Effect of liquid metal microstructure on properties and performance of soft composites

Abstract

<p>Soft multifunctional composites are enabling for soft and flexible materials in soft robotics, energy harvesting, actuation, and multifunctional devices. These applications require highly stretchable and conformal capabilities to maintain device operation while it undergoes any structural change. One emerging approach to create these composites is through the incorporation of liquid metal (LM) droplets such as eutectic gallium indium (EGaIn) in highly deformable elastomers. The microstructural aspects of LM droplets such as the size, shape, and particle-particle interaction (networks) in such systems is critical to their performance. However, in these soft composites techniques to control microstructures are lacking, their influence on properties is not well known, and how these enable diverse soft matter engineering applications depends on understanding their structure and ultimate properties. Here, we present a fabrication approach to create liquid metal-elastomer composites with independently controllable and highly tunable droplet size (100 $nm \leq D \leq 80$ ${\mu m}$) and volume loading ($0 \leq \phi \leq 80\%$).</p> <p>These materials display a high relative permittivity of 60 (16x the unfilled elastomer) in a composite with $\phi$ = 80\%, a low tan $\delta$ of 0.02, and a significant dependence on $\phi$ and minor dependence on $D$.</p> <p>Temperature response and stability are determined using dielectric spectroscopy through temperature and frequency sweeps and with DSC. These results demonstrate a wide temperature stability of the liquid metal phase (crystallizing $<$ -85 $^{\circ}$C for $D<$ 20 $\mu m$).</p> <p>Additionally, all composites are electrically insulating across a wide frequency (0.1 Hz - 10 MHz) and temperature (-70$^{\circ}$C to 100$^{\circ}$C) range even up to $\phi$ = 80\%.</p> <p>We highlight the benefit of LM microstructure control by creating all soft matter stretchable capacitive sensors with tunable sensitivity. These sensors are further integrated into a wearable sensing glove where we identify different objects during grasping motions.</p> <p>This surprising yet favorable insulating properties pose significant challenges for achieving high thermal conductivity in these material systems. Soft materials with high thermal conductivity are critical for flexible electronics, energy storage and transfer, and human-interface devices and robotics. We perform a systematic study of soft composites with solid, liquid, and solid-liquid multiphase metal fillers dispersed in elastomers that reveals key strategies to tune the thermal-mechanical response of soft materials. Experiments supported by thermodynamic and kinetic modeling demonstrate that multiphase systems quickly form intermetallics that solidify and degrade mechanical response with modest gains in thermal conductivity. In contrast, liquid metal inclusions provide benefits over solid and multiphase fillers as they can be loaded up to 80\% by volume with the composites being electrically insulating, soft ($<$ 1 MPa modulus), and highly thermally conductive ($k = 6.7 \pm 0.1 \; W\!\cdot m^{-1}\!\cdot K^{-1}$). The thermal-mechanical response of the composites is summarized and quantitative design maps are presented for soft, highly thermally conductive materials. This leads to soft materials with unique thermal-mechanical combinations, highlighted by a liquid metal composite with a high thermal conductivity of $ 11.0 \pm 0.5 \; W\!\cdot m^{-1}\!\cdot K^{-1}$ when strained.</p> <p>To create programmable microstructures in stress-free materials, we take advantage of the thermoplastic nature of the SIS matrix and the elongation of LM droplets in a strain field.</p> <p>This enables LM loadings up to 70\% by volume with prescribed particle aspect ratios and orientation, enabling control of microstructure throughout the bulk of the material.</p> <p>Through this microstructural control in soft composites we demonstrate a material which simultaneously achieves an unprecedented thermal conductivity as high as 13.0 Wm\textsuperscript{-1}K\textsuperscript{-1} ( $>$ 70 $\times$ increase over polymer matrix) with low modulus ($<$1.0 MPa) and high stretchability ($>$ 750 \% strain) in stress-free conditions. Such properties are required in applications that demand extreme mechanical flexibility with high thermal conductivity, which we demonstrate in soft electronics, wearable robotics, and electronics integrated into 3D printed materials.</p> <p>As highlighted above, soft-matter technologies have a pivotal role in emerging applications that require highly compliant and elastic materials. However, these technologies are largely composed of soft materials that are susceptible to damage and loss of functionality when exposed to real-world loading conditions. To address this critical challenge, we present a soft responsive material that, like natural nervous tissue, is able identify, compute, and signal damage in real-time. The soft composite material contains liquid metal droplets dispersed in an elastomer matrix that rupture when mechanical damage occurs (\textit{e.g.} compression, fracture, or puncture), creating electrically conductive pathways. The resulting change in local conductivity can be actively sensed and coupled with actuation, communication, and computation in a manner that presents new opportunities to identify damage, calculate severity, and respond to prevent failure within soft material systems. When placed on the surface of a soft, humanoid-like inflatable structure, the skin can detect puncture damage and control the operation of an embedded fan to prevent deflation.</p> <p>Finally, we explore a unique smart material system comprising of undercooled LM particles in elastomer. Recent developments in smart responsive composites have utilized various stimuli including heat, light, solvents, electricity, and magnetic fields to induce a change in material properties. We report a thermodynamically driven mechanically responsive composite, exploiting irreversible phase-transformation (relaxation) of metastable undercooled liquid metal core shell particle fillers. Thermal and mechanical analysis reveals that as the composite is deformed, the particles transform from individual liquid droplets to a solid metal network, resulting in a 300\% increase in Young’s modulus. In contrast to previous phase change materials, this dramatic change in stiffness occurs autonomously under deformation, is insensitive to environmental conditions, and does not require external energy sources such as heat, light, or electricity. We demonstrate the utility of this approach by transforming a flat, flexible composite strip into a rigid, 3D structure that is capable of supporting 50x its own weight. The ability for shape change and reconfiguration are further highlighted, indicating potential for multiple pathways to trigger or tune composite stiffness.</p> <p>To summarize, the research presented in this thesis determined the LM microstructure and it's relation to the bulk macroscopic properties and performance characteristics to enable programmable LM composites for soft matter systems. Creative fabrication avenues, characterization techniques with custom build experimental setups, analysis of multiple parameters and their coupled effects, thermodynamic simulations, theoretical predictions and design maps, and appealing application demonstrations highlight the potential of the LM-elastomer composite system in the field of soft matter. These materials and approach enable diverse applications specifically for soft robotics and stretchable electronics where flexibility and tunable functional response are critical.</p>