1. Introduction

Efficient and controllable liquid manipulation has recently attracted considerable attention due to its broad range of applications in fields such as chemical synthesis and analysis [1, 2], energy and environmental governance [3], health diagnosis and treatment [4-6], and advanced manufacturing [7, 8]. To achieve precise control over liquid motion, a variety of strategies have been developed, which are generally categorized into passive and active approaches. Passive approaches predominantly focus on engineering surface wettability gradients [9-12] and Laplace pressure differences induced by asymmetric structures [13, 14] to manipulate liquid movement, without requiring external energy inputs and are straightforward to implement. Meanwhile, active approaches provide greater versatility and precision by utilizing external stimuli, such as light [15], electric fields [16-18], acoustic fields [19], thermal gradients [3, 20], magnetic fields [21-23], and pH changes [24]. These stimuli enable a wide range of dynamic and programmable manipulation capabilities, including on-demand liquid motion, mixing, and flow regulation, making them particularly attractive for advanced applications in microfluidics, biomedical technologies, and industrial processing.

Among these stimuli, magnetic actuation has emerged as a particularly promising method for liquid manipulation due to its unique advantages, such as instantaneous response, remote operation, environmental robustness, and excellent biocompatibility [22, 25-28]. Furthermore, magnetic actuation can be combined with other external stimuli or tailored to trigger specific physicochemical responses, enabling highly efficient and multifunctional fluid management [29, 30]. However, conventional magnetically responsive liquid manipulation has primarily focused on droplet movement on open surfaces [31-33]. These strategies often involve using magnetic additives to render nonmagnetic liquids magnetically active [34, 35], or magneto-responsive surfaces that induce motion through structural or morphological changes [36, 37]. While these techniques are effective for certain applications, open-surface systems face inherent challenges, such as liquid evaporation, restricted liquid volumes, and undesired contamination risks arising from particle-liquid separation. These limitations hinder their scalability and applicability, particularly in complex or sensitive environmental conditions.

To address these limitations, emerging magnetic drive technologies have been developed to manipulate liquid behaviors in confined systems, such as microfluidic devices and enclosed frameworks. The mechanisms underlying liquid manipulation in these systems can be attributed to the interplay of magneto-thermal effects and magnetic force-driven effects, which give rise to physicochemical pathways that govern liquid behavior [25, 38]. For example, magnetically induced chemical modifications can change the molecular structures and chemical properties of confined interfaces, dynamically altering their wettability behavior or pore size [29]. Magnetic forces can deform elastomeric or gel-based solid materials and mix fluid materials, enabling precise structural control over liquid pathways [39-41]. Moreover, magnetically guided drag force in the transported droplets themselves represents another essential aspect, enabling efficient droplet capture and mixing [9, 35]. These mechanisms, whether acting independently or synergistically, provide programmable fluid manipulation capabilities in enclosed environments. Despite these advances, the field remains fragmented, with most reviews either focusing on open-surface systems or emphasizing the role of external stimuli other than magnetism in microfluidic applications [42-45]. A comprehensive overview specifically addressing the physicochemical design principles and control mechanisms for manipulating nonmagnetic liquids via magnetic stimulation is still lacking. Most of these processes occur at liquid-solid or liquid-liquid interfaces within microscale pores and channels, triggered by magnetic fields. We refer to these as magneto-responsive confined interfaces.

This review aims to bridge this gap by providing a detailed overview of recent advances in magnetically induced physicochemical-driven liquid manipulation in confined interface systems. The focus is placed on three core mechanisms: magnetically induced chemical changes, magnetically induced structural deformations, and magnetically induced dragging motions. We highlight key applications in liquid valves, liquid mixing, liquid flow regulation, and liquid pumping. At last, we outline existing challenges and propose future directions for developing scalable, precise, and multifunctional fluid manipulation systems tailored for closed environments.

