Maglev-like droplet-based transporters co-regulated by wettability and magnetic field

doi:10.21203/rs.3.rs-5054916/v1

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Maglev-like droplet-based transporters co-regulated by wettability and magnetic field

https://doi.org/10.21203/rs.3.rs-5054916/v1

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The adhesion behavior of liquid droplets on solid surfaces is critical for natural transportation and industrial production. In particular, microrobots based on adhesion at solid-liquid interfaces have important applications in chemical reactions, cargo transport, and energy harvesting. However, current droplet-based manipulation strategies primarily depend on the structures or reactions between solid-liquid interfaces to achieve programmable droplets movement, while robust carrying capacity and precise synergistic operation of multiple droplets remain challenging. Herein, we propose a maglev-like droplet-based transporter system (MDTs) with adjustable structures to achieve precise droplets manipulation for robust cargo transportation and energy conversion. By controlling the magnetic fields and the droplet adhesion at the solid-liquid interfaces, the MDTs can precisely transport, climb and rotate, as well as efficiently convert mechanical energy into electricity for motion monitoring and self-powered devices. These findings deepen the understanding of force transport co-directed by surface wettability and magnetic fields, and broaden the utilization of these systems for cargo transportation and energy conversion, offering a great potential for contactless blind operation, lossless transfer and reactions, and powerless micro-electromechanical systems (MEMS).

Physical sciences/Materials science/Nanoscale materials/Magnetic properties and materials

Physical sciences/Energy science and technology/Energy harvesting/Devices for energy harvesting

From cargo transportation in space to drug delivery within the human body, mechanical energy transfer and conversion systems are ubiquitous in our daily lives, serving as one of the most fundamental and extensively studied fields^1,2. Particularly, controllable and precise transport of objects in clustered, narrow, and constrained spaces^3,4, such as optically opaque and deeply enclosed small spaces, has attracted much attention among researchers. Compared to micro-tethered robots^5,6, the droplet-based robots actuated by electricity^7–9, light^10–12, mechanical vibration^13,14, and other stimuli^15,16, tend to exhibit a lower energy loss and interesting dynamic behaviors at superwetting interfaces, including climbing at different angles, rotation, and directional transport. For example, electrically actuated microrobots have been shown to manipulate droplets to navigate with high velocity and long transport distance by surface charge density (SCD) method, and used for enabling cargo carriers with droplet wheels with a preset SCD gradient path⁷. Li et al. have created a wavy dielectrophoretic force field by photopyroelectric microfluidics for on-demand navigation, fusion, pinch, and cleavage of fluids with ultra-low mass loss, which has been utilized for sequential transportation of solid objects and organics¹⁰. Xue et al. have developed a droplet-based mechanical transducer system based on the synergistic effect of surface tension and solid-liquid adhesion, which can be used for vibration management, object transport, and laser modulation with the swarm effect of the arrayed droplets under a fixed mechanical vibration¹³. Such microdroplets actuated by these stimuli could directly achieve high-speed transport or large angle climb. However, due to the limitations of non-addressability and shaping ability of individual droplets, existing droplet-based robots have weak carrying capacity and can only transfer tiny objects with low transport velocity and accuracy, which fails to accomplish complex cooperative tasks.

Contrast with the above mentioned, magnetically actuated droplets (e.g., ferrofluids¹⁷) are much softer and gentler and have more advanced features¹⁸. Due to their liquid properties, they can potentially reduce the surface coefficient of the contact interfaces, and their linear negotiation abilities surpass that of non-magnetic liquid, as well as can be reconfigured into different shapes and stiffness by an externally controlled magnetic field. The physics of ferrofluidic droplet properties at micro- and macro-scale has been intensively studied and reported in fundamental studies^19–21. Recent works have also demonstrated utilizing these magnetic droplets for engineering applications^22–27. For example, magnetically actuated droplets have been controlled to achieve complex shape-morphing behaviors and planar motions by programming magnetic fields spatiotemporally and reconfigure into different shapes for cargo delivery in narrow channels much smaller than their sizes²⁷. 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. However, such magnetically actuated droplet robots can only carry tiny cargo smaller and lighter than their sizes and are impossible to achieve pollution-free delivery of cargo.

