Fundamentals of soft thermofluidic system design.

The soft composition of many natural thermofluidic systems allows them to effectively move heat and control its transfer rate by dynamically changing shape (e.g. dilation or constriction of capillaries near our skin). So far, making analogous deformable "soft thermofluidic systems" has been limited by the low thermal conductivity of materials with suitable mechanical properties. By remaining soft and stretchable despite the addition of filler, elastomer composites with thermal conductivity enhanced by liquid-metal micro-droplets provide an ideal material for this application. In this work, we use these materials to develop an elementary thermofluidic system consisting of a soft, heat generating pipe that is internally cooled with flow of water and explore its thermal behavior as it undergoes large shape change. The transient device shape change invalidates many conventional assumptions employed in thermal design making analysis of this devices' operation a non-trivial undertaking. To this end, using time scale analysis we demonstrate when the conventional assumptions break down and highlight conditions under which the quasi-static assumption is applicable. In this gradual shape modulation regime the actuated devices' thermal behavior at a given stretch approaches that of a static device with equivalent geometry. We validate this time scale analysis by experimentally characterizing thermo-fluidic behavior of our soft system as it undergoes axial periodic extension-retraction at varying frequencies during operation. By doing so we explore multiple shape modulation regimes and provide a theoretical foundation to be used in the design of soft thermofluidic systems undergoing transient deformation.

Role of Scale Wettability on Rain-Harvesting Behavior in a Desert-Dwelling Rattlesnake.

During storms in the southwestern United States, several rattlesnake species have been observed drinking rain droplets collected on their dorsal scales. This process often includes coiling and flattening of the snake's body, presumably to enhance water collection. Here, we explored this rain-harvesting behavior of the Western Diamond-backed Rattlesnake (Crotalus atrox) from the perspective of surface science. Specifically, we compared surface wettability and texture, as well as droplet impact and evaporation dynamics on the rattlesnake epidermis with those of two unrelated (control) sympatric snake species (Desert Kingsnake, Lampropeltis splendida, and Sonoran Gopher Snake, Pituophis catenifer). These two control species are not known to show rain-harvesting behavior. Our results show that the dorsal scales of the rattlesnake aid in water collection by providing a highly sticky, hydrophobic surface, which pins the impacting water droplets. We show that this high pinning characteristic stems from surface nanotexture made of shallow, labyrinth-like channels.

Oxide-Mediated Formation of Chemically Stable Tungsten-Liquid Metal Mixtures for Enhanced Thermal Interfaces.

Modern microelectronics and emerging technologies such as wearable devices and soft robotics require conformable and thermally conductive thermal interface materials to improve their performance and longevity. Gallium-based liquid metals (LMs) are promising candidates for these applications yet are limited by their moderate thermal conductivity, difficulty in surface-spreading, and pump-out issues. Incorporation of metallic particles into the LM can address these problems, but observed alloying processes shift the LM melting point and lead to undesirable formation of additional surface roughness. Here, these problems are addressed by introducing a mixture of tungsten microparticles dispersed within a LM matrix (LM-W) that exhibits two- to threefold enhanced thermal conductivity (62 ± 2.28 W m-1 K-1 for gallium and 57 ± 2.08 W m-1 K-1 for EGaInSn at a 40% filler volume mixing ratio) and liquid-to-paste transition for better surface application. It is shown that the formation of a nanometer-scale LM oxide in oxygen-rich environments allows highly nonwetting tungsten particles to mix into LMs. Using in situ imaging and particle dipping experimentation within a focused ion beam and scanning electron microscopy system, the oxide-assisted mechanism behind this wetting process is revealed. Furthermore, since tungsten does not undergo room-temperature alloying with gallium, it is shown that LM-W remains a chemically stable mixture.

Droplet-train induced spatiotemporal swelling regimes in elastomers.

In this work, we perform a combined experimental and numerical analysis of elastomer swelling dynamics upon impingement of a train of solvent droplets. We use time scale analysis to identify spatiotemporal regimes resulting in distinct boundary conditions that occur based on relative values of the absorption timescale and the droplet train period. We recognize that when either timescale is significantly larger than the other, two cases of quasi-uniform swelling occur. In contrast, when the two timescales are comparable, a variety of temporary geometrical features due to localized swelling are observed. We show that the swelling feature and its temporal evolution depends upon geometric scaling of polymer thickness and width relative to the droplet size. Based on this scaling, we identify six cases of localized swelling and experimentally demonstrate the swelling features for two cases representing limits of thickness and width. A finite element model of local swelling is developed and validated with experimental results for these two cases. The model is subsequently used to explore the swelling behavior in the rest of the identified cases. We show that depending upon the lateral dimension of the sample, swelling can locally exhibit mushroom, mesa, and cap like shapes. These deformations are magnified during the droplet-train impact but dissipate during post-train polymer equilibration. Our results also show that while swelling shape is a function of lateral dimensions of the sample, the extent of swelling increases with the elastomer sample thickness.

Microscale Mechanism of Age Dependent Wetting Properties of Prickly Pear Cacti (Opuntia).

Cacti thrive in xeric environments through specialized water storage and collection tactics such as a shallow, widespread root system that maximizes rainwater absorption and spines adapted for fog droplet collection. However, in many cacti, the epidermis, not the spines, dominates the exterior surface area. Yet, little attention has been dedicated to studying interactions of the cactus epidermis with water drops. Surprisingly, the epidermis of plants in the genus Opuntia, also known as prickly pear cacti, has water-repelling characteristics. In this work, we report that surface properties of cladodes of 25 taxa of Opuntia grown in an arid Sonoran climate switch from water-repelling to superwetting under water impact over the span of a single season. We show that the old cladode surfaces are not superhydrophilic, but have nearly vanishing receding contact angle. We study water drop interactions with, as well as nano/microscale topology and chemistry of, the new and old cladodes of two Opuntia species and use this information to uncover the microscopic mechanism underlying this phenomenon. We demonstrate that composition of extracted wax and its contact angle do not change significantly with time. Instead, we show that the reported age dependent wetting behavior primarily stems from pinning of the receding contact line along multilayer surface microcracks in the epicuticular wax that expose the underlying highly hydrophilic layers.