2. Physicochemical mechanisms of magnetically driven confined nonmagnetic liquids 2.1. Magnetically induced chemical modifications

A magnetic field can induce chemical changes on the inner surface of microchannels by altering the properties of magneto-chemical substances. These changes are usually related to the shrinking/swelling behavior of thermo-responsive polymers driven by the magnetothermal effect. The shrinking/swelling processes involve the formation and breaking of hydrogen bonding (O···H) between the amide groups of the polymer (-CO–NH-) and water molecules (H₂O). This dynamic hydrogen-bonding interaction influences both the effective pore size and the interfacial physicochemical properties of the microchannel, enabling precise regulation of nonmagnetic liquid transport [46]. As illustrated in Fig. 1A(ⅰ), integrating superparamagnetic Fe₃O₄ nanoparticles with thermo-responsive polymers enables reversible control over membrane pore size, dramatically modulating water permeability–from restricted flow to enhanced flux [29]. A representative example is poly(N-isopropylacrylamide) (PNIPAAm), which exhibits a lower critical solution temperature (LCST) of 32 ℃ in water [47, 48]. Below this temperature, the isopropyl groups (-CH(CH₃)₂) on PNIPAAm chains form hydrogen bonds with water molecules, rendering the polymer brushes hydrophilic and causing it to swell, thereby trapping water and limiting flow. Under a magnetic field, localized heating from magnetic nanoparticles raises the membrane temperature, triggering a conformational transition in PNIPAAm chains on the pore walls. This transition results in polymer deswelling, increasing the effective pore size and enhancing water flux [49]. Therefore, by leveraging the magnetothermal effect to modulate temperature, the chemical properties of the membrane's inner surface can be dynamically adjusted, allowing precise control of microscale fluid behavior through magnetic field manipulation.

In addition to pore size regulation, magnetic field can also regulate the arrangement of magnetic surfactants, altering surface tension and enhancing emulsion stability. Magnetic surfactants, which incorporate magnetic components into their structures, adsorb at the interfaces such as liquid-gas or liquid-solid interfaces [9, 35]. For instance, ionic liquid-based surfactants exhibit reduced surface tension under a magnetic field, effectively stabilizing emulsions [50]. Another type involves magnetic nanoparticle-based surfactants, which control emulsions destabilization for rapid phase separation. The magnetic nanoparticles covering droplet interfaces provide a strong steric barrier preventing the coalescence of the droplets [51]. However, applying a magnetic field gradient introduces a magnetic force that disrupts the equilibrium, attracting nanoparticles to the magnet and facilitating phase separation (Fig. 1A(ⅱ)).

2.2. Magnetically induced structural deformations

Under the influence of an external magnetic field, magnetically induced deformation enables the directional transport of nonmagnetic liquids by altering the internal interface structure of microchannels. These deformations can be classified into three key types: (1) Volumetric deformation of gel microvalves, (2) asymmetric conical deformation of solid-liquid interfaces, and (3) symmetric cylindrical deformation of liquid-liquid interfaces.

Magnetic-responsive gels integrated into microchannels function as microvalves to manage fluid transport. These gels, typically consisting of thermosensitive polymers like PNIPAAm combined with magnetic nanoparticles, undergo volume changes in response to magnetic stimulation. In the absence of a magnetic field, the polymer network remains swollen below its LCST, blocking liquid flow by filling the microchannel cross-section. Upon exposure to a magnetic field, localized heat generated by the embedded magnetic nanoparticles raises the temperature above the LCST, triggering a phase transition in the polymer. This transition disrupts hydrogen bonding, leading to the polymer rapidly collapsing into a dehydrated state, reducing the gel volume and opening a pathway for liquid transport (Fig. 1B(ⅰ)). This coupling of magnetic energy conversion and polymer phase transitions, enables precise and reversible control of liquid transport within microchannels [52].

