Fluorescent Liquid Metal As a Transformable Biomimetic Chameleon

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Fluorescent liquid metal as transformable biomimetic chameleon Shuting Liang, Wei Rao, Kai Song, and Jing Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17233 • Publication Date (Web): 08 Dec 2017 Downloaded from http://pubs.acs.org on December 10, 2017

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ACS Applied Materials & Interfaces

Fluorescent liquid metal as transformable biomimetic chameleon

Shuting Liang,1 Wei Rao, 2 Kai Song 2 and Jing Liu *1,2

1

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

2

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

Address for correspondence: Dr. Jing Liu Department of Biomedical Engineering, School of Medicine, Tsinghua University, Beijing 100084, China E-mail: [email protected]

ABSTRACT: Liquid metal (LM) is of core interest for a wide variety of newly 1

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emerging areas. However, the functional materials thus made so far by LM only could display a single silver-white appearance. In this study, colorful LM marbles working like transformable biomimetic robot were proposed for the first time and fabricated from LM droplets through encasing them with fluorescent nano-particles. We demonstrated that this unique LM marble can be manipulated into various stable magnificent appearances as one desires, and then split and merge among different colors. Such multifunctional LM is capable of responding to the outside electric-stimulus and realizing shape transformation and discoloration behaviors as well. Farther more, the electric-stimuli has been successfully introduced to trigger the release of nano/micro-particles from the LM, and the mechanism lying behind was clarified. The present fluorescent LM was expected to offer important opportunities for diverse applications, especially in a wide range of functional smart material areas. KEYWORDS: Fluorescent liquid metal; Discoloration; Colorful marble; Biomimetic robot; Soft machine.

INTRODUCTION Nature has created a wonderful world full of beautiful colors. As an interesting animal, the chameleon owns the remarkable ability to rapidly change its skin colors during the interactions like courtship, camouflage and male contests1. In engineering area, quite a few of the bionic-robots have been fabricated to imitate the color-changing creatures in nature. Liquid metal (LM, EGaIn: GaIn24.5), owns distinctive high flexibility, 2


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deformability, electrical conductivity, and nontoxicity.2,3 This enables it a multitude of unique virtues in a wide range of applications including the flexible bionic nano-robot4-5, drug carrier6, flexible electronics7-11, 3D metal printing12,13, sensors14 or batteries15, skin electronics16-19, microfluidic channels20-22, functional devices23,24, and medical field25,26 etc. At the present stage, the LM EGaIn has a high reflectivity as most of other metals own in the optical aspect. Thus it often presented a silver-white hue.27 After utilizing the LM for the fabrication, the functional commodities usually got single appearances as well. From conceptual innovation aspect, here we envision that a big challenge in the field would come to the actualization of LM for fluorescent color appearances, which could be applied to many future practical fields, especially in those multifunctional and colorful soft robots. As everyone knows the LM marble can be oxidized quickly in an oxygen environment28 and constructed as a thin Ga2O3 oxide film with a thickness of 0.5~2.5nm29,30. The oxide film (consists of Ga2O3) has been described with an outstanding adhesion behavior which led it to strongly adhere to a majority of surfaces 31-33. According to the previous reports, the oxidation films of LM marbles coated with WO3, TiO2, In2O3, C nano-tube and semiconductor nano-material28,34 have a sea of sensitive electrochemical35 and photochemical36 properties. And some of these white LM marbles coated with hydrophobic coverings can even float on the water. Besides, a magnetic LM marble was acquired by covering an LM with nano-sized Fe particles37, which has a large potential in the magnetic field-driven manipulation38,

or

electrical

switching

field39.

Generally,

some

metal 3



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micro/nano-particles (Al3, Cu40, Mg) appear more likely to react with pure LM (EGaIn), and a new phase like CuGa2 can be formed. It has also been proven that LM can “phagocytose” copper micro/nano-particles under electrical stimulation41. These aforementioned Interesting researches had ever tried to load unique functions nano-particles into the LM to make desired materials34-41, although the color is not of their topic. Clearly, the coating of the fluorescent grains on the superficies of the LM marbles remained to be another key ingredient. Herein we made the first ever trial and described the primary instance of a spontaneous adhesion of stabilized fluorescent particulates on the superficies of LM droplets to achieve colorizing the LM droplet. The basic idea to make multifunctional fluorescent LM was proposed. The evidence suggested that this fluorescent LM marble was stable, and it can be merged and split between each other. Interestingly, the electric-stimuli has been further successfully introduced to trigger the release of nano-particles from the LM to realizing discoloration, and the specific release mechanism was interpreted in depth.

