@article{joshipura_persson_oh_kong_vong_ni_alsafatwi_parekh_zhao_dickey_2021, title={Are Contact Angle Measurements Useful for Oxide-Coated Liquid Metals?}, volume={37}, ISSN={["0743-7463"]}, DOI={10.1021/acs.langmuir.1c01173}, abstractNote={This work establishes that static contact angles for gallium-based liquid metals have no utility despite the continued and common use of such angles in the literature. In the presence of oxygen, these metals rapidly form a thin (∼1-3 nm) surface oxide that adheres to many surfaces and mechanically impedes its flow. This property is problematic for contact angle measurements, which presume the ability of liquids to flow freely to adopt shapes that minimize the interfacial energy. We show here that advancing angles for a metal are always high (>140°)-even on substrates to which it adheres-because the solid native oxide must rupture in tension to advance the contact line. The advancing angle for the metal depends subtly on the substrate surface chemistry but does not vary strongly with hydrophobicity of the substrate. During receding measurements, the metal droplet initially sags as the liquid withdraws from the sac formed by the skin and thus the contact area with the substrate initially increases despite its volumetric recession. The oxide pins at the perimeter of the deflated sac on all the surfaces are tested, except for certain rough surfaces. With additional withdrawal of the liquid metal, the pinned angle gets smaller until eventually the oxide sac collapses. Thus, static contact angles can be manipulated mechanically from 0° to >140° due to hysteresis and are therefore uninformative. We also provide recommendations and best practices for wetting experiments, which may find use in applications that use these alloys such as soft electronics, composites, and microfluidics.}, number={37}, journal={LANGMUIR}, author={Joshipura, Ishan D. and Persson, K. Alex and Oh, Ji-Hyun and Kong, Minsik and Vong, Man Hou and Ni, Chujun and Alsafatwi, Mohanad and Parekh, Dishit P. and Zhao, Hong and Dickey, Michael D.}, year={2021}, month={Sep}, pages={10914–10923} }
 @article{cook_parekh_ladd_kotwal_panich_durstock_dickey_tabor_2019, title={Shear-Driven Direct-Write Printing of Room-Temperature Gallium-Based Liquid Metal Alloys}, volume={21}, ISSN={["1527-2648"]}, DOI={10.1002/adem.201900400}, abstractNote={Gallium-based metal alloys have high electrical conductivity in the liquid state at room temperature. These liquid metal conductors inspire unique electronic applications such as reconfigurable circuits and stretchable components with extremely high strain tolerance. Previously, liquid metals have been successfully patterned via direct-writing, yielding metallically conductive features on-demand at room temperature that do not require post-processing, down to a resolution of ≈10 μm. While most direct-write processes extrude materials from a nozzle via pressure or volumetric displacement, liquid metal is instead printed here by a shear-driven mechanism that occurs when the oxide-coated meniscus of the metal adheres to the printing substrate and is “pulled” from the nozzle at pressures that are well-below that needed to extrude the metal in the absence of shear. Herein, the key operating parameters that enable shear-driven printing of liquid metals including dispensing pressure, choice of substrate, print height, the surrounding environmental conditions, and the speed and acceleration of the print head are elucidated. A guide to the best practices as well as limitations for implementing shear-driven printing of liquid metals at room temperature is provided in these studies.}, number={11}, journal={ADVANCED ENGINEERING MATERIALS}, author={Cook, Alexander and Parekh, Dishit P. and Ladd, Collin and Kotwal, Gargee and Panich, Lazar and Durstock, Michael and Dickey, Michael D. and Tabor, Christopher E.}, year={2019}, month={Nov} }
 @article{cooper_joshipura_parekh_norkett_mailen_miller_genzer_dickey_2019, title={Toughening stretchable fibers via serial fracturing of a metallic core}, volume={5}, ISSN={["2375-2548"]}, url={https://doi.org/10.1126/sciadv.aat4600}, DOI={10.1126/sciadv.aat4600}, abstractNote={Stretchable fibers dissipate energy via the sequential fracturing of a metallic core held together by an elastomeric shell.}, number={2}, journal={SCIENCE ADVANCES}, publisher={American Association for the Advancement of Science (AAAS)}, author={Cooper, Christopher B. and Joshipura, Ishan D. and Parekh, Dishit P. and Norkett, Justin and Mailen, Russell and Miller, Victoria M. and Genzer, Jan and Dickey, Michael D.}, year={2019}, month={Feb} }
