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• W3037259968 abstract "•An anisotropic conductive elastomer with both rigid and soft fillers was created•Magnetically aligning the ferromagnetic particles imparts anisotropy to composites•The composite exhibits opposite piezoconductive effects along different directions•Potentially valuable in developing future flexible sensors and soft electronics Elastic composites with conductive fillers combine the advantages of elastomers and conductors, among which anisotropic composites have aligned fillers that enhance properties in certain directions. Here, we created an anisotropic conductive composite filled with magnetic microparticles and liquid metal microdroplets. By magnetically aligning the particles, the composite becomes thousands of times more conductive. More importantly, this composite changes its conductivity in response to deformation in a remarkable way. Specifically, it exhibits both piezoconductive and piezoresistive effects within the same sample, depending on the direction of measurement relative to the direction of particle alignment. This unique property makes it possible to tune the degree of anisotropy with a large dynamic range simply by straining the material (compressing or elongating it). This material may find applications in smart sensors, flexible electronics, wearable devices, and beyond. Anisotropic elastic composites—that is, elastomers containing aligned fillers—often have enhanced properties in the direction of alignment. Depending on the fillers, these composites can have desirable electrical, thermal, or mechanical properties. Here, a silicone composite filled with both solid magnetic metal microparticles and liquid metal microdroplets has been developed. Aligning the solid particles within a magnetic field during curing imparts anisotropy in several properties. Thus, the composite is called an anisotropic liquid metal-filled magnetorheological elastomer (ALMMRE). Compared with isotropic liquid metal-filled composites, the conductivity of the ALMMRE is significantly enhanced in all directions. The ALMMRE also exhibits anisotropic piezoconductivity and a significantly enhanced electrical anisotropy under mechanical deformation; these properties are not observed in conventional anisotropic composites. The ALMMRE also shows anisotropic mechanical, thermal, and magnetic properties and demonstrates its several proof-of-concept applications. The sensitivity of the ALMMRE's properties to strain may help advance future flexible sensors and soft electronics. Anisotropic elastic composites—that is, elastomers containing aligned fillers—often have enhanced properties in the direction of alignment. Depending on the fillers, these composites can have desirable electrical, thermal, or mechanical properties. Here, a silicone composite filled with both solid magnetic metal microparticles and liquid metal microdroplets has been developed. Aligning the solid particles within a magnetic field during curing imparts anisotropy in several properties. Thus, the composite is called an anisotropic liquid metal-filled magnetorheological elastomer (ALMMRE). Compared with isotropic liquid metal-filled composites, the conductivity of the ALMMRE is significantly enhanced in all directions. The ALMMRE also exhibits anisotropic piezoconductivity and a significantly enhanced electrical anisotropy under mechanical deformation; these properties are not observed in conventional anisotropic composites. The ALMMRE also shows anisotropic mechanical, thermal, and magnetic properties and demonstrates its several proof-of-concept applications. The sensitivity of the ALMMRE's properties to strain may help advance future flexible sensors and soft electronics. Anisotropic composites are materials that exhibit directionally dependent physical and chemical properties. Among these, anisotropic conductive composites (ACCs) are used widely as high-performance electrodes,1Billaud J. Bouville F. Magrini T. Villevieille C. Studart A.R. Magnetically aligned graphite electrodes for high-rate performance Li-ion batteries.Nat. Energy. 