The wettability of the microchannel's inner surface, characterized by the contact angle (θ), plays a crucial role in determining the initial state of nonmagnetic liquids [53]. For an ideal flat solid surface, θ is determined by the interfacial tensions of the solid-gas interface (γ_SG), solid-liquid interface (γ_SL), and liquid-gas interface(γ_LG),, as described by the Young equation (Eq. 1) [54]:

$ \gamma_{\mathrm{LG}} \cos \theta=\gamma_{\mathrm{SG}}-\gamma_{\mathrm{SL}} $

(1)

In a hydrophilic microchannel (Fig. 1B(ⅱ)), nonmagnetic liquids form symmetrical menisci with equal Laplace forces in the absence of a magnetic field maintaining mechanical equilibrium. When an external magnetic field is applied, the microchannel deforms into a wedge-like structure with an asymmetric conical geometry, disrupting the menisci symmetry, and leading to a Laplace pressure difference (∆P) (Eq. 2):

$ \Delta P \approx 2 \gamma\left(1 / R_2-1 / R_1\right) $

(2)

here, γ denotes the surface tension of the liquid, R₁ and R₂ are the radii of curvature of the menisci. Given that R₁ > R₂, the resulting capillary force (F_C) drives the liquid droplet toward the narrow end of the channel (Eq. 3):

$ F_{\mathrm{C}} \approx \pi \alpha \gamma L \cos \theta $

(3)

In this equation, α is the apex angle of the wedge-like structure, L is the length of the microfluid droplet, and X ≫ L, where X is the distance between the precursor and the apex. This mechanism enables controlled self-transport of nonmagnetic liquids under magnetic stimulation. The deformation of the solid-liquid interface is often achieved using magnetoelastic materials, such as polydimethylsiloxane (PDMS) embedded with carbonyl iron powder [13]. Under a magnetic field, the magnetic particles within the PDMS matrix align along the magnetic induction lines and further deform the profile of the microchannel providing a robust platform for manipulating liquid flow in confined systems.

Liquid-liquid interfaces formed by magnetic fluids provide another mechanism to regulate liquid transport. In the absence of a magnetic field, the magnetic fluid within a confined space is randomly distributed, impeding the flow of nonmagnetic liquids. When a specific magnetic field distribution is applied, magnetic colloids align to form chain-like structures along the field direction, forming a soft liquid interface that enables the passage of nonmagnetic liquids through the microchannel (Fig. 1B(ⅲ)). Under a quadrupolar magnetic field, the interface stabilizes into a liquid-in-liquid configuration [41], confined by competing effects of magnetic pressure and surface energy (σ). The equilibrium diameter of the soft channel can be derived from Bernoulli's equation (Eq. 4) [55]:

$ d=4 \sigma /\left(2 \mu_0 \bar{M} H_1+\mu_0 M_1^2\right) $

(4)

here, H₁, M₁ represent the magnetic field strength and magnetization value at the interface, μ₀ is the permeability of free space, and M is the field-averaged magnetization of the fluid. This dynamic adaptation of ferrofluid interfaces to magnetic stimuli enables tunable liquid transport.

2.3. Magnetically induced dragging motions

The application of an external magnetic field enables directional transport of nonmagnetic liquids by dragging and moving them through magnetic additives introduced into the transported liquids. To achieve efficient transport of nonmagnetic liquids in confined spaces, minimizing the adhesion forces between droplets and additives is essential. Consequently, the hydrophobic inner surface of the microchannel is often constructed (Fig. 1C). Under the influence of a magnetic field, nonmagnetic liquids can undergo active deformation and exhibit directional movement, guided by magnetic additives in response to rapid changes in magnetic field strength. The motion modes of micro-droplets in such systems are primarily governed by the competition between the adhesion force (F_A) and the lateral adhesion force (F_LA). Thereinto, F_A exerted on the droplet by the three-phase contact line between the droplet andadditives could be expressed as Eq. 5 [9]:

$ F_{\mathrm{A}} \sim \gamma \pi D \sin \frac{\beta}{2} \cos \alpha $

(5)

Meanwhile, the F_LA can be described as Eq. 6 [34]:

$ F_{\mathrm{LA}} \sim k \gamma L\left(\cos \theta_{\mathrm{r}}-\cos \theta_{\mathrm{a}}\right) $

(6)

here, γ is the droplet surface tension; β is the position angle of the stable three-phase contact line, α is the angle between the move direction and the tangent line of the stable three-phase contact line, k is the dimensionless factor, L is the droplet contact width, θ_r is the receding contact angle, and θ_a is the advancing contact angle.

