Significance Efficient thermal transport is critical for applications ranging from electronics and energy to advanced manufacturing and transportation; it is essential in emerging domains like wearable computing and soft robotics, which require thermally conductive materials that are also soft and stretchable. However, heat transport within soft materials is limited by the dynamics of phonon transport, which results in a trade-off between thermal conductivity and compliance. We overcome this by engineering an elastomer composite embedded with elongated inclusions of liquid metal (LM) that function as thermally conductive pathways. These composites exhibit an extraordinary combination of low stiffness (<100 kPa), high strain limit (>600%), and metal-like thermal conductivity (up to 9.8 W⋅m−1⋅K−1) that far exceeds any other soft materials. Soft dielectric materials typically exhibit poor heat transfer properties due to the dynamics of phonon transport, which constrain thermal conductivity (k) to decrease monotonically with decreasing elastic modulus (E). This thermal−mechanical trade-off is limiting for wearable computing, soft robotics, and other emerging applications that require materials with both high thermal conductivity and low mechanical stiffness. Here, we overcome this constraint with an electrically insulating composite that exhibits an unprecedented combination of metal-like thermal conductivity, an elastic compliance similar to soft biological tissue (Young’s modulus < 100 kPa), and the capability to undergo extreme deformations (>600% strain). By incorporating liquid metal (LM) microdroplets into a soft elastomer, we achieve a ∼25× increase in thermal conductivity (4.7 ± 0.2 W⋅m−1⋅K−1) over the base polymer (0.20 ± 0.01 W⋅m−1·K−1) under stress-free conditions and a ∼50× increase (9.8 ± 0.8 W⋅m−1·K−1) when strained. This exceptional combination of thermal and mechanical properties is enabled by a unique thermal−mechanical coupling that exploits the deformability of the LM inclusions to create thermally conductive pathways in situ. Moreover, these materials offer possibilities for passive heat exchange in stretchable electronics and bioinspired robotics, which we demonstrate through the rapid heat dissipation of an elastomer-mounted extreme high-power LED lamp and a swimming soft robot.

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High thermal conductivity in soft elastomers with elongated liquid metal inclusions.

Stretchable high-dielectric-constant materials are crucial for electronic applications in emerging domains such as wearable computing and soft robotics. While previous efforts have shown promising materials architectures in the form of dielectric nano-/microinclusions embedded in stretchable matrices, the limited mechanical compliance of these materials significantly limits their practical application as soft energy-harvesting/storage transducers and actuators. Here, a class of liquid metal (LM)-elastomer nanocomposites is presented with elastic and dielectric properties that make them uniquely suited for applications in soft-matter engineering. In particular, the role of droplet size is examined and it is found that embedding an elastomer with a polydisperse distribution of nanoscale LM inclusions can enhance its electrical permittivity without significantly degrading its elastic compliance, stretchability, or dielectric breakdown strength. In contrast, elastomers embedded with microscale droplets exhibit similar improvements in permittivity but a dramatic reduction in breakdown strength. The unique enabling properties and practicality of LM-elastomer nanocomposites for use in soft machines and electronics is demonstrated through enhancements in performance of a dielectric elastomer actuator and energy-harvesting transducer.

A Liquid-Metal-Elastomer Nanocomposite for Stretchable Dielectric Materials.

This work discusses the attributes, fabrication methods, and applications of gallium-based liquid metal particles. Gallium-based liquid metals combine metallic and fluidic properties at room temperature. Unlike mercury, which is toxic and has a finite vapor pressure, gallium possesses low toxicity and effectively zero vapor pressure at room temperature, which makes it amenable to many applications. A variety of fabrication methods produce liquid metal particles with variable sizes, ranging from nm to mm (which is the upper limit set by the capillary length). The liquid nature of gallium enables fabrication methods-such as microfluidics and sonication-that are not possible with solid materials. Gallium-based liquid metal particles possess several notable attributes, including a metal-metal oxide (liquid-solid) core-shell structure as well as the ability to self-heal, merge, and change shape. They also have unusual phase behavior that depends on the size of the particles. The particles have no known commercial applications, but they show promise for drug delivery, soft electronics, microfluidics, catalysis, batteries, energy harvesting, and composites. Existing challenges and future opportunities are discussed herein.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/advs.202000192

Attributes, Fabrication, and Applications of Gallium-Based Liquid Metal Particles.

Natural biopolymer-based conductive hydrogels, which combine inherent renewable, nontoxic features, biocompatibility and biodegradability of biopolymers, and excellent flexibility and conductivity ...

Recent Progress in Natural Biopolymers Conductive Hydrogels for Flexible Wearable Sensors and Energy Devices: Materials, Structures, and Performance

In this review, we attempt to give an overview on the recent progress made on biosurfactants, surface-active biomolecules produced by microorganisms, which are a sustainable alternative to synthetic surfactants. Different biosurfactants, their production techniques, and their physical and chemical properties are discussed. There is a focus on recent studies related to surface properties and rheology of biosurfactants, both being properties which affect their ability to take part in a stable formulation. Biosurfactants can have applications in multiple different industrial sectors, such as agriculture, medicine, personal care, food, petroleum, etc. The specific properties important for applications in these sectors are discussed in detail.

Biosurfactants: Formulations, properties, and applications

Production and characterization of sophorolipids from stearic acid by solid-state fermentation, a cleaner alternative to chemical surfactants

Room-temperature liquid metals, such as eutectic gallium–indium–tin (galinstan), dispersed in a polymer matrix present unique potential as conductors that may have minimal influence on the host polymer mechanical performance while providing enhanced electrical performance Work described herein systematically evaluates the influence of uncured polydimethylsiloxane (PDMS) viscosity and galinstan loading on final dispersion viscosity and cured modulus Dispersions of up to 80 vol% galinstan were obtained with relative permittivity values up to 170 that otherwise exhibited similar uncured rheological changes to a solid filler Cured galinstan-in-PDMS dispersions, however, exhibited a reduced stiffness increase with respect to the host polymer relative to a solid filler At a critical PDMS viscosity and metal, loading phase inversion to a conductive PDMS-in-metal dispersion was observed It is anticipated that this work will enable the development of liquid metal polymer composites with independently controlled mechanical and electrical properties for a wide variety of stretchable electronic applications

Deformable liquid metal polymer composites with tunable electronic and mechanical properties

Sophorolipids (SLs), produced by Candida bombicola, are of interest as potential replacements for hazardous commercial surfactants. For the first time, a series of molecularly edited SLs with ethyl (EE), n-hexyl (HE), and n-decyl (DE) esters were evaluated at an oil (almond oil)–water interface for their ability to reduce interfacial tension (IFT) and generate stable emulsions. An increase in the n-alkyl ester chain length from ethyl to hexyl resulted in a maximum % decrease in the IFT from 86.1 to 95.3, respectively. Furthermore, the critical aggregation concentrations (CACs) decreased from 0.035 to 0.011 and 0.006 mg/mL as the ester chain length was increased from ethyl to n-hexyl and n-decyl, respectively. In contrast, the CAC of natural SL, composed of 50/50 acidic and LSL, is 0.142 mg/mL. Dynamic IFT analysis showed significant differences in diffusion coefficients for all SLs studied. Almond oil emulsions with up to 200:1 (by weight) oil/SL-DE were stable against oil separation for up to 1 week with...

Effect of Sophorolipid n-Alkyl Ester Chain Length on Its Interfacial Properties at the Almond Oil-Water Interface.

Biosurfactants from Waste: Structures and Interfacial Properties of Sophorolipids Produced from a Residual Oil Cake

Lactonic sophorolipids (LSL) are glycolipid biosurfactants produced in large quantity by yeast fermentation. Chemical or enzymatic modification of naturally produced sophorolipids is an effective route to improve their functional properties. For the first time, ring-opening cross metathesis (RO/CM) was used to convert natural LSL to a unique family of modified sophorolipids. Reaction conditions for RO/CM of LSL with n-alkyl acrylates, trans-3-hexene, 1-hexene and ethylene were investigated. For the RO/CM of n-alkyl acrylates with LSL, %-conversions greater than 95% within 1 h resulted from conducting reactions in THF at 60°C using 5 mol% of Grubbs second generation ruthenium based catalyst (M2). The RO/CM reaction of LSL with ethylene performed at 3 bar under an ethylene atmosphere using Grubbs first generation (G1) (10 mol%) as the catalyst in dichloromethane (room temperature, 5 h) gave complete conversion of LSL to the corresponding ring-opened product. Ethanolysis of LSL RO/CM products generated a series of medium chain (C10-C14) SL-surfactants and fatty acid co-products. Values of surface tension reduction at the air-water interface versus Log (C) for modified SLs were measured by the Wilhelmy plate method. Minimum surface tension values varied as a function of the hydrophobic character of modified SLs. The modified SL from RO/CM with 1-hexene (SL-14) gave the largest surface tension reduction and lowest CMC (to 34 mN/m and 0.15 mM, respectively) and showed a similar surface tension reduction behavior as n-dodecyl-β-D-maltoside (Mal-C12). Increasing the number of carbons in the hydrophobic segment for the homologous series of n-alkyl sophorosides results in an almost linear decrease in log(CMC), with B = 0.18 ± 0.03. This number is smaller than that of other related surfactants such as alkyl-β-D-glycosides and alkyl-β-D-maltosides.



Practical applications: Modified sophorolipids can be used in a wide variety of applications such as stabilization of oil-in-water dispersions, antimicrobials and various cleaning operations.







Ring-opening cross-metathesis was used to expand structural diversity of natural sophorolipid surfactants.

Amanda Koh

Papers

Production and characterization of sophorolipids from stearic acid by solid-state fermentation, a cleaner alternative to chemical surfactants

Deformable liquid metal polymer composites with tunable electronic and mechanical properties

Effect of Sophorolipid n-Alkyl Ester Chain Length on Its Interfacial Properties at the Almond Oil-Water Interface.

Biosurfactants from Waste: Structures and Interfacial Properties of Sophorolipids Produced from a Residual Oil Cake

Sophorolipids: Expanding structural diversity by ring‐opening cross‐metathesis