2016; 1: 16097Crossref Scopus (348) Google Scholar conductive adhesives,2Yim M.J. Paik K.W. Recent advances on anisotropic conductive adhesives (ACAs) for flat panel displays and semiconductor packaging applications.Int. J. Adhes. Adhes. 2006; 26: 304-313Crossref Scopus (171) Google Scholar motion sensors,3Chun J. Kang N.-R. Kim J.-Y. Noh M.-S. Kang C.-Y. Choi D. Kim S.-W. Wang Z.L. Baik J.M. Highly anisotropic power generation in piezoelectric hemispheres composed stretchable composite film for self-powered motion sensor.Nano Energy. 2015; 11: 1-10Crossref Scopus (104) Google Scholar and electromagnetic shielding materials.4Zeng Z. Jin H. Chen M. Li W. Zhou L. Zhang Z. Lightweight and anisotropic porous MWCNT/WPU composites for ultrahigh performance electromagnetic interference shielding.Adv. Funct. Mater. 2016; 26: 303-310Crossref Scopus (566) Google Scholar ACCs are mostly composed of an insulating elastic matrix and conductive fillers. Aligning the conductive fillers significantly enhances physical properties of the composite in the alignment direction, such as electrical conductivity, thermal conductivity, and mechanical strength.5Erb R.M. Libanori R. Rothfuchs N. Studart A.R. Composites reinforced in three dimensions by using low magnetic fields.Science. 2012; 335: 199-204Crossref PubMed Scopus (503) Google Scholar,6Liu P. Jin Z. Katsukis G. Drahushuk L.W. Shimizu S. Shih C.-J. Wetzel E.D. Taggart-Scarff J.K. Qing B. Van Vliet K.J. Layered and scrolled nanocomposites with aligned semi-infinite graphene inclusions at the platelet limit.Science. 2016; 353: 364-367Crossref PubMed Scopus (107) Google Scholar The shape, size, volume fraction, and other physical properties of the fillers, as well as the properties of the matrix, have a significant effect on the anisotropy and functionality of the composite.7Sancaktar E. Dilsiz N. Anisotropic alignment of nickel particles in a magnetic field for electronically conductive adhesives applications.J. Adhes. Sci. Technol. 1997; 11: 155-166Crossref Scopus (50) Google Scholar, 8Ajayan P.M. Tour J.M. Materials science: nanotube composites.Nature. 2007; 447: 1066Crossref PubMed Scopus (743) Google Scholar, 9Ladani R.B. Wu S. Kinloch A.J. Ghorbani K. Zhang J. Mouritz A.P. Wang C.H. Improving the toughness and electrical conductivity of epoxy nanocomposites by using aligned carbon nanofibres.Compos. Sci. Technol. 2015; 117: 146-158Crossref Scopus (128) Google Scholar The most critical step in the fabrication of anisotropic materials is the directional alignment of the fillers in the matrix. A common approach is to align the filler in a liquid matrix and then solidify the composite. One of the most effective methods is to apply an external electric field in the direction of alignment.10Goh P. Ismail A. Ng B. Directional alignment of carbon nanotubes in polymer matrices: contemporary approaches and future advances.Compos. A. Appl. Sci. Manuf. 2014; 56: 103-126Crossref Scopus (177) Google Scholar This method is often used to align conductive fibers, such as carbon nanotubes. A DC electric field can polarize fillers to generate torque that aligns the fibers with the electric field. In an AC electric field, the conductive fibers form a more uniform anisotropic structure based on the dielectrophoretic effect.11Martin C. Sandler J. Windle A. Schwarz M.-K. Bauhofer W. Schulte K. Shaffer M. Electric field-induced aligned multi-wall carbon nanotube networks in epoxy composites.Polymer. 2005; 46: 877-886Crossref Scopus (448) Google Scholar Alternatively, magnetic fields can effectively orient paramagnetic fibers or granular fillers with high magnetic permeabilities.12Steinert B.W. Dean D.R. Magnetic field alignment and electrical properties of solution cast PET-carbon nanotube composite films.Polymer. 