EXPERIMENTAL The liquid metal (eutectic gallium indium, EGaIn) was made by high-purity Gallium and Indium metals (with purity of 99.9%) as raw materials. Gallium and Indium were weighted with a ratio of 75.5:24.5 in a line of the chemical compositions of GaIn24.5 alloy. The weighted Gallium and Indium metals were mixed together in a beaker and heated to 50oC until the metals were fused completely. Different kinds of commercial fluorescent powders were adopted as the coating materials. The 4


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composition of rare earth phosphors is red phosphor: YVO4: Eu3+; yellow phosphor: Y2Al5O12: Ce3+; green phosphor: (Ba, Si)2SiO4: Eu2+, and blue phosphor: BaMgAl10O17: Eu3+. Commercial fluorescent powders and the copper electrodes were all purchased from Sinopharm Chemical Group Co., Ltd, and the purity of the two materials were all as high as 99.9%. The result of fluorescent LM droplet was characterized through a field emission scanning electron microscope (QUANTA FEG 250). The surface of the fluorescence LM was obtained by the SEM images. The optical property of the fluorescent LM droplet was analyzed via diffuse reflectance spectroscopy (UV-vis), which was performed using a Hitachi (U-3010) Spectrophotometer in the wavelength range of 300-780nm employs barium sulfate as a reference. Fluorescence spectra measurements were performed on a FLSP-920 fluorescence spectrophotometer. The fluorescence marbles were measured at room temperature. The color performance data of fluorescence LM was reported using the CIE-L*a*b* (1976) colorimetric system. The electric field was imposed by applying a DC between two inert copper rods as electrodes. The DC voltages were generated by a DC power supply (RIGOL, DG 1032Z 30-MHz Dual Channel, China) while the ramping voltages were obtained by a lab view controlled source meter with data acquisition function. High-speed videos of the autonomous locomotion were captured using a camera (Canon XF-305, Japan), and the transient images were extracted from these videos.

RESULTS AND DISCUSSION 5



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An elaboration procedure has been devoted to fabricating the fluorescent LM droplets: As illustrated in Fig. 1 (a), firstly, the pure LM was agitated in a vessel for 30 minutes. After being mixed and fully stirred, the naturally oxidized LM has a viscoelastic oxidative tegument. It was amorphous and not easy to flow, like a solid. Then, the fluorescent powder of 2g was mixed with this viscoelastic LM of 20g in a beaker, which was maintained under continuous stirring at 40oC for more than 30 minutes. The fluorescent powders were substantially adhered to the LM droplet’s oxidation tegument after being fully stirred, and the theory of the adhesion process of the oxide film has been established42. After that, agitating the above mixtures again with another liquid metal of 20g in a vessel for at least 10 minutes, and the fluorescent LM was prepared. Take a spoonful of the mixture of fluorescent LM, then dealt it with hydrogen chloride (HCl) solution. As an example: A fluorescent LM droplet of 2g would be treated with 4~5 drops of HCl (6% wt.%). After this processing, the shape of the fluorescent LM was totally transformed into a “sphere”43, and the physical properties of this marbles have changed from wetting to non-wetting state. The size of the LM droplets depended on the volume of the removal. Figure 1 (b-1) shows a bare LM galinstan droplet (R=2.04 cm) with a shiny silver tone. The naturally oxidized LM droplet with a dull gray appearance was presented in figure 1 (b-2) as well. Under UV excitation, this oxide skin was presented with a purple hue, as disclosed in fig. 1 (b-3). And the fluorescent LM droplet with an orange appearance was given in figure 1 (b-4). The current discoveries 6


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presented a novel opportunity for the implementation of full-color appearances of LM.

