FSVPy: A Python-based Package for Fluorescent Streak Velocimetry (FSV)

Predictive constitutive equations that connect easy-to-measure transport properties (e.g., viscosity and conductivity) with system performance variables (e.g., power consumption and efficiency) are needed to design advanced thermal and electrical systems. In this work, we explore the use of fluorescent particle-streak analysis to directly measure the local velocity field of a pressure-driven flow, introducing a new Python package (FSVPy) to perform the analysis. Fluorescent streak velocimetry (FSV) combines high-speed imaging with highly fluorescent particles to produce images that contain fluorescent streaks, whose length and intensity can be related to the local flow velocity. By capturing images throughout the sample volume, the three-dimensional velocity field can be quantified and reconstructed. We demonstrate this technique by characterizing the channel flow profiles of several non-Newtonian fluids: micellar Cetylpyridinium Chloride solution, Carbopol 940, and Polyethylene Glycol. We then explore more complex flows, where significant acceleration is created due to micro-scale features encountered within the flow. We demonstrate the ability of FSVPy to process streaks of various shapes, and use the variable intensity along the streak to extract position-specific velocity measurements from individual images. Thus, we demonstrate that FSVPy is a flexible tool that can be used to extract local velocimetry measurements from a wide variety of fluids and flow conditions

Scalable Manufacturing of Liquid Metal for Soft and Stretchable Electronics

Next-generation soft robots, wearable health monitoring devices, and humanmachine interfaces require electronic systems that can maintain their performance under deformations. Thus, researchers have been developing materials and methods to enable high-performance soft electronic systems in diverse applications. While a variety of solutions have been presented, development of stretchable materials with a combination of high stretchability, electrical conductivity, cyclic stability, and manufacturability is still an open challenge. Throughout this dissertation, gallium-based liquid metal alloy is used as the conductive material, leveraging its high conductivity and intrinsic stretchability for maintained performance under deformations. This dissertation presents both new liquid metal-based conductive materials and scalable manufacturing methods for the development of a diverse range of flexible and stretchable electronic circuits. First, a laser sintering method was developed to coalesce liquid metal micro/nanoparticles into soft, conductive structures enabled by oxide rupturing. The fast, non-contact, and maskless laser sintering technique, in combination with large-area spray-printing deposition, and high-throughput emulsion processing, provided a methodology to create different physical manifestations of liquid metal-based soft, stretchable, and reconfigurable electronics. Second, a liquid metalbased biphasic material was created using a thermal processing technique, yielding a printable, mechanically stable, and extremely stretchable conductor. This material’s compatibility with existing scalable manufacturing methods, robust interfaces with off-the-shelf electronic components, and electrical/mechanical cyclic stability enabled direct conversion of established circuit board assemblies to stretchable forms. The work presented in this dissertation paves the way for future mass-manufacturing of soft, stretchable circuits for direct integration into smart garments or soft robots

Stretchable Shape‐Sensing Sheets

Soft robot deformations are typically estimated using strain sensors to infer change from a nominal shape while taking a robot‐specific mechanical model into account. This approach performs poorly during buckling and when material properties change with time, and is untenable for shape‐changing robots that don't have a well‐defined resting (unactuated) shape. Herein, these limitations are overcome using stretchable shape sensing (S3) sheets that fuse orientation measurements to estimate 3D surface contours without making assumptions about the underlying robot geometry or material properties. The S3 sheets can estimate the shape of target objects to an accuracy of ≈3 mm for an 80 mm long sheet. The authors show the S3 sheets estimating their shape while being deformed in 3D space and also attached to the surface of a silicone three‐chamber pneumatic bladder, highlighting the potential for shape‐sensing sheets to be applied, removed, and reapplied to soft robots for shape estimation. Finally, the S3 sheets detecting their own stretch up to 30% strain is demonstrated. The approach introduced herein provides a generalized method for measuring the shape of objects without making strong assumptions about the objects, thus achieving a modular, mechanics model‐free approach to proprioception for wearable electronics and soft robotics

Static-state particle fabrication via rapid vitrification of a thixotropic medium

Upscale fabrication of functionalized microparticles is a pending challenge. Here, Kim et al. exploit the rheology of a thixotropic medium to grind sizeable amounts of raw material into well-defined colloidal dispersions, physically stabilized for further production steps

Different Shades of Oxide: From Nanoscale Wetting Mechanisms to Contact Printing of Gallium-Based Liquid Metals

Gallium-based liquid metals are of interest for a variety of applications including flexible electronics, soft robotics, and biomedical devices. Still, nano- to microscale device fabrication with these materials is challenging because, despite having surface tension 10 times higher than water, they strongly adhere to a majority of substrates. This unusually high adhesion is attributed to the formation of a thin oxide shell; however, its role in the adhesion process has not yet been established. In this work, we demonstrate that, dependent on dynamics of formation and resulting morphology of the liquid metal–substrate interface, GaInSn adhesion can occur in two modes. The first mode occurs when the oxide shell is not ruptured as it makes contact with the substrate. Because of the nanoscale topology of the oxide surface, this mode results in minimal adhesion between the liquid metal and most solids, regardless of substrate’s surface energy or texture. In the second mode, the formation of the GaInSn–substrate interface involves rupturing of the original oxide skin and formation of a composite interface that includes contact between the substrate and pieces of old oxide, bare liquid metal, and new oxide. We demonstrate that in this latter mode GaInSn adhesion is dominated by the intimate contact between new oxide and substrate. We also show that by varying the pinned contact line length using varied degrees of surface texturing, the adhesion of GaInSn in this mode can be either decreased or increased. Lastly, we demonstrate how these two adhesion modes limit microcontact printing of GaInSn patterns but can be exploited to repeatedly print individual sub-200 nm liquid metal drops

