Junfeng Zhaoa,
Xu Biab and
Han Dai*ab
aLaboratory of Advanced Light Alloy Materials and Devices, Yantai Nanshan University, Longkou 265713, China. E-mail: daihan1985@189.cn
bYulong Petrochemical Co., Ltd., Longkou 265700, China
First published on 3rd October 2022
Novel non-wetted/wetted floatable polyethylene/Cu and porous-Ni/Cu (P–Ni/Cu) coatings have been designed and fabricated for anti-combination of gallium-based liquid metal alloy (LM) marbles in solutions. Both coated LM pairs show strong anti-combination resistances even under a large extrusion ratio. Additionally, both coatings also show strong bonding forces with LMs and are floatable on the surfaces of LMs. Driven by electric or magnetic fields, floatable polyethylene/Cu or P–Ni/Cu coatings on LM surfaces are guided by these external fields, and then restore the original arrangement by the surface tension of the LMs and buoyancy of the coatings themselves after removing external fields, by which these coated LM marble or LM marble pair exhibit the revisable conductivity transitions and magnetic driven motion applications. This work should present a new way for the clustering and functional application of LMs in solutions.
The organic coating method is very effective and widely used to avoid agglomeration of nano-scale gallium-based LMs particles.7 However, at the macro-scale, micron-thick organic coatings generally make LM marbles become insulators.8 Although highly conductive coatings like graphene can effectively improve the conductivity of LM marbles, these organic or graphene coatings will fall off the surface of LM marbles rapidly when they are submerged in the acid or alkaline solutions due to the dissolution of the viscous oxide layer on LM marbles.9 As a result, both the conductivity and the bonding stability to LMs of the anti-combination coatings should also be considered in the solution.
As reported, Cu, Ag, Ge et al. can be highly wetted by gallium-based LMs in acid or alkaline solutions.10,11 Especially, Cu can easily react with liquid Ga as CuGa2, which greatly enhances the bonding between Cu and LM alloys.12–14 However, as reported, such intermetallic wetting between Cu, Ag and gallium-based LMs will inevitably cause irreversible internalization,15 which strictly limits their usage as coating materials for LMs.
Inspired from amphiphilic molecules used in ordinary detergent, herein novel polyethylene/Cu and porous Ni/Cu non-wettable/wettable structures have been designed and fabricated for anti-combination of the gallium-based LM marbles in solutions. The inner Cu layer (wettable) provides bonding forces with the LMs, while the outer layer of polyethylene (non-wettable) is to achieve the anti-combination function. It is found that both pairs of LMs with polyethylene/Cu or P–Ni/Cu coatings show good anti-combination properties even at a large extrusion ratio. What's more, LM marbles coated with polyethylene/Cu or P–Ni/Cu also realized other functions, such as reversible conductivity transformation and motion driven by electric or magnetic fields, due to their highly floatable properties. Owing to the reasons above, this work realizes the anti-combination of the gallium-based LM marbles in solution and shows great potential applications in various gallium-based LM devices by changing the appropriate functional outer layers.
Fig. 2 shows the combination resistance of polyethylene/Cu, P–Ni/Cu coatings with various diameters. A scheme and two optical images of combination resistance tests of LMs marble pairs with polyethylene/Cu, P–Ni/Cu coatings are presented in Fig. 2(a)–(c). The compression rate here is kept at about 1 mm s−1 for each test. The maximum compression sets before the combination of coated LMs marble pairs are presented in Fig. 2(d). For polyethylene/Cu coatings with a diameter of 0.3 cm/0.2 cm, the LMs marble pairs can tolerate nearly 20% extrusion ratio without combination. Since combination resistance is mainly due to physical isolation caused by external polyethylene layers, stronger combination resistance is obtained by polyethylene layers with larger diameters, for they have more redundant area to protect the marbles. Comparatively, the whole combination resistance of P–Ni/Cu coatings is stronger (with a minimum tolerance of nearly 40% deformation). P–Ni/Cu coatings feature in the electrochemical reaction enhanced combination resistance in HCl solution instead of physical isolation of polyethylene/Cu coatings. It is well known that the standard electrode potential of Ga (−0.55 V), being reduced after alloying with In and Sn, is much lower than that of Ni (−0.25 V) and Cu (0.34 V).16,17 Hydrogen bubbles generated by the electrochemical reactions (Fig. 1(e) and 2(c)) will converge on and surround the P–Ni/Cu coatings, and hence inhibiting the combination of LMs marbles. P–Ni/Cu coatings with smaller diameters generate more uniform bubbles and bring stronger combination resistance. Above all, both polyethylene/Cu and P–Ni/Cu coatings show strong combination resistance for LMs marble pairs in solution.