3. Applications of magneto-responsive confined interfaces in manipulating nonmagnetic liquids 3.1. Liquid valves

Liquid valves are critical for the precise regulation of nonmagnetic liquid transport in confined systems. By leveraging magnetically responsive chemical substances and their interaction mechanisms as described in the above section, strategies have been developed to dynamically control liquid flow through single channels or porous membranes. These valves enable on-demand opening and closing, flow regulation, and controllable liquid release, offering versatility for applications in microfluidics, drug delivery, and advanced chemical processes [56]. This subsection introduces various magnetically actuated liquid valve designs and mainly discusses their functionality and practical applications. By functionalizing microchannel walls with paramagnetic particles and thermo-responsive polymer, pore size can be dynamically regulated under magnetic fields, enabling reversible changes in liquid flow. For instance, Ulbricht et al. developed a magnetically responsive porous membrane by functionalizing track-etched polyethylene terephthalate (PET) membranes with Fe₃O₄ nanoparticles and temperature-sensitive PNIPAAm [29]. As shown in Fig. 2A, nanoparticle immobilization was achieved via peptide coupling (-CO–NH-) between the nanoparticles' carboxylic acid groups (-COOH) and the membrane's amino groups (-NH-), a process confirmed by SEM imaging. Flux measurements highlighted the reversible valve-like behavior, achieving a significantly higher flux (240 L m^-2 h^-1) under electromagnetic stimulation and returning to initial permeability levels when the field was turned off, showcasing precise and repeatable controllability.

Apart from surface modification, magneto-thermal effects provide another approach by inducing volume changes in thermo-responsive polymers, enabling precise microvalve control. Ghosh et al. explored the use of tunable magnetic nanoparticles embedded in cylindrical hydrogel materials for thermo-mechanical gating in microfluidic channels (Fig. 2B) [52]. Above the LCST, the polymer chains collapse, opening the valve, while below the LCST, they undergo a rapid reversible phase transition into extended hydrated chains that fill the channel and block the flow. An oscillating magnetic field triggers these transitions, causing volume contraction and compacting the nanomagnets within the polymer network, providing dynamic flow regulation. The deformation of magnetic solid porous membranes offers a versatile approach for controlled liquid release and storage, finding applications in gating valves and drug delivery. Chiao et al. enhanced the performance of magnetic PDMS surfaces by adsorbing bovine serum albumin to improve wettability, thereby facilitating water filling and drug dissolution in reservoirs [57]. This approach successfully enabled the delivery of rhodamine B as a model drug, achieving a tenfold increase in release rate under magnetic actuation compared to passive release methods. Building on this concept, Pisano et al. developed a refillable, valveless drug delivery device featuring a pear-shaped viscoelastic magnetic membrane [58]. When stimulated by an external magnetic field, the membrane is asymmetrically deflected, making contact with the dome-shaped drug reservoir to enable precise, on-demand drug release tailored for localized disease treatment. Meanwhile, in 2024, Li et al., developed magnetically driven capsules with a dual-layer ferromagnetic soft valve, enabling precise drug release and environmental interaction in biomedical applications [59]. As shown in Fig. 2C, the self-closing and magnetically controlled opening mechanism of the valve, ensures targeted and efficient drug delivery. When drug release efficiency is optimized at 30 Hz, where the magnetic field frequency achieves maximum deformation of the valve, highlighting the importance of precise frequency control. This special design advances the capabilities of magnetic soft robotics for controlled drug delivery and diagnostics in complex environments like the gastrointestinal tract.

