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The occasional use of chemical warfare agents (CWAs) by rogue states in current conflicts provides a reminder that these hazards are a real threat. Although hazmat suits made of fully impermeable b...

Liquid metal (LM)-based thermal interface materials (TIMs) have the potential to dissipate high heat loads in modern electronics and often consist of LM microcapsules embedded in a polymer matrix. The shells of these microcapsules consist of a thin LM oxide that forms spontaneously. Unfortunately, these oxide shells degrade heat transfer between LM capsules. Thus, rupturing these oxide shells to release their LM and effectively bridge the microcapsules is critical for achieving the full potential of LM-based TIMs. While this process has been studied from an electrical perspective, such results do not fully translate to thermal applications because electrical transport requires only a single percolation path. In this work, we introduce a novel method to study the rupture mechanics of beds composed solely of LM capsules. Specifically, by measuring the electrical and thermal resistances of capsule beds during compression, we can distinguish between the pressure at which capsule rupture initiates and the pressure at which widespread capsule rupture occurs. These pressures significantly differ, and we find that the pressure for widespread rupture corresponds to a peak in thermal conductivity during compression; hence, this pressure is more relevant to LM thermal applications. Next, we quantify the rupture pressure dependence on LM capsule age, size distribution, and oxide shell chemical treatment. Our results show that large freshly prepared capsules yield higher thermal conductivities and rupture more easily. We also show that chemically treating the oxide shell further facilitates rupture and increases thermal conductivity. We achieve a thermal conductivity of 16 W m

Self-regulating membranes that adjust mass transfer in response to environmental stimuli occur in nature and have a wide range of potential industrial applications. Inspired by the vapor transport regulation by stomata on cactus epidermal surfaces, a novel type of such membrane that regulates vapor transport based on the widening of nanofissures in a wax coating of a swelling material was recently introduced. Here it is shown that the network of epicuticular wax microfissures on the surface of Opuntia cactus cladodes exhibits equivalent seasonal hydration-induced morphology and vapor transport modulation. Using wettability measurements during controlled dehydration experiments and using microscale imaging of plants harvested during the wet and dry seasons, it is shown that the average external width of these microfissures decreases from approximately 15 μm down to 6 μm. Using simulations, it is estimated that dehydration decreases the vapor transport across the fracture network by 20–30%. Consequently, the plant-hydration-dependent surface morphological changes induce moderate vapor permeation modulation of the microfissure network but cannot be considered to have an on/off type of response. Nevertheless, these fissures act as secondary vapor pathways, providing an illustration of natural hierarchical design employed to mediate water vapor loss from the surface.

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.

Foaming of gallium-based liquid metals improves their processability and—seemingly in contrast to processing of other metal foams—can be achieved through shear-mixing in air without addition of solid microparticles. Resolving this discrepancy, systematic processing–structure–property characterization demonstrates that many crumpled oxide particles are generated prior to air bubble accumulation.

This work uses elementary theoretical arguments to estimate whether softening of the surface could be used, along with surface texture and chemistry, to control superheat required for onset of nucleate boiling. For an ideal, smooth surface a mild decrease of the required superheat is predicted. In turn, an approximate closed-form model of vapor trapping and bubble seeding from soft surface with conical cavities shows linear dependence between the required superheat and the substrate’s shear modulus. Based on these results, considerations involved in implementing soft coatings for boiling applications and relevant outstanding fundamental questions are also briefly discussed.

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.

A multi-fold enhancement in the rate of heat transfer can be achieved by promoting the dropwise condensation mode (DWC) over the filmwise condensation mode. Recent material developments are increasing the chances that DWC will transition into industrial applications. Consequently, the ability to quantitatively model heat transfer rate during DWC of water and low surface tension liquids will become increasingly important in design of condensers. DWC heat transfer models developed so far consider only conduction inside the condensate droplets. However, scaling analysis shows that, in contrast to buoyancy driven flow, thermocapillary flow could be present in a wide range of droplet sizes in industrially relevant conditions. In the present work, we theoretically quantify the effect of Marangoni convection on heat transfer across individual condensing droplets as well as its impact on the overall DWC heat transfer. Specifically, we use Finite Element simulations to estimate the change in heat transfer that thermocapillary flow induces in condensing drops with spherical cap geometry. Besides water, we also study heat transfer across drops of organic liquids including toluene, ethanol, and pentane. Our results indicate that heat transfer rates across droplets are higher in the conjugate heat transfer case than the conduction only case (up to 6-fold increase for large water droplets on hydrophobic and superhydrophobic surfaces under extreme subcooling of 50 K). However, irrelevant of fluid and contact angle, for smaller droplets with radius below 100 µm at most a twofold thermocapillary heat transfer enhancement was obtained. When integrated with the dropsize distribution, these multi-fold increases in heat transfer across individual drops translate in most cases to a minor 10% or lower increase in the overall dropwise condensation heat transfer coefficient. Thus, with exception of a few special cases, the Marangoni flow contribution to DWC heat transfer coefficient is on the order of typical experimental uncertainties and can be neglected.

