Anisotropic liquid metal–elastomer composites

Lifei Zhu , Yuzhen Chen , Wenhui Shang , Stephan Handschuh-Wang , Xiaohu Zhou , Tiansheng Gan , Qixing Wu , Yizhen Liu and Xuechang Zhou *
College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen, 518060, P. R. China. E-mail: xczhou@szu.edu.cn; Tel: +86-755-26536627

Received 16th June 2019 , Accepted 23rd July 2019

First published on 23rd July 2019


Anisotropic liquid metal (LM)–elastomer composites were fabricated by employing a sedimentation technique. The composite exhibits a remarkable electric anisotropy, heat transport anisotropy and mechanical anisotropy while being frozen. This composite can also be a shape morphing material to develop reconfigurable electronic circuits. This study opens up a powerful strategy for the design of LM–elastomer composites in responsive actuation, thermal regulation, and reconfigurable electronics.


Liquid metals (LMs) as well as their composite materials have been intensively investigated in recent years,1–5 owing to their remarkable features toward numerous emerging applications in electronics,6–10 soft robotics,11–13 energy harvesting and storage,14–17 catalysis,18,19 and medicine.20–23 In particular, LM–elastomer composites with remarkable properties, such as conductivity,24,25 dielectricity,26–29 enhanced mechanical stability,30,31 ability for self-healing32,33 and so on, have been developed towards a variety of applications.7,34–38 For example, Majidi and coworkers proposed an isotropic PDMS–LM composite that becomes permanently conductive by use of a mechanical load, which is ascribed to tearing or rupturing of (foam) walls between LM droplets and formation of a permanent network.24 On the basis of this approach, an autonomously and instantaneously electrically self-healing LM–elastomer composite was introduced with sustained sufficient conductivity even when exposed to extreme mechanical damage.7 Furthermore, such composites play a pivotal role for the generation of materials with improved properties, such as high-k dielectricity,26 mechanic robustness30 and tunable rigidity.31 Recently, Liu et al. reported reversible switching of an LM–elastomer composite between an insulator and a conductor by a thermal trigger.39 The composite is an insulator at room temperature and becomes conductive upon cooling to 212 K due to liquid–solid phase transition and expansion of the LM. Similarly, the liquid–solid phase transition of LMs can be utilized to alter the mechanical properties of an LM–elastomer composite. Thuo and coworkers reported a mechanically responsive composite.40 Liquid–solid phase-transition of undercooled LM particles was triggered by mechanical stress. The composition of most of the discussed composites is isotropic. As such, the electric, thermal and mechanical properties are isotropic. However, for advanced features, such as controlled heat transport and shape morphing ability, the presence of anisotropic elements is beneficial. Majidi et al. introduced a composite based on LM microdroplets and an elastomer. The thermal conductivity is increased due to the incorporation of LMs, and heat transport anisotropy is exhibited between the elastomer and the composite.41 In the electronic systems, on the other hand, a large number of artificial devices have been developed for applications with LM.8,42–46 Therefore, the combination of different functionalities within one material, that is, a material encoded with different anisotropic elements, should be of significance toward various emerging applications, such as electronic skin, reconfigurable electronics, and responsive actuation.

To advance this field, we report a feasible and reliable approach to fabricate LM–elastomer composites with spatially-distributed mechanical, electronic, and thermal properties. In this method, an LM (e.g., Gallium (Ga), melting temperature 29.8 °C) in its liquid state is mixed with elastomer prepolymers (Sylgard 184 and curing agent 10[thin space (1/6-em)]:[thin space (1/6-em)]1) by stirring. After several physical steps, i.e., degassing, sedimentation and curing, a bilayer Janus structure is obtained, where the Ga droplets are confined by the surrounding PDMS elastomer matrix. Such a Janus structure leads to several interesting features. Firstly, the mechanical elastic modulus is changed by an order of magnitude when the LM droplets undergo phase transition. As such, shape morphing into three-dimensional shapes can be achieved by freezing the deformed LM droplets as well as their LM–elastomer composites, and melting the frozen LM droplets to release the prestored stress. Secondly, electronic conductive paths can be encoded layer by layer into the LM–elastomer composites, where the LM-rich layer serves as a conductor, while the elastomer layer acts as an insulator. As such, electronic circuits can be fabricated within the elastomer toward cuttable circuits and electronics. Thirdly, anisotropic heat transport can be achieved in the composite, owing to the high thermal conductivity of the LM droplets. Spatially confined heat transport is realized within the LM–elastomer composite. Anisotropic thermal regulation, such as controlled thermal dissipation and heat transport, can be achieved.

