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Self-growing and Serpentine Locomotion of Liquid Metal Induced by Copper Ions Sen Chen, Xiaohu Yang, Yuntao Cui, and Jing Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07649 • Publication Date (Web): 22 Jun 2018 Downloaded from http://pubs.acs.org on June 23, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Applied Materials & Interfaces

Self-growing and Serpentine Locomotion of Liquid Metal Induced by Copper Ions Sen Chen 1, 2, Xiaohu Yang 1, 2, Yuntao Cui 1, Jing Liu 1, 2, 3* 1

Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China 2

School of Future Technology, University of Chinese Academy of Sciences, Beijing 100049, China

3

Department of Biomedical Engineering, School of Medicine, Tsinghua University, Beijing 100084, China

*Corresponding author. Email: [email protected]

KEYWORDS: Liquid metal; Interface phenomenon; Serpentine locomotion; Discretization effect; Bionics soft robots; Smart materials

ABSTRACT: The realization of serpentine locomotion has been a core goal pursued. Here, a straightforward approach was discovered to generate the serpentine locomotion based on a brand-new phenomenon observed on liquid metal (Ga67In21Sn12). The dynamic process that liquid metal can automatically produce and move like tremendous slim snakes was revealed and the underlying mechanisms were 1 ACS Paragon Plus Environment

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clarified and interpreted. It was found that the self-growing serpentine locomotion of liquid metal is driven by the localized surface pressure difference related to the unbalanced surface tension. The present work offers new insight and forms in developing future autonomous soft systems and bionic multifunctional robots.

Rigid-bodied robots may encounter the bottleneck of compliance due to their limited adaptability. Soft robots, which can deform their bodies in a continuous way, own the potential to break the bottleneck faced by traditional robots.1, 2 On the other hand, biology, which owns unique and exquisite features, has been serving as an important a source of inspiration for the manufacture of soft robots, 3-5 thus bionics has been an important strategy for the manufacture of soft robots. However, traditional methods are often facing with significant difficulties in achieving even a simple biological motion activity. For example, serpentine locomotion of the snake requires fine coordination of the muscles under complicated mechanism, 6 which is still a challenging task for many traditional machines. Although it is of great complexity, the serpentine locomotion has recently attracted numerous attentions in the investigation and development of soft devices. 7, 8 Recently, gallium-based liquid metal (LM: Ga67In21Sn12) with distinctive properties, including high flexibility, deformability, electrical conductivity, thermal conductivity and nontoxicity,

9-11

has attracted big attention among worldwide

researchers. A group of fascinating interfacial phenomena of LM systems have been discovered, such as self-running droplets, LM surface convection, self-actuation Cu wire in the LM droplets, etc.

12-15

These rich features enable it to be widely used in a 2

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wide range of applications

16-19

. Therefore, the LM, with outstanding versatile

capabilities and deformable behaviors, is expected to be applied to the future manufacture of soft robots. Until now, the combination of LM and soft robots is in progress.

20, 21

However, the movements and deformations of LM that have been

discovered so far are all integral shape transformation, which means that LM moves and deforms as a whole, such as surface convection and rolling, resulting in slow progress in the combination of LM and soft robots. Here, the large-scale global discrete deformation and serpentine locomotion had never been found before, which will significantly help update the form of movement and deformation of LM. Along this way, many complex motions or mechanisms of the soft robots could possibly be revealed by LM. In this article, the phenomenon of self-growing and serpentine locomotion of LM in the acidic copper sulfate (CuSO4) solution was first discovered. It was observed that the LM immersed in the acidic CuSO4 solution will spontaneously generate tremendous discrete slim protrusions. Such slim protrusions continuously grew and moved like a snake until all the LM was dispersed into thin strips. More intriguingly, the speed of the self-growing and serpentine locomotion can be controlled by acid strength and the dynamic process can be induced many times. The mechanisms lying behind the phenomena were revealed. The serpentine locomotion is driven by the localized surface pressure difference which is related with the myriad galvanic cell formed across the entire surface of LM. The phenomenon of self-growing serpentine

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locomotion of LM provides a straightforward method to realize snake-like movement, which is expected to generate a positive impact on the coming soft robot field. According to the principle of surface energy minimum, bulk LM tends to form an ellipsoid due to the larger surface tension. However, bulk LM immersed in acidic CuSO4 solution will generate tremendous discrete snake-like extensions (pseudopodia) spontaneously (Figure 1b and Movie S1). These pseudopodia continued to grow just like slim snakes until the entire bulk LM was consumed and became a lot of dendritic strips (Figure 1c). More intriguingly, the movement speed of the serpentine locomotion under strong acidic condition (Figure S1 and Movie S2) was about ten times faster than that of the weak acidic case (Figure 1c) through measuring the total length of the extended pseudopodia (Figure 1d), which means that the speed of serpentine locomotion can be regulated by adding different amounts of acid. In the experiment, it was observed clearly that particles appeared in the interface between the bulk LM and the solution during the experiments (Figure 1c), which was inferred as copper particles. From the previous literature, we can see that the electronegativity of gallium is weaker than that of the copper, 22 which means gallium has stronger reducibility than copper. Thus, the displacement reaction occurred in the interface between copper ion (Cu2+) and gallium (Ga), leading to the precipitation of copper particles on the LM surface. This replacement reaction is described as follows: 3Cu2++2Ga→2Ga3++3Cu