Different from solid-based pore/channel systems, liquid-based systems offer unique advantages for liquid manipulation. For example, a magnetic liquid that is immiscible with water can be used as a liquid-in-liquid fluidic valve, with its on/off state controlled by a magnetic field. Wall-less magnetic confinement valves represent an innovative design for magnetic fluidic control. Hermans et al. proposed a liquid wall using an extended quadrupolar flux source to create a null magnetic field along a central line, effectively forming a fluidic channel without a solid wall (Fig. 2D) [41]. The magnetic field is used to regulate the valve's opening and closing, providing precise control over liquid transport. Experimental data under 100 mbar pressure revealed that the flow rate through the valve is dependent on the number of applied magnetic fields, demonstrating a direct relationship between magnetic field strength and liquid transport behavior.

Liquid gating technology, which uses capillary-stabilized gating liquids (immiscible with the transported liquid) as pressure-driven and reversible gates, represents another important liquid-based system for controlling fluid behavior. This technology places a strong emphasis on the interactions at multiphase interfaces. Based on this principle, a multiphase fluid interface pressure testing analyzer (GIFT-001E) has been developed, capable of outputting interaction information at solid/liquid/liquid or solid/liquid/gas interfaces in the form of pressure data. Additionally, it facilitates the analysis of fluid behavior within mesoscale confined channels, offering a new tool for in-depth interface characterization and advancing the understanding of interfacial phenomena in confined systems [60]. The inherent fluidity and adaptability of liquid-liquid interfaces enable precise regulation of fluid transport, reduced fouling, and abundant responsiveness to external stimuli [12, 61]. Among them, magneto-responsive liquid gating membranes, driven by magnetic fields, can self-adjust the configuration of confined liquids, dynamically modulate pore size, and achieve precise control over fluid dynamics and transport efficiency, making them particularly effective for liquid valve applications. For instance, in 2022, Hou et al. introduced a self-driven liquid gating magnetoelastic porous membrane (LGMPM) that exhibits reversible meniscus-shaped deformations [62]. Under the same magnetic field, LGMPM enables easier liquid release compared to the magnetoelastic porous membrane, enhancing transport efficiency. By converting magnetic force into mechanical force that directly acts on the transported nonmagnetic liquid, the LGMPM can achieve self-driven liquid drug release under the influence of a magnetic field. Moreover, adjusting the magnetic field strength enables precise control over the mass of the released liquid (Fig. 2E).

Besides, the direction of the magnetic field further influences fluid transport behavior, enabling precise manipulation of flow dynamics. Utilizing the collective dynamics of confined magnetic colloids, a magnetically controlled fluid microvalve was reported [63]. When a perpendicular magnetic field is applied, the Stokes force shears the chains, increasing resistance to fluid transport. By adjusting the magnetic field direction, chemical reactions can also be precisely controlled, with the valve opening and chromogenic reaction occurring when the field aligns parallel to the flow (Fig. 2F). This multifunctional valve offers applications in smart lab-on-a-chip systems, where simultaneous control of liquid transport and chemical reactions is highly desirable, as well as in environmental monitoring and on-demand reagent release for industrial processes.

3.2. Liquid mixing

Liquid mixing, the process of achieving homogeneous and efficient integration of two or more liquid phases, is critical in various fields, such as chemical reactions, biomedical engineering, and environmental science [64, 65]. This becomes particularly significant in confined systems, where mixing efficiency is often a key factor in determining performance outcomes. Based on the methodology, magneto-responsive liquid mixing can be categorized into three main types: microdroplet mixing, liquid slug mixing, and liquid flow mixing, each offering distinct advantages for specific scenarios.