2009; 50: 898-904Crossref Scopus (120) Google Scholar,13Landa R.A. Soledad Antonel P. Ruiz M.M. Perez O.E. Butera A. Jorge G. Oliveira C.L. Negri R.M. Magnetic and elastic anisotropy in magnetorheological elastomers using nickel-based nanoparticles and nanochains.J. Appl. Phys. 2013; 114: 213912Crossref Scopus (34) Google Scholar Unlike electric fields, magnetic fields avoid large voltages and direct contact with the composite. The alignment effect is highly dependent on the magnetic permeability of the fillers, the viscosity of the uncured composite, and the strength of the magnetic field.14Ciambella J. Stanier D.C. Rahatekar S.S. Magnetic alignment of short carbon fibres in curing composites.Compos. B. Eng. 2017; 109: 129-137Crossref Scopus (39) Google Scholar Moreover, mechanical methods such as the “hot stretching” technique are also used in the preparation of fibrous filler-based anisotropic composites.15Jin L. Bower C. Zhou O. Alignment of carbon nanotubes in a polymer matrix by mechanical stretching.Appl. Phys. Lett. 1998; 73: 1197-1199Crossref Scopus (697) Google Scholar In addition to these conventional techniques, several new methods for manufacturing anisotropic materials also emerged in recent years, such as magnetically assisted slip casting,16Le Ferrand H. Bouville F. Niebel T.P. Studart A.R. Magnetically assisted slip casting of bioinspired heterogeneous composites.Nat. Mater. 2015; 14: 1172Crossref PubMed Scopus (254) Google Scholar electrically assisted three-dimensional (3D) printing,17Yang Y. Li X. Chu M. Sun H. Jin J. Yu K. Wang Q. Zhou Q. Chen Y. Electrically assisted 3D printing of nacre-inspired structures with self-sensing capability.Sci. Adv. 2019; 5: eaau9490Crossref PubMed Scopus (167) Google Scholar layer-by-layer sedimentation,18Zhu L. Chen Y. Shang W. Handschuh-Wang S. Zhou X. Gan T. Wu Q. Liu Y. Zhou X. Anisotropic liquid metal-elastomer composites.J. Mater. Chem. C. 2019; 7: 10166-10172Crossref Google Scholar and oriented assembly.19Nai J. Guan B.Y. Yu L. Lou X.W.D. Oriented assembly of anisotropic nanoparticles into frame-like superstructures.Sci. Adv. 2017; 3: e1700732Crossref PubMed Scopus (123) Google Scholar Most of the current anisotropic composites use carbon-based fillers, including carbon black, graphene, carbon fiber, and carbon nanotubes.20Zhang Y.-C. Dai K. Tang J.-H. Ji X. Li Z.-M. Anisotropically conductive polymer composites with a selective distribution of carbon black in an in situ microfibrillar reinforced blend.Mater. Lett. 2010; 64: 1430-1432Crossref Scopus (41) Google Scholar, 21Stankovich S. Dikin D.A. Dommett G.H. Kohlhaas K.M. Zimney E.J. Stach E.A. Piner R.D. Nguyen S.T. Ruoff R.S. Graphene-based composite materials.Nature. 2006; 442: 282-286Crossref PubMed Scopus (11445) Google Scholar, 22Gupta P. Rajput M. Singla N. Kumar V. Lahiri D. Electric field and current assisted alignment of CNT inside polymer matrix and its effects on electrical and mechanical properties.Polymer. 2016; 89: 119-127Crossref Scopus (73) Google Scholar Carbon-based fillers, especially carbon nanotubes, can enhance the electrical and thermal conductivities and mechanical strength of the composite in the fillers alignment direction.22Gupta P. Rajput M. Singla N. Kumar V. Lahiri D. Electric field and current assisted alignment of CNT inside polymer matrix and its effects on electrical and mechanical properties.Polymer. 2016; 89: 119-127Crossref Scopus (73) Google Scholar,23Gao J. He Y. Gong X. Effect of electric field induced alignment and dispersion of functionalized carbon nanotubes on properties of natural rubber.Results Phys. 2018; 9: 493-499Crossref Scopus (36) Google Scholar However, due to their low magnetic permeability, they require enormous magnetic fields24Kimura T. Ago H. Tobita M. Ohshima S. Kyotani M. Yumura M. Polymer composites of carbon nanotubes aligned by a magnetic field.Adv. Mater. 