(1)


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Figure 1. (a) Schematic illustration of the experiment. (b) Bird view of the snake-like protruding pseudopodia. (c) Time-lapse images of the serpentine locomotion. (d) A graph of the length of all pseudopodia over time. Here, strong acidic condition means 6 wt% HCl and weak acidic means 3.6 wt% HCl in the mixed solution. The amount of LM is 5ml. (e) Analysis of the concentration of various elements in the solution before and after the experiment. Scale bars, 10 mm. 5 ACS Paragon Plus Environment


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In order to confirm the occurrence of replacement reaction between Ga and Cu2+, inductive coupled plasma emission spectrometer (ICP) test that focuses on the analysis of elemental concentrations was utilized. As is shown in Figure 1e, the solution contained a large number of Cu (10953 mg/L) before start of the serpentine locomotion. After the end of serpentine locomotion, the solution contained a large amount of Ga (4323.6 mg/L) and a small amount of Cu (6.5 mg/L), as well as a very small amount of In (0.79 mg/L) and Sn (2.38 mg/L). The test results show that copper element was consumed and a large amount of gallium element was produced during the dynamic process, which indicated that the replacement reaction occurred between the gallium and the copper ion. The displacement reaction between Cu2+ and Ga is vital due to the fact that it not only provides copper particles to form countless galvanic cells but also guarantees the uniqueness of this system. Here, an experiment of locally adding CuSO4 particles was conducted in order to prove the occurrence of galvanic reaction. Based on theoretical analysis, it is deduced that Ga loses electrons in the system. Then electrons outflow to the solution through the Cu particles (Figure 2a). At the Cu anode, hydrogen ion (H+) receives those electrons, thus hydrogen is produced, which can be observed in the bulk LM during the experiment. Furthermore, the surface potential will be altered due to the formation of the galvanic cell, resulting in the appearance of the surface tension gradient. Such surface tension gradient induced surface flow and the flow was initiated from the interfacial region with lower surface tension towards the region with higher surface tension(Figure 2a), which belonged to 6 ACS Paragon Plus Environment

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the liquid metal Marangoni effect and was also observed in other studies.

23, 24

Furthermore, the liquid metal Marangoni effect could not only induce the surface convection but also cause the deformation of liquid metal, which has been clarified in detail by previous researchers.13 In this experiment, CuSO4 particles were locally added into the surface of LM immersed in the HCl solution. Experiment found that the surface convection of bulk LM took place with the adding of CuSO4 particles. Besides, it can be observed clearly that the surface flow of bulk LM began in the low surface tension region where the bulk LM contacted CuSO4 particles and ended in the high surface tension region, eventually forming a convective circulation (Figure 2b and Movie S3), which was consistent with our theoretical analysis. Therefore, one can draw such a conclusion that locally adding CuSO4 particles will lead to unbalanced surface tension originating from the formation of the Cu-Ga galvanic cells, causing surface convection of LM. As for the serpentine locomotion described in this paper, the reason why the serpentine motion occurs lies in that countless galvanic reactions occur throughout the entire interface. To reveal the mechanism lying behind the serpentine locomotion, the surface electric potential of bulk LM was measured by a millivolt voltmeter (against a saturated calomel electrode, SCE) (Figure 2c). The surface potential of LM existed due to the fact that the electric double layer (EDL) will form when the LM was placed in the solution due to the inherent properties of matter, which can be modeled as a



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charged capacitor. Based on the Lippmann’s equation, 25 the surface tension is related to the voltage of the EDL. Lippmann’s equation is as follows: 1 γ = γ 0 - CU 2 2

where,

γ0

(2)

is the original surface tension of the bulk LM, C and U is the capacitance

and voltage across the EDL. The change of U will greatly alter the surface tension of the bulk LM, leading to significant influence on the shape of bulk LM. Therefore, modifying the LM behavior by adjusting the EDL is completely practicable.


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Figure 2. (a) Diagram of Cu-Ga galvanic cell and the direction of surface convection of the bulk LM. (b) Time-lapse images of the surface convection of the bulk LM induced by CuSO4 granules. The direction of LM surface flow is indicated by white arrows. (c). Schematic setups of the bulk LM surface electric potential test. (d) Surface electric potential V of the bulk LM during the dynamic process. Here, the volume ratio of HCl to CuSO4 is prescribed as 1:4.