In microdroplet mixing, emulsifying behavior is largely governed by the wettability and responsiveness of surfactants. Magneto-responsive surfactants, in particular, revolutionized stable polyphase mixing during emulsion preparation. Eastoe et al. first introduced ionic liquid surfactants with magneto-responsiveness, enabling the perturbation of liquid emulsions via external magnetic fields [66]. Building on this concept, Guo and colleagues developed magneto-responsive Janus emulsions stabilized by [H-G-C₁₆]FeCl₄ surfactants using two immiscible oils, which exhibit paramagnetic properties (Fig. 3A(ⅰ)) [50]. In the presence of an external magnetic field, the pendant droplet elongation is observed along with a decrease in surface tension (from 38.6 mN/m to 35.4 mN/m). This magnetic field-driven phenomenon ensures the stable formation of emulsion. Additionally, the saturation magnetization increases by 44.5% as the surfactant concentration rises from 4 wt% to 12 wt%, highlighting the tunable properties of these systems. With the stability of the above magnetic surfactants, two types of emulsion droplets are formed. Among them, the vegetable oil/aqueous (VO/aq) interface exhibit higher surface adsorption and adjustable magnetic responsiveness compared to the dimethyl phthalate/aqueous (DMP/aq), owing to its enhanced adsorption capacity and enlarged surface area. Expanding on these findings, Loewe et al. further developed a surfactant-free and straightforward technique for continuously generating double emulsion droplets composed of an organic solvent and a paramagnetic ionic liquid, using a two-step T-junction microfluidic system [67]. The first T-junction alternates slugs of organic solvents and paramagnetic ionic liquids in the inner capillary, and the second T-junction enables the slugs to be encapsulated into double emulsion droplets within a coaxial capillary-in-tube flow system. Adjusting the magnetic field allows for precise control of the microdroplet mixing process in the microfluidic system, as demonstrated by the merging of differently colored organic phases into a homogeneous green solution (Fig. 3A(ⅱ)).

Alternatively, magnetic nanoparticles have also gained considerable attention as emulsifiers for preparing Pickering emulsions due to their irreversible adsorption, low toxicity, and cost-effectiveness. Moreover, their use as demulsifiers enables rapid phase separation, leveraging their superparamagnetic behavior and magnetic responsiveness (Fig. 3A(ⅲ)). For example, Sun et al. reported a unique composite Pickering emulsifier consisting of Fe₃O₄ as the magnetic core, silica as a protective intermediate layer, and chitosan of varying molecular weights to provide surface activity and magnetic-responsive properties [68]. These emulsifiers can be easily recovered using a magnetic field, enabling reusability and efficient phase separation. Similarly, Yang et al. developed switchable magnetic emulsions stabilized by 3-aminopropyltriethoxy silane-coated nanoparticles, which exhibited excellent long-term stability over several days and achieved complete on-demand demulsification within minutes, highlighting their adaptability for controlled emulsion processes [51]. In confined environments, Mendiratta et al. synthesized "smart" hydroxyapatite-based magnetic nanoparticles to create corresponding stimuli-responsive Pickering emulsions [69]. Using a microfluidic platform, they demonstrated that magnetic fields could reduce emulsion stability and promote droplet coalescence, providing precise control over emulsion behavior in restricted spaces.

For larger-size liquid slug mixing, magnetically induced reversible deformations of magnetoelastic membranes provide an effective strategy. The deformation of magnetoelastic channels (Fig. 3B) enabled controlled mixing through the self-transport of liquid slugs [39]. This approach supports stepwise chemical reactions within a specially designed channel featuring two asymmetrical sections: cylindrical ends, a flattened middle portion, and an oblong hole. These structures allow for precise control over reaction dynamics and mixing efficiency, making them ideal for applications requiring sequential or continuous mixing.

In confined flow systems, liquid flow mixing can achieve enhanced efficiency through the manipulation of restricted channels. Hermans et al. demonstrated this concept using magnetic anti-tube principles (Fig. 2D). In their Y-junction microfluidic setup, blue and pink dye solutions introduced at the inlets (flow rate: 300 µL/min) merged and mixed instantaneously upon contact at the junction [41]. By applying a magnetic field, they achieved rapid and homogeneous mixing of the two-phase solution before it exited the outlet (Fig. 3C). This approach highlights the ability of magnetic fields to induce efficient and instantaneous liquid flow mixing in microfluidic systems.