Promoting dropwise condensation (DWC) could improve the efficiency of many industrial systems. Consequently, a lot of effort has been dedicated to finding durable materials that could sustainably promote DWC as well as finding routes to enhance the heat transfer rate during this phase change process. Motivated by previous reports of substrate softening increasing droplet nucleation rate, here we investigated how mechanical properties of a substrate impact relevant droplet-surface interactions and DWC heat transfer rate. Specifically, we experimentally quantified the effect of hydrophobic elastomer's shear modulus on droplet nucleation density and shedding radius. To quantify the impact of substrate softening on heat transfer through individual droplets, we combined analytical solution of elastomer deformation induced by droplets with finite element modeling of the heat transfer process. By substituting these experimentally and theoretically derived values into DWC heat transfer model, we quantified the compounding effect of the substrate's mechanical properties on the overall heat transfer rate. Our results show that softening of the substrates below a shear modulus of 500 kPa results in a significant reduction in the condensation heat transfer rate. This trend is primarily driven by additional thermal resistance of the liquid posed by depression of the soft substrate.

This work reports new evidence of the versatility of soft and environmentally responsive micron-sized colloidal gel particles as stabilizers at ionic liquid-water droplet interfaces. These particles display a duality with properties ascribed typically to both polymeric and colloidal systems. The utilization of fluorescently labeled composite microgel particles allows in-situ and facile visualization without the necessity of invasive sample preparation. When the prepared particles form monolayers equilibrated at the ionic liquid-water interface on fully covered droplets, the colloidal lattice re-orders itself depending on the surface charge of these particles. Finally, we demonstrate that the spontaneously formed and densely packed layer of microgel particles can be employed for extraction applications, as the interface remains permeable to small active species.

The degradation of corrosion preventative coatings contributes to the high cost and time requirements for maintaining structures in harsh environments. However, the development of new hydrophobic coatings holds the promise of extending the usable life of structures in marine environments. In this work, we quantify the barrier properties and corrosion resistance of two novel highly hydrophobic polysiloxane formulations and the legacy silicone alkyd topcoat used on the topside of Navy’s ships, all with haze gray pigmentation. Based on FTIR-ATR and EIS measurements of the pristine coatings, both the single- (1K) and the two-component (2K) polysiloxane provide significantly improved barrier characteristics (lower water diffusion coefficient and capacitance) than the silicone alkyd. These results were confirmed through a 3-month-long immersion corrosion test, which also showed that the 1K and 2K polysiloxane coatings had comparable degradation characteristic and remained highly hydrophobic.

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.

Focused ion beam and scanning electron microscope (FIB-SEM) instruments are extensively used to characterize nanoscale composition of composite materials, however, their application to analysis of organic corrosion barrier coatings has been limited. The primary concern that arises with use of FIB to mill organic materials is the possibility of severe thermal damage that occurs in close proximity to the ion beam impact. Recent research has shown that such localized artefacts can be mitigated for a number of polymers through cryogenic cooling of the sample as well as low current milling and intelligent ion beam control. Here we report unexpected nonlocalized artefacts that occur during FIB milling of composite organic coatings with pigment particles. Specifically, we show that FIB milling of pigmented polysiloxane coating can lead to formation of multiple microscopic voids within the substrate as far as 5 μm away from the ion beam impact. We use further experimentation and modelling to show that void formation occurs via ion beam heating of the pigment particles that leads to decomposition and vaporization of the surrounding polysiloxane. We also identify FIB milling conditions that mitigate this issue.

