Ultrarobust, Highly Sensitive and Swinging‐Independent Droplets‐Spatial Manipulator Enabled via Laser‐Printed Magnetically Actuated Shape‐Morphing Microshutters

Spatial manipulation of liquid droplets holds great significance in various areas from laboratory research to everyday life. Unfortunately, current manipulators are limited to lower responsivity and unreliable durability together with swinging‐dependent manner. Herein, an ultrarobust, sensitive, and swinging‐independent droplet‐spatial manipulator is reported, namely, magnetically actuated shape‐morphing microshutters (MASM) enabled by laser printing and soft transfer technique. Leveraging the loading/discharging of a remote magnet, the MASM can be reversibly switched between a bending mode and an erect mode within 1 s, thereby modulating the three‐phase contact line (TCL). The underlying hydrodynamics is that B‐mode MASM renders a longer TCL and an exaggerated adhesion force (Fa), thereby pinning the surface droplets under the assistance of a magnetic field. Once the magnetic trigger is removed, the bent shutters reverse back to the E‐mode so as to grant a shorter TCL and smaller Fa. Based on this phenomenon, the droplet's capture/release can be readily realized in response to magnetic stimuli. Last but not least, by taking advantage of an optimized MASM, pattern‐free configuration and droplets on‐demand marriage and targeted chemical reaction together with programmable circuit remedy are deployed. This work provides impetus for researchers in constructing intelligent droplet manipulation systems for multidisciplinary areas.