To tackle this challenge, we propose a ferrofluid droplet-based robot (the aforementioned MDTs) with high transport capacity achieved through controllable adhesion and external magnetic fields. We first report the hybrid mechanism to control contactless cargo transport by exploiting the capillary adhesion forces between solid-liquid interfaces in conjunction with magnetic fields, which provides these droplets with greater rigidity for loading and force transfer compared to existing droplet-based robots. Using the ferrofluid droplets as the intermediate medium, we enhance the adhesion of the magnetic liquid through a superhydrophilic pattern, reduce the friction of the magnetic fluid on the surface by introducing superhydrophobicity, and in addition, enhance the stability of the force transfer of the droplets with the help of a magnetic field. Then, the controllable support ability of the MDTs can be moderated by its structure and ferrofluid droplet volume. Furthermore, the ferrofluid droplet-based robots can perform multiple controllable functionalities, such as migrating, climbing, and rotating for cargo delivery, navigating through narrow tubular structures in confined spaces, and harvesting mechanical energy for motion monitoring and self-powered devices. In summary, our proposed programmable MDTs could provide a wide range of opportunities to realize unprecedented functionalities that are critical for advanced microsystem and energy conversion application.

2.1 Design of multifunctional MDTs

Our transport platform is simply fabricated by closely sandwiching ferrofluid droplets between a hybrid wetting surface (upper plate) and a superhydrophobic surface (lower plate) under an external magnetic field (Fig. 1a). Here, aluminum-based hydrophilic/hydrophobic substrates are prepared by laser ablation and low surface energy modification (Fig. S1, S2, Supplementary Information). The magnetic field can guide the movement of the ferrofluid droplets, which allows the movement of the upper plate, thus enabling contactless transportation control of the objects. For the bottom system, the shape and the movement of ferrofluid droplet can be easily controlled by an external magnetic field on a superhydrophobic surface (Fig. 1b). For the top hybrid wetted surface, the ferrofluid droplets can be firmly trapped by the superhydrophilic patterned regions due the strong adhesion between the patterned regions and such droplets, whereas the opposite is true for the superhydrophobic surfaces, allowing the droplets to easily roll off the surfaces (Fig. 1c). By aligning the patterned region of the upper plate with the ferrofluid droplets and gently placing it down, the upper plate self-assembles with the ferrofluid droplets due to its self-weight and the attraction of the superhydrophilic patterned regions to the ferrofluid droplets (Fig. 1d). Interestingly, if cleverly design the array and structure of such self-assembled structures (FDSS) based on individual ferrofluidic droplets, the unique capabilities of MDTs consisting of four FDSSs for cargo transportation and energy conversion can be realized. The MDTs can not only deliver cargo on plane, but also transfer on narrow and long tubular structures, with high transportation velocity and accuracy (Fig. 1e). Furthermore, the MDTs can realize energy conversion from mechanical energy to electricity based on Faraday’s law of electromagnetic induction, and the output electrical signals in turn can be used to monitor its movement (Fig. 1f).

2.2 Conceptualization of FDSS and its mechanical stability and wettability

Superhydrophobicity is common in nature, and the superelasticity and low sliding resistance usually exhibited by droplets on their surfaces. Through clever surface design, the kinetic energy of the droplets can be converted into other forms of motion and energy output^30,31. For example, two superhydrophobic structures can be used to drive the movement of a droplet through compression and release (model 1, 2, Fig. S3a, Supplementary Information). Here, two fluids are used for comparison, a water droplet and a ferrofluid droplet. However, it is not possible to achieve directional transport through droplet manipulation. Accordingly, structures with hybrid wettable surfaces can be manipulated in a restricted domain for droplets (Model 3, 4, Fig. S3a, Supplementary Information), which is expected to enable droplet motion control. Compared to Model 3, Model 4 is prone to failure when an external force is applied, in which case the minimum gap (h_min) between the upper and lower plates after compression is maintained at 0.2 mm. Besides, structures (Model 3) that are superhydrophobic on one side and hybrid wettable surfaces on the other are more resistant to compression than dual superhydrophobic surfaces (Model 1 & 2) and dual hybrid wettable surfaces (Model 4), as shown in Fig. S3b, Video S1, Supplementary Information. Thus, Model 3 is expected to realize the restricted domain control of droplets for transport.

Compared to water droplets, ferrofluid droplets are more controllable by external magnetic field, especially on super-sliding surfaces³². When the ferrofluid droplets is subjected to an external magnetic field, it is automatically shaped and its stiffness is increased. Therefore, based on model 3, we propose a hybrid model (model 5, the aforementioned FDSS) with capillary adhesion and magnetic actuation that can easily achieve rapid recovery after extrusion (Fig. S3a, Supplementary Information). Compared to Model 3, FDSS has greater stress resistance and recovery. When both models are subjected to external pressure and compressed to their limit positions (h_min <0.1 mm), the adhesive forces are unable to maintain the confinement of the droplets, causing the droplets in model 3 to be extruded (Fig. 2a), while the droplets in FDSS can be easily recovered (Fig. 2b). As for FDSS, the vibration between the maximum spreading diameter (d) of the ferrofluid droplet on the lower plate and the distance (h) between the top and lower plates is plotted in Fig, 2c, which shows that the compression and recovery curves basically overlap, indicating that the FDSS has a good deformation recovery capacity. The maximum pressure that can be withstood by the FDSS is ~ 248 mN, which is ~ 10.8 times that of the model when h is varied by 400 µm·s^− 1 to 0.2 mm and then recovered (Fig. 2d). Furthermore, our hybrid adhesive is reusable and highly durable. As evidenced by Fig. 2e, the weight retention of the ferrofluid droplet is sustained over 97.5% after 60 cycles.

The robustness of FDSS depends not only on the guidance and reinforcement of ferrofluid droplets by the magnetic field, but also on the adhesion of aqueous ferrofluid droplets by different wettability substrates. Here, the hybrid wetting and superhydrophobic surfaces are fabricated by laser ablation and low surface energy modification. After laser processing, the aluminum surfaces are covered with micro-nanoclusters (Fig. S2, Supplementary Information). After low surface energy modification, the static contact angle (CA) of the ferrofluid droplets on superhydrophobic surfaces is ~ 154°, while after secondary laser patterning, the CA of the ferrofluid droplet on the patterned region is ~ 24° (Fig. 2f). The hydrophilic pattern has a strong capillary adhesion that firmly trapped the ferrofluid droplets and prevents them from falling off the inclined surfaces, whereas in superhydrophobic region, the ferrofluid droplets roll off easily. Furthermore, ferrofluid droplets can be shaped and stiffened by external magnetic field to overcome the instability of the liquid. Therefore, the combination of hybrid wettability and magnetic field guidance would be a good candidate for robust cargo transportation, in which ferrofluid droplets can not only be shaped and slide rapidly over the superhydrophobic surfaces when actuated by an external magnetic field, but also can drive the upper plate for stable transport.

2.3 Mechanical performance of MDTs

To achieve robust cargo transportation, we propose a synergistic transport system (MDTs) consisting of four sets of FDSS, as shown in Fig. 3a. Here, the surface magnetic strength of the magnet used is ~ 302.6 mT (Fig. 3b). Load stability is important for microrobot to carry cargo. As schemed in Fig. 3c, we design two main aspects for probing the stability of MDTs, including the ferrofluid droplet volume and the structure of the system. The vibration curve of the droplet volume on the pressure resistance is shown in Fig. 3d. The system’s tolerance does not increase with increasing droplet volume, but reaches a maximum of ~ 930 mN at 18 µL and then decreases with increasing volume (18 to 36 µL). Besides, in the absence of an external force, the almost linear increase in pristine spread diameter (L) and pristine ferrofluid height (H) with increasing volume and the decreasing rate of change in between top and lower plates (Fig. 3e), which are mainly attributed to the fact that the further away from the magnetic field, the weaker the magnetic field's ability to bind the droplet, leading to a decrease in the droplet's stiffness. Here, the spacing (D) of adjacent superhydrophilic patterns, the superhydrophilic pattern diameter (d) and the minimum gap after applying pressure are 13 mm, 4 mm and 0.3 mm, respectively.