According to test results (Figure 2d), it can be obtained clearly that the surface electric potential of bulk LM showed a shift from about 0 V to -0.15V when serpentine locomotion was induced. The surface electric potential of the bulk LM placed in CuSO4 solution was nearly 0V due to the oxidized surface and it became -0.15V instantly with the adding of HCl solution. Meanwhile, LM pseudopodia stretched from the bulk LM spontaneously and moved like a slim snake. As the serpentine locomotion of LM ceased, the surface potential returned to near 0V, which showed that there existed a good correspondence between the surface electric potential and the motion behavior of the bulk LM. The measurement of the surface electric potential clearly points out the change of surface potential (V) is responsible for the dramatic altering of the surface tension ( γ ) according to the Lippmann’s equation. In addition, the surface potential measured here was generally stable, indicating that copper particles can be stably precipitated and swallowed, which was crucial for the generation of serpentine locomotion. More experiments illustrated that the acidity of the solution owned an extremely important influence on the serpentine locomotion. In the case of strong acidity, the 9 ACS Paragon Plus Environment


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surface electric potential can be divided into three stages (Figure S2), including the unsteady surface convection (Movie S7), serpentine movement and motion stopping. During the dynamics process, the transition from convection to serpentine locomotion occurred (Figure S3 and Movie S8), which was different from the case of weak acidity. However, the serpentine locomotion no longer took place in the acid-free condition, which means that acidity is a prerequisite for this phenomenon (Figure S4). More discussion can be found in the supporting information. From the above discussion, we know that the serpentine locomotion was induced by unbalanced surface tension. According to the Yang-Laplace equation, 26 the surface pressure difference between electrolyte and LM can be described as follows: P = γ (

1 1 + ) R1 R2

(3)

where, R1 and R2 are the principal radii of curvature at the interface, respectively. The surface pressure difference is related to the LM surface tension and surface curvature. Therefore, curvature is another important factor affecting the surface pressure difference, which was also verified by our measurement. Experimental results showed that the serpentine locomotion took place easily when the bulk LM leaned against the wall of the dish (Figure 3a and Movie S4), which certified that the protruding of pseudopodia was related to the surface curvature of the bulk LM. The force analysis is adopted to better expound the effect of curvature (Figure 3c).


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Figure 3. (a) Time-lapse images showing the serpentine locomotion of the bulk LM owning the large curvature. (b) Close-up view of the corner. (c) The force analysis of the bulk LM placed in the acid CuSO4 solution. Here, the γ represents the surface tension of LM and

∆p

indicates surface pressure difference. C means corner and S

refers to smooth region. R1 and R2 are the principal radii of curvature at the interface, respectively. (d) Artificially formed curvature induced serpentine locomotion.

The LM was in the quasi-steady state when the serpentine locomotion was going to take place. At the smooth region, θ′ was obviously larger than that at the corner (θ), resulting in cos θ′ < cos θ. Thus it was deduced that the surface pressure difference at 11 ACS Paragon Plus Environment


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the smooth margin ( ∆pc ) was smaller than that at the corner ( ∆ps ) based on Yang-Laplace equation. In other words, the corners which own large surface curvature will be driven by a greater pressure difference. Therefore, the serpentine locomotion took place easily at those deformed corners (Figure 3b), which further formed the slender and long pseudopodia. Artificially induced curvature can also stimulate the spontaneous serpentine movement, which further proved the reasonableness of the curvature analysis based on the Yang-Laplace equation (Figure 3d and Movie S5). In a conclusion, the difference in curvature greatly influenced the serpentine motion. Besides, in order to achieve continuous serpentine locomotion, particle internalization between LM and Cu articles also plays a crucial role in LM serpentine locomotion. Particles internalization means precipitated Cu particles will be swallowed by the bulk LM in the acidic solution. 27 Experimental results showed that the precipitated Cu particles were constantly swallowed by the bulk LM, which can be observed clearly on the surface of the bulk LM immersed in the acidic CuSO4 solution (Movie S6). The stable precipitation of copper particles across the entire surface ensures the continued formation of the galvanic cell and the phagocytosis of copper particles ensures that the copper particles will not accumulate on the surface of LM. Such two processes maintained the dynamic balance, which was the vital feature of the unique environment and the essential reason of the continuous serpentine motion.


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Figure 4. (a) SEM image of the bulk LM after swallowing copper particles. (b) Higher magnification of the SEM image. (c) Composition analysis of bulk LM after the end of serpentine locomotion through EDS. (d) XRD patterns of the obtained samples that appear on the LM surface.