 @article{bharambe_parekh_ladd_moussa_dickey_adams_2018, title={Liquid-Metal-Filled 3-D Antenna Array Structure With an Integrated Feeding Network}, volume={17}, ISSN={["1548-5757"]}, url={https://doi.org/10.1109/LAWP.2018.2813309}, DOI={10.1109/lawp.2018.2813309}, abstractNote={This letter describes the fabrication and characterization of a microstrip patch array and a three-dimensional (3-D) coaxial feed network embedded within a 3-D printed part. Internal cavities within the acrylic structure are filled with a gallium-based liquid metal alloy using a vacuum-driven process to form conducting elements. In this way, four rectangular patch elements and a feeding network, including power dividers and vertical transitions, are embedded within a single 3-D printed acrylic geometry. Simulations and measurements of a 6 GHz array show that the array produces a matched response and moderate gain at the design frequency. This procedure can be employed to integrate numerous radiating elements and their corresponding feeding networks into a single monolithic acrylic structure, eliminating the need for separate fabrication of printed-circuit-board-based antennas and feeds. The procedure can serve as a convenient approach for rapid prototyping of complex array designs that exploit the additional spatial degrees of freedom to enhance their electromagnetic performance. Furthermore, manipulating the liquid-phase metallization inside these acrylic cavities can potentially be used to produce frequency- or pattern-reconfigurable arrays in the future.}, number={5}, journal={IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Bharambe, Vivek and Parekh, Dishit P. and Ladd, Collin and Moussa, Khalil and Dickey, Michael D. and Adams, Jacob J.}, year={2018}, month={May}, pages={739–742} }
 @article{andrews_mondal_neumann_cardenas_wang_parekh_lin_ballentine_dickey_franklin_et al._2018, title={Patterned Liquid Metal Contacts for Printed Carbon Nanotube Transistors}, volume={12}, ISSN={["1936-086X"]}, url={https://doi.org/10.1021/acsnano.8b00909}, DOI={10.1021/acsnano.8b00909}, abstractNote={Flexible and stretchable electronics are poised to enable many applications that cannot be realized with traditional, rigid devices. One of the most promising options for low-cost stretchable transistors are printed carbon nanotubes (CNTs). However, a major limiting factor in stretchable CNT devices is the lack of a stable and versatile contact material that forms both the interconnects and contact electrodes. In this work, we introduce the use of eutectic gallium-indium (EGaIn) liquid metal for electrical contacts to printed CNT channels. We analyze thin-film transistors (TFTs) fabricated using two different liquid metal deposition techniques-vacuum-filling polydimethylsiloxane (PDMS) microchannel structures and direct-writing liquid metals on the CNTs. The highest performing CNT-TFT was realized using vacuum-filled microchannel deposition with an in situ annealing temperature of 150 °C. This device exhibited an on/off ratio of more than 104 and on-currents as high as 150 μA/mm-metrics that are on par with other printed CNT-TFTs. Additionally, we observed that at room temperature the contact resistances of the vacuum-filled microchannel structures were 50% lower than those of the direct-write structures, likely due to the poor adhesion between the materials observed during the direct-writing process. The insights gained in this study show that stretchable electronics can be realized using low-cost and solely solution processing techniques. Furthermore, we demonstrate methods that can be used to electrically characterize semiconducting materials as transistors without requiring elevated temperatures or cleanroom processes.}, number={6}, journal={ACS NANO}, publisher={American Chemical Society (ACS)}, author={Andrews, Joseph B. and Mondal, Kunal and Neumann, Taylor V. and Cardenas, Jorge A. and Wang, Justin and Parekh, Dishit P. and Lin, Yiliang and Ballentine, Peter and Dickey, Michael and Franklin, Aaron D. and et al.}, year={2018}, month={Jun}, pages={5482–5488} }
 @article{roh_parekh_bharti_stoyanov_velev_2017, title={3D Printing by Multiphase Silicone/Water Capillary Inks}, volume={29}, ISSN={["1521-4095"]}, DOI={10.1002/adma.201701554}, abstractNote={3D printing of polymers is accomplished easily with thermoplastics as the extruded hot melt solidifies rapidly during the printing process. Printing with liquid polymer precursors is more challenging due to their longer curing times. One curable liquid polymer of specific interest is polydimethylsiloxane (PDMS). This study demonstrates a new efficient technique for 3D printing with PDMS by using a capillary suspension ink containing PDMS in the form of both precured microbeads and uncured liquid precursor, dispersed in water as continuous medium. The PDMS microbeads are held together in thixotropic granular paste by capillary attraction induced by the liquid precursor. These capillary suspensions possess high storage moduli and yield stresses that are needed for direct ink writing. They could be 3D printed and cured both in air and under water. The resulting PDMS structures are remarkably elastic, flexible, and extensible. As the ink is made of porous, biocompatible silicone that can be printed directly inside aqueous medium, it can be used in 3D printed biomedical products, or in applications such as direct printing of bioscaffolds on live tissue. This study demonstrates a number of examples using the high softness, elasticity, and resilience of these 3D printed structures.}, number={30}, journal={ADVANCED MATERIALS}, author={Roh, Sangchul and Parekh, Dishit P. and Bharti, Bhuvnesh and Stoyanov, Simeon D. and Velev, Orlin D.}, year={2017}, month={Aug} }
