Are Liquid Metals Bulk Conductors?

Stretchable electronics have potential in wide-reaching applications including wearables, personal health monitoring, and soft robotics. Many recent advances in stretchable electronics leverage liquid metals, particularly eutectic gallium-indium (EGaIn). A variety of EGaIn electromechanical behaviors have been reported, ranging from bulk conductor responses to effectively strain-insensitive responses. However, numerous measurement techniques have been used throughout the literature, making it difficult to directly compare the various proposed formulations. Here, the electromechanical responses of EGaIn found in the literature is reviewed and pure EGaIn is investigated using three electrical resistance measurement techniques: four point probe, two point probe, and Wheatstone bridge. The results indicate substantial differences in measured electromechanical behavior between the three methods, which can largely be accounted for by correcting for a fixed offset corresponding to the resistances of various parts of the measurement circuits. Yet, even accounting for several of these sources of experimental error, the average relative change in resistance of EGaIn is found to be lower than that predicted by the commonly used bulk conductor assumption, referred to as Pouillet's law. Building upon recent theories proposed in the literature, possible explanations for the discrepancies are discussed. Finally, suggestions are provided on experimental design to enable reproducible and interpretable research.

Soft Actuators Made of Discrete Grains.

Recent work has demonstrated the potential of actuators consisting of bulk elastomers with phase-changing inclusions for generating high forces and large volumetric expansions. Simultaneously, granular assemblies have been shown to enable tunable properties via different packings, dynamic moduli via jamming, and compatibility with various printing methods via suspension in carrier fluids. Herein, granular actuators are introduced, which represent a new class of soft actuators made of discrete grains. The soft grains consist of a hyperelastic shell and multiple solvent cores. Upon heating, the encapsulated solvent cores undergo liquid-to-gas phase change, inducing rapid and strong volumetric expansion of the hyperelastic shell up to 700%. The grains can be used independently for micro-actuation, or in granular agglomerates for meso- and macroscale actuation, demonstrating the scalability of the granular actuators. Furthermore, the active grains can be suspended in a carrier resin or solvent to enable printable soft actuators via established granular material processing techniques. By combining the advantages of phase-change soft actuation and granularity, this work presents the opportunity to realize soft actuators with tunable bulk properties, compatibility with self-assembly techniques, and on-demand reconfigurability.

In Situ and Real-Time Studies, via Synchrotron X-ray Scattering, of the Orientational Order of Cellulose Nanocrystals during Solution Shearing.

In this manuscript, we report on the ordering of the cellulose nanocrystals (CNCs) as they experience shear forces during the casting process. To achieve these measurements, in situ and in real time, we used synchrotron-based grazing incidence wide-angle X-ray scattering (GIWAX). We believe that the GIWAX technique, although not commonly used to probe these types of phenomena, can open new avenues to gain deeper insights into film formation processes and surface-driven phenomena. In particular, we investigated the influence of solution concentration, shear-cast velocity, and drying temperature on the ordering of cellulose nanocrystals (CNCs) using GIWAXS. The films were prepared from aqueous suspensions of cellulose nanocrystals at two concentration values (7 and 9 wt %). As the films were cast, the X-ray beam was focused on a fixed position and GIWAXS patterns were recorded at regular time intervals. Structural characterization of the dry films was carried out via polarized optical microscopy and scanning electron microscopy. In addition, a rheological study of the CNC suspensions was performed. Our results show that the morphology of the CNC films was significantly influenced by shear velocity, concentration of the precursor suspension, and evaporation temperature. In contrast, we observed that the orientation parameter of the films was not significantly affected. The scattering intensity of the peak (200) was analyzed as a function of time, following a sigmoidal profile, hence indicating short- and long-range interactions within the anisotropic domains as they reached their final orientation state. A model capable of describing the resulting film morphologies is also proposed. The results and analysis presented in this manuscript provide new insights into the controlled alignment of cellulose nanocrystals under shear. This controlled alignment has significant implications in the development of advanced coatings and films currently used in a myriad of applications, such as catalysis, optics, electronics, and biomedicine.

Oriented Growth of α-MnO2 Nanorods Using Natural Extracts from Grape Stems and Apple Peels.

We report on the synthesis of alpha manganese dioxide (α-MnO2) nanorods using natural extracts from Vitis vinifera grape stems and Malus domestica 'Cortland' apple peels. We used a two-step method to produce highly crystalline α-MnO2 nanorods: (1) reduction of KMnO4 in the presence of natural extracts to initiate the nucleation process; and (2) a thermal treatment to enable further solid-state growth of the nuclei. Transmission electron microscopy (TEM) and field emission scanning electron microscopy (FESEM) images provided direct evidence of the morphology of the nanorods and these images were used to propose nucleation and growth mechanisms. We found that the α-MnO2 nanorods synthesized using natural extracts exhibit structural and magnetic properties similar to those of nanoparticles synthesized via traditional chemical routes. Furthermore, Fourier transform infrared (FTIR) shows that the particle growth of the α-MnO2 nanorods appears to be controlled by the presence of natural capping agents during the thermal treatment. We also evaluated the catalytic activity of the nanorods in the degradation of aqueous solutions of indigo carmine dye, highlighting the potential use of these materials to clean dye-polluted water.

Nanowire-functionalized cotton textiles.

We show the general functionalization of cotton fabrics using solution-synthesized CdSe and CdTe nanowires (NWs). Conformal coatings onto individual cotton fibers have been achieved through various physical and chemical approaches. Some involve the electrostatic attraction of NWs to cotton charged positively with a Van de Graaff generator or via 2,3-epoxypropyltrimethylammonium chloride treatments. Resulting NW-functionalized textiles consist of dense, conformal coatings and have been characterized for their UV-visible absorption as well as Raman activity. We demonstrate potential uses of these functionalized textiles through two proof-of-concept applications. The first entails barcoding cotton using the unique Raman signature of the NWs. We also demonstrate the surface-enhancement of their Raman signatures using codeposited Au. A second demonstration takes advantage of the photoconductive nature of semiconductor NWs to create cotton-based photodetectors. Apart from these illustrations, NW-functionalized cotton textiles may possess other uses in the realm of medical, anticounterfeiting, and photocatalytic applications.