An official website of the United States government
Material extrusion (MEX) of soft, multifunctional composites consisting of liquid metal (LM) droplets can enable highly tailored properties for a range of applications from soft robotics to stretchable electronics. However, an understanding of how LM ink rheology and print process parameters can reconfigure LM droplet shape during MEX processing for in-situ control of properties and function is currently limited. Herein, the material (ink viscosity, and LM droplet size) and process (nozzle velocity, height from print bed, and extrusion rate) parameters are determined which control LM microstructure during MEX of these composites. The interplay and interdependence of these parameters is evaluated and nearly spherical LM droplets are transformed into highly elongated ellipsoidal shapes with an average aspect ratio of 12.4 by systematically tuning each individual parameter. Material and process relationships are established for the LM ink which show that an ink viscosity threshold should be fulfilled to program the LM microstructure from spherical to an ellipsoidal shape during MEX. Additionally, the thin oxide layer on the LM droplets is found to play a unique and critical role in the reconfiguration and retention of droplet shape. Finally, two quantitative design maps based on material and process parameters are presented to guide MEX additive manufacturing strategies for tuning liquid droplet architecture in LM-polymer inks. The insights gained from this study enable informed design of materials and manufacturing to control microstructure of emerging multifunctional soft composites.
more »
« less
On‐Demand Programming of Liquid Metal‐Composite Microstructures through Direct Ink Write 3D Printing
Abstract Soft, elastically deformable composites with liquid metal (LM) droplets can enable new generations of soft electronics, robotics, and reconfigurable structures. However, techniques to control local composite microstructure, which ultimately governs material properties and performance, is lacking. Here a direct ink writing technique is developed to program the LM microstructure (i.e., shape, orientation, and connectivity) on demand throughout elastomer composites. In contrast to inks with rigid particles that have fixed shape and size, it is shown that emulsion inks with LM fillers enable in situ control of microstructure. This enables filaments, films, and 3D structures with unique LM microstructures that are generated on demand and locked in during printing. This includes smooth and discrete transitions from spherical to needle‐like droplets, curvilinear microstructures, geometrically complex embedded inclusion patterns, and connected LM networks. The printed materials are soft (modulus < 200 kPa), highly deformable (>600 % strain), and can be made locally insulating or electrically conductive using a single ink by controlling the process conditions. These capabilities are demonstrated by embedding elongated LM droplets in a soft heat sink, which rapidly dissipates heat from high‐power LEDs. These programmable microstructures can enable new composite paradigms for emerging technologies that demand mechanical compliance with multifunctional response.
more »
« less
- PAR ID:
- 10367401
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Advanced Materials
- Volume:
- 34
- Issue:
- 20
- ISSN:
- 0935-9648
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Thermal management in electric vehicles, electronics, and robotics requires the systematic ability to dissipate and direct the flow of heat. Thermally conductive soft composites are promising for thermal management due to their high thermal conductivity and mechanical flexibility. However, composites typically have the same microstructure throughout a film, which limits directional and spatial control of thermal management in emerging systems that have distributed heat loads. Herein, directional and spatially tunable thermal properties are programmed into liquid metal (LM) soft composites through a direct ink writing (DIW) process. Through the local control of LM droplet aspect ratio and orientation this programmable LM microstructure has a thermal conductivity in the direction of LM elongation of 9.9 W m−1·K−1, which is ∼40 times higher than the unfilled elastomer (0.24 W m−1·K−1). The DIW process enables LM droplets to be oriented in specific directions with tunable aspect ratios at different locations throughout a continuous film. This introduces anisotropic and heterogeneous thermal conductivity in compliant films to control the direction and magnitude of heat transfer. This methodology and resulting materials can provide designed thermal management solutions for rigid and soft devices.more » « less
-
Abstract Liquid metal (LM) elastomer composites offer promising potential in soft robotics, wearable electronics, and human‐machine interfaces. Direct ink write (DIW) 3D printing offers a versatile manufacturing technique capable of precise control over LM microstructures, yet challenges such as interfilament void formation in multilayer structures impact material performance. Here, a DIW strategy is introduced to control both LM microstructure and material architecture. Investigating three key process parameters–nozzle height, extrusion rate, and nondimensionalized nozzle velocity–it is found that nozzle height and velocity predominantly influence filament geometry. The nozzle height primarily dictates the aspect ratio of the filament and the formation of voids. A threshold print height based on filament geometry is identified; below the height, significant surface roughness occurs, and above the ink fractures, which facilitates the creation of porous structures with tunable stiffness and programmable LM microstructure. These porous architectures exhibit reduced density and enhanced thermal conductivity compared to cast samples. When used as a dielectric in a soft capacitive sensor, they display high sensitivity (gauge factor = 9.0), as permittivity increases with compressive strain. These results demonstrate the capability to simultaneously manipulate LM microstructure and geometric architecture in LM elastomer composites through precise control of print parameters, while maintaining geometric fidelity in the printed design.more » « less
-
Soft machines will require soft materials that exhibit a rich diversity of functionality, including shape morphing and photoresponsivity. The combination of these functionalities enables useful behaviors in soft machines that can be further developed by synthesizing materials that exhibit localized responsivity. Localized responsivity of liquid crystal elastomers (LCEs), which are soft materials that exhibit shape morphing, can be enabled by formulating composite inks for direct ink writing (DIW). Gold nanorods (AuNRs) can be added to LCEs to enable photothermal shape change upon absorption of light through a localized surface plasmon resonance. We compared LCE formulations, focusing on their amenability for printing by DIW and the photoresponsivity of AuNRs. The local responsivity of different three-dimensional architectures enabled soft machines that could oscillate, crawl, roll, transport mass, and display other unique modes of actuation and motion in response to light, making these promising functional materials for advanced applications.more » « less
-
Abstract Current stretchable conductors, often composed of elastomeric composites infused with rigid conductive fillers, suffer from limited stretchability and durability, and declined conductivity with stretching. These limitations hinder their potential applications as essential components such as interconnects, sensors, and actuators in stretchable electronics and soft machines. In this context, an innovative elastomeric composite that incorporates a 3D network of liquid metal (LM), offering exceptional stretchability, durability, and conductivity, is introduced. The mechanics model elucidates how the interconnected 3DLM architecture imparts softness and stretchability to the composites, allowing them to withstand tensile strains of up to 500% without rupture. The relatively low surface‐to‐volume ratio of the 3DLM network limits the reforming of the oxide layer during cyclic stretch, thereby contributing to low permanent strain and enhanced durability. Additionally, the 3D architecture facilitates crack blunting and stress delocalization, elevating fracture resistance, while simultaneously establishing continuous conductive pathways that result in high conductivity. Notably, the conductivity of the 3DLM composite increases with strain during substantial stretching, highlighting its strain‐enhanced conductivity. In comparison to other LM‐based composites featuring 0D LM droplets, the 3DLM composite stands out with superior properties.more » « less