2002; 14: 1380-1383Crossref Scopus (422) Google Scholar or additional alignment methods.25Wu S. Zhang J. Ladani R.B. Ghorbani K. Mouritz A.P. Kinloch A.J. Wang C.H. A novel route for tethering graphene with iron oxide and its magnetic field alignment in polymer nanocomposites.Polymer. 2016; 97: 273-284Crossref Scopus (35) Google Scholar The conductivity of carbon-based ACCs is also generally lower than that of metal-based ACCs and less sensitive to mechanical deformation.21Stankovich S. Dikin D.A. Dommett G.H. Kohlhaas K.M. Zimney E.J. Stach E.A. Piner R.D. Nguyen S.T. Ruoff R.S. Graphene-based composite materials.Nature. 2006; 442: 282-286Crossref PubMed Scopus (11445) Google Scholar In addition, some ACCs use metal fillers with a better conductivity or magnetic permeability, such as gold microwires or iron (Fe) microparticles.26Zhang H. Hussain I. Brust M. Butler M.F. Rannard S.P. Cooper A.I. Aligned two- and three-dimensional structures by directional freezing of polymers and nanoparticles.Nat. Mater. 2005; 4: 787Crossref PubMed Scopus (661) Google Scholar,27Chen L. Gong X. Li W. Microstructures and viscoelastic properties of anisotropic magnetorheological elastomers.Smart Mater. Struct. 2007; 16: 2645Crossref Scopus (201) Google Scholar An ACC filled with ferromagnetic particles is also referred to as an anisotropic magnetorheological elastomer (AMRE). The AMRE forms regular particle chains in the polymer matrix by solidifying the composite within a magnetic field, thus achieving electrical and mechanical anisotropy.28Carlson J.D. Jolly M.R. MR fluid, foam and elastomer devices.Mechatronics. 2000; 10: 555-569Crossref Scopus (1075) Google Scholar In this work, we created and studied an anisotropic liquid metal-filled magnetorheological elastomer (ALMMRE) using the magnetic field alignment method. Different from traditional ACCs filled with only single solid fillers, the ALMMRE uses both solid metal magnetic microparticles and liquid metal eutectic gallium-indium (EGaIn, 75% gallium [Ga] and 25% indium) microdroplets. Owing to their high deformability and conductivity, EGaIn microdroplets find wide uses in electrical conductive elastomer,29Yun G. Tang S.-Y. Sun S. Yuan D. Zhao Q. Deng L. Yan S. Du H. Dickey M.D. Li W. Liquid metal-filled magnetorheological elastomer with positive piezoconductivity.Nat. Commun. 2019; 10: 1300Crossref PubMed Scopus (190) Google Scholar, 30Bartlett M.D. Fassler A. Kazem N. Markvicka E.J. Mandal P. Majidi C. Stretchable, high-k dielectric elastomers through liquid-metal inclusions.Adv. Mater. 2016; 28: 3726-3731Crossref PubMed Scopus (249) Google Scholar, 31Markvicka E.J. Bartlett M.D. Huang X. Majidi C. An autonomously electrically self-healing liquid metal–elastomer composite for robust soft-matter robotics and electronics.Nat. Mater. 2018; 17: 618-624Crossref PubMed Scopus (527) Google Scholar, 32Wang J. Cai G. Li S. Gao D. Xiong J. Lee P.S. 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Petersen P. Mitchell A. Khoshmanesh K. Continuous transfer of liquid metal droplets across a fluid–fluid interface within an integrated microfluidic chip.Lab Chip. 2015; 15: 2476-2485Crossref PubMed Google Scholar, 45Tang S.Y. Ayan B. Nama N. Bian Y. Lata J.P. Guo X. Huang T.J. On-chip production of size-controllable liquid metal microdroplets using acoustic waves.Small. 2016; 12: 3861-3869Crossref PubMed Scopus (65) Google Scholar, 46Khoshmanesh K. Tang S.-Y. Zhu J.Y. Schaefer S. Mitchell A. Kalantar-Zadeh K. Dickey M.D. Liquid metal enabled microfluidics.Lab Chip. 2017; 17: 974-993Crossref PubMed Google Scholar, 47Zhu L. Wang B. Handschuh-Wang S. Zhou X. Liquid metal–based soft microfluidics.Small. 