To validate the as-made anisotropic LM droplet–elastomer, multiple layered LM droplet–elastomer composite structures were fabricated for thermal regulation. Subsequently, we demonstrated morphing of the composite into various three-dimensional shapes as well as shape morphable circuits for flexible electronics. Finally, we demonstrated the application of the as-made thermal and mechano-regulated LM–PDMS elastomer for light-triggered reconfigurable electronics.

The fabrication of the anisotropic LM droplet–elastomer structures is illustrated in Fig. 1a. Briefly, PDMS and liquefied gallium were mixed with a blender until the LM droplets were uniformly distributed. Subsequently, the composite was transferred to a mold. The filled mold was allowed to stand for 10 min to enable sedimentation of the LM. Then, the composite was degassed and cured at 65 °C. The as-cured composite exhibits electrical, thermal and mechanical anisotropy. Multilayer composites can be generated on the basis of the anisotropic composite by over coating the composite with a layer of PDMS/gallium and repeating the sedimentation, degassing and curing steps. Such a multilayer composite is shown in Fig. 1c, where the bright and dark layers denote for PDMS-rich and gallium-rich layers, respectively. Not limited to rectangular geometry, anisotropic composite structures can be fabricated into copious shapes, such as Janus spheres, as shown in Fig. 1b. Fig. 1d shows the cross-section of a shape morphing composite measured by scanning electron microscopy. In the SEM micrograph, the PDMS appears dark while the gallium droplets are bright. The SEM micrograph illustrates that the sedimentation approach affords a high separation efficiency, as two layers, a PDMS-rich (left) and a gallium-rich layer, are discernable. The magnification in Fig. 1e shows a section of the cross-section of the gallium-rich layer from Fig. 1d, signifying well-dispersed gallium particles with sizes of 10–100 μm in PDMS. The thickness of the gallium-rich layer is dependent on the vol% of the employed gallium.


image file: c9tc03222c-f1.tif
Fig. 1 The gallium/PDMS composite. (a) Schematic illustration of the synthesis strategy of an anisotropic (multilayer) gallium/PDMS composite. (b) A single layer gallium/PDMS composite in a spherical shape. The gallium-rich layer is pointing upwards. (c) An anisotropic composite with five layers. The dark layers denote gallium-rich layers, while the bright layers denote PDMS-rich layers. (d and e) SEM micrographs of the cross-section of a single layer gallium/PDMS composite. The bright areas denote gallium droplets dispersed in the PDMS elastomer.

The lowest thickness of the LM-rich layer was around 44.1 μm (at 2.13 vol% gallium). By increasing the vol% of gallium, the thickness of the gallium-rich layer increased nearly linearly to 780.9 μm (34.4 vol%), as shown in Fig. 2a and Fig. S1 (ESI). The thinnest composite does not exhibit electrical conductivity (no percolation network formed). With increasing vol% of gallium, at first the resistance of the composite decreases from 0.5 Ω mm−1 (4.18 vol% gallium) to 0.1 Ω mm−1 (8.02 vol% gallium). Further increase in gallium content does not significantly alter the resistance of the composite, as shown in Fig. 2b. Interestingly, the variation of the gallium vol% influences the mechanical properties of the layered composite. The gallium-rich layer is stiff and fractures at relatively small strain. In contrast, the PDMS-rich layer is soft. In Fig. 2c, the first region in the stress–strain curve is thus ascribed to the stiff gallium-rich layer of the composite, which gives rise to the second region, where the moderate slope is related to the modulus of the PDMS-layer. Therefore, mechanical anisotropy is established in the composite, which can be exploited for shape morphing. With increasing gallium content, the Young's modulus (parallel to the layers) of the composite (solidified gallium) increased from 7.6 ± 1.5 MPa (14.9%) to 20.4 ± 2.9 MPa (34.4%) while the fracture strain decreased from 250% to around 150% (see Fig. 2d and Movie S1, ESI). This decline may be related to the thinner PDMS-rich layer and more abundant gallium particles in the PDMS-rich layer, serving as nuclei for fracture.


image file: c9tc03222c-f2.tif
Fig. 2 (a) The thickness of the gallium-rich layer, (b) the resistance of the LM–PDMS composite, (c) stress–strain curves of the LM–PDMS composite and (d) the Young's modulus of composites with different liquid metal vol%.