As heatwaves become more frequent and intense, personal cooling becomes increasingly important for maintaining outdoor activities and for individuals without access to air conditioning. For about one-third of the current global population living in drylands, evaporating water from clothing is the simplest method of augmenting natural thermoregulation. To cool off, one can simply wear a water-soaked cotton shirt or a highly water-absorbing commercial cooling garment. However, of the stored water, the vast majority is wasted if such apparel is exposed to solar radiation or even slow air flow. Here I show that this issue can be mostly mitigated by incorporating sun and wind shading elements over surface of the cooling garment. First, to enable rational design of these multifunctional shading elements, I develop and benchmark a comprehensive multiphysics finite element model. This model couples conductive, convective, evaporative, and radiative heat transfer with mass transport in natural or forced laminar flow. In the case of natural convection, the model accounts for air buoyancy induced by both temperature and water vapor concentration, which in conditions of interest have a competing effect that can induce flow reversal. Second, I use the model to quantify the impact of geometry and radiative properties of louver and slitted shades on the performance of an evaporative cooling vest in hot and arid conditions. Under natural convection conditions, wearer cooling and water usage efficiency are optimized by introducing about 1.5 cm ventilation gap between the vest surface and the shading structures. In forced convection conditions, however, such a gap results in excessive evaporation rates that are highly wind-speed dependent. Based on these results, I propose a slitted shade design with a collapsible ventilation gap that can provide nearly sun and wind independent moderate cooling rate. If required due to high wearer exertion rate, the intelligently shaded evaporative vest could also provide a higher cooling rate by maintaining the gap. This shaded evaporative vest design concept can minimize the weight of the water stored in the garment and/or significantly increase its cooling period.

Thermal contact resistance at the finger-object interface plays a significant role in our thermal perception and is an important parameter for the design of a myriad of electronics and thermal devices. Currently, its value is measured experimentally or, more commonly, is estimated using a semi-empirical model. This model was developed by Cooper, Mikic, and Yovanovich (CMY) in the 1960s for predicting contact resistance of metal-metal interfaces in a vacuum. In this work, it is shown that measured value of finger-object contact resistance is better predicted by a more recent correlation by Prasher and Matayabas (PM) that was developed by fitting contact resistance data for silicone gel-metal surface interfaces in microelectronic applications. Furthermore, it is show that the functional form of the empirical PM correlation can be derived using scale analysis of the finger-solid contact scenario, consequently can be considered a physics-based model. Comparing the two models against two previously published experimental data sets demonstrates that the PM model predicts well the thermal resistance between finger and variety of materials over a wide range of contact pressures. Specifically, for finger contact with significantly more conductive materials (thermal conductivity above 1 Wm−1K−1) including aluminum, BaF2 crystal, and marble a good prediction of contact resistance can be attained. For skin contact with less conductive materials, such as wood, both models become highly sensitive to the substrate’s thermal conductivity value and provide only an order of magnitude estimate. The main implications of these results and relevant outstanding questions are also briefly discussed.

Room-temperature liquid metals (LMs) are attractive candidates for thermal interface materials (TIMs) because of their moderately high thermal conductivity and liquid nature, which allow them to conform well to mating surfaces with little thermal resistance. However, gallium-based LMs may be of concern due to the gallium-driven degradation of many metal microelectronic components. We present a three-component composite with LM, copper (Cu) microparticles, and a polymer matrix, as a cheaper, noncorrosive solution. The solid copper particles alloy with the gallium in the LM, in situ and at room temperature, immobilizing the LM and eliminating any corrosion issues of nearby components. Investigation of the structure-property-process relationship of the three-component composites reveals that the method and degree of additive blending dramatically alter the resulting thermal transport properties. In particular, microdispersion of any combination of the LM and Cu additives results in a large number of interfaces and a thermal conductivity below 2 W m

Inhibition of frost formation is important for increasing efficiency of refrigeration systems and heat exchangers, as well as for preventing the rapid icing over of water-repellant coatings that are designed to prevent accumulation of rime and glaze. From a thermodynamic point of view, this task can be achieved by either increasing hydrophobicity of the surface or decreasing the concentration of water vapor above it. The first approach has been studied in depth, but so far has not yielded a robust solution to the problem of frost formation. In this work, we systematically explore how frost growth can be inhibited by controlling water vapor concentration using bilayer coatings with a porous exterior covering a hygroscopic liquid-infused layer. We lay the theoretical foundation and provide experimental validation of the mass transport mechanism that governs the integral humidity sink effect based on this coating platform as well as reveal intriguing sizing effects about this system. We show that the concentration profile above periodically spaced pores is governed by the sink and source concentrations and two geometrical parameters: the nondimensional pore size and the ratio of the pore spacing to the boundary layer thickness. We demonstrate that when the ratio of the pore spacing to the boundary layer thickness vanishes, as for the nanoporous bilayer coatings, the entire surface concentration becomes uniform and equal to the concentration set by the hygroscopic liquid. In other words, the surface concentration becomes completely independent of the nanopore size. We identified the threshold geometrical parameters for this condition and show that it can lead to a 65 K decrease in the nucleation onset surface temperature below the dew point. With this fundamental insight, we use bilayer coatings to nanoengineer the integral humidity sink effect to provide extreme antifrosting performance with up to a 2 h delay in nucleation onset at 263 K. The nanoporous bilayer coatings can be designed to combine optimal antifrosting functionality with a superhydrophobic water repelling exterior to provide coatings that can robustly prevent frost, rime, and glaze accumulation. By minimizing the required amount of antifreeze, this anti-icing method can have minimal operational cost and environmental impact.

