Spontaneous Dispersion and Large-Scale Deformation of Gallium

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Spontaneous Dispersion and Large-Scale Deformation of Liquid Metal Induced by Ferric Ions Sen Chen, and Jing Liu J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b12115 • Publication Date (Web): 19 Feb 2019 Downloaded from http://pubs.acs.org on February 23, 2019

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The Journal of Physical Chemistry

Spontaneous Dispersion and Large-Scale Deformation of Liquid Metal Induced by Ferric Ions Sen Chen1, 2，Jing Liu1, 2, 3* 1

Beijing Key Lab of Cryo-Biomedical Engineering and Key Lab of Cryogenics, Technical

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

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

100049 3

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

100084 Corresponding Author * Electronic mail: [email protected].

ACS Paragon Plus Environment

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ABSTRACT.

Gallium-based liquid metal owns the largest interfacial tension among all the room temperature liquid, which gives it strong deformability and promises its role in the field of soft machines. Paradoxically, such a material always remains nearly spherical in solution due to large interfacial tension, which in turn hinders the construction of liquid metal-based soft machines. Consequently, it is of significant theoretical and practical value to regulate the interfacial tension of liquid metal in order to carry out richer deformation. In this study, spontaneous dispersion and large-scale deformation of the bulk liquid metal were disclosed to be induced by ferric ions. It was found that the bulk liquid metal immersed in the FeCl3 solution can spontaneously disperse into a large amount of droplets. And the dispersed liquid metal droplets could move and deform through increasing the concentration of the solutions or adding acids. The mechanisms lying behind the untraditional phenomena lie in the non-uniform interfacial tension over the entire surface of liquid metal, which is associated with the space-time distribution of FeCl3 solutions. Further, directional locomotion and periodic oscillation occur due to the non-uniform interfacial tension, which leads to the autonomous dispersion and deformation of liquid metal. Overall, the unique redox reactions between liquid metal and FeCl3 solutions play an essential role in ensuring the continuity of deformation. The present spontaneous dispersion and deformation capability of liquid metal signify a paradigm shift and open up new possibilities for making the chemistryenabled soft machines in the coming time.


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INTRODUCTION With high thermal conductivity, electrical conductivity, non-toxicity and excellent fluidity, Gabased liquid metal (LM) is attracting extensive attention from the scientific community in many fields, including functional materials, 1-3 soft electronics, 4-7 thermal management, 8-11 biomedical applications,

12-14

chemical reaction environments,

15-19

etc. A common problem in these

applications is to regulate the interfacial tension of LM. This is because that the large interfacial tension of LM is often unfavorable for practical use. On the other hand, it is precisely because of the huge interfacial tension that the significant interfacial tension gradient thus forms, which promotes a series of deformations and movements on the LM. 20-27 Owing to current progresses, LM-based soft machines are becoming an emerging frontier of robotics science, which is gradually turning from concept to reality. Therefore, it is of significance to flexibly regulate the interfacial tension of LM, whether in industrial development or academic research. Generally, it is quite difficult to regulate the interfacial tension of the liquid without changing the component. Fortunately, we notice that LM belongs to metallic liquid that possesses the unique properties of metals, which means that the LM can form a primary battery with other metals. This process will greatly change the interfacial tension of the LM, which in turn enables the movement and deformation of the LM. Over the past few years, researches has made some progresses along this path, including LM self-driven motion by swallowing Al film, 21 surface convection induced by the Cu particles,28 surface Marangoni flow through contacting the Cu plate, 29 etc. Among them, one can notice that the movements and deformations of LM that have been discovered so far are almost all in integral shape transformation. Such phenomena are caused by the fact that the LM still exists in the form of an entire ellipsoid when placed in a solution environment, which may


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hinder the development of LM-based soft machines. Hence, whether the LM can be spontaneously dispersed reserved to be a long-term thinking of researchers in the field and has been pursued for a long time. To solve the problem, one notices especially that the contact between LM and other metals belongs to point or multi-point contact, regardless of whether aluminum foil, copper wire or copper particles are used by carefully analyzing the above LM deformations. Therefore, instead to the point contact, we try to construct a full surface contact between LM and other metals to change the interfacial tension over the entire surface, thereby achieving completely different behaviors. Fortunately, we have successfully identified an alternative strategy to realize the surface contact and tackle the spontaneous dispersion issue of droplets. In this work, it was found that the bulk LM immersed in FeCl3 solution spontaneously disperses into considerable droplets. The size Further, through increasing the concentration of the FeCl3 solution, the dispersed LM droplets would turn to oscillate and deform. At low concentrations, the locomotion and deformation of the LM could also be achieved by adding an appropriate amount of acid. Besides, the solution after the reaction could preserve the LM for a long time for subsequent applications. The spontaneous dispersion and deformation of LM realized in FeCl3 solution are expected to promote further development of LM-based soft machines.