Introduction
Controllable steering over small quantities of fluids has aroused considerable interest and research due to their great potential in fog collection, [1] condensation, [2] chemical reaction, [3] medical diagnosis, [4] biological analysis, [5] and so forth.To date, maneuvering the microdroplets on 2D smart surfaces under the assistance of light, [6] thermo, [7] electric, [8] magnetism, [9] vibration, [10] mechanical stretching, [11] wettability gradients [1a,12] has been extensively explored.For example, by mimicking the natural cactus stem, Jiang et al. developed the first paradigm of pyramid-structured fibers with concave curved surfaces, which was accountable for the rapid self-propulsion of droplets under the assistance of wettability gradient force (F wet-grad ) arising from the Laplace pressure difference (P ΔLap ). [13]Inspired by Nepenthes plant, Yao et al. fabricated the state-of-the-art elastic slippery liquidinfused porous surface (SLIPS), which grants the locomotion control of diverse liquids between pinning and sliding leveraging on mechanical stretching. [14]eyond nature, Li et al. conferred a hydrophilic magnetic-actuated robot to realize the complex mobility maneuvering of liquids including transport, split, release, and rotation. [15]In brief, although with many successes, these classical dropletsactuators were difficult to accurately manipulate droplets toward a targeted location or cross-regional locations over a long distance.In this regard, blooming a new fashion of intelligent surfaces responsible for droplets-spatial manipulation (DSM) is thus a timely need.
With this in mind, Sun et al. proposed a light-responsive DSM actuator by emulsifying graphene oxide (GO) hybrid N-isopropylacrylamide (NIPAM) hydrogel solution into silica nanoparticlesdispersed ethoxylated trimethylolpropane triacrylate (ETPTA) phase. [16]Owing to the photothermal effect of GO, the encapsulated hydrophilic hydrogel arrays could shrink back into the holes to expose their hydrophobic surface with near-infrared (NIR) irradiation, thereby achieving the programmable release and effective transfer of droplets.In another paradigm, Song et al. presented a stretchable DSM actuator by seeding the magnetic microcolumn array on elastic polydimethylsiloxane platform, where the selective capture/release over droplets could be readily realized through tuning the grasping gaps of adjacent microcolumn arrays upon mechanical stretch. [17]More recently, Jing et al. prepared a magneto-driven DSM actuator consisting of magnetic Fe-doped soft micropillars, which enables the droplets to move in a planar direction and especially release depending on the swinging inertia force. [18]Although ceaseless endeavors have been dedicated to the droplet actuators, several blockages have arisen DOI: 10.1002/aisy.202300375Spatial manipulation of liquid droplets holds great significance in various areas from laboratory research to everyday life.Unfortunately, current manipulators are limited to lower responsivity and unreliable durability together with swingingdependent manner.Herein, an ultrarobust, sensitive, and swinging-independent droplet-spatial manipulator is reported, namely, magnetically actuated shapemorphing microshutters (MASM) enabled by laser printing and soft transfer technique.Leveraging the loading/discharging of a remote magnet, the MASM can be reversibly switched between a bending mode and an erect mode within 1 s, thereby modulating the three-phase contact line (TCL).The underlying hydrodynamics is that B-mode MASM renders a longer TCL and an exaggerated adhesion force (F a ), thereby pinning the surface droplets under the assistance of a magnetic field.Once the magnetic trigger is removed, the bent shutters reverse back to the E-mode so as to grant a shorter TCL and smaller F a .Based on this phenomenon, the droplet's capture/release can be readily realized in response to magnetic stimuli.Last but not least, by taking advantage of an optimized MASM, pattern-free configuration and droplets on-demand marriage and targeted chemical reaction together with programmable circuit remedy are deployed.This work provides impetus for researchers in constructing intelligent droplet manipulation systems for multidisciplinary areas.
subsequently. 1) Photothermal release of droplets requires tens of seconds to a few minutes, signifying an unsatisfactory responsivity. [16]2) The repeated mechanical stretching would initiate poor longevity and deteriorate the steering performance. [17]3) The magnetic release of droplets is highly dependent on the swinging cycles, the swinging angles as well as the swinging velocity, resulting in a complex and time-consuming operation. [18]As a result, developing a robust, sensitive, and swing-independent dropletsspatial manipulator is highly desirable yet challenging to date.
To address the above challenge, we developed a droplet-spatial manipulator namely magnetically actuated shape-morphing microshutters (MASM) by combining the femtosecond laser ablation and soft transfer technique.Via loading/discharging a remote magnet, the capture or the release of a targeted droplet by using MASM could be realized within 1 s, which is more convenient and sensitive than the previously-reported manipulators.Furthermore, a comprehensive investigation has been conducted to analyze the correlation between the steering performance and the topography of the shutters.The fundamental basis of interfacial hydrodynamics enables us to uncover the steering mechanism.Significantly, analogous to a mechanical hand, the current robust MASM is competent for spatially delivering diverse liquids toward any platform in an accurate manner.Last but not least, by leveraging on MASM, the heterogeneous chemical reaction together with the circuit board controller is successfully deployed.The ultrafast liquid-handling capability of the MASM holds potential in the fields of microfluidics, biological detection, targeted therapy, and so.Fe-doped gel infusing, thermo-triggered cross-linking, and a subsequent demolding process (Figure 1A).Concretely, i) by employing the femtosecond (fs) laser, we could punch through the highly arrayed tunnels on the PTFE substrate with thickness of 0.8 mm.Thereafter, ii) the as-prepared uniform matrix of Fe nanoparticles (NPs) hybridized in polydimethylsiloxane (PDMS) was poured onto the above grooved PTFE mold, which could uniformly spread and penetrate into the grooves under the help of spin-coating technique (1000 rpm, 60 s) and a subsequent vacuumizing operation (1.5 h).Wherein, iii,iv) a thermo-curing method was applied to cross-link the Fe-doped gel and then a piece of MASM could be successfully harvested by a careful demolding craft, where the typical height (h) and the half-width (w) and the shutters interval were characterized as 0.76, 0.12, and 0.47 mm (Figure 1B,C), respectively.Notably, compared to the conventional chemical method, fs laser is more adaptive for texturing a diversity of superwetting micro-/nanostructures owing to its ultra-high peak power and low thermal-effect merits. [19]hanks to the air-pockets existing among the microshutters, the current laser-printed MASM unfolds a superhydrophobic apparency with a water contact angle (WCA) of %154 o , signifying its waterproof feature (Figure 1D).Once the magnetic field was remotely loaded, the erect shutters reversed back to the bending state within 1 s, wherein the WCA decreased to %154 o arising from the extrusion of partial air pockets.Moreover, by loading/discharging a remote magnet, the droplet (%10 μL) on the current dual-mode MASM could be reversibly switched between the pinning state (dwelling at a tilt angle of 90°) and slippery state (sliding angle: %18°) (Figure 1E).Upon harnessing the above wetting and dewetting effect rather than a repeat swinging manner assigned to the previously reported manipulator, the current MASM is responsible for facilely maneuvering a water drop (%10 μL) including the fast capture (with magnet) and the targeted release (without magnet) in situ.As a comparison, a tilt microshutters array without Fe nanoparticles is also responsible for capturing the drop by virtue of its giant adhesive force yet the release of the drop is not adaptive (Figure S1, Supporting Information).In short, this part unfolds an ultrasensitive and swing-independent DSM function by levering on current MASM (Figure 1F; Movie S1, Supporting Information).