Figure 1. (a) Fluorescent LMs were prepared by stirring a LM marble with nano-fluorescent powders in a beaker; (b) The color-changing of LM: (1) Galinstan droplet treated with 0.5mol/L NaOH; (2) Images of a galinstan droplet with naturally formed native oxide layer in ambient air; (3) Images of galinstan droplet with naturally formed native oxide skin under UV excitation; (4) The orange color performance of fluorescent LM was observed under UV excitation. (c) The NIR reflectance and no absorption spectra of bare LM and oxidized LM. The color of an object depends on its physics. Objects can display the color of the light leaving their surfaces, which normally depends on the reflectance properties of the surface. Therefore, by testing the Vis-reflectivity of different LM marbles, the optical properties can be known. An excellent Vis reflectance (81%) of the bare LM marble with a specular reflection was exhibited in fig. 1 (c), while the Vis reflectance of the oxidized LM marble with an oxide skin diminished to 72%. The oxidized LM has a lower reflectance of light, so its color was darker when compared with the bare 7



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LM. A reflectance fluctuation appears between the wavelength of 300 and 400 nm for LM droplets, which may be due to that the surface Ga2O3 have a unique transmission of the UV light44. In the following, the adhesion behaviour between the fluorescent particulates and LM marbles was investigated. Fig. 2 (a-1~2) presented the images of the native oxide tegument which was naturally developed on the surface of a puny micro-fluorescent LM marble (R=480µm) in the ambient air. The oxidized LM droplet has a draped and irregular oxide skin, which protected the metal from further oxidation. The LM droplet was fast oxidized even in a mere 0.2% oxygen environment42. It has been revealed that the “tearing morphology” was constructed from the expansion of the LM marble under vacuum in the SEM system as well29. The SEM images of the green and orange fluorescent LM marbles were presented in figure 2 (b)~(c). It can be observed that the stable fluorescent LM marbles were multifaceted systems composed of LM cores and solid fluorescent particulates shell. The green and orange fluorescent particles have dimensions from 800 nm to 20 µm, respectively. The native oxide covering played a paramount role in adhesive bonding between the external functional particles and the bare LM marbles, as demonstrated in figure 2 (c-2). The result indicated that the fluorescent particles only existed on the surface of the oxide layer, and the particles could not adhere to the smooth and bare LM droplet.

8


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Figure 2. (a) (1-2) The SEM of oxidized LM with a draped and viscoelastic oxide layer. (b) The SEM images of a green fluorescent LM droplet: (1-2) LM droplet encapsulated in coating of 800 nm green fluorescent particles; (c) The SEM images of an orange fluorescent LM droplet: (1-2) LM droplet encapsulated in coating of 20 µm orange fluorescent particles.

Different synthesized fluorescent LM marbles were manufactured with either relatively small or large sizes, as clearly exhibited in Figure 3 (a), such as multi-coloured fluorescence, fluorescent orange, pink, to blue colors etc, and the color performances of these marbles were shown in Table 1. The oxide film makes it much easier for LM to adhere to the solid surface and also stabilizes LMs into different shapes. When constructed, these fluorescent LM marbles were “sphere” in shape, but occasionally adopted an “ellipsoidal” geometry due to their dissimilar surface tensions. Besides, the addition of fluorescence has not changed the particular physical-chemistry properties of the LM droplets, which has been explained in the previous paper45. Figure 3 (b) presented a photograph of the originally separated LM droplets with pink and green appearances. A straw was used to merge the two 9



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different colored LM droplets together, which resulted in a biggish ultimate multicolor LM marble. In figure 3 (c), one entire fluorescent LM marble could be divided into a couple of slight LM marbles by utilizing a blade. If this cutting manipulation was performed more than once and one huge fluorescent LM marble was transformed into thousands of multicolor LM marbles or merge together between distinctive droplets. When different bands of lights were applied to excite the fluorescent LM, the fluorescent LM emitted different colors of fluorescence in the dark. As shown in figure 3 (d), when the emission spectrum manifested narrow emission peaks, the fluorescent blue, orange, and pink LM marbles displayed excitation peaks around 358nm, 364nm, and 535nm, respectively. The fluorescent emission spectrum of these blue, orange, and pink marbles manifested narrow and symmetric emission peaks at 467nm, 517nm, and 600nm, respectively. Since the fluorescent particulates are self-luminous materials, the intense bright luminescence of dissimilar fluorescent LM marbles could be perceived by the naked eyes in the dark.

