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Gallium-based liquid metals (LMs) combine metallic properties with the deformability of a liquid, which makes them promising candidates for a variety of applications. To broaden the range of physical and chemical properties, a variety of solid additives have been incorporated into the LMs in the literature. In contrast, only a handful of secondary fluids have been incorporated into LMs to create foams (gas-in-LM) or emulsions (liquid-in-LM). LM foams readily form through mixing of LM in air, facilitated by the formation of a native oxide on the LM. In contrast, LM breaks up into microdroplets when mixed with a secondary liquid such as silicone oil. Stable silicone oil-in-LM emulsions form only during mixing of the oil with LM foam. In this work, we investigate the fundamental mechanism underlying this process. We describe two possible microscale mechanisms for emulsion formation: (1) oil replacing air in the foam or (2) oil creating additional features in the foam. The associated foam-to-emulsion density difference demonstrates that emulsions predominantly form through the addition of oxide-covered silicone oil capsules to the LM foam. We demonstrate this through density and surface wettability measurements and multiscale imaging of LM foam mixed with varied silicone oil contents in air or nitrogen environments. We also demonstrate the presence of a continuous silicone oil film on the emulsion surface and that this oil film prevents the embrittlement of contacting aluminum.

Preliminary human studies show that reduced skin temperature minimises the risk of mechanically-induced skin damage. However, the mechanisms by which cooling enhances skin tolerance to pressure and shear remain poorly understood. We hypothesized that skin cooling below thermo-neutral conditions will decrease friction at the skin-material interface. To test our hypothesis, we measured the friction coefficient of a thermally pre-conditioned index finger sliding at a normal load (5N) across a plate maintained at three different temperatures (38, 24, and 16□). To quantify the temperature distribution of the skin tissue, we used 3D surface scanning and Optical Coherence Tomography to develop an anatomically-representative thermal model of the finger. Our data indicated that the sliding finger with thermally affected tissues (up to 8mm depth) experienced significantly (p

Radiation accounts for a significant fraction of the human body and environment heat exchange and strongly impacts thermal comfort and safety. The direct radiative exchange between an individual and a source or sink can be quantified using the effective (f

Thermal comfort research has utilized various sensors and models to estimate the mean radiant temperature (MRT) experienced by a human, including the standard black globe thermometer (SGT), acrylic globe thermometers (AGT), and cylindrical radiation thermometers (CRT). Rather than directly measuring radiation, a temperature is measured in the center of these low-cost sensors that can be related to MRT after theoretically accounting for convection. However, these sensors have not been systematically tested under long-term hot and clear conditions. Further, under variable weather conditions, many issues can arise due to slow response times, shape, inaccuracies in material properties and assumptions, and color (albedo, emissivity) inconsistencies. Here, we assess the performance of MRT produced by various heat transfer models, with and without new average surface temperature ( $$ {\overline{T}}_{\mathrm{s}} $$ ) correction factors, using five instruments—the SGT (15 cm, black), tan and black CRTs, gray and black 38 mm AGTs—compared to 3D integral radiation measurements. Measurements were taken on an unobscured roof throughout summer-to-early-fall months in Tempe, Arizona, examining 58 full-sun days. Deviations without correcting for asymmetrical surface heating—found to be the main cause of errors—reached ± 15–20 °C MRT. By accounting for asymmetric heating through $$ {\overline{T}}_{\mathrm{s}} $$ calculations, new corrective algorithms were derived for the low-cost sensor models. Results show significant improvements in the estimated MRT error for each sensor (i.e., ∆MRTmodel − IRM) when applying the $$ {\overline{T}}_{\mathrm{s}} $$ corrections. The tan MRTCRT improved from 1.9 ± 6.2 to −0.1 ± 4.4 °C, while the gray AGT and SGT showed improvements from −1.6 ± 7.2 to −0.4 ± 6.3 °C and − 6.6 ± 6.4 to − 0.03 ± 5.7 °C, respectively. The new corrections also eliminated dependence on other meteorological factors (zenith, wind speed). From these results, we provide three simple equations for CRT, AGT, and SGT correction for future research use under warm-hot and clear conditions. This study is the most comprehensive empirical assessment of various low-cost instruments with broad applicability in urban climate and biometeorological research.