Switchable Hydrodynamics over Droplets on MASM
According to Young's equation, the wetting and dewetting properties of MASM should be very related to its roughness factor and the air-pockets content.In light of this, we first investigate the evolution of the bending angle (α) of the microshutters with the change of Fe-doped content, where the plateau (α = 52°) is found to be located at 60 wt% (Figure 2A and S2, Supporting Information).Further, by utilizing this optimized MASM (Fe-doped ratio: 60 wt%), regardless of w (a constant: 0.12 mm) and h (a constant: 0.76 mm), we systematically study the effect of shutters interval on the wetting performance including the water contact angle (WCA) and the water sliding angle (WSA).As the d values increased from 0.4 to 0.8 mm, the WCA for dual-state MASM were, respectively, recorded as 158°, 154°, 145°, 140°, 135°, 119°, 121°, 123°, 151°, and 152°(Figure 2B).As the interval d between the shutters increases, the hydrophobicity of the Estate MASM decreases, while the water-repellency of the B-state MASM improves.As such, With the elevation of d values, the WSA for an E-state MASM evolved from 22°to 18°, 12°, 7°a nd then 7°, while the droplets tend to pin on the B-state MASM with a tilt angle of 90°(Figure 2C).Notably, the enhanced d and the enlarging air-pockets contribute to a smaller adhesion force between the surface droplet and the E-state MASM, which could be evidenced by calculating the force values according to the empirical equation of F a// equals to ρgV Â sinα (Figure 2D).On the above basis, current MASM with tunable wettability could be adaptive for the dynamic control of air-pockets buffering as well as drops migration.

Maneuvering Principle over Droplets-Spatial-Transport
To clarify the droplets-special-transfer hydrodynamics, under the assistance of a high-frame-rate CCD, we in situ monitor the wetting behavior of a drop (10 μL) on the inverted dual-state MASM (Figure 3A).The results display that the erect shutters unfold a limited contact area with the drop, signifying that E-state MASM has a smaller three-phase contact line (TCL).In sharp contrast, once the shutters switched to a bending morphology with the trigger of a remote magnet, the TCL was immensely elongated to wet the surface drop.According to F a equals to γ multiplied by TCL, through regulating the magnitude of the resultant force between the adhesion force F a and the drop's gravity G, the MASM could be readily maneuverable between a sticky mode and a nonsticky mode accountable for the capture and the release of a targeted drop.To verify the rationality of the proposed steering principle, via harnessing a homemade force-tester, we harvest the adhesion force assigned to dual-state MASM as 0.3 Â 10 À6 and 0.8 Â 10 À6 N (Figure 3B), respectively.The result manifests that B-state MASM unfolds an exaggerated adhesive force in comparison with that of E-state MASM, contributing to the controllable transport (capture/release) over droplets by using the current mechanical hand.Significantly, the MASM exhibits a long-term switching capability over the drops wetting and dewetting behavior, which was evidenced by the sustaining stable wetting performance even after suffering from the repeat capture/release operation for 20 cycles (Figure 3C).