EXPERIMENT Over the current experiments, the LM was Ga75.5In24.5, which was prepared by stirring and heating gallium (Ga, 75.5wt%, 99.99% purity), indium (In, 24.5wt %, 99.99% purity) together at 200℃ for 2 h until metal alloy was formed. Different concentrations of FeCl3 solutions were used to provide a suitable environment, which were prepared by dissolving solid FeCl3 particles in deionized water. Here, several ferric salts and concentrated hydrochloric acid were purchased from


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Beijing Lan Yi Chemical Products Limited Liability Company (Beijing, China). In this study, the surface electric potentials (V) of the LM and substrates were measured by a millivolt voltmeter with one electrode (copper wire) inserted in the LM and the reference electrode (SCE: Saturated calomel electrode) in the solution. The temperature of the chemical reaction system was measured with two T type thermocouples to ensure the accuracy of the measured temperature. The imaging data used in this study were recorded by the Sony Digital Video (FDR-AX40). The contact angle of LM was tested by the contact angle instrument (POWEREACH JC2000D3, China). The infrared images were recorded by the far-infrared thermograph imaging system (FLIR SC620, FLIR Systems Inc, USA). After the end of the reaction, the element distributions of LM were evaluated by using Energy Dispersive Spectrometry (EDS; 6853-H, HORIBA, Ltd.). The concentration of ions in the solution was tested by means of the Inductive Coupled Plasma Emission Spectrometer (ICP; Varian725-ES).

RESULTS AND DISCUSSIONS Basics of the spontaneous dispersion and large-scale deformation Over the present experiments, the bulk LM (Ga75.5In24.5) was placed into the FeCl3 solution. Immediately, the bulk LM would spread into the shape of a pancake with a gray surface due to the fact that the precipitated particles were scattered on the surface of the LM (Figure 1a). Subsequently, the spreading LM will shrink continuously and the surface color would turn from gray to bright. Surprisingly, the large-area of LM was cut to form substantial droplets during the contraction. Experimental results indicate that the number of small droplets formed by complete spontaneous dispersion gradually increased and then stabilized for several hours (Figure 1b). To the best knowledge of the authors, such intriguing phenomenon has not been reported before. Clearly, understanding the role of FeCl3 in the spontaneous dispersion is essential for exploring


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the mechanisms behind the phenomenon. For this reason, the experiments about the effect of FeCl3 particles on LM were investigated. Over the experiments, the FeCl3 particles (6g) were placed into the deionized water (40mL) and the LM was placed at the edge of the FeCl3 particles. Instantly, the LM immersed in the deionized water moved just like mollusks toward the side of the FeCl3 particles (Figure 1c and Movie S1), which is very similar to the chemotaxis of the organism. The amoeba-like movement of LM was accompanied by the production of LM pseudopods, which separated the entire LM into individual droplets. Ultimately, when the FeCl3 particles were all dissolved into the deionized water, a picture similar to Figure 1a was formed. The experiment about FeCl3 particles visually demonstrates the formation of dispersed LM droplets, which proves that the interaction between FeCl3 particles and LM promotes the spontaneous dispersion of the bulk LM.


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Figure 1. The dispersion and deformation of LM. a. The spontaneous dispersion of the bulk LM in FeCl3 solution (2mol/L). b. The number of LM droplets gradually increased and then remained stable during the process of LM dispersion. c. The amoeba-like locomotion of LM induced by FeCl3 particles. d. Intense dispersion and deformation of LM in high concentration FeCl3 solution (5mol/L). e. Particle size of LM droplets in FeCl3 solutions with different concentrations. Here, the total volume of LM is 10mL. f. Periodic oscillation and deformation of LM in FeCl3 solution (2mol/L) with the addition of acid (2moL/L). Here, the amounts of FeCl3 and HCl were 40ml and 2ml, respectively. Scale bars, 10 mm.

Besides, the system involved here is a typical chemically reacting system, which exhibits interesting unsteady-state and dynamic behaviors and is susceptible to the concentration of FeCl3 solutions. The experimental findings show that the LM placed therein no longer keeps stable as the concentration of the FeCl3 solutions increases. Here, the LM was injected into the FeCl3 solution (5mol/L). Quickly, the bulk LM was dispersed into a large number of droplets and the dispersed LM droplets intensely oscillated and moved just like living creature, eventually evolving into overall large-scale deformation (Figure 1d and Movie S2). Typically, the moving LM constantly blended with LM droplets next to it and eventually became a whole (Figure S1). When the

concentration

increased

to

6mol/L,

the

dispersion

and

deformation

of

LM

turned to be vehement, which only took 12 seconds for the whole process (Figure S2 and Movie S3). At the same time, it was found that the size of LM droplets formed after spontaneous dispersion was closely related to the concentrations of FeCl3 solution. To explore this relationship in detail, a series of experiments were carried out. In these experiments, each group of experiments was repeated three times to ensure the reliability of the results. Specifically, the particle size


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marked in the Figure 1e was obtained by dividing the total volume of LM (10mL) by the number of droplets generated (average value of three experiments). The error bar represents the deviation between the actual size of LM droplets and the calculated average value. As shown in Figure 1e, with the increase of the concentration of FeCl3 solution, the particle size of LM droplets tended to decrease. On the other hand, the experimental results also indicate that the distribution of particle size is not uniform because of the inherent non-equilibrium of the system. Further, the LM deformation was achieved by adding the acid into the stable LM droplets. Experiment found that the LM moved and deformed immediately from where moderate HCl was added. Specifically, the movements of LM were mainly composed of periodic oscillation and large-scale deformation (Figure 1f and Movie S4). Besides, the experimental results show that that the acidity had a great effect on the phenomena. On one hand, the FeCl3 solution itself is acidic owing to the hydrolysis of iron ions, which avoids the oxidation of LM. On the other hand, the added acid would also have a huge impact on the experimental phenomena, which leads to the periodic oscillation and large-scale deformation. Notably, the amount of added acid is a vital factor affecting the phenomenon. As illustrated in Figure S3, the LM placed in the ellipsoidal glassware became bright with the addition of moderate amount of HCl. Along with the surface convection of LM, the pseudopod appeared constantly. Meanwhile, the snake-like locomotion occurred when the LM was moved to an open space (Figure S3). However, the LM no longer dispersed and deformed when the acidity of solution was too strong or the added acid was too much. Meanwhile, that large numbers of bubbles were observed due to the production of hydrogen (Figure S3 and Movie S5). The directional movement and periodic oscillation


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To further explore the mechanisms behind the magical phenomena, we designed and implemented a series of experiments involving the motion direction of LM. Among the experiments, the behaviors of LM drops were found to depend delicately on the concentration c of FeCl3 solution and its gradient. The experimental device with two cavities was shown in Figure 2a. One of the cavities was filled with LM and the other cavity was used to place FeCl3 particles, which were all immersed in deionized water. Experimental results indicate that the LM drop moved toward the FeCl3 particle with the particle gradually dissolved (Figure 2a). After the LM contacted the FeCl3 particle, the surface convection direction of the LM was heading the FeCl3 particle, which was consistent with the results of another experiment carried out in ellipsoidal glassware (Figure 2b). In such test, the LM was immersed in the ellipsoidal glassware with a 3cm diameter and the amount of deionized water was 40ml. Experimental results indicate that the direction of the LM surface convection was also directed to the FeCl3 particles in the early part of the experiment, which was induced by the concentration gradient. However, the direction of the LM surface flow no longer pointed to the FeCl3 particles, but from the periphery towards the center after about three minutes of reaction. During the process, with the continuous dissolution of FeCl3 particles, the concentration gradient of Fe3+ in the solution gradually became lower, which was responsible for the change of the convection direction of the LM surface. Meanwhile, the periodic contraction and relaxation of the LM drop occurred spontaneously (Figure 2b and Movie S6). Therefore, one can draw a conclusion that the directional movement towards the FeCl3 particles and periodic oscillation were two main forms of LM deformation induced by ferric ion.


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Figure 2. The direction of LM deformation and locomotion. a. LM drop moved toward FeCl3 particle. b. The direction of surface convection of LM. Surface convection direction changed over time. c. Schematic diagram of two motion forms. (i) Directional locomotion due to the ion concentration gradient; (ii) Oscillatory motion owing to the periodic formation and diffusion of the electric double layer (EDL). d. Schematic diagram of a device for testing the surface potential of LM. e. Test results of the surface potential of LM. The results indicate that the change in surface potential and the deformation of LM were synchronized. Here, the arrows represent the direction of LM movement. Scale bars, 10mm.


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We noticed that the directional movement and periodic oscillation also existed in the spontaneous dispersion of LM in FeCl3 solution. Therefore, explaining the mechanisms behind these two deformations is vital for understanding the intriguing phenomenon. To interpret the physical mechanisms of the directional motion, one can consider Figure 2c (i) for more details. Here, the unbalanced Young’s force (dF) can be given by the following equations. 𝑑𝑑𝑑𝑑 = 𝐹𝐹𝐻𝐻 + 𝐹𝐹𝐿𝐿

𝐹𝐹𝐻𝐻 = �𝛾𝛾𝑠𝑠𝑠𝑠(𝐻𝐻) − 𝛾𝛾𝑠𝑠𝑠𝑠(𝐻𝐻) �𝑑𝑑𝑑𝑑 = −𝛾𝛾𝑙𝑙𝑙𝑙(𝐻𝐻) cos 𝜃𝜃𝑅𝑅 𝑑𝑑𝑑𝑑 𝐹𝐹𝐿𝐿 = −�𝛾𝛾𝑠𝑠𝑠𝑠(𝐿𝐿) − 𝛾𝛾𝑠𝑠𝑠𝑠(𝐿𝐿) �𝑑𝑑𝑑𝑑 = 𝛾𝛾𝑙𝑙𝑙𝑙(𝐿𝐿) cos 𝜃𝜃𝐿𝐿 𝑑𝑑𝑑𝑑

(1) (2)

(3)

where 𝛾𝛾𝑠𝑠𝑠𝑠 , 𝛾𝛾𝑠𝑠𝑠𝑠 , 𝛾𝛾𝑙𝑙𝑙𝑙 refers to the interfacial tension on the solid/LM interface, solid/aqueous

interface and LM/aqueous interface, respectively (the subscripts H and L are used to distinguish

the forces on both sides). Here, 𝜃𝜃𝐻𝐻 and 𝜃𝜃𝐿𝐿 represent the contact angles of the LM droplet. The

surface of the LM placed in deionized water will be oxidized to form an oxide film, which causes

the interfacial tension (𝛾𝛾𝑙𝑙𝑙𝑙 ) to be greatly reduced. Besides, the FeCl3 solution capable of being hydrolyzed displays strong acidity, which could dissolve the oxide film of the LM. Initially, the LM remained stable because of the bilaterally symmetrical interfacial tension (dF=0). Subsequently, the FeCl3 solution diffused to the edge of LM, dissolving the oxide film on the surface. Hence, the interfacial tension of LM in the high concentration of FeCl3 solution (𝛾𝛾𝑙𝑙𝑙𝑙(𝐻𝐻) ) was larger than that in the low concentration of FeCl3 solution ( 𝛾𝛾𝑙𝑙𝑙𝑙(𝐿𝐿) ). From Equations 1-3, one

can conclude that the side of the LM contacting the high concentration FeCl3 solution would be subjected to a larger force than the other side (|FH|>|FL|). Therefore, the resultant force of the LM droplet pointed to the high concentration side, causing the LM droplet to orient to the place where the concentration of FeCl3 was high. Such directional movement can be used to drive LM droplets (Figure S4 and Movie S7).


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In addition, experimental observations show that LM droplets gradually oscillated when the apparent concentration gradient of FeCl3 solutions disappeared. The physical mechanisms behind it lie in the periodic formation of surface gallium chloride layer and periodic dissolution of surface gallium chloride layer, which will alter the interfacial tension related with electric double layer (EDL) on LM surface. The relationship between the EDL and the interfacial tension can be understood by the Lippmann equation which reads as: 𝑐𝑐

𝛾𝛾𝜑𝜑 = 𝛾𝛾0 ― 2 (𝜑𝜑 ― 𝜑𝜑0 )2

(4)

where c denotes for EDL capacitance per unit area. 𝛾𝛾0 and 𝛾𝛾𝜑𝜑 represent the interfacial tension

when the surface potential is 𝜑𝜑0 and 𝜑𝜑 , respectively. Therefore, the periodic generation and dissolution of the oxide layer of the LM will change the EDL, thus resulting in periodic change in

surface tension, which is responsible for the rhythmic oscillation. Here, the oxide layer can be dissolved because of the acidity of the solution. When the acidity of the solution is too weak to dissolve the oxide layer quickly, the dynamic equilibrium of the system will be broken and the LM remains relatively stable. Hence, we speculated that the addition of an appropriate amount of acid can equally achieve the oscillation and deformation of LM droplets, which has been proven by the related experiments. Further, the surface potentials was measured to characterize the change in interfacial tension. This method is effective, which lies in the dependence of the interfacial tension on the electrochemical surface potential at the aqueous/LM boundary based on the Equation 4. As shown in Figure 2d, the surface potential of LM was measured with respect to the saturated calomel electrode (SCE). Test results suggest that the surface potential of LM all exhibited a drastic change with the addition of HCl regardless of whether the surrounding environment was FeCl3 particles or FeCl3 solution (Figure 2e). Meanwhile, the LM also turned into an active state, exhibiting


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periodic oscillations and large-scale deformation. After the reaction, the surface potential of the LM became 0.75V. At this point, the LM remained stable and no longer moved or deformed. Then, the FeCl3 particles were added to the solution again. Surprisingly, the movement reoccurred and the surface potential of LM changed back to 0.5, which confirms that the change in surface potential and the movement of LM were synchronized. Temperature change in the chemical reaction system Up to now, one can draw a conclusion that the spontaneous dispersion of the bulk LM was induced by the directional movement and periodic oscillation owing to non-uniform distribution of interfacial tension over the LM surface. Moreover, it was found that the temperature of the system increased significantly as the bulk LM dispersed and deformed (Figure 3a). To clearly characterize the temperature distribution, the temperature maps during the deformation of the LM were recorded by the infrared camera. In this experiment, the FeCl3 particle (10g) with a triangular shape was added into the deionized water (40ml). Then, the LM (5ml) was placed in the solution. As stated before, the LM moved just like amoeba. At the same time, the LM temperature was relatively low and uniform. Then, 2ml HCl (2mol/L) was added into the solution and the oscillation and deformation of LM occurred. Meanwhile, it can be clearly seen that the temperature rose significantly where the HCl was added (Figure 3b). Further, the intense oscillation of LM induced by the addition of HCl caused the flow of water, which could be clearly seen in the infrared images. As for the FeCl3 solution, such a clear map of temperature distribution was also observed. As shown in the Figure 3b (ii), the red and white parts of the infrared image owned the higher temperature, which corresponded to where the LM oscillated and deformed. Experimental results suggest that the temperature rose from the position where the LM deformed.


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Figure 3. Temperature changes in the chemical reaction system. a. The spontaneous dispersion of LM in 4mol/L FeCl3 solution. b. Infrared images during LM locomotion and deformation. i. The convection of LM induced by the FeCl3 particles. The deformation of LM accompanying temperature change occurred when the HCl was added into the solution, causing the flow of water; ii. The deformation of LM in the FeCl3 solution (5 mol/L). The temperature rose from the position where the LM deformed until it expanded to the whole. c. Schematic diagram of testing temperature using the thermocouple. Here, two thermocouples were used for more accurate measurements. d. The temperature change of the system during the deformation. The addition of HCl (2 mol/L, 2mL) accelerated the deformation of LM accompanied by a sharp rise in temperature.


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Quantitatively, the temperature of the chemical reaction system was measured (Figure 3c). Here, it can be clearly seen that the temperature of the FeCl3 solution rose obviously with the continuous evolution of this system (Figure 3d). For the high concentration of FeCl3 solution (5mol/L), the temperature of solution gradually increased with the bulk LM spontaneously dispersing into droplets. After about 400s of reaction, the temperature of solution rose sharply. Correspondingly, the dispersed LM droplets began to move and deform spontaneously. However, the temperature rose slowly and eventually tended to stabilize in the 2mol/L FeCl3 solution. Then, the 2ml HCl (2mol/L) was added into the solution and the large-scale oscillation and deformation occurred with a steep increase in temperature. Further, long-term temperature change trend of LM placed in low concentration FeCl3 has also been investigated (Figure S5). Such facts indicate that the deformation and movement of LM were companied with the great rise of the temperature, which means that the chemical reactions between LM and solution continue to occur and have a great impact on the dispersion and deformation of the LM. Chemical reactions occurring in this system Temperature tests confirm that the deformations of LM were related to the chemical reactions. For this reason, we have consulted the standard reduction potentials to reveal the chemical reactions that occurred lying behind the deformations. Based on the Table 1, the standard electrode potential of Ga is lower than that of In, suggesting that Ga has stronger reducibility than In and will participate in chemical reactions preferentially. Besides, because of the very negative standard potential of the Ga0/Ga3+ (ε=−0.549 V) redox couples, galvanic replacement reaction with metallic ions of higher standard potential (Fe0/Fe3+,ε=-0.037) was shown to spontaneously occur in adequate solutions of metallic salts. Therefore, the chemical reaction can be expressed as follows:


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Fe3+ + Ga → Ga3+ + Fe

Page 16 of 33

(5).

However, the precipitated Fe is difficult to stable due to the face that the FeCl3 solution is acidic and the precipitated Fe will be converted into Fe2+, expressed as follows: Fe + 2H + → Fe2+ + H2 ↑ Fe + 2Fe3+ → 3Fe2+

(6) (7)

Besides that, it can be seen that the standard electrode potential of Fe3+/Fe2+ is the highest, which means that the conversion easily happened in the redox reaction. Therefore, one can conclude that the chemical reactions occurring in this system appeared as follows: Fe3+ + Ga+2H+ → Ga3+ + Fe2+ + H2 ↑

(8)

After the Fe3+ was consumed completely, another reaction occurred: 3Fe2+ + 2Ga → 2Ga3+ + 3Fe

(9).

On the basic of chemical reaction analyzed above, the uniqueness of the ferric ion solution is that the precipitated Fe is neither swallowed nor covered on the surface of the LM, which ensures the continuity of the deformation while avoiding the change of the physical properties of the LM itself. As for other ions, such conditions (including excellent hydrolysis capacity, proper oxidation, acidity and metal activity) are difficult to meet. Therefore, the environment of FeCl3 solution is crucial for the spontaneous dispersion and large-scale deformation of LM. Table 1. The standard reduction potentials, εvalues, at 298.15 K (25°C), and at a pressure of 101.325 kPa (1 atm). 30 Reaction

ε/V

Fe2+ + 2 e ⇌ Fe

–0.447

Fe3+ + 3 e ⇌ Fe

–0.037


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Fe3+ + e ⇌ Fe2+

+ 0.771

Ga+ + e ⇌ Ga

0.2

In+ + e ⇌ In

–0.14

Ga3+ + 3 e ⇌ Ga

–0.549

In3+ + 3 e ⇌ In

–0.3382

According to theoretical analysis, we speculated that the elemental iron would be more likely to precipitate and adhere to the surface of LM for the low concentration of FeCl3 solutions. As illustrated in Figure 4a, the color of the FeCl3 solution (0.5 mol/L) turned white and a series of broken black fragments appeared. The experimental results indicate that these black fragments could be attracted by the magnet and moved along with the magnet. Further, XRD tests show that the precipitated black fragments are elemental iron (Figure S6), which is consistent with previous theoretical analysis. For the high concentration of FeCl3 solution (3mol/L), the surface of LM remained bright during the deformation process (Figure 4b). Besides, for the FeCl3 solutions without the addition of acid, experimental results show that the color of FeCl3 solution turned light green and the surface of LM was covered by the precipitated Fe particles after 24 hours of reaction (Figure 4b(iii)). Surprisingly, the color of FeCl3 solution still remained light yellow for a long time when the acid was added into the solution (Figure 4b (iv)).


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Figure 4. Chemical reactions between LM and FeCl3 solution. (a) Behaviors of LM in low concentration FeCl3 solution (0.5mol/L). i. After ten minutes of reaction. ii. After 24 hours of reaction. Fe particles were precipitated on the surface of LM. (b) Optical images of FeCl3 solution. i. Top view. ii. Side view. iii. Side view after 24 hours of reaction without adding acid. iv. Side view after 30 days of reaction with adding acid. Different colors reflect changes in ions in solution. (c) Test results of inductively coupled plasma (ICP) spectrometer for FeCl3 solution with adding acid. (d) Energy Dispersive Spectrometer (EDS) diagram of LM after 30 days of reaction with adding acid. ICP test and EDS test show that the precipitated iron particles were not swallowed by LM. Scale bars, 10mm.


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Further, we quantitatively tested the elemental composition of the solution for the latter case. The ICP test results show that there was almost no change in the content of iron element with a steady increase in gallium element (Figure 4c). Such stable state did not change significantly after even one month, including no changes in color and precipitation of substances in the solution. Also, the energy dispersive spectrometer (EDS) test on LM was carried out. The EDS test results indicate that no iron element was found in the LM (Figure 4d). These tests suggest that only the conversion reaction between Fe3+ and Fe2+ occurred in the acid-containing system, which proves the occurrence of the previously analyzed reaction. This phenomenon is mainly because the externally added acid promotes the conversion of Fe3+ into Fe2+ and also inhibits the conversion of Fe2+ into Fe, thereby ensuring that this state can exist stably for a long time. The behaviors of LM in other ferric salt solution To further investigate the mechanisms lying behind the phenomena, the experiments about other ferric salts were carried out. In this experiment, the Fe2(SO4)3 was chose as the representative of other iron salts. However, the spontaneous dispersion and deformation of the bulk LM failed to take place in such a situation. Experimental results show that only the spread of LM can be observed when the LM was exposed to the Fe2(SO4)3 particles and the Fe2(SO4)3 particles cannot cause the amoeba-like locomotion of LM (Figure 5a). Similarly, the bulk LM gradually spread over the entire glass beaker when the LM was immersed in the Fe2(SO4)3 solution (Figure 5b). In both cases, the bulk LM was no longer dispersed into a large amount of LM droplets. On the other hand, the large surface tension of the LM makes it difficult to keep a planar shape in solution. Thus, such excellent plane formed by the spontaneous spreading of LM is quite favorable for many subsequent applications, including the preparation of gallium-based two-dimensional materials (Ga2(SO4)3 and Ga(NO3)3), 31 LM catalyst systems.32


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Figure 5. The behaviors of LM in other ferric salt solution. a. The slow spreading of LM induced by the Fe2(SO4)3 particles. b. The LM spread over the entire glass dish in Fe2(SO4)3 solution (2mol/L). c. The results of the contact angle measurement of LM in different ferric salt solution (2mol/L). i. FeCl3 solution; ii. Fe(NO3)3 solution; iii. Fe2(SO4)3 solution. Significantly different from the spreading behaviors of LM in Fe2(SO4)3 solution and Fe(NO3)3 solution, the LM maintained a relatively large contact angle in HCl solution. Here, the volume of the solution is 40 ml and the mass of ferric salt particles is 10g. Compared to the untraditional behaviors in FeCl3 solution, the behaviors of LM in Fe2(SO4)3 solution were totally different. The reasons lying behind the different phenomena were due to the


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surface oxide layer of LM, which can be well disclosed by the contact angle experiments. As illustrated in the Figure 5c, the LM immersed in the FeCl3 solution remained ellipsoid with a large contact angle. Distinct with the behavior, the LM immersed in the Fe2(SO4)3 displayed a pancake shape, which was responsible for the spread of the bulk LM mentioned before. As for the Fe(NO3)3 solution, the similar experimental phenomena to Fe2(SO4)3 were observed on the contact angle experiments. As a result, we predicted that bulk LM will only spread in the Fe(NO3)3 solution without dispersion and deformation, which was confirmed by the related experiments. Actually, the essential reason lying behind these different phenomena is that the solubility of gallium salts is different. In the case of Fe(NO3)3 and Fe2(SO4)3, insoluble salt layer formed at the surface of the LM drop could inhibit long lasting deformation, thus preventing the spontaneous dispersion and large-scale deformation. As for the FeCl3 solution, as previously analyzed, the surface soluble oxide layer will continue to be produced and dissolved, thus ensuring the continuation of deformation. Discussion The spontaneous dispersion of LM induced by ferric ions no longer requires the injection of external energy and gets rid of the dependence on external equipments, which is significantly different from traditional methods of droplet dispersion, including acoustic waves, 33 mechanical vibrations,

34

microfluidic channels,35 liquid-based nebulization.36 In addition, the Fe particles

precipitated in this system can be well utilized to form LM ferrofluid for magnetocalor, based magnetoactive slurries,

38

37

LM-

gallium-based LM amalgams.39 More importantly, the

spontaneous dispersion and large-scale deformation of LM will facilitate the development of soft machines based on LM due to the uniqueness of this reaction system. Currently, LM-based soft machines are gradually becoming the frontier of research because of its excellent deformability. It


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is extremely important to create a system that can be used to stably store LM and generate motion and deformation whenever needed. However, the problem is yet to be solved. The LM serpentine locomotion previously achieved in copper salt solutions is a representative development in this area.26 However, the LM immersed in the CuSO4 solution will swallow the precipitated Cu particles, changing its own physical and chemical properties, which affects its subsequent deformability. In the FeCl3 system currently under study, this problem is well solved due to the fact that the precipitated Fe will not be swallowed by LM.40 Meanwhile, the precipitated Fe can be converted into Fe2+, which ensures the continuous reaction and deformation. More importantly, this system can be stably stored for a long time in the acid-containing system until the LM is exposed to the ferric ions again (Movie S8), which is extremely valuable for future application of LM-based soft machines. In the near future, further experimental and the theoretical studies are needed to the quantitative understanding of the deformation of a chemically reacting LM drop in ferric salt solution. The present finding adds new knowledge for the LM deformation from the perspective of chemistry. 41 More explorations are requested along this direction.