Furthermore, to demonstrate the mechanical performance of the system, we systematically investigate the dependence of the pressure tolerance on the MDTs structure, and summarize the results in Fig. 3f-h. Here, the structure of the robot is quantified by the spacing (D), the diameter (d) and the distance (h), and the droplet volume used and the minimum gap after applying pressure are 18 µL and 0.3 mm, respectively. Firstly, the system with a d of 4 mm can support a ~ 920 mN downforce when D ≤ 13 mm, while the ferrofluid may spill from the gap, due to the limited area of the upper substrate (Fig. 3f). Secondly, in general, increasing d contributes to the increase the load carrying capacity of the robot, while spill of the ferrofluid droplets occurs when enlarging d by more than 6 mm, as shown in Fig. 3g. Finally, when increasing h to 0.3 mm, the load capacity of the robot is maximized, but decrease as continues to increase (h > 0.3 mm), as shown in Fig. 3h. Besides, the MDT has excellent mechanical stability under long-term compression, as shown in Video S2, Supplementary Information.

Dynamic stability is also important for transporters. When the MDTs are impacted by glass beads (weight: 0.65g) falling from a height of 30 cm on the surface, they can be stabilized and restored to their original state without loss (Fig. S4, Supplementary Information). More importantly, regardless of whether the MDTs are loaded or unloaded, after being subjected to a certain range of lateral forces, they can automatically recover due to the action of capillary adhesion forces, as shown in Fig. S5, Video S2, Supplementary Information, which further prove the stability of the MDTs.

2.4 Applications

Static and dynamic stability of MDTs demonstrates its potential for cargo transportation in practice. For cargo transportation, the lower plate is designed to be large enough to allow the MDTs to move easily over the surface. The load supported by the upper plate can be moved along a predefined trajectory by programmable movement of the bottom magnet array, as shown in Fig. 4a. Here, the MDTs are named robot. To demonstrate the robot’s transport behaviors, we used two important evaluation metrics, load weight (W) and transport velocity (V), to systematically investigate their effects on the robot's transport performance, and summarize the results in Fig. 4b. The thickness of the superhydrophobic lower plate is 0.3 mm and the upper plate weight is ~ 0.435 g.

In general, stable cargo transportation is achieved with moderate W and V; stable transport also occurs when reducing W and/or V, while increasing W and/or V so that the robot will be in a transition state between stabilization and failure (i.e., unstable transport, Fig. S6, Supporting Information); the robot will fail to transport when continuing to W and/or V. For example, the robot is capable of transporting a solid cargo of W ≤ 8 g on the surface at a speed of V ≤ 80 mm/s, while a unstable transport state will occur if the load enlarges to 9 ~ 12 g. When increasing W to 15 g or V to 130 mm/s, the cargo, along with the upper plate, will slide off the surfaces of the ferrofluid droplets due to the fact that the friction driving the transport of the cargo will be greater than the adhesion of the superhydrophilic region to the ferrofluid droplets (Fig. S6, Supporting Information). Besides, the robot can transport the largest loads (W = 15.5 g) when the velocity V ≤ 40 mm/s, which is ~ 35.6 times the mass of the upper plate used to carry the load, indicating excellent cargo transportation capabilities. Physically, cargo transportation behavior of the robot is directly related to solid-liquid contact interface angles. When the robot is loaded, the load angle (𝛼₁ and 𝛼₂) hardly changes with the change of load mass or transportation state due to the solid-liquid interface property ^33,34. During robot loading and transportation, the advancing angle (𝜃₂) decreases as the transportation speed (V) increases, and the receding angle (𝜃₁) remains constant, which is attributed to the shaping and strengthening of the magnetic droplets by the magnetic field, as shown in Fig. S7, Supporting Information.