Multiple-Droplets Spatial Manipulation via Dual-Mode MASM
In addition, as a drop's mechanical hand, the MASM's feasibility in executing the capture-release operation over the drops with different sizes was systematically examined.In parts of Figure 4A,B, we showcase a phase map for assessing the capability of capturing and releasing the drops with different sizes by using diverse MASM with different Fe-doped content and various d values.Accordingly, we deduced that an optimized MASM should have a 40 wt% Fe-doped content and a 0.5 mm interval.Leveraging on this optimized MASM, we could spatially manipulate three drop species for their capture, delivery, release, and merging on-demand (Figure 4C, Movie S2, Supporting Information).In detail, 1) the MASM was mounted on the A drop and applied with gentle pressure when a magnetic field was loaded for bending the shutters so as to stick the drop depending on its longer TCL.2) The A drop could be captured and uplifted toward the top location of B drop, where the A drop could be released from MASM by discharging the magnet trigger and then merged with B drop.By the same token, 3) the ternary A þ B þ C hybrid could be mixed by delivering A þ B mixture to C drop.Also, by accurately steering a diversity of drops, various patterns could be readily configured by using current magnetictriggered MASM (Figure 4D).On the above basis, two potential applications by using current sensitive MASM were in sequence implemented, including the chemical reaction and the circuit remedy.As shown in Figure 5A, by loading a remote magnet beside the MASM, a 10 μL NaOH drop (dissolved in water: 10 wt%) was captured and delivered to the top side of a 10 μL phenolphthalein drop (dissolved in glycerol: 5 wt%) on a soft polydimethylsiloxane substrate.Thereafter, the magnetic field was immediately removed to make the shutters reverse back to the erect states, thereby releasing the NaOH drop onto the phenolphthalein drop (Movie S3, Supporting information).It is noteworthy that, differing from the previously explored droplet manipulators, the current droplet special delivery process was independent of the repeat swinging operation, suggesting its facile and highly effective merit.As such, another paradigm is that we built two isolated open electric circuits composed of a power source and two disconnected copper electrodes together with LED.NaCl droplets (10 μL; dissolved in water: 10 wt%) served as the conductive media to remedy the circuit in a programmable manner (Figure 5B, Movie S4, Supporting information).Through loading a magnetic field beside the MASM, we could capture a NaCl droplet and then transport the drop toward the upside of the disconnect point of two copper wires.Wherein, the NaCl drop was in situ liberated from the MASM by virtue of removing the magnet stimuli, thereby lighting the green LED for a logical circuit (1,0) attributing to the ionic conduction.By the same token, another NaCl droplet could be spatially steered to remedy the circuit that regulates the blue LED so as to reach a logical circuit (1,1).That is, by taking advantage of the current MASM, the functional droplets could be freely delivered to any destination for their targeted functionality (e.g., chemical reaction, drug delivery).

Conclusion
In summary, we proposed an ultrarobust, ultrasensitive, and swinging-independent MASM responsible for droplet-spatial manipulation by combining the fs laser ablation and soft lithography method.Owing to its remote steering manner, MASM could be reversibly switched between sticky state and nonsticky state within 1 s in response to a magnetic trigger, which is far more sensitive than the previously explored actuators.The underlying mechanism is that the erect shutters render a longer TCL and a larger adhesive force F a over droplets, resulting in a water-repellent apparency.Once the remote magnetic field was loaded, the shutters reversed back to the bending state leading to the elongation of TCL and the elevation of F a , thereby tightly capturing the droplet.Taking advantage of the above hydrodynamics assigned to MASM, spatially maneuvering functional droplets for chemical reactions and programmable circuit salvage were successfully deployed.This work is envisioned to provide guidance for the researchers occupied in microfluidics, biological detection, targeted therapy, and so forth.