Different Shades of Oxide: From Nanoscale Wetting Mechanisms to Contact Printing of Gallium-Based Liquid Metals

Gallium-based liquid metals are of interest for a variety of applications including flexible electronics, soft robotics, and biomedical devices. Still, nano- to microscale device fabrication with these materials is challenging because, despite having surface tension 10 times higher than water, they strongly adhere to a majority of substrates. This unusually high adhesion is attributed to the formation of a thin oxide shell; however, its role in the adhesion process has not yet been established. In this work, we demonstrate that, dependent on dynamics of formation and resulting morphology of the liquid metal–substrate interface, GaInSn adhesion can occur in two modes. The first mode occurs when the oxide shell is not ruptured as it makes contact with the substrate. Because of the nanoscale topology of the oxide surface, this mode results in minimal adhesion between the liquid metal and most solids, regardless of substrate’s surface energy or texture. In the second mode, the formation of the GaInSn–substrate interface involves rupturing of the original oxide skin and formation of a composite interface that includes contact between the substrate and pieces of old oxide, bare liquid metal, and new oxide. We demonstrate that in this latter mode GaInSn adhesion is dominated by the intimate contact between new oxide and substrate. We also show that by varying the pinned contact line length using varied degrees of surface texturing, the adhesion of GaInSn in this mode can be either decreased or increased. Lastly, we demonstrate how these two adhesion modes limit microcontact printing of GaInSn patterns but can be exploited to repeatedly print individual sub-200 nm liquid metal drops

Laser Sintering of Liquid Metal Nanoparticles for Scalable Manufacturing of Soft and Flexible Electronics

Soft, flexible, and stretchable electronics are needed to transmit power and information, and track dynamic poses in next-generation wearables, soft robots, and biocompatible devices. Liquid metal has emerged as a promising material for these applications due to its high conductivity and liquid phase state at room temperature; however, surface oxidation of liquid metal gives it unique behaviors that are often incompatible with scalable manufacturing techniques. This paper reports a rapid and scalable approach to fabricate soft and flexible electronics composed of liquid metal. Compared to other liquid metal patterning approaches, this approach has the advantages of compatibility with a variety of substrates, ease of scalability, and efficiency through automated processes. Nonconductive liquid metal nanoparticle films are sintered into electrically conductive patterns by use of a focused laser beam to rupture and ablate particle oxide shells, and allow their liquid metal cores to escape and coalesce. The laser sintering phenomenon is investigated through comparison with focused ion beam sintering and by studying the effects of thermal propagation in sintered films. The effects of laser fluence, nanoparticle size, film thickness, and substrate material on resistance of the sintered films are evaluated. Several devices are fabricated to demonstrate the electrical stability of laser-patterned liquid metal traces under flexing, multilayer circuits, and intricately patterned circuits. This work merges the precision, consistency, and speed of laser manufacturing with the material benefits of liquid conductors on elastic substrates to demonstrate decisive progress toward commercial-scale manufacturing of soft electronics

Laser Sintering of Liquid Metal Nanoparticles for Scalable Manufacturing of Soft and Flexible Electronics

Soft, flexible, and stretchable electronics are needed to transmit power and information, and track dynamic poses in next-generation wearables, soft robots, and biocompatible devices. Liquid metal has emerged as a promising material for these applications due to its high conductivity and liquid phase state at room temperature; however, surface oxidation of liquid metal gives it unique behaviors that are often incompatible with scalable manufacturing techniques. This paper reports a rapid and scalable approach to fabricate soft and flexible electronics composed of liquid metal. Compared to other liquid metal patterning approaches, this approach has the advantages of compatibility with a variety of substrates, ease of scalability, and efficiency through automated processes. Nonconductive liquid metal nanoparticle films are sintered into electrically conductive patterns by use of a focused laser beam to rupture and ablate particle oxide shells, and allow their liquid metal cores to escape and coalesce. The laser sintering phenomenon is investigated through comparison with focused ion beam sintering and by studying the effects of thermal propagation in sintered films. The effects of laser fluence, nanoparticle size, film thickness, and substrate material on resistance of the sintered films are evaluated. Several devices are fabricated to demonstrate the electrical stability of laser-patterned liquid metal traces under flexing, multilayer circuits, and intricately patterned circuits. This work merges the precision, consistency, and speed of laser manufacturing with the material benefits of liquid conductors on elastic substrates to demonstrate decisive progress toward commercial-scale manufacturing of soft electronics