3.3. Liquid flow regulation

In confined systems, controlling the flow rate of nonmagnetic liquids is critical for various applications, and both passive and active strategies have been explored. The passive approaches often rely on constructing surface wettability gradients or creating asymmetric structures that induce Laplace pressure gradients to drive liquid motion. While effective, these methods are inherently limited by significant resistance to motion, resulting in low transport velocities, typically ranging from several micrometers to a few millimeters per second. To overcome these constraints, active strategies utilizing external stimuli, such as magnetic fields, have been explored, enabling more efficient directional liquid transport. Recently, Lei et al. reported a simple, additive-free fabrication of magnetic tubular micro-actuators (MTMAs), which leverage magnetic-induced asymmetric deformation to manipulate liquid droplets with controllable velocity and direction [13]. This system achieved a remarkable transport speed of 10 cm/s without disengaging from the magnet, setting a new benchmark in confined liquid transport. As shown in Fig. 4A(ⅰ), the MTMA deforms into a cone-like geometry, and its diameter is dependent on the magnetic fields. This deformation generates an adjustable capillary force that propels the liquid toward the magnet. Building on this innovation, Xia et al. introduced hydrophobic modifications to the MTMA system, creating a magnetic superhydrophobic tubular PDMS actuator. This modification enhanced the liquid transport speed up to 16.1 cm/s [25]. Additionally, they engineered the magnetic tubular actuators with slippery inner surfaces (MTLA-SIS), incorporating submillimeter-sized through-hole pore structures (Fig. 4A(ⅱ)) [70]. These pores allow the liquid droplet to undergo chemical or physical interactions during transport, providing multifunctionality. The MTLA-SIS system also enables the droplet to act as an electrical switch for integrated circuit control. Transport efficiency analyses demonstrated that the MTLA-SIS system moderately outperformed other active systems in nonmagnetic liquid transport, making it a promising platform for diverse applications.

In contrast to the previously mentioned active self-transport processes, Cheng et al. developed a polymer tube with a lubricated magnetically responsive gel surface, which synergistically combines active and passive mechanisms to enable in situ control of droplet moving and ceasing transitions [39]. As shown in Fig. 4B(ⅰ), the tube features an asymmetric shape and a lubricated inner surface with exceptionally low contact angle hysteresis, allowing for passive, directional self-transport of nonmagnetic droplets. By applying or removing an external magnetic field, the resistance generated by the gel can be dynamically adjusted, enabling precise, reversible transitions between droplet movement and cessation. Fig. 4B(ⅱ) demonstrates this concept, with a water droplet undergoing round-trip self-transport by adjusting the asymmetrical orientation of the tube and the magnetic field. This innovation thus highlights a promising direction for versatile and adaptable magneto-responsive confined interfaces in droplet manipulation.

3.4. Liquid pumping

Liquid pumping is an essential component of liquid manipulation systems, enabling a wide range of applications from microfluidics to large-scale liquid transport. Conventional liquid pumps, such as syringes and pneumatic pumps, rely on mechanical or pneumatic force to transfer or pressurize liquids by effective [71], these systems are usually faced with challenges including high energy consumption, potential liquid contamination due to residual fluids, and limitations in precise flow control. In recent years, magneto-responsive confined interfaces have emerged as a promising alternative for liquid pumping, leveraging innovative microchannel designs and magnetic fluid deformation techniques. These approaches enable non-contact and efficient manipulation of liquids driven by magnetic fields. For example, Aizenberg et al. demonstrated a centimeter-scale liquid pumping system by utilizing the interplay between magnets and ferrofluid confined within microporous structures [72]. In this system, pairs of magnets dynamically extract the ferrofluid plugs along the circular pipe, effectively driving the flow of nonmagnetic liquids. As shown in Fig. 5A, an ethanol solution of rhodamine B is continuously pumped from the right vial to the left vial, showcasing the feasibility and efficiency of this magnetic pumping approach. Expanding on this approach, Hermans et al. developed a liquid-in-liquid fluidic channel design for magneto-static pumping, enabling liquid manipulation by moving permanent magnets within confined channels without physical contact, generating a flow rate of up to 32.7 mL/min (Fig. 5B(ⅰ)) [41]. Notably, this approach demonstrated the capacity to pump complex biological fluids, such as whole human blood. Compared to traditional peristaltic pumping, which mechanically compresses blood through plastic tubes, this method achieved a one-order-of-magnitude reduction in haemoglobin, decreasing it from 130 mg/dL to 12 mg/dL (Fig. 5B(ⅱ)). The ability to minimize mechanical damage to biological samples highlights the potential of magneto-responsive pumping systems in biomedical and diagnostic applications.