Experimental Section
Materials: The carbonyl iron powder with the diameter of 3-5 μm (≥99.5% purity) was purchased from Nangong Rui Teng Alloy Material Co., Ltd.Liquid polydimethylsiloxane (Sylgard 184, Dow Corning) was first mixed with carbonyl iron powder in a weight ratio of 40%, after thoroughly mixed, the cross-linker was added at a ratio of 10:1 (w/w) and thoroughly mixed again.The PTFE with a thickness of 800 μm as the template source was purchased from Yangzhong Xingfuda Rubber & Plastic Co., Ltd.The magnets used in the experiment were purchased from Shanghai Ze He Mechanical & Electrical Co., Ltd.(40 Â 40 Â 20 mm).Phenolphthalein, glycerol, NaOH, and NaCl were obtained from Sinopharm Chemical Reagent Co., Ltd.Distilled water (H 2 O, 1 g cm À3 density) served as contact-angle test materials.
Laser-Writing PTFE Template: The line-by-line scanning routine over PTFE templates was ablated by using a regenerative amplified Ti: Sapphire femtosecond laser system (Legend Elite-1 K-HE, Coherent, USA).Typically, the scanning power, scanning speed, scanning length, and repeated scanning cycles were fixed as 350 mW, 2 mm s À1 , 2 cm, and 30 times, where the intervals of lines were set as 0.4, 0.5, 0.6, 0.7, and 0.8 mm, respectively.The obtained template was a micro-groovearrayed topography for a subsequent soft transfer process.
Characterization: Morphology of microshutters was characterized by using a field-emission scanning electron microscope (JSM-6700 F). 3D images for monitoring the topography of OLR-MARS were captured by a 3D profile meter (VK-X100, Keyence Corporation, Japan).The sliding angles of the water droplets were measured using a CA10°C contact-angle system (Innuo) at 10% humidity and 20 °C.All digital photos were shot by a mobile phone (iphone-8 plus, 7 mega-pixel).
Figure 1.Facile fabrication of dual-state MASM.A) Schematic diagram for the preparation of MASM including laser grooving, Fe-doped matrix impregnation, thermo-triggered cross-linking, and a subsequent demolding process.B) Top view (upper image) and sectional view (lower image) by SEM for displaying the morphology of laser-printed MASM; the scale bar is 0.2 mm.C) Line-scanning 3D profile for recording the evolution of shutter topography as a function of distance.The comparison of D) apparent WCA and E) WSA over the droplets (%10 μL) on dual-state MASM.F) Digital clips for evidencing a typical droplet-spatial steering process (%10 μL) by a remote magnetic field, including a capture and release operation.

Figure 2 .
Figure 2. Switchable wettability over droplets on dual-state MASM.A) The change of shutter's bending angle α with the Fe-doped ratio.The variation of B) WCA and C) WSA for the water drop (%10 μL) on dual-state MASM with different intervals d.D) The corresponding adhesion force between the drop and the E-mode MASM evolving with the shutter's interval d.

Figure 3 .
Figure3.Underlying hydrodynamics over droplets-spatial-maneuvering.A) Schematic diagram for clarifying the evolution of TCL in response to a magnetic field.Without the magnet, MASM maintains the erect state so as to decrease the drop's contact area, thereby displaying a nonsticky state.Once the magnet is remotely loaded, the MASM switches to a bending morphology, resulting in an enhanced TCL and a sticky state.B) Comparison of the adhesion force F a for the droplets on dual-mode MASM.The result manifests that B-mode MASM exhibits a larger adhesive force relative to that of the droplet on E-mode MASM.C) Longevity test by virtue of cyclic pinning/sliding and capture/release operation via dual-mode MASM.No deterioration has been observed for both WSA and F a , signifying its good durability.

Figure 4 .
Figure 4. Multiple droplets spatial manipulation via dual-mode MASM.The phase map for studying the droplets steering capability by MASM with variable A) Fe-doped content and B) intervals d.C) Digital clips for displaying the spatial delivery of diverse liquid species for their controllable marriage through dual-mode MASM.D) Photos over various pattern configurations via on-demand steering droplets by MASM in an accurate fashion.The results showcase that the current dual-mode MASM is competent for the fast, sensitive, and accurate capture/release of diverse drops toward any destination.

Figure 5 .
Figure 5. Proof-of-concept potential in droplets-spatial-delivery for chemical reaction and circuit remedy.A) Digital clips for spatially maneuvering NaOH solution toward a targeted phenolphthalein solution for their metachromatic reaction by using current dual-mode MASM.B) Photos for displaying the MASM potential in harnessing the logical circuit.The conductive droplets of NaCl solution could be selectively mounted on any circuit for its remedy through loading/discharging the magnetic field over MASM.