Table 1. The color performances of LM marbles. Color performance

Sample L*

a*

b*

Bare LM

95.39

-0.27

0.09

Oxide LM

88.05

-0.26

0.7

Orange LM

79

21

65

Pink LM

52

25

-12

Bule LM

78

-17

-32

10


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Figure 3. The fluorescent liquid metal marbles: (a) Luminescence of fluorescent LM marbles observed by the naked eye upon excitation with UV light from left to right: (1) Optical image of fluorescent multicolored LM marble (R=1.7cm), (2) Orange LM marble (R=2.3cm), (3) Pink LM marble (R=3.09cm), and (4) Blue LM marble (R=3.55cm). (b) The merged together color from different fluorescent LM marbles: (1) A fluorescent pink LM marble and a fluorescent green LM marble; (2) The composite fluorescent LM marble constructed from the pink and green LM marbles. (c) The split of fluorescent LM marble: (1) A fluorescent LM marble; (2) Cutting the fluorescent LM marble; (d) The excitation spectra (1) and emission spectra (2) of three different fluorescent LM marbles in the figure (a 2-4). The loading capacity could also be calculated. The high loading capacity depends on the large specific surface area (4πr2) of the epidermis of LM (volume 4/3πr3). The volume of the fluorescence reads as ≈ 4πr2×20µm, as the nano/micro phosphor were only 0.8~20 µm thick. Thus, the loading capacity of the fluorescent

11



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LM can be calculated as: =(4πr2×20µm)/(4/3πr3) =60/r(µm). So the loading capacity of fluorescence LM droplet of 2cm is about 60*2/20000= 0.6%. In the following, we spread a drop of 2cm diameter fluorescent LM on the plastic substrate. The coating nano-particles could chemically tolerate both basic and acidic environments. The major phenomenon in the NaOH electrolytic solution and HCl electrolytic solution (about 0.6 mol/L) were summarized. When the fluorescent LM was contacted with the cathode (-) of a copper electrode at the different voltages (from 2V to 15V), it was found that the LM marble slowly released the fluorescent nano/micro-particles. Typical snapshots of this discoloration are presented in figure 4. (also see the Supporting Movie S1 and Movie S2). The fluorescence firstly disappeared and the bare LM surface occurred near the cathode at 293 seconds. The LM changed appearance from its fluorescent orange color into a whole naked one at 380 seconds. As a consequence, the fluorescent particulates were constantly released from the tegument.

12


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Figure 4. Electrically induced discoloration of the fluorescent LM marbles in a HCl electrolyte: The mechanism of the process to release the fluorescence and the schematic of the electric field force influencing the discoloration of the fluorescent LM marble in a basic HCl solution. The main reason was due to electrochemistry mechanism. The electrons migrated from the anode to the oxide layer of fluorescent LM. Undergo the reduction reaction, oxidation of gallium (Ga2O3) was removed on the surface of LM marble, producing metal Ga and indicating significant variations. Subsequently, the hemisphere of LM facing the anode appears to be clear of fluorescent nano-particles. Through control over the surface oxide of a droplet, which in turn dominates the strong adhesion behavior in the substrate, the fluorescent particulates were constantly released from the tegument. As the cathode was inserted into the fluorescent LM, in the release process, a part of bare metal EGaIn was exposed, the electricity charged the conductive surface of the LM, and the cations accumulated near the droplet, which caused the formation 13



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of an electrical double layer (EDL)46,47. According to Lippman’s equation and Young-Laplace equation37,4, one has: Surface tension γ(V)= γ0-C/2*V2

(1)

Pressure P=γ(2/r)

(2)

Here, the voltage drops across the EDL became less at the right hemisphere, and the surface tension and pressure went higher, as shown in figure 4. The pressure difference also induced “Marangoni flow” outside the LM droplet, resulting in the motion and release of fluorescent nano-particles which stands towards the side far from the anode. Using electric stimulus, by control over the surface oxide of LM droplets, which in turn dominates the adhesion and surface tension on the LM substrate. It was also found that through increasing the applied voltage, the speed of discoloration gets increased, and the fluorescent LM exhibited a relatively fast release property at a high voltage. This increase could also be attributed to a larger (electrical double layer) EDL induced surface tension difference between the poles of the LM droplet.

14


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Figure 5. Movement of the fluorescent LM marbles in a basic electrolyte when a step voltage of 15V is applied across the two copper electrodes (the length and the width of the channel were 14cm and 0.62cm, respectively). The mechanisms of the process of the fluorescent LM marble in an electric field and the actuation of fluorescent LM marbles were shown.

