Gallium-Based Liquid Metal Amalgams: Transitional-State Metallic

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Gallium-Based Liquid Metal Amalgams: TransitionalState Metallic Mixtures (TransMixes) with Enhanced and Tunable Electrical, Thermal, and Mechanical Properties 2

Jianbo Tang, Xi Zhao, Jing Li, Rui Guo, Yuan Zhou, and Jing Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10256 • Publication Date (Web): 26 Sep 2017 Downloaded from http://pubs.acs.org on September 28, 2017

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ACS Applied Materials & Interfaces

Gallium-Based Liquid Metal Amalgams: Transitional-State Metallic Mixtures (TransM2ixes) with Enhanced and Tunable Electrical, Thermal, and Mechanical Properties Jianbo Tang*,† Xi Zhao,‡ Jing Li,§ Rui Guo,† Yuan Zhou,‡ and Jing Liu*,†,‡ †

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

‡

Key Laboratory of Cryogenics, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China

§

Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China

KEYWORDS: liquid metals, metallic dispersions, direct-printing electronics, interface materials, soft materials ABSTRACT: Metals are excellent choices for electrical- and thermal-current conducting. However, either the stiffness of solid metals or the fluidity of liquid metals could be troublesome when flexibility and formability are both desired. To address this problem, a reliable two-stage

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route to improve the functionalities of gallium-based liquid metals is proposed. A series of stable semiliquid/semisolid gallium-based liquid metal amalgams with well-controlled particle packing ratios, which we call TransM2ixes, are prepared and characterized. Through effectively packing the liquid metal with copper particles (which are found to turn into intermetallic compound, CuGa2, after dispersing), remarkable enhancements in electrical conductivity (6 × 10 S m , ~80% increase) and thermal conductivity (50 W m K , ~100% increase) are obtained, making the TransM2ixes stand out from current conductive soft materials. The TransM2ixes also exhibit appealing semiliquid/semisolid mechanical behaviors such as excellent adhesion, tunable formability, self-healing ability. As a class of highly-conductive yet editable metallic mixtures, the TransM2ixes demonstrate potential applications in fields like printed and/or flexible electronics, thermal interface materials, as well as other circumstances where the flexibility and conductivity of interfaces and connections are crucial.

1. INTRODUCTION From tasty coffee and ice cream to colourful ink and paint, and to cement paste that shapes the world, liquid-particle mixtures are ubiquitous in life around us. The material properties as well as the governing mechanics of these semiliquid/semisolid mixtures are quite different from those associated with their basic-state ingredients, either liquid or solid.1-4 Starting from this basis, the simple route of mixing liquid and solid particles together has generated a vast number of transitional-state mixtures with enhanced properties as well as synthetic features.5-8 In fields like printed and/or flexible electronics,9-15 thermal management,16,17 soft robotics,18,19 etc., materials integrating electrical conductivity, thermal conductivity, flexibility are strongly desired. Low-melting-point gallium-based liquid metals (eutectic alloys consist of


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gallium, indium, tin, etc.) are emerging as such candidates in recent years.20-27 As a matter of fact, restricted by the intrinsic properties of their alloy compositions, both the electrical conductivity and the thermal conductivity of this class of liquid metals are relatively small when compared to that of the widely used materials, taking copper for instance. Besides, the fluidity and surface tension of the liquid metals tend to complicate their handling and processing. Therefore, it is of particular interest to add suitable ingredients to make liquid metal mixtures (or hybrids) which are easy-handling and at the same time, superior in performance. Adding metallic ingredients to a liquid metal base is contrary to incorporating the liquid metal into less conductive elastomers and organic bases.22-25 Both the electrical conductivity and the thermal conductivity of the latter are much smaller than the liquid metal base, although these combinations produce other benefits. The history of clinical dentistry has witnessed the rise and fall of mercury amalgams as restorative dental materials. However, the specific application of mercury amalgams inevitably directs the focus of previous practice to problems such as durability and safety.28 It has been shown that highly-conductive liquid metal paste and magnetic liquid metal can be prepared through directly mixing copper particles and nickel particles, respectively, with the liquid metal base.29,30 However, such direct-mixing methods, which has been successfully applied in making mercury amalgams, are unsatisfactory when it comes to gallium-based liquid metals due to their easily-passivated surface.31,32 So, despite the apparent practical significance, effective methods to disperse metal particles into gallium-based liquid metals have not been proposed until recently and there still remains a major challenge to prepare stable liquid metal-particle mixtures.33,34 Here, we propose a two-stage route to prepare stable gallium-based liquid metal amalgams, a series of transitional-state metallic mixtures (TransM2ixes) with enhanced electrical