4. Conclusion and perspective

Magneto-responsive confined systems have emerged as a transformative approach for precise liquid manipulation, enabling dynamic control over nonmagnetic liquids in microfluidics, chemical reactions, and biomedical diagnostics. Physicochemical interface designs that integrate magnetothermal and magnetic force-driven effects underpin their ability to perform tasks such as liquid transport, mixing, flow regulation, and pumping with exceptional precision. While some progress has been achieved, challenges remain in both scientific research and practical application, which should be addressed to fully realize their potential. (1) Magneto-responsive materials, particularly those undergoing repeated physicochemical transformations (e.g., wettability changes and structural deformations), often suffer from performance degradation over time. (2) The coupling of magnetic fields with thermal, fluidic, and surface interactions in confined systems introduce significant complexity, requiring a deeper understanding of these multiphysics phenomena at both macroscopic and microscopic scales. (3) Ensuring compatibility with diverse liquid chemistries, such as multiphase or reactive fluids, while maintaining contamination-free operation and robustness under harsh environments (e.g., extreme pH or temperature conditions), further complicates practical implementation.

Future efforts should focus on refining the design and functionality of magneto-responsive confined interfaces to address these challenges and fully harness the potential of these systems. Firstly, there is a need for advanced materials with improved durability and responsiveness, such as magnetic components-embedded durable polymers, self-healing elastomers, or hybrid composites, which exhibit robustness under repeated physicochemical transformations, including wettability changes or structural deformations. Surface functionalization strategies, such as chemical grafting or self-assembled monolayers, can be employed to tailor hydrophilic or hydrophobic properties, thereby enabling precise and versatile fluid control in complex multiphase environments.

Another promising direction lies in the integration of advanced computational tools, such as multiphysics simulations coupled with machine learning algorithms, to optimize magnetic field parameters and predict liquid behavior in real-time. These approaches not only facilitate adaptive and intelligent fluid manipulation, but also provide deeper insights into the interplay between magnetic forces, fluid dynamics, and confined interfacial properties. Additionally, combining magnetic actuation with other external stimuli, such as light or electric fields, could unlock new functionalities, including spatially and temporally resolved liquid mixing, separation, or programmable flow regulation.

Finally, to bridge the gap between laboratory-scale and real-world applications, scalable manufacturing techniques, such as soft lithography, soft lithography, or 3D printing, should be leveraged to fabricate microfluidic devices with embedded magnetic elements. These systems should be designed with modularity and adaptability in mind, allowing for customization to meet the demands of diverse industrial or biomedical applications, such as point-of-care diagnostics or smart drug delivery platforms. Furthermore, the incorporation of real-time monitoring and feedback control systems could enhance the precision and reliability of fluid manipulation. By advancing these areas, magneto-responsive confined systems hold immense potential to drive innovation across a wide range of fields, including frontier scientific research, industrial processing, and biomedical technologies, ultimately paving the way for transformative applications in both academia and industry.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

CRediT authorship contribution statement

Jing Liu: Writing – review & editing, Writing – original draft, Methodology, Investigation, Formal analysis. Ming Li: Writing – review & editing, Writing – original draft, Visualization, Investigation, Formal analysis. Jian Zhang: Writing – review & editing, Writing – original draft, Visualization, Formal analysis. Xinyu Li: Writing – review & editing, Visualization, Formal analysis. Yuqing Zheng: Writing – review & editing, Visualization, Formal analysis. Xu Hou: Writing – review & editing, Supervision, Methodology, Conceptualization.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Nos. 52025132, U24A20205, 52303373, 21621091, 22021001, and 22121001), the China Postdoctoral Science Foundation (No. 2024M763174), the 111 Project (Nos. B17027, B16029), the Natural Science Foundation of Fujian Province of China (No. 2022J02059), and the New Cornerstone Science Foundation through the Xplorer Prize.