Obviously, the strength of the bonding force between coatings and LMs is another important parameter for their quality evaluation. A lifting device as shown in Fig. 3(a) has been used to measure the bonding force between coatings and LMs through recording the maximum lifting forces. Herein, only the bonding force of polyethylene/Cu has been presented in Fig. 3(b) due to the same bonding layer of P–Ni/Cu coatings. It can be clearly observed that the maximum lifting forces between polyethylene/Cu coatings with LMs are similar (about 0.05 N cm−2) in hydrochloric acid or NaOH solutions, but become lower in deionized water (around 0.03 N cm−2), which are calculated by the ratio of measured tensions F and contact area S of the coatings. The possible reason is that the bonding force between coatings and LMs is much higher than the surface tension of LMs. Through the lifting, the measured maximum forces are originally from the surface tension of LMs, and thus showing similar values no matter in hydrochloric acid or NaOH solutions. When LMs are submerged in deionized water, the instantaneously formed oxide layer will significantly reduce the surface tension of LMs, and then the maximum lifting forces will be greatly reduced accordingly. Furthermore, the serious deformation of LMs marbles, induced by the direct and vertical pulling of the polyethylene/Cu and P–Ni/Cu coatings and the LMs fully adhered to Cu surfaces (inset in Fig. 3(a), (c) and (d)), further proves the strong bonding force between coatings and LMs. Apparently, the high bonding force between the polyethylene/Cu, P–Ni/Cu and LMs is beneficial for its coating application in solution.
As above, the strong bonding force between the non-wettable/wettable coatings and LMs mainly originates from the surface tension of LMs. Owing to this, the bonding of these non-wettable/wettable coatings should be very small in the tangential direction, which makes the shifts of these coatings on the surface of LMs fairly easy. In other words, the non-wettable/wettable coatings can be considered as floating on the surface of LMs. Fig. 4(a) and (b) show that the polyethylene/Cu coatings are pushed opposite to the moving direction of LMs marbles by the 4 V electric field induced surface flow.18 When the electric field is removed, the polyethylene/Cu coatings are quickly restored to the cover of LMs marble by the surface tension of LMs and their own buoyancy. Because the polyethylene/Cu coatings are non-conductive, such reversible shifts of polyethylene/Cu coatings are expected to realize the electric field induced switching of the circuit in solution (Fig. S1†). In addition, the thermal conductivity of LMs changes from about 15 ± 5 W m−1 K to 30 ± 10 W m−1 K during the close/open of polyethylene/Cu coatings, which also shows their potential usage as changeable thermal conductivity materials. Apparently, the function above requires larger polyethylene/Cu coatings to obtain better performance. However, the electric field induced motion tests show that polyethylene/Cu coatings with larger diameters exhibit greater motion resistance, as shown in Fig. 4(c). Therefore, it is necessary to balance the motion abilities with the improvement of other functions of LMs. As shown in Fig. 4(d) and (e), the reversible shifts of P–Ni coatings on the surface of LMs marbles can be driven similarly by magnetic field and can be restored by the surface tension of LMs and buoyancy of porous Ni when the magnetic field is removed. The difference between the two is that, in the electric field, the coatings are driven to move by the LMs marbles, while in magnetic field (Fig. S2†), the LMs marbles are driven to move by the coatings. What's more, both coatings show high durability (Fig. S3†), which guaranteed their usage in the solutions. Above all, such non-wettable/wettable and floatable coating design concept provides a new design strategy for the functional applications of LMs marble based devices.
Apparently, this non-wetted/wetted floatable coating design efficiently avoids the combination between LMs marbles. And the functional outer layers realize the reversible conductivity transformation and magnetic motion of LMs marbles. In addition, from the perspective of the functional properties of the surface of LMs marbles themselves, such as the platinum catalysis,19 the interfacial bismuth precipitation on the surface of LMs,20 such eclectic or magnetic field induced reversible spreading and closing of the coatings should be beneficial for their processes control and protect internal functional materials in the non-working state.
Footnote |
† Electronic supplementary information (ESI) available. See https://doi.org/10.1039/d2ra04706c |
This journal is © The Royal Society of Chemistry 2022 |