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conductivity , thermal conductivity , as well as appealing semiliquid/semisolid mechanical behaviours such as excellent adhesion, tunable formability, self-healing ability, etc. We show that the method of dispersing large amount of particles in the liquid metal base in a solution environment with the assistance of an electrical polarization is robust, while to obtain durable products, a follow-up vacuum drying process is crucial. In order to reach a comprehensive understanding of the TransM2ix framework, we further investigate their chemical compositions, electrical/thermal properties, as well as mechanical behaviours. Moreover, their potential uses as printable and mouldable electronic and/or thermal interface materials are demonstrated. 2. MATERIALS AND METHODS 2.1. Materials. GaIn alloy (75.5 wt% gallium, 24.5 wt% indium, = 3.4 × 10 S m and = 26 W m K ), is used as the metallic liquid base. Copper (Cu) particles (Purchased from Beijing DK Nano Technology Co., Ltd., see Figure S1) are chosen as the packing material for its superior conductivity ( = 60.6 × 10 S m and = 400 W m K ) and lower cost compared with the liquid metal. NaOH aqueous solution (1.0 mol L ) is used during particle internalization. 2.2. TransM2ix Preparation. The first stage to prepare the TransM2ixes involves an electricalpolarization-assisted particle-internalization process performed in the NaOH solution (Figure 1a). The removal of the oxide layer on the liquid metal and Cu particles, which are two prerequisites for particle internalization, can be achieved by exerting an external voltage in an alkaline solution.33 Pre-weighed GaIn and Cu particles, together with the NaOH solution (GaInNaOH solution volume ratio ~1: 1), are transferred to a flat container. The cathode and anode (graphite sticks) connecting to a DC power source are fixed in the liquid metal and the solution, respectively. A 5 V polarization is applied and the liquid metal is stirred gently to accelerate


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particle internalization. It takes about 30 min to 60 min to fully internalize the particles, depending on the packing ratio of different cases. The packing ratio is defined as

=

!"# /!%&'( , where !"# and !%&'( represent the mass of the copper particles and the liquid metal, respectively.

Figure 1. (a) The two-stage process for TransM2ix preparation. The wet intermediate products obtained from the first stage differ largely from the vacuum-dried TransM2ixes. (b) Intensive cavities generate on the surface of the intermediate product (left) and such situation can be avoided by a vacuum drying process (right). (c) TransM2ix samples of different packing ratio

photographed two months after preparation.

The first-step wet-processing stage is found to introduce water along with the particles to the liquid metal. The presence of water in the intermediate products will produce intensive


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cavities when the samples are put under SEM (Figure 1b). Since water reacts slowly but constantly with the liquid metal (corrosion of the liquid metal), the obtained intermediate products are not durable. The fact that the contact area between the liquid metal and the particles is proportional to the square of the particle diameter indicates that, for the same amount of particles, a decrease in particle diameter will result in a significant increase of the contact area, which in return increases the water content. For such concerns, micro copper particles (See Figure S1 of Supporting Information) are used in the present study, although micro and nano particles can both be internalized. Given the same reason, a second-stage vacuum drying is further performed right after the first stage to exhaust the water content. Stable TransM2ixes can be obtained after vacuum drying the intermediate products at 60~80℃ for 3 hours. Following these two procedures, a series of TransM2ixes with different

have been prepared, which look

much like the frequently encountered suspension, cream and paste but have intrinsic metallic nature (Figure 1c). 2.3. Impacting Test. To demonstrate the liquid-like to solid-like transition of the TransM2ixes as the packing ratio increases, the impacting behaviours of the samples are investigated. The samples are dropped on a glass plate from 0.5 m above and the sequential images are captured by a high-speed camera (IDT NR4 S3 High-Speed Data Acquisition Instrument). The impact duration and the expanding rim of the droplet for different packing ratios are compared. 2.4. Chemical Composition Characterization. X-ray diffraction (XRD) tests are firstly performed for TransM2ix samples with different packing ratios. Then the sample of