2019; 16: 1903841Crossref PubMed Scopus (101) Google Scholar Although the liquid metal has been used in anisotropic thermally conductive materials in recent research,48Bartlett M.D. Kazem N. Powell-Palm M.J. Huang X. Sun W. Malen J.A. Majidi C. High thermal conductivity in soft elastomers with elongated liquid metal inclusions.Proc. Natl. Acad. Sci. U S A. 2017; 114: 2143-2148Crossref PubMed Scopus (353) Google Scholar its potential in developing anisotropic electrically conductive composites has not yet been fully investigated. The conductivity of most previously reported liquid metal conductive elastomers is very low in the relaxed state. Although the metal is highly conductive, elastomer tends to fill the spaces between liquid metal particles, rendering it insulating. In this study, we solved this problem by magnetically aligning co-filler particles with the liquid metal microdroplets (note that the liquid metal does not respond to the magnetic field, only the co-filler). Our previous work on a liquid metal-filled magnetorheological elastomer (LMMRE) did not use external magnetic field alignment. The LMMRE becomes orders of magnitude more conductive under any small mechanical loadings (i.e., it has an increased conductivity under tensile strain and is therefore unique relative to most conductive composites, which shows an increased resistivity).29Yun G. Tang S.-Y. Sun S. Yuan D. Zhao Q. Deng L. Yan S. Du H. Dickey M.D. Li W. Liquid metal-filled magnetorheological elastomer with positive piezoconductivity.Nat. Commun. 2019; 10: 1300Crossref PubMed Scopus (190) Google Scholar Here, we show that magnetically aligning the particles prior to curing these composites produces different results, namely, the conductivity of the ALMMRE—in the absence of deformation—improves significantly by several orders of magnitude in all directions compared with the isotropic LMMRE with the same composition. In addition, previous studies on ACCs lack a rigorous analysis of the strain response of their electrical properties, which is important for their applications in developing sensors and flexible devices. Here, we not only analyzed the resistivity changes of the ALMMRE under mechanical deformation but also explored its unique and anisotropic piezoconductive properties. Unlike conventional composites, its electrical anisotropy changes significantly with strain. Furthermore, we discovered that the mechanical, magnetic, and thermal properties of the ALMMRE also exhibit significant anisotropy. Harnessing the unique properties of ALMMRE, we demonstrated several applications such as an exponentially adjustable rheostat, soft tactile logic devices, and an improved intelligent heating device. Before proceeding with results, we summarize the terminologies used in this article. (1) Negative piezoconductivity: most conventional conductive composites exhibit negative piezoconductivity, whereby their electrical conductivity decreases upon stretching (positive strain) and increases upon compression (negative strain). Negative piezoconductivity often occurs on the basis of changes in the spacing between conductive particles in the matrix. (2) Positive piezoconductivity: if conductive composites exhibit a positive piezoconductive effect, their electrical conductivity increases upon stretching and decreases upon compression. Figure 1A shows the fabrication schematic and 3D microstructure of the ALMMRE. This composite consists of three raw materials: polydimethylsiloxane (PDMS), carbonyl Fe ferromagnetic microparticles (diameter 2–5 μm), and EGaIn. They were firstly mixed with a high-speed electric stirrer and vacuumed to remove air bubbles, then poured into a mold. Next, we placed the mold in a uniform magnetic field (1 T, see Experimental Procedures for details of the process). Owing to its high magnetic permeability, the Fe particles align in the direction of the magnetic field. Thereafter, the PDMS was cured at 70°C. In this way, the Fe particles can form stable chain structures in the matrix. Figure 1B shows the optical images of the ALMMRE