Dual-bionic superwetting gears with liquid directional steering for oil-water separation

Developing an effective and sustainable method for separating and purifying oily wastewater is a significant challenge. Conventional separation membrane and sponge systems are limited in their long-term usage due to weak antifouling abilities and poor processing capacity for systems with multiple oils. In this study, we present a dual-bionic superwetting gears overflow system with liquid steering abilities, which enables the separation of oil-in-water emulsions into pure phases. This is achieved through the synergistic effect of surface superwettability and complementary topological structures. By applying the surface energy matching principle, water and oil in the mixture rapidly and continuously spread on preferential gear surfaces, forming distinct liquid films that repel each other. The topological structures of the gears facilitate the overflow and rapid transfer of the liquid films, resulting in a high separation flux with the assistance of rotational motion. Importantly, this separation model mitigates the decrease in separation flux caused by fouling and maintains a consistently high separation efficiency for multiple oils with varying densities and surface tensions.


Comment 1
We wonder if the hydrophilic gears in Fig. 1d and f are coated with TiO2 NPs. If the TiO2 coating was processed on the hydrophilic gears, please include the illustration of the TiO2 coating process in the 'fabrication process' scheme because TiO2 coating is essential for super-hydrophilicity. Comment 2 On page 7, lines 153-154, the author argued 'Our motivation is to utilize the synergies of superwettability, centrifugal force and extrusion force synergies ~'. Please explain and add in detail what are the synergies of superwettability, centrifugal force, and extrusion force in the separation process, respectively. Comment 3 On page 11, line 225, the sentence starting 'Form macro-imaging process, teeth and cavities ~' has a typing error. Please correct the typos. Comment 4 In Fig. 3h, the authors used hexadecane and tetrachloromethane with different densities for showing the use of various oils. Please specify each density of oils next to the oil name in the graph.
Comment 5 In Fig. 4c, the collected water looks like a dyed state. To avoid confusion about the image result and separation efficiency that did not separate oil and water perfectly, please supplement the residual components of clean water and the collected water using UV-Vis spectroscopy.
Reviewer #3 (Remarks to the Author): This manuscript aims to achieve oil-water separation in a more sustainable way. This problem becomes much trickier considering the weak antifouling ability of the existing membrane or sponge separation system and the low feasibility of multi-scale oil-water systems. In their manuscript, the authors take inspiration from the surface morphologies of the cat-tongue and pitcher plant to devise an oil-water separation system. This work is demonstrated in two parts. First, the authors experimentally compare the parameters by the simple model to screen the structural design in multi-scales. Second, the biomimetic gear's peculiar geometry is assumed to be favorable to the long-termed oil-water separation process. The manuscript combines model experiments with theoretical considerations to rationalize the optimality of the gear design. The authors then test their contraption in real life to prove that their oil-water separation system outperforms existing techniques.
The results they provide are rather convincing. I recommend the publication of this manuscript in Nature Communications.
One issue that needs addressing, is providing more informative descriptions in the caption of Figure 4 and the methods part.
Another point to address is the duration of gears in the long-term experiment.
Reviewer #4 (Remarks to the Author): The manuscript reports the design and development of a new oil-water separation system based on gears systems with different wettability features that allows oil-water separation via films' spreading-formation-breaking mechanism. The design was well illustrated and the performance investigations were thoroughly performed. Overall, the work offers novelty compared to the established literature, and provides significant contribution to the field of oil spill cleanup and wastewater treatment. Below are some comments and recommendations for revisions to help improve the manuscript: 1. Abstract: Line 29, the wetting is performed before the separation process, please revise this statement according to Lines 109-110.
2. In the introduction section, the authors focused on the existing materials/membranes for oilwater separation but failed to mention and discuss the progress made in scalable oil-water separation systems, that have been recently developed/designed, such as functionalized trawling nets (Environ. 4. Preferentially, the characterization of the prepared CTP and PCP surface (e.g., wettability features) should be discussed before the oil-water separation application for more coherency and to allow readers better establish structure-property-performance relationship.
5. Please explain the cause of the superhydrophilicity and underwater superoleophobicity of CTP. The same for the superoleophilicity and underwater superhydrophobicity of PCP.
6. The chemical characterization (FTIR, XPS, or NMR, …) of the upper layers/surface of the two gears (CTP and PCP) needs to be performed to investigate the surface chemistry that is vital for the wetting behavior. Please revise accordingly. 7. Movie 1 does not show the entire separation process. It shows the before and after, and the magnified gears connection. The authors need to provide a large-frame and full (sped up) video showing the oil-water separation process (for 1h: ~99 mL of clean water and ~1 mL of red-dyed n-hexadecane, as stated in Lines 119-120).
8. How were the oil-water emulsions prepared before the separation experiments? What is the average size of the oil droplets? 9. The work did not address the simultaneous separation of co-existing heavy and light oil, as pointed out in Lines 320-342. Please provide additional data (using know mixtures of different oil types in water emulsions) or revise this point about the device performance.
10. The recycling stability of the gears should also be demonstrated by the stability of the gears materials and coatings after a long operating time, such as checking the wettability both gears, and the possibility of TiO2 nanoparticles leaching that might cause secondary pollution.
11. The effect of temperature on the device's performance is worth investigating to show the impact of oil viscosity change and heating on the proposed films' spread-formationseparation/breaking mechanism. Please revise accordingly.
12. Also, the effect of the gears' rotation speed on the separation efficiency is a very important parameter to investigate to show the effect of residence time needed for efficient films' formationbreaking. Please revise accordingly.
13. What is the composition of the simulated kitchen wastewater?
14. Please add the type of emulsions separated in previous works as an additional column in Extended Data Table 2 to allow a more accurate and relevant comparison. 18. The authors can use a simpler and more straightforward language through the text to better convey their message to the readers. Example (Title): "co-design" or "design"? what is "multiscaled" referring to? Please revise.