= 0.15 is

scanned at intermittent time points after its preparation (All the samples are stored under ambient conditions with no special cares taken). And one sample is also frozen using liquid nitrogen before the test to reveal the liquid compositions. Energy dispersive spectrometer (EDS) equipped


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scanning electron microscopy (SEM, Hitachi S-4800) is further used to investigate the particle sizes, element distributions, as well as the metallic interaction that takes place in the TransM2ix framework. 2.5. Electrical- and Thermal-Conductivity Measurement. The electrical conductivity of the TransM2ix samples are measured using a standard four-point technique. The samples are filled inside a 750 mm-long groove which has a rectangular cross section of 5.1 ± 0.03 mm × 4.1 ± 0.06 mm . The thermal conductivity of the TransM2ix samples is measured by a thermal constants analyser (Hot Disk TPS 2500S). And an oil bath (Julabo PRESTO® A30) is used for temperature control. The temperature range from 20~100℃ is investigated and a long-term (~3 weeks) thermal conductivity measurement is also performed to validate the durability of the samples. See Supporting information for details. 2.6. DSC Measurement. The influences of particles on the phase change behaviours (melting and solidification) of the TransM2ix samples are investigated using differential scanning calorimetry (DSC, Equipment model: NETZSCH DSC 200F3 Maia). The samples are scanned between a temperature range of −70 ℃ ~ 70 ℃. The same speed of 10 ℃ min is used for temperature control during sample cooling and heating, and a 5-min isothermal stage is maintained between cooling and heating. DSC/TG Pan Al2O3 crucibles are used as sample holders. Each sample is scanned with two repeated cycles and three samples are measured for each packing ratio. 2.7. Mechanical Property Measurement. The response of the TransM2ixes under mechanical stress is investigated with the assistance of a sensitive stress gauge (DCAT11, DataPhysics Instruments GmbH). A glass rod (cross-section radius: 1 mm , length: 60 mm ), which is connected to a fixed stress sensor, is used as the stress probe. Each sample is held by a container


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which can be vertically adjusted. The probe is firstly pushed in the TransM2ix sample and then pulled out at controlled speeds during a complete test. The force acting on the probe is recorded during the whole process. The radius of the cylindrical TransM2ix sample is 16 mm and the filling height is kept > 20 mm. Special cares are taken to make sure the sample surface is flat, especially for samples of high packing ratios. The inserting depth D is set as 5 mm for all the cases. The moving speed of the displacement adjuster is set as 0.05 mm s during all the stages except for the stage before the probe reaches the sample surface, during which an approaching speed of 0.5 mm s is used. Pushing force (negative) that exceeds 40 mN is out of range and thus not recorded. 3. RESULTS 3.1. Transitional State Characterization. The transitional (semiliquid/semisolid) state of the TransM2ixes is the iconic feature of the materials and it is also crucial for their handling and processing in real applications. Therefore, we firstly characterize their transitional-state features. To do so, the fluidity and rigidity (two opposite properties that identify liquid and solid, respectively) of the TransM2ix samples are compared through impacting tests. As shown in Figure 2a, low-packing-ratio droplet (e.g. while high-packing-ratio droplet (e.g. As a step increase of

= 0.05) reveals a liquid-like impacting behaviours

= 0.20) shows evident paste-like impacting behaviours.

from 0.05 to 0.20, the impacting duration and the expanding ratio of the

droplet rim d(t)/d5 both decrease gradually (Figure 2b), which gives a clear indication of fluidity decrease (rigidity increase) as the liquid metal is packed more and more heavily (Figure 2c). The impacting tests also imply that the transitional state of the materials can be adjusted on demand by a proper control of .


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Figure 2. (a) Time-lapse images of dropping TransM2ix samples. The impacting tests reveal liquid-like to solid-like transition of their impacting behaviours as

increases. (b) Comparison of the time-dependent

impacting parameter d(t)/d0. (c) A schematic demonstration of fluidity decrease and rigidity increase of the TransM2ixes as

increases.