Gallium based liquid metals (LM) have prospective biomedical, stretchable electronics, soft robotics, and energy storage applications, and are being widely adopted as thermal interface materials. The danger of gallium corroding most metals used in microelectronics requires the cumbersome addition of "barrier" layers or LM break-up into droplets within an inert matrix such as silicone oil. Such LM-in-oil emulsions are stabilized by native oxide on the droplets but have decreased thermal performance. Here we show that mixing of the silicone oil into an LM-air foam yields emulsions with inverted phases. We investigate the stability of these oil-in-LM emulsions through a range of processing times and oil viscosities, and characterize the impact of these parameters on the materials' structure and thermal property relationships. We demonstrate that the emulsion with 40 vol% of 10 cSt silicone oil provides a unique thermal management material with a 10 W m-1 K-1 thermal conductivity and an exterior lubricant thin film that completely prevents corrosion of contacting aluminum.

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.

Polymer matrix composites containing room temperature liquid metal (LM) microdroplets offer a unique set of thermo-mechanical characteristics that makes them attractive candidates for high performance thermal interface materials. However, to achieve the desired level of the composite thermal conductivity, effective bridging of such fillers into interconnected percolation networks needs to be induced. Thermal percolation of the LM microdroplets requires two physical barriers to be overcome. First, the LM microdroplets must directly contact each other through the polymer matrix. Second, the native oxide shell on the LM microdroplet must also be ruptured. In this work, we demonstrate that both physical barriers can be penetrated to induce ample bridging of the LM microdroplets and thereby achieve higher thermal conductivity composites. We accomplish this through a synergistic combination of solid silver and LM fillers, tuning of the silicone oil "matrix" viscosity, and sample compression. We selected silver as the solid additive because it rapidly alloys with gallium to form microscale needles that could act as additional paths that aid in connecting the LM droplets. We systematically explore the impact of the composition (filler type, volume fraction, and matrix oil viscosity) and applied pressure on the thermal conductivity and multiscale structure of these composites. We reveal the microscopic mechanism underlying the macroscopic experimental trends and also identify an optimal composition of the multiphase Ag-LM-Silicone oil composite for thermal applications. The identified design knobs offer path for developing tunable LM-based polymer composites for microelectronics cooling, biomedical applications, and flexible electronics.

The understanding of heat conduction during finger contact with cooler or hotter objects is important for designing many electronic devices and for setting safety standards in a variety of occupational settings. In the most common experimental approach to study this process, a micro-thermocouple is placed at the finger-object interface. However, the interpretation of what this measurement corresponds to is not clear. To this end, we develop a three-dimensional thermal simulation of the finger-thermocouple-substrate configuration. The model predictions match finger cooling measurements in eight distinct cases available in prior literature (finger pressed with 1 N or 9.8 N against a steel or an aluminum substrate held at -2 °C or -10 °C). We demonstrate that the thermocouple can be represented accurately as a truncated sphere with emerging cylindrical wires while a multilayer block model of the finger provides similar results to an anatomically representative model. Our simulations show that in the eight previously studied cooling cases, the average surface temperature of skin that is in contact with the substrate follows nearly the same but offset cooling trend as the thermocouple tip temperature. The value of the offset is predominantly determined by the substrate material, with the thermocouple tip temperature being lower than the average skin surface temperature by 1-5 °C and 3-10 °C for steel and aluminum substrate cases, respectively. This temperature difference results in a moderate to an extreme thermocouple underprediction of the time necessary for the skin surface to reach the experimental safety threshold of 1 °C. Consequently, from the perspective of the safety related applications the thermocouple measurement provides a conservative limit on the contact duration and thereby is suitable for such purposes, but for applications requiring accurate skin temperature measurements alternative experimental approaches should be used.

Thermal comfort research has utilized various sensors and models to estimate the mean radiant temperature (MRT) experienced by a human, including the standard black globe thermometer (SGT), acrylic globe thermometers (AGT), and cylindrical radiation thermometers (CRT). Rather than directly measuring radiation, a temperature is measured in the center of these low-cost sensors that can be related to MRT after theoretically accounting for convection. However, these sensors have not been systematically tested under long-term hot and clear conditions. Further, under variable weather conditions, many issues can arise due to slow response times, shape, inaccuracies in material properties and assumptions, and color (albedo, emissivity) inconsistencies. Here, we assess the performance of MRT produced by various heat transfer models, with and without new average surface temperature ([Formula: see text]) correction factors, using five instruments-the SGT (15 cm, black), tan and black CRTs, gray and black 38 mm AGTs-compared to 3D integral radiation measurements. Measurements were taken on an unobscured roof throughout summer-to-early-fall months in Tempe, