A wearable patch for continuous monitoring of sweat electrolytes during exertion.

Implementation of wearable sweat sensors for continuous measurement of fluid based biomarkers (including electrolytes, metabolites and proteins) is an attractive alternative to common, yet intrusive and invasive, practices such as urine or blood analysis. Recent years have witnessed several key demonstrations of sweat based electrochemical sensing in wearable formats, however, there are still significant challenges and opportunities in this space for clinical acceptance, and thus mass implementation of these devices. For instance, there are inherent challenges in establishing direct correlations between sweat-based and gold-standard plasma-based biomarker concentrations for clinical decision-making. In addition, the wearable sweat monitoring devices themselves may exacerbate these challenges, as they can significantly alter sweat physiology (example, sweat rate and composition). Reported here is the demonstration of a fully integrated, wireless, wearable and flexible sweat sensing device for non-obtrusive and continuous monitoring of electrolytes during moderate to intense exertion as a metric for hydration status. The focus of this work is twofold: 1- design of a conformable fluidics systems to suit conditions of operation for sweat collection (to minimize sensor lag) with rapid removal of sweat from the sensing site (to minimize effects on sweat physiology). 2- integration of Na+ and K+ ion-selective electrodes (ISEs) with flexible microfluidics and low noise small footprint electronics components to enable wireless, wearable sweat monitoring. While this device is specific to electrolyte analysis during intense perspiration, the lessons in microfluidics and overall system design are likely applicable across a broad range of analytes.

Influence of substrate elasticity on droplet impact dynamics.

Droplet impact dynamics is vital to the understanding of several phase-change and heat-transfer phenomena. This work examines the role of substrate elasticity on the spreading and retraction behavior of water droplets impacting flat and textured superhydrophobic substrates. Experiments reveal that droplet retraction on flat surfaces decreases with decreasing substrate elasticity. This trend is confirmed through a careful measurement of droplet impact dynamics on multiple PDMS surfaces with varying elastic moduli and comparison with impact dynamics on hard silicon surfaces. These findings reveal that surfaces tend to become more wettable upon droplet impact as the elastic modulus is decreased. First-order analyses are developed to explain this reduced retraction in terms of increased viscoelastic dissipation on soft substrates. Interestingly, superhydrophobic surfaces display substrate-elasticity-invariant impact dynamics. These findings are critical when designing polymeric surfaces for fluid-surface interaction applications.

Freezing of water next to solid surfaces probed by infrared-visible sum frequency generation spectroscopy.

Ice formation next to solid surfaces is important in many biological, materials, and geological phenomena and may be a factor in how they impact various technologies. We have used sum frequency generation (SFG) spectroscopy to study the structure of ice as well as the freezing and melting transition temperatures of water in contact with sapphire substrates. We have observed that the structure of ice and water are a function of pH and the surface charge of the sapphire substrate. At low pH, we observed an increase in the SFG signal subsequent to ice formation. Contrary to expectations, at pH 9.8, corresponding to a negatively charged surface, the intensity of the ice SFG signal is about 10 times lower than that of water. Recent simulation studies have suggested that charge transfer is important for the high intensity of the ice peak at the ice-air interface. We believe that the segregation of sodium ions next to the negatively charged sapphire substrate may be responsible for disrupting the charge transfer and stitching bilayer at high pH, providing a plausible explanation for the experimental observations. Even though the structure of water and ice are affected by pH, the freezing and melting transition temperatures are independent of the surface charge. This report offers a unique insight on how ions next to solid surfaces could influence the structure of ice.

Dynamics of ice nucleation on water repellent surfaces.

Prevention of ice accretion and adhesion on surfaces is relevant to many applications, leading to improved operation safety, increased energy efficiency, and cost reduction. Development of passive nonicing coatings is highly desirable, since current antiicing strategies are energy and cost intensive. Superhydrophobicity has been proposed as a lead passive nonicing strategy, yet the exact mechanism of delayed icing on these surfaces is not clearly understood. In this work, we present an in-depth analysis of ice formation dynamics upon water droplet impact on surfaces with different wettabilities. We experimentally demonstrate that ice nucleation under low-humidity conditions can be delayed through control of surface chemistry and texture. Combining infrared (IR) thermometry and high-speed photography, we observe that the reduction of water-surface contact area on superhydrophobic surfaces plays a dual role in delaying nucleation: first by reducing heat transfer and second by reducing the probability of heterogeneous nucleation at the water-substrate interface. This work also includes an analysis (based on classical nucleation theory) to estimate various homogeneous and heterogeneous nucleation rates in icing situations. The key finding is that ice nucleation delay on superhydrophobic surfaces is more prominent at moderate degrees of supercooling, while closer to the homogeneous nucleation temperature, bulk and air-water interface nucleation effects become equally important. The study presented here offers a comprehensive perspective on the efficacy of textured surfaces for nonicing applications.

Adhesion of liquid droplets to rough surfaces.

We study the adhesion of liquid droplets to rough surfaces, focusing on how adhesion changes with surface chemistry and roughness. For hydrophobic surfaces (equilibrium contact angle θ(e)>90°), although increasing surface roughness augments apparent contact angle, it does not necessarily always reduce adhesion. In a domain defined by roughness and equilibrium contact angle, this study identifies regions where adhesion increases or decreases with increasing roughness. The two regions do not border at θ(e)=90°. It is found that making surfaces with low roughness ratio (close to 1) does not reduce adhesion unless the surface material is highly hydrophobic (θ(e)>120°). In other words, to reduce adhesion for existing hydrophobic materials (90°<θ(e)<120°), high roughness ratios are needed. Additionally, to reduce adhesion, the geometry of microstructures should be designed such that wetted fraction decreases with increasing roughness ratio. This study is of particular importance for the design of textured superhydrophobic surfaces.

Multiscale modeling of the surfactant mediated synthesis and supramolecular assembly of cobalt nanodots.

Multiscale simulations are used to bridge the surfactant templated assembly of individual approximately 1-10 nm cobalt dots, to their ordering into supramolecular arrays. Potential energy surfaces derived from ab initio calculations are input to lattice Monte Carlo simulations at atomic scales. By this process we quantitatively reproduce the experimental cobalt nanoparticle sizes. Crucially, we find that there is an effective short range attraction between pairs of nanodots. Mesoscale simulations show that these attractive interdot potentials are so short ranged that the dots can assemble only into orientally ordered hexatic phases as in the experiments.