3.2. Chemical Composition Characterization. To answer the question what happens after the particles being internalized, further inspections on the chemical compositions of the TransM2ixes are made. As shown in Figure 3a, the XRD tests reveal an unambiguous existence of an intermetallic phase, CuGa2, in all the TransM2ix samples. And the results collected at different time points further suggest a continuous formation of the intermetallic phase after their preparation (Figure 3b). The existence of CuGa2 phase is found even at early stages (day 1 and day 2) but the characteristic peaks of Cu cannot be detected. Since the formation of the intermetallic compounds between Ga and Cu is not expected to be rapid enough to consume all


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the Cu phase under our experimental conditions,35 it is speculated that the absence of Cu peaks shortly after the preparation is a result of the inner Cu phase being overwhelmed by the liquid metal. Note that the test on day 1 is conducted ~1 h after vacuum drying the sample. To identify the previously undetectable liquid metal compositions, i.e., liquid (non-crystalline) Ga and In, a frozen sample ( = 0.15) is further tested (Figure 3c). Through this frozen XRD test, the complete compositions of the TransM2ixes are revealed and their semiliquid/semisolid nature under usual conditions is also confirmed.

Figure 3. XRD characterization of the TransM2ix samples (a) with different packing ratio (tested on day 30 after preparation) and (b) at different time points ( = 0.15). (c) A sample frozen by liquid nitrogen to identify


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Ga and In. Standard intensity cards of different compositions are provided for comparison. Note that due to the crystallization of ambient moisture, characteristic peaks of ice also become measurable in the frozen sample.

The existence of the intermetallic compound CuGa2 is in agreement with the Cu-Ga binary phase diagram in the room-temperature range.36,37 But microscopic views from the SEM and EDS tests suggest that the actual intermetallic reaction in the present case (Cu particles being immersed in bulk liquid metal) is a rich problem. At early stages, irregular particles (Figure 4a) similar to the original Cu particles (Figure S1 of Supporting Information), are dispersed in the liquid metal. Meanwhile, small nuclei are found to form on the particles due to point-specific crystallization of CuGa2 (Figure 4a, magnified image). As the process proceeds, the irregular particles become untraceable and the nuclei grow into larger tetragonal blocks (Figure 4b). The results suggest that the dissolving of Cu and the formation of CuGa2 are not in-situ. Instead, the Cu phase in direct contact with the liquid metal is firstly dissolved in the liquid metal, resulting in a Cu-rich layer near the surface region of the original Cu particles. At the small CuGa2 nuclei sites, the dissolved Cu crystallizes with Ga to build up the CuGa2 particles, which decreases the concentration of dissolved Cu and creates a concentration gradient near the surface region. Driven by such concentration gradient, Cu is continuously transferred by the movable liquid metal phase along the near-surface region to the nuclei sites. As a result, the observed tetragonal CuGa2 particles are produced by such continuous intermetallic reaction of the system. Element distribution mapped by EDS shows the existence of Cu and Ga, and the absence of In in the tetragonal particle area, which further confirm the chemical compositions of the particles (Figure 4c). Given that the formation of intermetallic phase usually follows a power law37,38 and the liquid metal is excessive, all the Cu particles are expected to turn into the CuGa2 compound. Based on this point of view and the above-mentioned characterizations, conclusion can be made


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that the TransM2ix framework contains a single crystalline phase of CuGa2, and liquid phase GaIn (and their oxides) the rest. The mass ratio of the CuGa2 (7) for each

can thus be

estimated, which gives 7 = (1 8 2M%& /M"# )⁄(1 8 ) , where M%& and M"# are the Molar mass of Ga and Cu, respectively.

Figure 4. (a) Irregular particles dispersed in the liquid metal and the formation of small tetragonal nuclei (magnified image) on the particles observed on day 2. (b) Tetragonal blocks observed on day 30. (c) Element distribution mapped by EDS with brightness proportional to element abundance.

3.3. Electrical conductivity and Thermal Conductivity Measurement. The electrical conductivity and thermal conductivity are two key parameters when the materials are used as electronic materials or thermal interface materials. Being aware of the chemical compositions of the TransM2ixes, we further show that the incorporation of the liquid metal and the CuGa2 particles can induce a significant enhancement in and . As shown in Figure 5a and 5b, both and show gradual increases as 7 increases. And for the most-heavily packed sample