Moreover, we demonstrate the dynamic programmability and applicability of the robot for transportation in planar/curved environments, as most of the reported droplet-based manipulation systems for cargo transportation become invalid due to load carrying capacity, uneven operating surfaces, or limited space. Figure 4c displays the programmable cargo transportation control for robot on flat surfaces. Firstly, the robot can be simply assembled in the self-assembly area when subjected to the influence of the magnetic field on the ferrofluid droplets and the adhesion of the superhydrophilic region to the ferrofluid droplets. Secondly, the robot can be dragged and moved to the loading zone for cargo loading by using a magnetic field to orient the ferrofluid droplets. In addition, the magnetic field can be directed to control the robot, including migration, rotation, and movement on an inclined plane, as shown in Fig. S8, S9, Supporting Information. After loading, the cargo is transported along a preset programmed path to the unloading area and complete rotation, commutation and unloading. Finally, the robot returns to its original position after a single transport, the process that takes only 27 s (Video S3, Supplementary Information). Furthermore, the robot can also be applied in limited spaces, such as narrow tubular structures and boxes, which is important in in the field of contactless handling and transportation. By manipulating the movement of the magnets, the robot can easily move and robustly transport cargo inside the PEF tube without mass loss (Fig. 4d, S9, Video S3, Supporting Information). Here, the lower plate is designed as a curved FEP tube with a thickness of 0.8 mm, and the superhydrophilic pattern of the upper plate and the ferrofluid droplets can be designed to match very well for adhesion on the curved surface. Besides, inverted MDTs are also suitable for underwater transportation. The robot can not only directionally and stably tow underwater objects, but also perform programmable rapid movement of cargo (Fig. S10, Video S4, Supporting Information).

Accurate motion monitoring and energy conversion of kinetic energy are notable in contact-free manipulation and transportation, especially in confined unsupervised spaces, which is widely used in microchannel delivery, chemical reactions, and biotechnology. For illustration, we design a unique platform for energy conversion and velocity motion state detection based on Faraday's law of electromagnetic induction³⁵, as shown in Fig. 5a. The lower plate of the platform consists of a double layer, and multiple array coils are placed in the middle of the double layer substrate with a coil spacing of 25 mm. Here, the coils used have a diameter of 20 mm, a thickness of 0.2 mm, an internal resistance of 34 Ω, and a coil number of 400 turns, and the overall thickness of the lower plate is 0.8 mm. As the robot moves rapidly across the platform, the magnetic flux through the coil changes rapidly (Fig. 5b), resulting in an open-circuit voltage (V_OC) that can be recorded by an oscilloscope, thus realizing the conversion of kinetic energy to electrical energy for driving external electronic devices such as LED arrays “DUTs” (Fig. 5c, Video S5, Supplementary Information). The external circuit used is shown in Fig. S11, Supplementary Information. Besides, when the robot moves at different speeds and generate different voltages. In general, the faster the robot is transported, the higher the V_OC is (Fig. 5d). For example, when robot transports at a speed of 2.8 cm/s, the resulting peak V_OC is 24.6 mV, while at a speed of 34.2 cm/s, the V_OC is 94.8 mV.

Furthermore, the V_OC generated by the different coils can be used to monitor the robot’s motion, such as the instantaneous speed and motion direction. By programmable manipulation of the ferrofluid droplets using a magnetic field, we perform sequential kinematic state monitoring of the robot. As shown in Fig. 5e, the V_OC generated by the coil at positions 1, 2, and 5 is ~ 25.0 mV, at positions 6, 9, 8 is 30.2 mV, and at positions 7 and 4 is 99.6 mV, which indicates that the robot move along positions 1, 2, and 5 to position 6 at a uniform speed of V₁ = 3.0 cm/s, and then move along positions 9 and 8 to position 7 at a uniform speed of V₂ = 12.0 cm/s, finally, move to position 4 at a uniform speed of V₃ = 34.5 cm/s. Therefore, our platform provides a versatile pathway for automated control and monitoring of contactless droplet manipulation and transportation. In practice, the durability of the system must be given priority. To verify its durability, the robot is performed by continuously moving back and force linearly with a speed of V = 14.3 cm/s for 40 s (Fig. 5f). The robot can output a sufficiently stable electric response over long periods after the move, and no significant mechanical damage or performance degradation is observed.