The gallium particles form a conductive percolation network, as the gallium-rich layer (34.4 vol%) is conductive (∼0.09 ± 0.04 Ω mm−1), as shown in Fig. 2b. The PDMS-rich layer, however, is an insulator because the percolation threshold of the conductor (gallium) is not reached. The fact that the different layers are either conductor or insulator endows the composite with electrical anisotropy. For example, the composite behaves like an insulator when different gallium layers are probed (here, layer two and three) and when measured perpendicular to the layers. In contrast, applying a current in the plane of the gallium-rich layer(s) shows that the gallium-rich layers are conductive (see Fig. 3).


image file: c9tc03222c-f3.tif
Fig. 3 Anisotropic electric conductivity of the (triple layer) composite. (a and b) Experiments showing that the PDMS layers are insulators, inhibiting conductivity between different gallium-rich layers, i.e. lower layer and middle layer (a) and lower layer and top layer (b). (c) Schematic illustration of the anisotropy of the electric conductivity. (d and e) Experiments showing the conductivity of the composite in the plane of the gallium-rich layer, i.e. showing the conductivity of the middle gallium-rich layer (d) and all gallium-rich layers (e).

Furthermore, the composite exhibits interesting thermal transport properties. Notably, PDMS is a thermal insulator, while gallium is a thermal conductor. The heat dissipation experiment in Fig. 4a and Movie S2 (ESI) shows the difference between the two materials. For this experiment, a quintuple layered composite was heated with a hairdryer to around 50 °C, and the heat dissipation process was observed for 75 s with the aid of an IR camera. After 50 s, the thermogram of the composite (side view) clearly signifies that the temperature of the PDMS-rich layer is around 45 °C higher than the temperature of the gallium-rich layer, which is ascribed to the slow heat dissipation of the elastomer (Fig. 4a). Moreover, the composite exhibits heat transport anisotropy. The heat transport is dependent on the direction relative to the layer plane and the heated location (side) of the composite (Fig. 4b–d). Local heating of the cross-section of the multi-layer composite affords fast heat transfer along with the gallium-rich layers (heat transfer in z- and y-axis direction), while the heat transfer in the x-axis direction is impeded by the thermal insulating elastomer layer. A temperature difference of around 20 °C could be observed at equidistant points (∼3.6 mm) on the x-axis (30 °C, perpendicular to LM layer) and z-axis (50 °C, across the LM layer) after irradiation for 50 s, as shown in (Fig. S3a, ESI). Thus, the heat is spreading parallel to the incident heat source (Fig. 4b and Movie S3, ESI). In contrast, the heat transfer is perpendicular to the incident light source when the incident light (z-axis) is perpendicular to the gallium-rich layer of the composite, as shown in Fig. 4c and Movie S4 (ESI). In this case, the temperature transfer along the x-axis is faster than the z-axis (see Fig. S3b, ESI). However, while the incident light (z-axis) is perpendicular to the PDMS-rich layer, the heat is concentrated at a small area due to the poor heat transport properties of the PDMS and the strong local heating of finely dispersed LM droplets in the PDMS rich layer, temperatures higher than 150 °C were measured by the IR camera while smoke developed and the sample was irreversibly destroyed in a few seconds. Therefore, the heat transfer real-time recording in this case can only be shown for a few seconds, as shown in Fig. 4d and Movie S5 (ESI).


image file: c9tc03222c-f4.tif
Fig. 4 Anisotropic thermal conductivity of the composites. (a) Thermograms (side view) of a quintuple layered composite during cooling for 45 seconds, clearly showing the layered structure of the composite. (b–d) Heat transport during localized heating with a laser: (b) laser irradiation parallel to the plane of layers, directed at the cross-section, (c) laser irradiation directed on the gallium-rich layer perpendicular to the layer planes and (d) laser irradiation directed on the PDMS-rich layer perpendicular to the layer planes.

Interestingly, by making use of the solid–liquid phase transition of gallium (melting temperature, Tm = 29.5 °C), the ability for supercooling down to a temperature of −28 °C (in this experiment −11.7 °C, see Fig. S5, ESI) and the change in mechanical properties due to this phase transition, the composite can be employed as a shape morphing material. Fig. 5a illustrates schematically the strategy for shape morphing of the composite. The composite is stretched (40% tensile strain) while the gallium in the composite is in its liquid form. Still under tensile strain, the gallium in the composite is solidified by freezing with liquid nitrogen. After the release of the applied stress at room temperature (RT), three dimensional folded structures are formed, owing to the mechanical mismatch between the frozen LM in the LM-rich layer and the PDMS-rich layer. Indeed, by changing the vol% of gallium in the composite not only can the mechanical properties be altered but also the degree of deformation (bending angle) can be altered, as shown in Fig. S4 (ESI). By lowering the vol% of gallium from 34.4% to 14.9%, the bending angle could be increased from 140 to 265°. However, a good compromise between bending angle and mechanical stability of the shape morphing material is 34.4 vol%, and therefore, all subsequently shown shape morphing experiments were executed with this vol%. Importantly, the folded structures can be relaxed to their original shape by heating with the aid of a hot plate above the melting temperature of the LM droplets (Fig. 5b and Movie S6–S9, ESI). The reversibility of the shape morphing can be ascribed to two effects. Firstly, during pre-stretching, the LM droplets are deformed (elongated, see Fig. S4, ESI). This elongation is fixed during freezing of the gallium. After melting of gallium, the high surface tension of gallium affords beading up, thus relaxing the composite. Secondly, during solid–liquid phase transition of gallium in the composite, the mechanical properties of the composite decline drastically. These lowered mechanical properties together with the surface tension driving force allow for the reversal of shape morphing.