Referees' Comments:
Summary Liu et al. developed an oil-water separation device by engaging the two gears with different superwetting features. The oil-water mixture is separated into continuous liquid phases when the mixture droplet is cast on the rotating pre-wetted gears. Authors claim that the critical factors of oil-water separation by this device are (1) the use of two different bionic superwetting surfaces, (2) combining opposite superwetting surfaces in one device, and (3) mechanical separation by gear rotation. This work addressed the flux degradation issues in oil-water separation technology.

Recommendation
This work can reach the standards published in Nature communications after major revision.
There is research in applying the bionic directional spreading surfaces for oil-water separation (e.g., Angew. Chem. Int. Ed. 2017, 56, 13623.; Science 2021, 373, 1344.) or developing the separation devices combining opposite superwetting surfaces (ACS Appl. Mater. Interfaces 2015, 7, 18915.). However, the idea of using gears to separate emulsions mechanically is new. The study of the liquid curvature effect and mechanism of microdroplet squeezing is interesting. The manuscript quality is excellent. However, several points should be addressed before publication that I listed thereafter.

Response:
We thank the referee for the positive comments and insightful suggestions to improve the quality of our manuscript. We have updated the manuscript and included a point-to-point response to each individual comment as below.
Comments for the authors  Figure S3.

Response:
We greatly appreciate the referee for the valuable suggestions. Topology bionic features are useful and necessary for oil-water separation.
Firstly, complementary topology bionic features can provide rapid transport paths for oil and water separately while ensuring the liquid is squeezed. If there are no topology bionic features, the contact surface of two gears with flat surfaces will be completely closed, greatly reducing the liquid flux, and resulting in liquid spillover. To support our demonstration, we also performed the control experiment with two gears without bionic features. Even if the two gears have the same superwettabilities as the bionic gears, they will still lose the ability to separate oil and water (Fig. R1). Fig. 5). Time sequence images of two gears without topology bionic features for the oil-water micro-drop separation process.

Figure R1(Extended Data
Secondly, the directional transport performance of the topological oil-water paths speeds up the transport rates of oil and water, respectively, thus improving the refresh speed of the two liquid surfaces on gears. This not only increases the flow rates and separation rate but also prevents the deposition of pollutants. The revised manuscript demonstrates the directional water and oil spreading on CTP and PCT (Fig. R2). Fig. 3). Directional transport of CTP and PCP. a, The wetting of CTP and PCP in a water-oil system. b, Directional transport of water on periodically arranged. c, Directional transport of oil on PCT.

Figure R2 (Extended Data
Thirdly, the directional transport performance based on topology bionic features can also ensure that the liquid films on bionic gears are uniform and dispersed. Therefore, topology bionic features are essential. Super-lubricity is the key to achieving the separation of oil and water, but it is also inspired by the Nepenthes peristome. In conclusion, both superwettability and topology bionic features are indispensable for successful and rapid sustainable multi-scaled oil-water separation.  Secondly, PDMS gear swelling can reduce the gap between the two gears so that it can better bite, increasing the emulsion's extrusion efficiency. The swollen PDMS gear has good flexibility (Fig. 3d). In the long-term experiment, the flexible PDMS gear can reduce the wear to the rigid resin gear, thus extending the device's service life (Fig. R4). Fig. 1d, 1f). The microscope images and SEM photographs of the two gears before and after separation. a-b, Microscope image of CTG before separation (a) and after separation (b). c-f, SEM image of CTG and nano-TiO2 superhydrophilic coating before separation (c, e) and after separation (d, f). g-h,

Figure R4 (Extended Data
Microscope image of PDMS gear before separation (g) and after separation (h).
In general, PDMS gear swelling is helpful for oil and water separation. The collocation of rigid and flexible complementary materials is also one of the innovation points of our experimental design. We hope that our supplement and explanation can satisfy the reviewers.