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(

= 0.20 and 7 = 0.54 ), the measured and , respectively, exceed 6 × 10 S m and

50 W m K , corresponding to a remarkable near 80% and 100% increase compared with the liquid metal base. Such increases in conductivity should be attributed to the CuGa2 particles which are more conductive than the liquid metal.39 And such parabolic development in thermal conductivity is consistent with other systems with heavily packed particles (See Ref. 4 and the references therein). A good agreement in at 7=1 (pure CuGa2) is found between the value predicted by the fitting equation in Figure 5b ( =105.90 W m K ) and that calculated from Ref. 39 ( =103.64 W m K ). Moreover, since the electrical conductivity of a metal is proportional to it thermal conductivity (Wiedemann-Franz law), a parabolic developing trend of leads to a similar parabolic developing of .40 The liquid metal already outperforms many soft and/or printable materials due to its metallic nature.5, 9-13,17,23 The TransM2ixes produced here further raise the bar to a much higher level. Therefore, they are expected to bring straightforward benefits to systems which have being seeking for lower dissipation and higher efficiency during conducting electric and/or thermal current. One difference between metals of liquid and solid states is the temperature-dependence of , which reveals as opposite temperature coefficients ; (slope of the − < curves) of the two. Liquid metals usually show positive temperature coefficients while that of solid metals have an opposite sign.41 Figure 5c shows that, in the TransM2ix framework, the combination of liquid GaIn and solid intermetallic compound CuGa2 induces a liquid-to-solid transition of as a step increase of

. For TransM2ixes of

= 0.05 and

= 0.10, liquid-metal-like (positive) ; are

measured, which meets with that of the pure liquid metal ( = 0). As accordingly and when

increases, ; decreases

reaches 0.15 or higher, ; becomes solid-metal-like (negative).


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Therefore, the transitional state of the TransM2ix framework can be also classified thermodynamically by using ; as an indicator.

Figure 5. (a) Electrical conductivity ( ) and (b) thermal conductivity ( ) measurement of the TransM2ixes as a function of the mass ratio of CuGa2 (7) and Cu ( ), respectively. Each data point of represents the mean value of ten repeated tests and the error bar (uncertainty of the tests) has taken into consideration of the standard deviation and the errors rising from sample geometry, standard resistor resistance and instrumentation. And each data point of and its error bar represent the mean value of six repeated tests and the corresponding standard deviation, respectively. (c) Development of as a function of temperature from 20℃ to100℃. (d) Long-term (20 days) measurement of ( = 0.12, 20℃).

After careful evaluation of the measured packing-ratio dependence of ;, it occurs to us that it is possible to prepare a TransM2ix sample with near-zero ; by choosing a suitable 0.10 and 0.15. Starting from this basis, a TransM2ix sample of

between

= 0.12 is further prepared and

characterized. The sample validates our inference by showing a ; value of 0.017, which is a step closer to a highly-conductive yet thermally-dull soft material. A long-term thermal conductivity


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measurement is also conducted to test the durability of the material (Figure 5d). A gradual increase of is measured in the first 120 hours. And we attribute this increase to the formation of the intermetallic phase since a portion of Ga is crystallized during this process, given the timedependent growth of CuGa2 (Figure 3b). Thereafter, becomes stable during a two-week follow-up measurement, which means the sample is durable (no sedimentation and no degradation). The electrical conductivity and thermal conductivity measurements validate the reliability and reproducibility of our method. Also, they show that the TransM2ixes with enhanced and controllable properties are also very stable. 3.4. DSC Measurement. Typical curves of different TransM2ix samples are plotted in Figure 6a. During one complete scanning cycle, the tested samples start to melt when heated to an onset melting temperature T1. Due to the supercooling nature of the liquid metal, the samples will not freeze when they are cooled down back to T1. Instead, solidification takes place when the temperature further reaches T2. Characteristic peaks (negative values for melting and positive values for solidification) are detected during phase changes and the heat absorbed (released) during melting (solidification) of the samples are represented by the area of the peaks (A). It can be found from Figure 6b that the TransM2ix samples reveal a step increase of melting point with respect to the packing ratio. Since the change in the Ga/In ratio of the alloy also changes its melting point, the increase of T2 is attributed to the decrease of Ga (Ga is consumed during the formation of solid CuGa2 particles) in the GaIn. As also shown in Figure 6c, the supercooling behaviour of the liquid metal is suppressed by particle incorporation. In general, the supercooling behaviour of the liquid metal is influenced by its compositions (through affecting the structure of the lattice), thermal conductivity, and purity as well. Therefore, given that the compositions and the thermal conductivity of the TransM2ixes are significantly changed, and


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also the dispersed tiny particles can potentially provide numerous nuclei for crystallization, it is reasonable to find the TransM2ixes to solidify at higher temperature than the pure liquid metal. It may be also because the solidification of the liquid metal is determined by multiple factors, the measured T2 shows more significant fluctuation than T1. Moreover, since the amount of liquid metal in the TransM2ixes decreases as packing ratio increases, the heat exchanged during melting and solidification also shows a similar trend (Figure 6d).

