The use of liquid metals based on gallium for soft and stretchable electronics is discussed. This emerging class of electronics is motivated, in part, by the new opportunities that arise from devices that have mechanical properties similar to those encountered in the human experience, such as skin, tissue, textiles, and clothing. These types of electronics (e.g., wearable or implantable electronics, sensors for soft robotics, e-skin) must operate during deformation. Liquid metals are compelling materials for these applications because, in principle, they are infinitely deformable while retaining metallic conductivity. Liquid metals have been used for stretchable wires and interconnects, reconfigurable antennas, soft sensors, self-healing circuits, and conformal electrodes. In contrast to Hg, liquid metals based on gallium have low toxicity and essentially no vapor pressure and are therefore considered safe to handle. Whereas most liquids bead up to minimize surface energy, the presence of a surface oxide on these metals makes it possible to pattern them into useful shapes using a variety of techniques, including fluidic injection and 3D printing. In addition to forming excellent conductors, these metals can be used actively to form memory devices, sensors, and diodes that are completely built from soft materials. The properties of these materials, their applications within soft and stretchable electronics, and future opportunities and challenges are considered.

Stretchable and Soft Electronics using Liquid Metals.

The literature on drying sessile droplets and deposition of suspended material is reviewed including the simple explanation of the “coffee ring” deposit given by Deegan et al.1 Analytical and numerical solutions for the flow are given, including the effect of Marangoni stresses, pinning or movement of the contact line, and viscous, thermal, gravitational, and other effects. The solution space is explored using dimensionless groups governing mass, momentum, and heat transfer effects in the droplet, external gas, and substrate. The most common types of deposition patterns are summarized, including those produced by pinned contact lines, sticking-and-slipping contact lines, and Marangoni effects. The influence of contact-line deposits is also reviewed, and the effects of colloidal, polymeric, and other depositing materials. Advanced applications from ink-jet printing to disease diagnosis are discussed as well. The review helps readers take stock of what has been learned and what remains incompletely explained. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1538–1571, 2014

Transport and deposition patterns in drying sessile droplets

Binary mixtures of liquid metal (LM) or low-melting-point alloy (LMPA) in an elastomeric or fluidic carrier medium can exhibit unique combinations of electrical, thermal, and mechanical properties. This emerging class of soft multifunctional composites have potential applications in wearable computing, bio-inspired robotics, and shape-programmable architectures. The dispersion phase can range from dilute droplets to connected networks that support electrical conductivity. In contrast to deterministically patterned LM microfluidics, LMPA- and LM-embedded elastomer (LMEE) composites are statistically homogenous and exhibit effective bulk properties. Eutectic Ga-In (EGaIn) and Ga-In-Sn (Galinstan) alloys are typically used due to their high conductivity, low viscosity, negligible nontoxicity, and ability to wet to nonmetallic materials. Because they are liquid-phase, these alloys can alter the electrical and thermal properties of the composite while preserving the mechanics of the surrounding medium. For composites with LMPA inclusions (e.g., Field's metal, Pb-based solder), mechanical rigidity can be actively tuned with external heating or electrical activation. This progress report, reviews recent experimental and theoretical studies of this emerging class of soft material architectures and identifies current technical challenges and opportunities for further advancement.

Soft Multifunctional Composites and Emulsions with Liquid Metals

The wetting of solid surfaces using liquid droplets has been studied since the early 1800s. Thomas Young and Pierre-Simon Laplace investigated the wetting properties, as well as the role of the contact angle and the coupling of a liquid and solid, on the contact angle formation. The geometry of a sessile droplet is relatively simple. However, it is sufficiently complex to be applied for solving and prediction of real-life situations (for example, metallic inks for inkjet printing, the spreading of pesticides on leaves, the dropping of whole blood, the spreading of blood serum, and drying for medical applications). Moreover, when taking into account both wetting and evaporation, a simple droplet becomes a very complex problem, and has been investigated by a number of researchers worldwide. The complexity is mainly due to the physics involved, the full coupling with the substrate upon which the drop is deposited, the atmosphere surrounding the droplet, and the nature of the fluid (pure fluid, bi- or multi-phase mixtures, or even fluids containing colloids and/or nano-particles). This review presents the physics involved during droplet wetting and evaporation by focusing on the evaporation dynamics, the flow motion, the vapour behaviour, the surface tension, and the wetting properties.

Recent advances in droplet wetting and evaporation

Wearable antennas have gained much attention in recent years due to their attractive features and possibilities in enabling lightweight, flexible, low cost, and portable wireless communication and sensing. Such antennas need to be conformal when used on different parts of the human body, thus need to be implemented using flexible materials and designed in a low profile structure. Ultimately, these antennas need to be capable of operating with minimum degradation in proximity to the human body. Such requirements render the design of wearable antennas challenging, especially when considering aspects such as their size compactness, effects of structural deformation and coupling to the body, and fabrication complexity and accuracy. Despite slight variations in severity according to applications, most of these issues exist in the context of body-worn implementation. This review aims to present different challenges and issues in designing wearable antennas, their material selection, and fabrication techniques. More importantly, recent innovative methods in back radiations reduction techniques, circular polarization (CP) generation methods, dual polarization techniques, and providing additional robustness against environmental effects are first presented. This is followed by a discussion of innovative features and their respective methods in alleviating these issues recently proposed by the scientific community researching in this field.

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Wearable Antennas: A Review of Materials, Structures, and Innovative Features for Autonomous Communication and Sensing

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Evaporation of sessile drops under combined diffusion and natural convection

A miniaturized wideband interferometry-based sensor for complex dielectric spectroscopy of liquid chemicals is presented in this paper. Two composite right-/left-hand (CRLH) transmission lines (TLs) are employed in a direct downconversion architecture. The material under test (MUT) is placed in direct contact with the printed interdigital capacitor of the artificial TL as the sensing component. Since the employed CRLH TL provides nonlinear dispersion characteristics, improved sensitivity with wideband (4.2–8 GHz) operation is achieved compared with resonator- or TL-based interferometers. The sensor circuit board is realized based on common printed circuit board technology, and the fluidic polydimethylsiloxane channels are fabricated and bonded to the circuit board using 3-D printing and soft lithography techniques. The measured results of pure liquid chemical characterization suggest the mean squared errors of ~1.1% and ~1.6% for $\epsilon ^{\prime }_{r}$ and $\epsilon ^{\prime \prime }_{r}$ , respectively, compared with the MUT’s theoretical model. Moreover, binary mixture characterization is carried out based on $\epsilon ^{\prime }_{r}$ measurements with an accuracy of 1%.

A Metamaterial-Inspired Wideband Microwave Interferometry Sensor for Dielectric Spectroscopy of Liquid Chemicals

Microfluidic channels filled with liquid metal are used to realize miniature and reconfigurable CPW folded slot antennas. The method is based on employing the reactive loading effect of fluid metal bridges on top of the CPW slot antenna. As a result of this reactive loading, the frequency of the antenna reduces and the antenna is miniaturized by a factor of 85%. Also, by changing the configuration of filled and empty channels, each channel can be used as a switch. By using two pairs of microfluidic channels, three different frequency bands of 2.4, 3.5, and 5.8 GHz can be achieved. This translates to a switching ratio ( $\bm{f}_{\bm{TR}}=\bm{f}_{\mathbf{2}}/\bm{f}_{\mathbf{1}}$ ) of more than 2.5. The antenna is realized using common PCB techniques for the antenna circuit board and three-dimensional (3-D) printing technology for PDMS-based microfluidics structure. The antenna circuit board and the PDMS structure are bonded to each other using a very thin spin-coated PDMS layer. Design methodology, simulation, and measurement results of both antenna prototypes are presented. Both the miniature and reconfigurable antennas have similar radiation patterns to a normal CPW folded slot antenna and show low cross-polarization levels at all operating frequencies.

Miniature and Reconfigurable CPW Folded Slot Antennas Employing Liquid-Metal Capacitive Loading

In this paper, a microfluidically reconfigurable coplanar waveguide (CPW) filter is presented with a tuning range of $\sim$ 1.6:1 and four different states. The passband frequency of the filter is changed based on employing the capacitive loading effect of a liquid metal placed on top of each CPW resonator using three parallel micro-channels. In addition, because of the loading effect of the metal bridges, miniaturization by a factor of 40% is achieved. The filter is digitally tuned from 3.4 to 5.5 GHz with an insertion loss of less than 5.0 dB and a relative bandwidth of 5 $\pm$ 0.35%. The RF power-handling capability of the filter is characterized using a customized measurement setup. It is observed that the filter can be used for input RF powers of up to $\sim$ 20 W for short-duration excitation conditions and 10 W for high-average-power excitation conditions. The filter is realized using common printed circuit board technology and a polydimethylsiloxane structure. Design methodology, simulation, and measurement results of the filter prototype are presented.

A Miniaturized Microfluidically Reconfigurable Coplanar Waveguide Bandpass Filter With Maximum Power Handling of 10 Watts

Reconfiguration of filled and empty microchannels is used to realize a switchable dual-band slot antenna whose both bands can be controlled independently. Reactive loading effect of Galinstan bridges is used to shift down each slot antenna's frequency of operation. Using five microchannels results in a 5-bit reconfigurable antenna. Any combination of frequencies within the ranges of 1.8-3.1 GHz (first band) and 3.2-5.4 GHz (second band) can be obtained. This translates into a discrete tuning ratio of ~ 1.7:1 for both bands and overall ratio of 3:1 (1.8-5.4 GHz) for the antenna. The antenna circuit board is realized using common printed circuit board (PCB) techniques, and the microchannels' polydimethylsiloxane (PDMS) substrate is fabricated using 3-D printing technologies. The circuit board and the PDMS substrates are bonded to each other using a thin layer of spin-coated PDMS. The performance of the antenna is measured, and close agreement between the simulation and measurement is observed.

Jaskirat Singh Batra

Papers

Evaporation of sessile drops under combined diffusion and natural convection

A Metamaterial-Inspired Wideband Microwave Interferometry Sensor for Dielectric Spectroscopy of Liquid Chemicals

Miniature and Reconfigurable CPW Folded Slot Antennas Employing Liquid-Metal Capacitive Loading

A Miniaturized Microfluidically Reconfigurable Coplanar Waveguide Bandpass Filter With Maximum Power Handling of 10 Watts

A Microfluidically Reconfigurable Dual-Band Slot Antenna With a Frequency Coverage Ratio of 3:1