7
Based on the referee's question, a complementary characterization of the durability of gears has been made. After a long separation experiment, we found no obvious reduction of TiO2 nanoparticles on the surface of the superhydrophilic resin gear.
Our modification to the manuscript: On page 6, lines 127-129, we added described of gear nanostructure: "Moreover, we found that after a 1h separation experiment, no obvious reduction or damage was observed on the superhydrophilic resin gear or PDMS gear (Extended Data Fig. 1d and 1f)".   6. If the hydrophilic gear is first wetted by oil, is it replaced by water?

In
Response: Oil cannot be replaced by water because the swelling oil can protect the gear from wetting by the water phase. In the fabrication process, we pre-wet each gear with water or oil before separating. In the same way, the hydrophilic gear will not be replaced with oil when it is wetted by water. So, our dual-bionic superwetting gears device can achieve long time oil-water separation without any loss of efficiency.
9 7. The authors demonstrated the separation of low oil content mixture throughout the study.
Is it possible to separate the high-oil content mixture? In addition, is it possible to separate water-in-oil emulsion or that containing surfactant?
9. In the discussion section, the authors should explain the limitation of this strategy and perspective.
Response: Thanks for the referee's valuable comment and professional question.
Comments #7 and #9 are very meaningful, which can make our paper more objective, and at the same time, can give readers more inspiration, thus encouraging future research.
Response to comment #7: Improving the oil content for the mixture of oil-water microdroplets will not affect the separation efficiency and speed. However, it is a big challenge to separate emulsions containing surfactants. The separation efficiency was not as high as expected, so we did not present these results in detail in this paper. This is a limitation of our study. At the same time, this is a research project that we will focus on in future research. Following the reviewer's suggestion, we rewrote the Discussion section to highlight our research's limitations.
Response to comment #9: We rewrote the Discussion section in the revised manuscript according to the referee's suggestion. Moreover, we present some perspectives to guide researchers to make breakthroughs based on our work in the future. 8. In Figure 4B, how long does it take to separate the oil-water mixture?
Response: Thanks a lot for the referee's comments. We have modified this part in our revised manuscript, as described below: Our modification to the manuscript: On Page 16, Line 335-336, we added a discussion of the separation time; "The separation rate was set at 40 μL s -1 , and the total separation time was about 420 minutes." 11 Responses to Referee # 2

Referees' Comments:
Comments to the Author This manuscript reports the dual-bionic super-wetting gears with liquid steering abilities to separate oil-water micro-drops and oil-in-water emulsions into oil and water, respectively. The authors fabricated the fancy automated system with gears that have different topological structures, facilitating the liquid transfer. In particular, the designing of the cat tongue plane (CTP) and peristome-inspired cavity plane (PCP) has a positive effect on separation efficiencies, flux, and applicability of various oils with different density because the structures of CTP and PCP fit perfectly to separate oil and water with minimum separation error. To understand the basic principle of oil-water separation, the authors carried out the pre-experiments using pads with the same structure. Based on the variation of the setup construction including pad position, R, and α, the authors also found the optimized condition that can separate the oil/water solution into two solutions (oil and water). As a result, this is a well-organized manuscript, and this work will be of great interest to the related community for further studies of oil/water separators. Therefore, we recommend the manuscript for publication in Nature Communication if the following minor comments can be addressed properly.

Response:
We thank the referee for these positive comments. We have updated the manuscript and included a point-to-point response to each individual comment as below.
1. We wonder if the hydrophilic gears in Fig. 1d     14 Responses to Referee # 3

Referees' Comments:
This manuscript aims to achieve oil-water separation in a more sustainable way. This problem becomes much trickier considering the weak antifouling ability of the existing membrane or sponge separation system and the low feasibility of multi-scale oil-water systems. In their manuscript, the authors take inspiration from the surface morphologies of the cat-tongue and pitcher plant to devise an oil-