After the end of serpentine locomotion, it was found that there were many small particles inside the LM (Figure 4a). The elemental analysis of regions I and II showed that the small particles in the interior contained Cu element (Figure 4b and Figure 4c), which indicated that precipitated Cu particles were successfully swallowed by the bulk LM. Finally, the copper gallium alloy (CuGa2) will be formed after the end of self-growing serpentine locomotion. Such CuGa2 owns excellent wetting properties 13 ACS Paragon Plus Environment


and application prospects

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, which can be reused after the end of serpentine

locomotion. Furthermore, the X-ray diffraction (XRD) test was implemented to verify the structure of the particles. Test results proved that the precipitated particles were indeed Cu particles (Figure 4d), and more discussion about the test can be seen in the supporting information. From the above interpretations, we can know that copper granules are precipitated on the surface of LM due to the potential difference between copper and gallium. Such granules can form countless tiny Cu-Ga galvanic cells across the entire interface and alter the surface tension of LM, leading to unbalanced surface pressure difference and symmetric deformation. Based on the mechanism analysis, the system composed of acidic copper salt solution is unique to the serpentine locomotion (see the Supporting Information). The additional point to explain is that the self-growing serpentine locomotion can be triggered many times after the end of the movement to increase the durability of the motion. The triggering methods include not only the artificial curvature but also the addition of copper ions (Figure S5 and Movie S9). Besides, serpentine locomotion could also be excited on small droplets (50μL), although a bulk of liquid metals is quite feasible for realizing complex serpentine locomotion as described in this article. Because surface tension plays a more important role in small dimensions, the size of the droplets will affect the phenomenon, leading to a faster recovery for the LM from serpentine to ellipsoid shape (Movie S10).


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Furthermore, the salt solution used in current article is CuSO4 solution and it should be noted that CuCl2 can also achieve the same effect, which was verified by related experiments (Movie S11). Besides, different kinds of acids will also affect the serpentine locomotion and the specific discussion is shown in the supporting information. As pointed out by the latest review article, 29 the chemistry of liquid metal is emerging as a rather important frontier in the area. Clearly, the present finding adds new knowledge for the chemistry therein. Further explorations are requested along this direction. In summary, the intriguing discrete self-growing and serpentine locomotion of LM driven by the surface pressure difference originating from surface tension imbalance was experimentally disclosed. It was clarified that during this process, the stable precipitation and phagocytosis of the copper particle across the entire interface are the essential reason of the continuous serpentine motion in this unique system, which is difficult to achieve by existing systems otherwise. Such large-scale discrete deformation and serpentine locomotion are no longer limited to single convection, rolling or integral deformation and significantly updated the form of movement and deformation of LM, which also refreshes the basic physical and chemical effects of LM and advances the understanding of electrical-capillary effect. LM which reacts with copper ions can be considered as a smart soft matter, which provides new insight into further exploration and has an important application value in many fields, such as, ionic smart response sensors, self-growing LM circuit, autonomous soft systems, etc. Moreover, this new finding on self-growing serpentine locomotion induced by copper 15 ACS Paragon Plus Environment


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ions provides a completely novel strategy to achieve the bionic serpentine locomotion, which has vital values in developing future potential soft bionic LM robots, especially those multifunctional and deformable robots which own the ability to self-grow, evolve and self-recover.

ASSOCIATED CONTENT

Supporting Information The following files are available free of charge. Supplementary explanation of the mechanism, fast serpentine locomotion, the surface electric potential of LM in the case of strong acidity, the transition from convection to serpentine locomotion, images in the acid-free conditions and copper ions once again excites serpentine locomotion. (PDF) Movie S1: Self-growing serpentine locomotion. (AVI) Movie S2: Fast serpentine locomotion in the case of strong acidity. (AVI) Movie S3: LM convective motion triggered by particles. (AVI) Movie S4: Curvature excited LM serpentine locomotion. (AVI) Movie S5: The artificially formed curvature stimulated the serpentine movement of LM. (AVI) Movie S6: Particle internalization between LM and Cu particles. (AVI) Movie S7: Unstable surface convection of the bulk LM. (AVI) Movie S8: The transition from convection to serpentine locomotion takes place. (AVI)


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Movie S9: The addition of copper ions once again excites serpentine locomotion. (AVI) Movie S10: Serpentine locomotion of a small volume of liquid metal. (AVI) Movie S11: Serpentine locomotion realized in the acidic CuCl2 solution and the deformation of LM induced by Cu2+ in H2SO4 solution. (AVI) AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]

Author Contributions S. Chen and J. Liu conceived the idea. S. Chen carried out all the experiments. All authors participated in the the discussion and the preparation of the manuscript. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS

This work is partially supported by NSFC Key Project under Grant No. 91748206, Dean’s Research Funding and the Frontier Project of the Chinese Academy of Sciences.

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