Figure 6. (a) Typical DSC curves of TransM2ix samples with different packing ratios. (b), (c) Influence of particle packing ratio on the onset temperature during melting T1 (b) and solidification T2 (c) of the TransM2ixes. (d) Comparison of the phase-change heat A of different samples (open circles-melting, filled circles-solidification).

3.5. Mechanical Property Characterization. After showing their enhanced and tunable electrical and thermal properties, we are now in the position to demonstrate the mechanical


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properties of the TransM2ixes, which guarantee simple and reliable handling and processing of the materials in multiple applications. A push-and-pull method is introduced to characterize the mechanical properties of the TransM2ixes (Figure 7a). During a continuous test, the sample is firstly moved toward the probe mounting over its surface (stage i). Upon contacting, the force and the position are automatically set as reference values (= = 0, ? = 0). Then the probe is inserted in the sample at a constant speed (stage ii). When a predefined maximum inserting depth (@) is reached, the probe is retracted as the sample being moved backwards (stage iii). The probe stays in contact with the sample surface when it is pulled out from ? = 0 (stage iv). And the final detachment takes place when the surface further moves down to a height of ∆B (stage v) below the probe. The force acting on the probe (=) throughout the process is recorded. And since certain amount of remainder of the sample will adhere to the probe, a small drag ∆= is measured after the detachment. The work needed to push in and pull out the probe thus can be represented by the filled areas C and CD in the = − ? curve, respectively. In Figure 7b, the results measured with different TransM2ix samples are presented and compared to that of the pure liquid metal ( =0.00). It can be seen that during the insert stage (stage ii), the force = that pushes (negative) on the probe increases faster for TransM2ix sample with higher , which means the force (or stress), as well as the work C required to induce the same insertion depth (strain) increases along with (

. Particularly, for high packing ratios

= 0.15 and 0.20 ), = increases sharply as the probe begins to penetrate the sample,

demonstrating solid-like stiffness. Similar pulling-force (positive) and pulling-work CD developing trends as the insert stage are found during the retract stages (stage iii). The main distinction among different samples also emerges when

reaches 0.15 or higher, in which a

force peak at the beginning of the retraction becomes evident and the peak further reaches about


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ten times higher when

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= 0.20. The peak occurs because the interactions between the particles

become dominant for heavily packed samples. During the inserting of the probe (stage ii), the particles inside the sample will be squeezed together, which increases the clamping force and the static friction between the particles.2 The squeezed particles aside the probe in return increase the friction acting on the probe. In principle, this friction will maintain its magnitude but change to the opposite direction when switching from the inserting stage (stage ii) to the retracting stage (stage iii). Therefore, unlike the lightly packed samples (

≤ 0.10 ) in which = is mainly

influenced by surface tension (distortion of the surface) and shows smooth transitions from stage ii to stage iii, a sudden shift of = to a peak value occurs at high packing ratio ( = 0.15 and 0.20).


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Figure 7. (a) Schematic drawings of the setups for the push-and-pull test and the characteristic development of F as a function of y during different stages (i-approach; ii-insert; iii, iv-retract; v-detach. Differentiated by colours). An abrupt detachment of pure liquid metal is indicated with dashed line. A typical = − ? curve featuring the detaching region is inserted to show the remainder induced force ∆=. (b) = − ? curves of samples with varied

measured during the push-and-pull test using the same setting.

In stage iv, different from the pure liquid metal which shows an abrupt detachment (abrupt drop of =), the TransM2ix samples detach the probe continuously (continuous drop of =). Based on the observation during the tests, such gradual detachment comes from the gradual breaking of a TransM2ix chain formed between the sample and the probe. The sample remainder adhering to the probe (typically 10~20 mg calculated from ∆=) is much more than that measured with the