Besides, the platform also can be used as a self-power device for wireless signal transmission. The robot can output a stable electrical response activated by a magnetic field, indicating that the electrical output performance of the robot can be evaluated by connecting different external loads. As shown in Fig. 5g, different external resistors are connected in series with the robot, and their peak voltage and power are tested. The output voltages of the robot increased with load resistance, while the corresponding power increases and then decreases, reaching a peak power of ~ 48 µW at R = 34 Ω, which indicates that the system can be used to supply power wirelessly based on the previous study^36–38.

In summary, we demonstrate a strategy for cargo transport control using adhesion between solid-liquid interfaces and magnetic field guidance. Distinct from previous methods that relied on structures or other stimuli to manipulate droplets, the discovery of such transport mechanism enriches our ability to control droplets for robust and precise object transport. The robot enables efficient energy conversion from mechanical to electrical energy for self-powered wireless signaling, in addition to transporting high-weight loads at high speeds. From a broader perspective, endowed with characteristic of low-cost, efficiency, and precision, such approach will open a new avenue for a wide range of applications in microsystems and energy harvesting.

4.1 Fabrication of superhydrophobic surfaces and hybrid wetted surfaces

The fabrication of the patterned aluminum (Al) substrate is shown in Fig. S1, Supplementary Information. First, the Al plates were ultrasonically cleaned with ethanol to remove impurities on the surface and dried. Then, a nanosecond laser system (SK-CX30, SAKE Laser Technology Co., Ltd., Shanghai, China) was used to construct rough structures on the Al substrates under one time laser ablation with 800 mm·s^− 1 scanning speed and 18 W power. Followed by cleaning in ethanol and drying, the Al surfaces with micro and nano structures were immersed in 1 wt% Fluoroalkylsilane [FAS, CF₃(CF₂)₅(CH₂)₂Si(OCH₃)₃] ethanol solution for 1 h and dried at 80°C for 30 min to obtain superhydrophobicity, which can be used as the lower plates. Finally, the superhydrophobic Al plates were ablated at the same parameters to fabricate wettability patterns. After cleaning the surfaces by blowing, the patterned wettable Al surfaces were successfully fabricated, which can be used as the upper plates. Unless otherwise stated, the thickness of the aluminum plates used was 0.3 mm and only one surface was laser ablated and modified. The contact angle (CA) and dynamic rolling process of the droplets were recorded and measured by an optical contact angle meter (SL200KS, KINO, USA). The volume of ferrofluid droplets used was 6 µL. The surface microstructures and elemental distribution of the samples were characterized by a scanning electron microscopy (SEM, JSM-6360LV, Japan) with an energy dispersive X-ray spectrometry (EDS) analysis.

4.2 Mechanical stability testing of structures supported by single droplet and droplet-magnetic field interaction

To verify the mechanical stability of structures supported droplet-magnetic field interaction, single droplet-based support structure was designed. Here, the diameter of the wettable pattern and the volume of the ferrofluid droplet were 4 mm and 18 µL, respectively. When the structures were subjected to vertical downward pressure, the minimum gap between upper plate and the lower plate was set to 0.2 mm. The time evolution of the force generated between two pates was recorded by a graphic push-pull gauges (SBT970TX, Simbatouch, China). The downward pressure feed rate was 400 µm·s^− 1.

4.3 Three-dimensional multi-droplet synergistic support structures

Utilizing the adhesion between solid-liquid interfaces and synergistic effect of droplet array, four ferrofluid droplets can stably support the upper plate for three-dimensional transport behavior. We designed different structural parameters for probing the mechanical stability of this support structure, including circular spacing (D = 9–15 mm), circular diameter (d = 2 ~ 6 mm), thickness of the lower plate (h = 0.2–0.6 mm), and volume of the ferrofluid droplets (Volume = 6 ~ 60 µL). First, superhydrophilic arrayed pattern was fabricated on the upper plate. Then four cylindrical NdFeB magnets (N52, Beijing Jiujiugaoke Magnetic Material Co., Ltd, China) were fixed to the bottom base, and their array spacing was kept consistent according to the upper plate pattern spacing. Four ferrofluid droplets were shaped and reinforced on the lower plate by magnetic fields. Finally, four ferrofluid droplets were automatically pinned by relevant superhydrophilic spots on the superhydrophobic upper plate, when assembling the upper and lower plates. The thickness of both the upper and lower plate covers is 0.3mm. The maximum size of the upper and lower plates used in the test was 25 mm × 25 mm with a chamfered edge length of 5 mm. The minimum gap between upper and the lower plates under pressure was set to 0.3 mm. The downward pressure feed rate was 400 µm·s^− 1. The center surface magnetic strength of the magnet used was 3026 Gs, tested by a commercial Teslameter (WT10A, WEITE Magnetic Technology Co., Ltd, China).