produced using PDMS (1 g, 23.5 wt %, 68.7 vol %, PDMS/curing agent ratio 7:1), carbonyl Fe microparticles (2 g, 47.1 wt %, 17.6 vol %), and EGaIn (1.25 g, 29.4 wt %, 13.7 vol %). Optical images of the AMRE prepared without EGaIn microdroplets (Fe/PDMS mass ratio 1:1, volume ratio 0.128:1) are shown in Figure 1C to clearly demonstrate the alignment of the Fe particles in the matrix. From the scanning electron microscopy (SEM) images in Figure 1D, we can see that the Fe particles align in a chain structure in the horizontal direction. However, these chains are not completely parallel but have some intersections. In addition, during mixing, EGaIn breaks into microdroplets (10–50 μm in diameter) and embeds between adjacent Fe particle chains. These intersections and EGaIn microdroplets connect Fe particle chains and form a conductive network in the PDMS matrix. Energy-dispersive X-ray spectroscopy (EDS) analysis of ALMMRE for the red-dashed area in Figure 1D is given in Figures 1E and 1F. The distribution of Fe particles, EGaIn, and PDMS is represented by their characteristic elements of Fe, Ga, and silicon (Si), respectively. The network structure formed by Fe particle chains and EGaIn droplets can be seen more clearly in Figure 1F. The Fe particle chain structures in the matrix impart anisotropy to the ALMMRE. Along the direction of the aligned Fe particle chains (IPCs), we expect the composite to have high electrical, magnetic, and thermal conductivities. We also reason that these conductivities will be significantly reduced perpendicular to the IPC alignment direction. In a Cartesian coordinate system, assuming that the IPC alignment direction is along the x axis, the ALMMRE is isotropic in the y-z plane, i.e., its physical properties are identical in any direction perpendicular to the IPC alignment direction. We studied variables that may affect the electrical properties of the composite and found that the main factors that affect the anisotropic properties of the ALMMRE are Fe particles and EGaIn content. Other factors such as EGaIn droplet size, curing agent/PDMS ratio, and curing temperature have the same effect on the resistivity of the ALMMRE parallel and perpendicular to the alignment direction, i.e., these factors should not affect the anisotropy of the ALMMRE. Besides, their effects on the physical properties of the ALMMRE are expected to have a similar trend to the case of isotropic LMMRE investigated in our previous work.29Yun G. Tang S.-Y. Sun S. Yuan D. Zhao Q. Deng L. Yan S. Du H. Dickey M.D. Li W. Liquid metal-filled magnetorheological elastomer with positive piezoconductivity.Nat. Commun. 2019; 10: 1300Crossref PubMed Scopus (190) Google Scholar Thus, this work mainly focuses on investigating the anisotropic properties of ALMMRE with different Fe particle contents. The ALMMRE in this work contains PDMS (1:7 mass ratio of curing agent to elastomer), Fe microparticles (2–5 μm), and EGaIn microdroplets (diameter of ∼20 μm), and the EGaIn/PDMS mass ratio is set at 1.25:1 (volume ratio 0.2:1). The ALMMRE samples have three different Fe/PDMS mass ratios of 1.5:1, 2:1, and 3:1 (volume ratio 0.192:1, 0.256:1, and 0.385:1), and are denoted as ALMMRE1.5, ALMMRE2, and ALMMRE3, respectively. To analyze the electrical property of the ALMMRE, we fabricated a set of ALMMRE2 samples (6 × 6 × 6 mm, Fe/PDMS mass ratio 2:1). We measured the resistivity horizontally and also applied compressive and tensile strain horizontally. The angle between the measurement direction (horizontal direction) and the magnetic alignment direction was defined as θ, as illustrated in Figure 2A. When θ is 0°, the measurement direction is parallel to the alignment direction. We obtained resistivity-strain curves at different θ (Figure 2B). Along the alignment direction (θ = 0°), the resistivity of the ALMMRE2 in the relaxed state is 1.34 kΩ∙m. As the θ increases to 