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pure liquid metal (typically G 0.1 mg). The push-and-pull test gives quantitative information about the mechanical properties of the TransM2ixes. Also, the tests confirm that the increase of brings better adhesion and formability to the TransM2ixes. An explanation for the mechanical response of the TransM2ix framework to external force is schematically presented in Figure 8. It is established that gallium-based liquid metals adhere to substrates mainly due to the formation of oxide layer on their surface.42 Dispersing solid particles in the liquid metal, on one hand, roughens the liquid metal surface and creates oxide-layer wrinkles (Figure 8 inset a). Given that most substrates also have surface textures, the resulted rugged yet conformable surface of the TransM2ix samples will significantly increase the contact area upon impacting (especially under external force loading), which enhances adhesion. On the other hand, the interactions between the particles through rigid contacts and liquid bridges (Figure 8 inset b) will reduce the influence of surface tension as well as the mobility of the liquid metal. Consequently, applied force could be directed and dissipated by the clamping force and static friction between the particles. Due to such reasons, the patterns made by the sticky TransM2ixes can combine strength, conformability and flexibility at the same time.


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Figure 8. A schematic drawing shows how the particle matrix in the TransM2ix framework increases contact area and dissipates applied force = . fI and fR represent the initiative forces and their counterforces, respectively). Insets: (a) rugged surface of TransM2ixes induced by solid particles (top view) and (b) liquid bridges formed between solid particles (cross-section view).

3.6. Adhesion-Guaranteed Direct Writing and Painting. Inspired by the push-and-pull test, we introduce a fast patterning strategy that uses dipping brushes for direct writing and painting of the TransM2ixes. For conceptual demonstration, we firstly show that the TransM2ixes shows much superior adhesion to the writing tool, a traditional Chinese brush whose head is made of soft animal hair and the painting tool, an ordinary painting brush whose head is shorter and harder than the Chinese brush. As can be seen from Figure 9a, while pure liquid metal hardly adheres to the brushes, a large amount of TransM2ix remainder will adhere to the brush head after a single dipping (See Movie S1 of Supporting Information). Taking advantage of such excellent adhesion of the TransM2ixes to the brushes, we show that TransM2ix patterns can be directly written or painted on different substrates. Using the Chinese brush, Chinese calligraphy can be fluently written on different substrates with the TransM2ix ink of

= 0.10 (Figure 9b,

see also Movie S2 of Supporting Information). In Figure 9c, it is further demonstrated that the TransM2ixes ( =0.10 and 0.15) can be readily painted into complex patterns using the painting brush. The characteristic thickness of the cross-section of the hand-writing patterns is on the scale of 100 μm while that of the painting patterns is about half thick, respectively (See Figure S4 of Supporting Information). Since the TransM2ixes are intrinsically conductive, the patterns can be further integrated into flexible circuits (Figure 9d). Such directly written and painted patterns cannot be made with pure gallium-based liquid metals alone. The demonstrated simple


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and fast processing methods can later be replaced by standardised ones to fabricated flexible circuits and other conducting patterns and films.

Figure 9. (a) Adhesion between the compared samples (pure liquid metal,

= 0.00 and the TransM2ix,

= 0.10 ) and two types of brushes during a single dip-in test. It can be seen that a large amount of TransM2ix remainder adheres to both brushes after the brushes being retracted from the sample but remainder is rarely observed when pure liquid metal is used. (b) TransM2ix-made Chinese calligraphies written with a traditional Chinese brush on different substrate materials ( = 0.10). (c) Two graphic works painted with a painting brush on plastic substrate using TransM2ixes of

= 0.10 (the upper) and

= 0.15 (the lower),

respectively. (d) Lighted LED connected by conductive patterns painted on flexible transparent substrate. Scale bars: 20 mm.


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3.7. Formability-Guaranteed Moulding. Besides favourable adhesion, the TransM2ixes also exhibit other semiliquid/semisolid mechanical behaviours which give variety to their processing strategies. As shown in the left panel of Figure 10a, pure liquid metal ( = 0.00) tends to form a round shape in HCl solution when geometrical constrains are removed due to its large surface tension. The acid solution is used to dissolve the surface oxide during the demonstrations since the existence of a solid oxide film on the TransM2ix surface in ambient air will complicate the influence of the particles on fluidity. However, the TransM2ix of

= 0.10 begins to exhibit

formability and can be readily moulded into predefined shapes in the same solution. Moreover, the TransM2ix letters are found to reshape their structures by clapping their “arms” (Figure 10b, see also Movie S3 of Supporting Information). This kind of behaviour can be used as a selfhealing strategy which, in one specific case, enables a fish-shaped TransM2ix to autonomously heal from multiple cuts (Figure 10c, see also Movie S4 of Supporting Information). The selfhealing mechanism is attributed to the torques generated by the liquid metal menisci formed between the “arms” as indicated in Figure 10a. Apparently, the unique combination of formability and partially reserved surface tension of the TransM2ixes are crucial for the selfhealing behaviour. And such self-healing behaviour is expected to find applications in smart systems where liquids or soft materials are involved. Although it has been shown in former studies that self-healing strategies can be achieved with the liquid metal alone,43,44 using the TransM2ixes may eliminate the engineered structures previously used to confine the liquid metal by taking advantage of their formability.