image file: c9tc03222c-f5.tif
Fig. 5 Laser (heat)-induced shape morphing. (a) Schematic illustration of shape morphing enabled by pre-stretching and change of the mechanical properties of the gallium-rich layer by freezing (liquid–solid phase transition) of gallium, and release at room temperature (RT). (b) Reversible shape morphing of diverse shapes, such as leaf, cross, blossom and hand. (c) Unfolding of gallium/PDMS composites by locally heating with a red laser.

The reversal of shape morphing can also be triggered by localized heating with light (laser), which is shown in Fig. 5c and Movies S10 and S11 (ESI) for two samples. A circular deformed composite is straightened by light-induced shape reversal. Here, the shape reversal begins at and near the incident light and with time the whole composite strip is straightened due to the excellent heat transport of the gallium-rich layer. Similarly, the second-closed hand shaped-composite can be straightened (opened) by successively irradiating single fingers with light and thus straightening them.

Not only limited to mechanical mismatch for temperature and light-triggered shape morphing, the composite can also be applied for reconfigured flexible electronics due to the anisotropy in electrical conductivity. As a proof-of-concept, we fabricated a temperature and light-triggered shape morphing conductor for the control of a circuit with three light-emitting-diode (LED) lamps connected in parallel. A schematic representation of such an experiment is shown in Fig. 6a, illustrating the possibility to switch ON one specific LED remotely controlled by light-induced shape morphing of a composite strip. Fig. 6b–f and Movie S12 (ESI) show the aforementioned circuit before, during and after light-induced shape morphing of all three “frozen” composite strips successively, signifying the reliability of the light and temperature triggered switches. It has to be mentioned that switching with light is executed with a time delay of around 60 seconds, as shown in the IV curve in Fig. 6g. However, this circuit might be fabricated by direct heating, i.e. with a heating plate (see Fig. S6 and Movie S13, ESI).


image file: c9tc03222c-f6.tif
Fig. 6 Laser-triggered shape morphing for reconfigured flexible electronics. (a) Schematic illustration showing the operation of a circuit by light-induced shape morphing. (b–f) Three LED lamps switched by localized heating with a laser. (g) It diagram of the circuit in (b–f) during switching on three LEDs in succession.

In conclusion, we have developed a simple and versatile strategy, so-called “enrichment by sedimentation” for the fabrication of shape morphing elastomers with anisotropic mechanical, electrical and heat transport properties. We validated this strategy through the fabrication of gallium–PDMS shape morphing materials with three-dimensional shapes. The as-made composite exhibited excellent shape morphing behavior, which was tunable by the vol% of employed gallium. Furthermore, the shape morphing was reversible by external triggers, such as heating and light, due to a solid–liquid phase transition of the gallium. Despite the proof-of-concept demonstration of conductive shape morphing materials for light-controlled circuits, we envision that the “enrichment by sedimentation” method is a simple, reliable and universal strategy for fabricating shape morphing materials with highly-defined, mechanical, thermal, and electrical anisotropy. This as-made anisotropic LM–elastomer composite may be employed for light-triggered switches or for protection against overheating, given that defined metals/alloys can tune the temperature window (melting and freezing temperature).

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We acknowledge the National Natural Science Foundation of China (21674064), Shenzhen Science and Technology Foundation (KQJSCX20170727100240033) for financial support. Y. Z. Chen acknowledges Special Funds for the Cultivation of Guangdong College Students' Scientific and Technological Innovation (pdjhb0434) and Graduate Innovation Development Project (PIDFP-ZR2018003).

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Footnote

Electronic supplementary information (ESI) available: Experimental details and characterization of mechanical properties of the LM–elastomer. See DOI: 10.1039/c9tc03222c

This journal is © The Royal Society of Chemistry 2019