Another point to address is the duration of gears in the long-term experiment.
Response: Thanks for the referee's suggestions. Based on the referee's suggestion, a complementary characterization of the durability of gears has been made. After a long separation experiment, we found no obvious reduction of TiO2 nanoparticles on the surface of the superhydrophilic resin gear. We also characterized the surface morphology of PDMS gear before and after a long time separation experiment, and found that there was little change, as shown in Figure R4: Figure R4 (Extended Data Fig. 1d, 1f). The microscope images and SEM photographs of two gears before and after separation. a-b, Microscope image of CTG before separation (a) and after separation (b). c-f, SEM image of CTG and nano-TiO2 16 superhydrophilic coating before separation (c, e) and after separation (d, f). g-h, Microscope image of PDMS gear before separation (g) and after separation (h). At the same time, we also characterized the changes in the wettability of the two gears after a long-time separation. The results of the contact angle test proved that the wettability of the two gears did not change.
In summary, our dual-bionic superwetting gears device has good durability and can be used for long time separation experiments.

Referees' Comments:
The manuscript reports the design and development of a new oil-water separation system based on gears systems with different wettability features that allows oil-water separation via films' spreading-formation-breaking mechanism. The design was well illustrated and the performance investigations were thoroughly performed. Overall, the work offers novelty compared to the established literature, and provides significant contribution to the field of oil spill cleanup and wastewater treatment. Below are some comments and recommendations for revisions to help improve the manuscript:

Response:
We thank the referee for insightful suggestions to improve the quality of our manuscript. We have updated the manuscript and included a point-to-point response to each individual comment as below.
1. Abstract: Line 29, the wetting is performed before the separation process, please revise this statement according to Lines 109-110.

Response:
Thanks for the referee's comments and suggestions. The pure water or oil phase is pre-wet on the gear surface before the separation process. Then, during the separation process, the water or oil phase in the mixture can steer to spread rapidly and continuously on the water or oil pre-wetted gear surface.

Our modification to the manuscript: On page 2, line 29, "the water and oil in the mixture
can steer to spread on the preferential gear surfaces rapidly and continuously." 2. In the introduction section, the authors focused on the existing materials/membranes for oil-water separation but failed to mention and discuss the progress made in scalable  Fig. 1.
Different from referee's comments, we did not display and analyze these characterizations in the Designed dual-bionic gears for oil-water separation section, mainly for two reasons: Firstly, we want to be able to introduce our design concept to readers in the most concise and clear sentences, from the plane to the curve to the gear. So, we don't present a concrete characterization here, just a result. Detailed characterization is, therefore, shown in the

Screening dual-bionic gears designed for sustained oil-water separation section;
Secondly, in Screening dual-bionic gears designed for sustained oil-water separation section, we present detailed experiments and discuss the effects of various parameters. It would be easier for the reader to understand. Just as the reviewer mentioned the "structureproperty-performance relationship", that's how our part of the logic works.
Thanks again for the referee's advice. We hope that our supplement can satisfy the referee.

5.
Please explain the cause of the superhydrophilicity and underwater superoleophobicity of CTP. The same for the superoleophilicity and underwater superhydrophobicity of PCP.
Response: Thanks for the referee's professional comments. This is a very professional question, and the supplement and explanation in this respect can make the basic theory of our paper more clearly. Therefore, we add relevant explanations and a schematic diagram ( Figure R10). For the superhydrophilic CTP, as we wrote in the Separation mechanism section, which was determined by the difference between f(γwcosθw -γocosθo) and γow. The ! ! can be obtained by formula (eq R1) 61 , Where ! = + = 0°， , < + , and considering the f < 1. We can calculate that ! ! > 150°. Therefore, CTP exhibits excellent underwater superoleophobicity.