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Figure 10. (a) After geometrical constrain being removed, pure liquid metal ( = 0.00) instantly adapts a round shape while the TransM2ix (

= 0.10) can maintain pre-designed structures. Liquid metal menisci,

which are responsible for the reshaping and self-healing behaviours to be shown in (b) and (c), are be found between the “arms”. (b) Moulded TransM2ix letters (left column) reshaping (right column) in HCl solution ( = 0.10). (c) Fish-shaped TransM2ix self-heals when cut ( = 0.10). (d) Concave letters made by stamping ( = 0.15). (e) Free-standing TransM2ix “sandcastles” made by moulding (left:

= 0.15; middle and right:

= 0.20). Scale bars: 10 mm.

For more densely packed TransM2ixes (

= 0.15 and 0.20) which exhibit high stiffness

and good formability, other methods such as stamping and moulding can also be applied to create engineered patterns (Figure 10d) and large-scale free-standing structures (Figure 10e), which cannot be created with pure liquid metal. Moreover, different from real “sandcastles”, the structures made with the TransM2ixes are highly conductive. As can be seen from these demonstrations, multiple simple yet reliable methods can be used to process the TransM2ixes


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owing to their transitional-state mechanical behaviours. Given their enhanced electrical and thermal properties, more diverse potentials of the materials in real applications are predictable. 4. CONCLUSIONS In summary, we have proposed a two-stage route to prepare a series of stable transitionalstate metallic mixtures by incorporating GaIn alloy and Cu particles. We have also characterized the compositions of the TransM2ixes and further present details regarding the intermetallic phase (CuGa2) formation of the system. Compared to pure gallium-based liquid metals, the TransM2ixes show remarkable increase in electrical conductivity, thermal conductivity as well as more favourable adhesion and formability. Such easyhandling TransM2ixes are excellent choices for fabricating highly-conductive patterns and structures that are at the same time flexible and conformable. This class of material represents a liquid metal-particle framework for the improvement of the functionalities of gallium-based liquid metals. Therefore, more possibilities of the TransM2ix framework are also within sight when other functional particles are introduced. ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Supporting Information: Characterization of the original copper particles, details of electrical conductivity and thermal conductivity measurement, and the characteristic thickness of the writing and painting patterns. (PDF) Movie S1: Adhesion of pure liquid metal and TransM2ix to dipping brushes. (avi) Movie S2: Direct-writing TransM2ix Chinese calligraphies. (avi) Movie S3: Autonomous reshaping of TransM2ixes. (avi)


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Movie S4: Self-healing behaviour of TransM2ixes. (avi) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected] Author Contributions J.T. and J. Liu conceived the idea. J.T. and X.Z. carried out most of the experiments. All authors participated in 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. ACKNOWLEDGMENT This work is partially supported by the Ministry of Higher Education Equipment Development Fund, Dean’s Research Funding of the Chinese Academy of Sciences and the Frontier Project of the Chinese Academy of Sciences, as well as Beijing Municipal Science & Technology Funding (Grant No. Z151100003715002). The authors acknowledge the help of Shang-juan Liu and Sen Liang for their kind help in preparation of the artworks. J.L acknowledges the support of NSFC (Grants No.61307065). REFERENCES (1) Brown, E.; Jaeger, H. M., Through Thick and Thin. Science 2011, 333 (6047), 1230-1231. (2) Mueller, S.; Llewellin, E. W.; Mader, H. M., The rheology of Suspensions of Solid Particles. Proc. R. Soc. A 2010, 466 (2116), 1201-1228. (3) Eastman, J. A.; Phillpot, S. R.; Choi, S.; Keblinski, P., Thermal Transport in Nanofluids.


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TOC Figure


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Gallium-Based Liquid Metal Amalgams: Transitional-State Metallic

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