90° (perpendicular to the alignment direction), the resistivity gradually increases to 13.2 kΩ∙m, demonstrating the anisotropic resistivity of the ALMMRE2 at zero strain. We observed that not only the initial resistivity but also the response of resistivity to deformation differs with respect to θ. When θ is 0°, the resistivity of the ALMMRE2 drops drastically by ∼1,000-fold to only 1.1 Ω∙m when applying a 10% compressive strain. However, with increasing θ, the slope of the resistivity-strain curve gradually becomes less steep. At the critical angle of ∼52°, the resistivity remains constant at ∼4.1 kΩ∙m when applying a compressive strain less than 12%. A conductive composite that does not change resistivity during deformation is unusual and of interest for circuits that do not change resistance with deformation. Once θ exceeds a critical angle, the resistivity of the ALMMRE2 increases when compressed horizontally at a low strain, exhibiting a unique positive piezoconductive effect. In contrast, compression of conventional composites moves the conductive particles closer together and thereby increases conductivity. The resistivity of the ALMMRE2 reaches a maximum at the critical strain and then decreases. We can also see from Figure 2B that the ALMMRE2 is more sensitive to mechanical deformation along the IPC alignment direction (θ = 0°). This is characterized by calculating the piezoconductive coefficient (PCC) of the ALMMRE2 at different directions (see Figure S1A). The PCC can be calculated as: PCC = Δσ/σ0ε, where Δσ and σ0 are the change in conductivity and unstrained conductivity, respectively, and ε is the strain of the composite. In summary, in addition to the anisotropic resistivity, the ALMMRE also exhibits anisotropic positive and negative piezoconductivity depending on θ. It should be noted that the LMMRE2 (isotropic, Fe/PDMS mass ratio 2:1), which is not cured in a magnetic field, is a poor conductor under any strain (resistivity exceeds 1 MΩ∙m). This indicates that the alignment of the Fe particles within the composite can enhance its electrical conductivity in both directions parallel and perpendicular to the alignment. The resistivity-strain curves of the LMMRE3 and ALMMRE3 (Fe/PDMS mass ratio 3:1) also support this hypothesis (see Figure S1B). In comparison with the LMMRE with a uniform distribution of Fe particles, the external field aligns the Fe particles within the ALMMRE during the curing process, leading to contact of the particles along the IPC alignment direction. This alignment increases the conductivity of the ALMMRE in this direction by nearly three orders of magnitude. In addition, adjacent IPCs can form conductive contacts through EGaIn droplets. This contact forms a conductive network perpendicular to the IPC alignment direction to reduce the overall resistivity (see the EDS image of Figure 1F). As a control experiment, we further characterized the anisotropic electrical properties of the composites prepared with and without EGaIn microdroplets. We found that the addition of EGaIn microdroplets to the composite is necessary for reducing the overall resistivity and altering the critical strains when the composites reach their maximum resistivity (see details in Figures S1C–S1E). Interestingly, the ALMMRE exhibits negative piezoconductivity at θ = 0° yet positive piezoconductivity at θ = 90°. Figures 2C and 2D shows the resistivity-strain curves of three ALMMRE samples along (θ = 0°) and perpendicular to (θ = 90°) the IPC alignment direction. For convenience, we reiterate that the ALMMRE samples with three different Fe/PDMS" @default.
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• W3037259968 date "2020-09-01" @default.
• W3037259968 modified "2023-10-16" @default.
• W3037259968 title "Liquid Metal Composites with Anisotropic and Unconventional Piezoconductivity" @default.
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