 @inproceedings{shen_aiken_abbasi_parekh_zhao_dickey_ricketts_2017, title={Rapid prototyping of low loss 3D printed waveguides for millimeter-wave applications}, DOI={10.1109/mwsym.2017.8058593}, abstractNote={Traditional hollow metallic waveguide manufacturing techniques are readily capable of producing components with high-precision geometric tolerances, yet generally lack the ability to customize individual parts on demand or to deliver finished components with low lead times. This paper proposes a Rapid-Prototyping (RP) method for relatively low-loss millimeter-wave hollow waveguides produced using consumer-grade stere-olithographic (SLA) Additive Manufacturing (AM) technology, in conjunction with an electroless metallization process optimized for acrylate-based photopolymer substrates. To demonstrate the capabilities of this particular AM process, waveguide prototypes are fabricated for the W- and D-bands. The measured insertion loss at W-band is between 0.12 dB/in to 0.25 dB/in, corresponding to a mean value of 0.16 dB/in. To our knowledge, this is the lowest insertion loss figure presented to date, when compared to other W-Band AM waveguide designs reported in the literature. Printed D-band waveguide prototypes exhibit a transducer loss of 0.26 dB/in to 1.01 dB/in, with a corresponding mean value of 0.65 dB/in, which is similar performance to a commercial metal waveguide.}, booktitle={2017 ieee mtt-s international microwave symposium (ims)}, author={Shen, J. Y. and Aiken, M. W. and Abbasi, M. and Parekh, D. P. and Zhao, X. and Dickey, Michael and Ricketts, D. S.}, year={2017}, pages={41–44} }
 @article{bharambe_parekh_ladd_moussa_dickey_adams_2017, title={Vacuum-filling of liquid metals for 3D printed RF antennas}, volume={18}, ISSN={["2214-7810"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85031777244&partnerID=MN8TOARS}, DOI={10.1016/j.addma.2017.10.012}, abstractNote={This paper describes a facile method to fabricate complex three-dimensional (3D) antennas by vacuum filling gallium-based liquid metals into 3D printed cavities at room temperature. To create the cavities, a commercial printer co-prints a sacrificial wax-like material with an acrylic resin. Dissolving the printed wax in oil creates cavities as small as 500 μm within the acrylic monolith. Placing the entire structure under vacuum evacuates most of the air from these cavities through a reservoir of liquid metal that covers a single inlet. Returning the assembly to atmospheric pressure pushes the metal from the reservoir into the cavities due to the pressure differential. This method enables filling of the closed internal cavities to create planar and curved conductive 3D geometries without leaving pockets of trapped air that lead to defects. An advantage of this technique is the ability to rapidly prototype 3D embedded antennas and other microwave components with metallic conductivity at room temperature using a simple process. Because the conductors are liquid, they also enable the possibility of manipulating the properties of such devices by flowing metal in or out of selected cavities. The measured electrical properties of fabricated devices match well to electromagnetic simulations, indicating that the approach described here forms antenna geometries with high fidelity. Finally, the capabilities and limitations of this process are discussed along with possible improvements for future work.}, journal={ADDITIVE MANUFACTURING}, publisher={Elsevier BV}, author={Bharambe, Vivek and Parekh, Dishit P. and Ladd, Collin and Moussa, Khalil and Dickey, Michael D. and Adams, Jacob J.}, year={2017}, month={Dec}, pages={221–227} }
 @article{trlica_parekh_panich_ladd_dickey_2014, title={3-D printing of liquid metals for stretchable and flexible conductors}, volume={9083}, ISSN={["1996-756X"]}, DOI={10.1117/12.2050212}, abstractNote={3-D printing is an emerging technology that has been used primarily on small scales for rapid prototyping, but which could also herald a wider movement towards decentralized, highly customizable manufacturing. Polymers are the most common materials to be 3-D printed today, but there is great demand for a way to easily print metals. Existing techniques for 3-D printing metals tend to be expensive and energy-intensive, and usually require high temperatures or pressures, making them incompatible with polymers, organics, soft materials, and biological materials. Here, we describe room temperature liquid metals as complements to polymers for 3-D printing applications. These metals enable the fabrication of soft, flexible, and stretchable devices. We survey potential room temperature liquid metal candidates and describe the benefits of gallium and its alloys for these purposes. We demonstrate the direct printing of a liquid gallium alloy in both 2-D and 3-D and highlight the structures and shapes that can be fabricated using these processes.}, journal={MICRO- AND NANOTECHNOLOGY SENSORS, SYSTEMS, AND APPLICATIONS VI}, author={Trlica, Chris and Parekh, Dishit Paresh and Panich, Lazar and Ladd, Collin and Dickey, Michael D.}, year={2014} }