When the electrodes were immersed in the electrolyte without contacting the fluorescent LM, the fluorescent LM marble would planar locomotion and motivate towards the anode at a speed of 0.5 body length per-second, as displayed in figure 5. A different surface tension could be induced according to Lippman’s equation. The formation of EDL on the surface of bare LM causes the locomotion. Meanwhile, the fluorescent nano-particles on the surface of the LM marble were changed to non-uniform. A bare LM with the uncoated area was formed, and the Ga2O3 integument full of fluorescent powders was created in the right hemisphere. (Also sees the Supporting Information Movie S3)

15



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Figure 6. Electrically induced transformation of the fluorescent LM marbles in a NaOH electrolyte.

Table 2. The phenomena of fluorescent LM marbles in different electrolytes. Case

Electrolyte

1 2

NaOH

Anode (+)

Cathode (-)

Not contacted

Contact LM

Contact LM

Not contacted

3

HCl

6

Not contacted

Contact LM

Contact LM

Not contacted

Phenomena Discoloration Transformation

Not contacted

4 5

(+)(-)

Planar locomotion Discoloration Transformation, turn black

Not contacted

Planar locomotion

When the anode (+) was attached to the fluorescent LM, the copper-wire cathode (-) was immersed in water, the colorful LM sphere quickly transformed into a flat film with a large area. Figure 6 illustrates the deformation of fluorescent green LM in the basic electrolyte. In the first 4 seconds, the LM extended gradually; while in the 10 seconds, the green LM moved close to the cathode, and the oxidation and deformation were observed on the bare surface of LM, which has also been explained in the previous paper4. Through changing the electrode arrangements and the electrolytes, various 16


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behaviours

of

fluorescent

LMs

were

investigated,

including

deformation,

discoloration, and the planar locomotion, respectively, as summarized in Table 2. Clearly, using this electric-stimuli can manipulate the LM shapes and colors, which would help simulate the interesting behaviours of the chameleon in nature. Besides, this is also a first-ever report for an electricity-responsive LM to achieve efficient releasing of fluorescent substances. One more point, at this stage, the current fluorescent LM was mainly fabricated in free space. In fact, additional manufacture tool can be developed along this direction. For example, we can combine the fabrication process of fluorescent LM with the microfluidic droplet generation systems as established before

48, 49

. Through

properly using these devices, one could probably produce many stable, and monodisperse fluorescent LM microspheres from the glycerol-PVA or glycerol-water model system. And the fluorescent LM droplets with a µm diameter could be continuously generated by the droplet generator. In the coming time, such kind of automatic fabrication strategy is also worth of pursuing to fulfil various needs.

CONCLUSIONS In summary, we have demonstrated a facile and rapid route to successfully fabricate fluorescent LM which could work as a transformable biomimetic chameleon. Such approach takes advantage of the inherent adhesion properties at the oxide surface of the LM marbles. The fluorescent particles offer extra capabilities, unique physical/chemical, and optical properties in comparison to bare single silvery-white 17



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LM. We disclosed that this fluorescent LM was able to respond to a particular electric stimulus, and changed their original shapes and colors just like a transformable biomimetic chameleon. Moreover, under the electric field stimulation, all these nano/micro materials in the outside layer of the LM can be completely released. Such electro-responsive materials are smarter and more intelligent than ordinary materials. The fluorescent LM possesses unique virtues in a wide variety of crucial areas, such as multifunctional and colorful soft machine, or micro-robot allowed in vivo fluorescence imaging, or controlled/targeted drugs delivery via electric stimulus in complex fluidics, monodisperse fluorescent LM microspheres generated by the microfluidic droplet systems, or biomimetic field.

SUPPORTING INFORMATION Supporting Information is available from the online library or from the author.

ACKNOWLEDGEMENTS This work is partially supported by the National Nature Science Foundation of China Project, the Ministry of Higher Education Equipment Development Fund, Dean’s Research Funding and the Frontier Project of the Chinese Academy of Sciences, as well as Beijing Municipal Science & Technology Funding (Grant No. Z151100003715002).

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Table of Contents

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Fluorescent Liquid Metal As a Transformable Biomimetic Chameleon

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