…For example, magnetically actuated droplets have been controlled to navigate on various terrains by forming self-assembled structures and disassemble into multiple smaller droplets to pass through a narrow gap (26), while the functionality of such collective droplets is simple and their ability to complete cooperative tasks is also restricted by the lack of addressability of individual droplets. In addition, other magnetically actuated droplet robots have also been proposed with promising proof-of-concept functionalities, such as transporting liquid samples and mixing chemicals in lab-on-a-chip applications (28,29). Yu et al (28) have proposed to employ ferrofluids as droplet robots for automated sample collection and transportation by controlling the Significance This work proposes reconfigurable multifunctional ferrofluid droplets as soft robots to overcome the limitation of existing magnetically actuated miniature soft robots based on elastomers, i.e., they cannot navigate inside very clustered and constrained spaces and reconfigure their shapes in situ for diverse tasks, due to limited deformability and predesigned shapes.…”

“…magnetic field produced by a millimeter-scale permanent magnet on top of an electromagnet array. Li et al (29) have developed magnetically actuated droplets by controlling two steel beads to manipulate a water or oil droplet, which has been utilized for sequential chemical reaction and simulated calculi removal. Despite recent advances in this field, existing magnetically actuated droplets can only have limited deformability, such as only simple elongation under a uniform magnetic field.…”