CONCLUSIONS In conclusion, fully utilizing the metallicity of Ga-based LM to regulate the interfacial tension of LM, we firstly observed that the LM immersed in the FeCl3 solution spontaneously disperses into a large number of LM droplets. Such phenomenon is contrary to the common sense that LM will retain ellipsoidal shape due to its large surface tension, which can be understood by nonuniform interfacial tension associated with the space-time distribution of FeCl3 solution. Further, directional locomotion and periodic oscillation occur due to the non-uniform interfacial tension, which leads to the autonomous dispersion and deformation of LM. Overall, the unique redox reactions between LM and FeCl3 solution play a crucial role in ensuring the continuity of


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deformation. Furthermore, the deformation speed of LM can be effectively controlled by changing the concentrations of FeCl3 solution. The LM deforms more severely in the high concentration FeCl3 solution. The size of the LM droplets is inversely proportional to the solution concentration. Besides, the acidity also has a great impact on this process. The addition of an appropriate amount of acid contributes to the movement and deformation of the LM. However, the addition of excess acid produces a large amount of air bubbles, causing the movement and deformation to stop prematurely. Quantitatively, the alter of interface potential confirms the change of the electric double layer during the dispersion and deformation of the LM, further confirming that the unbalanced surface tension causes the phenomenon. Temperature data obtained using infrared cameras and thermocouples also indicate the important role of chemical reactions in this process. Finally, we show that the spontaneous dispersion and deformation of LM cannot be realized in other iron salts (Fe2(SO4)3, Fe(NO3)3) and illustrate the possible reasons, which indicates the uniqueness of this system. The experimental findings and theoretical interpretations may refresh the basic understandings of the general interface science especially those surface physical and chemical properties of LM soft matter. Also, this work offers new insight in developing future autonomous soft systems, machines and robots, especially for the swarm robotics that can be spontaneously dispersed and reunited.

Supporting Information The following files are available free of charge. Experimental details, discussion on the acid, the locomotion induced by the ferric ions and the characterization of precipitated black particles. (PDF) Movie S1: Amoeba-like motion induced by FeCl3 particles. (AVI)


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Movie S2: The bulk LM dispersed and deformed spontaneously in FeCl3 solution. (AVI) Movie S3: The bulk LM dispersed quickly with intense reaction and deformation. (AVI) Movie S4: The oscillation and deformation of LM wih the addition of acid. (AVI) Movie S5: The effect of excessive acid on the deformation of LM. (AVI) Movie S6: The directional surface convection and periodic oscillation of LM. (AVI) Movie S7: The locomotion of LM induced by the ferric ions. (AVI) Movie S8: The re-motion of LM with the addition of ferric ions. (AVI) AUTHOR INFORMATION Notes The authors declare no competing financial interests. ACKNOWLEDGMENT 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. REFERENCES (1) Markvicka, E. J.; Bartlett, M. D.; Huang, X.; Majidi, C. An Autonomously Electrically SelfHealing Liquid Metal-Elastomer Composite for Robust Soft-Matter Robotics and Electronics. Nat. Mater. 2018, 17, 618. (2) Wang, H.; Yuan, B.; Liang, S.; Guo, R.; Rao, W.; Wang, X.; Chang, H.; Ding, Y.; Liu, J.; Wang, L. PLUS-M: a Porous Liquid-metal Enabled Ubiquitous Soft Material. Mater. Horiz 2018, 5, 222-229. (3) Chen, Y.; Zhou, T.; Li, Y.; Zhu, L.; Handschuh-Wang, S.; Zhu, D.; Zhou, X.; Liu, Z.; Gan, T.; Zhou, X. Robust Fabrication of Nonstick, Noncorrosive, Conductive Graphene‐Coated Liquid Metal Droplets for Droplet‐Based, Floating Electrodes. Adv. Funct. Mater. 2018, 28, 1706277.


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