4.4 Programmable object transportation test

The spacing and diameter of the circle pattern on the upper plate were 13 mm and 4 mm, respectively. Each ferrofluid droplets volume was 18 µL. The size of the lower plate used in the test was 100 mm×100 mm. The upper plate was placed on the ferrofluid droplets for supporting objects, which were placed on the center of the plate. As the magnets performed programmable movements, the objects on the upper plate moved quickly and steadily along with them, which was recorded by a camera to show the different transport capabilities of the transporter. In addition to the cargo transportation on the plane, we have also designed the transporter to migrate on curved surfaces. The commercial fluorinated ethylene propylene (FEP) tube used has an inner diameter of 19 mm and an outer diameter of 21 mm. The transporter was loaded with a weight of 4.8 g.

4.5 Movement monitoring and energy conversion test

The commercial coils with a diameter of 20 mm were placed in the middle of two identical upper plates (0.3 mm thickness) for signal acquisition and energy conversion. The internal resistance of the coil was 34 Ω measured by a digital multimeter (XDM1041, Fujian Lilliput Optoelectronics Technology Co., Ltd, China), and the wire diameter was 50 µm. The spacing of the array coils was 25 mm and the structural parameters of the adopted transmitter were the same as in section 4.4. An external circuit consisting of a passive rectifier (KBP206, MDD, China) and a current/voltage converter amplifier (AD620, Keruitai Electronic Technology Co., Ltd, China) in series was used to light the LEDs. When the magnetic flux through the coil changed, the output electrical signals were captured by a digital phosphor oscilloscope (DPO 2014B, Tektronix, America).

Competing interests:

The authors declare no competing interests.

Author contributions:

Y.Z. conceived the research. H.Z. and X.L. supervised the research. J.Z. and Y.L. prepared the samples. Y.Z., H.Z., J.Z., Y.L., S.M., Y.W., and Z.W. carried out the experiment. Y.Z., H.Z., and Z.Y. built the model. All the authors analyzed the data. X.L., Y.Z. and H.Z. wrote the paper.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (Grant No. 52475430), the Fundamental Research Funds for the Central Universities (Grant Nos. DUT19ZD202, DUT23YG118), and the Thematic Fund for Scientific Research of Talents Introduced to Dalian University of Technology (DUT24RC(3)040).

Data availability.

The data that support the findings of this study are available from the corresponding authors on reasonable request.

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There is NO Competing Interest.

SupplementaryInformation.docx
VideoS1.mp4
Compression of processes for different models under pressure.
VideoS2.mp4
Mechanical stability and lateral recovery of MDTs under long-term compression, which can be used for cargo transportation.
VideoS3.mp4
Transportation of MDTs on planar and curved surfaces.
VideoS4.mp4
Inverted MDTs for underwater cargo transportation.
VideoS5.mp4
Energy conversion from mechanical energy to electricity of MDTs.

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Maglev-like droplet-based transporters co-regulated by wettability and magnetic field

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Abstract

Figures

Introduction

Results and Discussion

2.1 Design of multifunctional MDTs

2.2 Conceptualization of FDSS and its mechanical stability and wettability

2.3 Mechanical performance of MDTs

2.4 Applications

Conclusion

Experimental Section

4.1 Fabrication of superhydrophobic surfaces and hybrid wetted surfaces

4.3 Three-dimensional multi-droplet synergistic support structures

4.4 Programmable object transportation test

4.5 Movement monitoring and energy conversion test

Declarations

Competing interests:

Author contributions:

Acknowledgments

Data availability.

References

Additional Declarations

Supplementary Files

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