20
For the PCP surface, similar results can be obtained by (eq R2). Here ! = 0°. Therefore, + ! will be decided by + and f. And we konw that + of PDMS is greater than 90 °.
On the other hand, in the experiment, the PDMS will first swell with oil. So, its surface is filled with oil, and there is no trapped air. When immiscible water droplets with high surface energy are introduced, the solid surface submerged by oil can support the oil-water interface, greatly reducing the water-solid contact area (f). This means the droplets float rather than invade and spread. Thus, PCP exhibits superhydrophobicity underoil.
Our modification to the manuscript: On Page 29, lines 651-659, FTIR spectra are shown in Extended Data Fig. 2  We hope that our modification can satisfy the reviewers.
7. Movie 1 does not show the entire separation process. It shows the before and after, and the magnified gears connection. The authors need to provide a large-frame and full (sped up) video showing the oil-water separation process (for 1h: ~99 mL of clean water and ~1 mL of red-dyed n-hexadecane, as stated in Lines 119-120).
Response: Thanks for the referee's comments. We have provided a large-frame and full  On page 16, lines 329-331, we added described the particle size of the emulsion: "In addition, the emulsion dispersing phase's particle size and distribution (Fig. 4c) were analyzed by the laser granularity instrument (Winner319C) and the average size of the oil droplets is ~100 μm." After a long separation experiment, we found TiO2 nanoparticles on the surface of the superhydrophilic resin gear were still there, and no obvious reduction was observed. We also characterized the surface morphology of PDMS gear before and after a long-time separation experiment, and found that there was little change, as described below:  At the same time, we also characterized the changes in the wettability of the two gears after a long-time separation experiment. The results of the contact angle test proved that the wettability of the two gears did not change.
To sum up, our dual-bionic superwetting gears device has good durability and can be used 27 for long time separation experiments.
11. The effect of temperature on the device's performance is worth investigating to show the impact of oil viscosity change and heating on the proposed films' spread-formationseparation/breaking mechanism. Please revise accordingly.
Response: Thanks for the referee's comments. First, we demonstrate that the wettability of our dual-bionic superwetting gears can remain stable within a certain temperature range.
We placed the two gears at 20℃, 50℃, and 80℃ for 2 hours, then measured their contact angles, respectively. It turns out that they still maintain good underwater superoleophobicity, and underoil superhydrophobicity as described below:  Then, we tested the viscosity of different oils at 20℃, 50℃ and 80℃ (Table R1). Next, 28 silicone oil-100/water emulsion via our dual-bionic superwetting gears at three different temperatures (30 ℃, 40 ℃, and 55 ℃) showed that the temperature rise has no negative effect on the separation efficiency ( Figure R15). We think this is because temperature changes have little effect on water viscosity. In addition, in general, the higher the temperature, the lower the viscosity of the oil phase. Therefore, less resistance and demulsification are easier to achieve in the extrusion process. We need to mention that a higher ambient temperature means faster liquid volatilization and more liquid loss. So, the oil and water separation in a high-temperature environment is of little significance for improving separation in our system. 12. Also, the effect of the gears' rotation speed on the separation efficiency is a very important parameter to investigate to show the effect of residence time needed for efficient films' formation-breaking. Please revise accordingly.

Response:
Thanks for the referee's comments. We add the test of the influence of rotation speed. We tested the separation efficiency of n-hexadecane/water emulsion at different rotation speeds within our device capacity. The results show that the efficiency of emulsion separation is very low when the rotational speed is very small. This may be because too much of the dispersed oil phase falls down the gear before it can be squeezed.
When the rotational speed is increased, the emulsion separation efficiency can be maintained at a very high level. The supplement of this question makes our research more systematic and comprehensive. Thank you very much for your help. Fig.6). Optical sequence images of two gears for the oilwater emulsion separation process at different rotation speeds. Here, we have learned from the calculation method of oil and water separation experiment by superhydrophobic meshes. We take the cross-section area where the liquid passes through the joint of the two gears as the effective surface area. As shown in Figure   R18, product is our definition of effective surface area. Response: Thanks for the referee's comments. We add the emulsion dispersing phase's particle size in Figure 4b. The distribution ( Figure R12) were analyzed by the laser granularity instrument (Winner319C).

Figure R16 (Extended Data
Our modification to the manuscript: On page 16, lines 329-331, we added described the particle size of the emulsion: "In addition, the emulsion dispersing phase's particle size and distribution (Fig. 4b) were analyzed by the laser granularity instrument (Winner319C) and the average size of the oil droplets is ~100 μm."

Figure R12
The particle size of the emulsion. a,c, Schematic of emulsion before separation (a) and after separation (c). b, Histogram of emulsion particle size distribution.
d, e, Optical and stereomicroscopic images of collected water (d) and collected oil (e).
18. The authors can use a simpler and more straightforward language through the text to better convey their message to the readers. Example (Title): "co-design" or "design"?
what is "multi-scaled" referring to? Please revise.

Response:
Thanks for the referee's comments. "Design" refers to a participatory approach to designing structures, in which the designed agricultures are treated as equal collaborators in the design process. In our system, the dual-bionic superwetting gears, including the combination of the cat-tongue biomimetic structures and Nepenthes-pitcherperistome cavities, could form complementary topological structures that can match together. The 'multi-scaled' means that the bionic gears can separate oil-water mixtures in different scales, for example, oil-water micro-drops on a millimeter scale, and oil-inwater emulsions on the micrometer scale.
Based on the referee's suggestion, we replaced the word 'co-design' with 'design'. A simpler and more straightforward language can better convey the message to the readers.
Thanks again for the referee's suggestion.