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.

Many varieties of the cabbage family have leaves covered with superhydrophobic epicuticular wax, which provides them with self-cleaning characteristics. Since the wax also lowers insect adhesion, rinsing of the leaves with water should be an effective way of removing the insects. Conversely, we report that superhydrophobicity of tuscan kale increases resistance of aphids to hydrodynamic removal. The exterior surface of the insects is also superhydrophobic and acts as an extension of the leaf's surface. As a result even at moderate impact velocities impinging water drops cannot penetrate under the pests. Consequently, liquid impact aids the insect's adhesion by increasing the normal compressive forces they experience. We show that on a hydrophilic arugula leaf this mechanism is absent, and aphids can be easily washed off with water, as it is able to penetrate underneath them. As for removal of aphids from Tuscan kale, we show that lower surface tension liquids, such as oils and soapy water are more effective, because they are able to wet both the plant and insect surfaces. We also show that aerodynamic removal of aphids consisting of simply exposing the invaded leaf to an air flow is most effective.

Insects of the order Embioptera, known as embiopterans, embiids, or webspinners, weave silk fibers together into sheets to make shelters called galleries. In this study, we show that silk galleries produced by the embiopteran Antipaluria urichi exhibit a highly hydrophobic wetting state with high water adhesion macroscopically equivalent to the rose petal effect. Specifically, the silk sheets have advancing contact angles above 150°, but receding contact angle approaching 0°. The silk sheets consist of layered fiber bundles with single strands spaced by microscale gaps. Scanning and transmission electron microscopy (SEM, TEM) images of silk treated with organic solvent and gas chromatography mass spectrometry (GC-MS) of the organic extract support the presence of a lipid outer layer on the silk fibers. We use cryogenic SEM to demonstrate that water drops reside on only the first layer of the silk fibers. The area fraction of this sparse outer silk layers is 0.1 to 0.3, which according to the Cassie-Baxter equation yields an effective static contact angle of ∼130° even for a mildly hydrophobic lipid coating. Using high magnification optical imaging of the three phase contact line of a water droplet receding from the silk sheet, we show that the high adhesion of the drop stems from water pinning along bundles of multiple silk fibers. The bundles likely form when the drop contact line is pinned on individual fibers and pulls them together as it recedes. The dynamic reorganization of the silk sheets during the droplet movement leads to formation of "super-pinning sites" that give embiopteran silk one of the strongest adhesions to water of any natural hydrophobic surface.

Rapid surface oxidation of gallium-based liquid metals complicates their manipulation but can also be used to stabilize them into 3D shapes. We show that GaInSn can readily flow within such structures. The oxide skin microchannel walls are flexible and, if ruptured, are restored through oxidation of exposed liquid metal. These flexible-wall microchannels can be repeatedly deflated and refilled with the liquid metal.

Condensation on liquids has been studied extensively in context of breath figure templating, materials synthesis and enhancing heat transfer using liquid impregnated surfaces. However, the mechanics of nucleation and growth on liquids remains unclear, especially on liquids that spread on the condensate. By examining the energy barriers of nucleation, we provide a framework to choose liquids that can lead to enhanced nucleation. We show that due to limits of vapor sorption within a liquid, nucleation is most favoured at the liquid–air interface and demonstrate that on spreading liquids, droplet submergence within the liquid occurs thereafter. We provide a direct visualization of the thin liquid profile that cloaks the condensed droplet on a liquid impregnated surface and elucidate the vapour transport mechanism in the liquid films. Finally, we show that although the viscosity of the liquid does not affect droplet nucleation, it plays a crucial role in droplet growth., MIT Energy Initiative, Masdar Institute of Science and Technology (Grant 69238330)

Controlling water droplet motion on a surface is important for facilitating or improving the efficiency of many processes. Irrespective of the external force inducing the motion, surface wettability plays a vital role in this process. In this work, we study the effect of changing the length scale of chemical heterogeneities on wetting and droplet dynamics during the impact and condensation on surfaces with alternating, equal sized hydrophilic and hydrophobic stripes. We show that as the width of the stripes decreases to nanoscale, the surface shows near isotropic wettability. Specifically, we demonstrate that the difference between the advancing contact angle, sliding angle, and contact angle hysteresis measured parallel and perpendicular to the stripes is negligible for the nano-striped surface. Moreover, we show that the droplet dynamics during the impact and condensation on the nano-striped surfaces are similar to those observed on a chemically homogeneous surface with equivalent wettability.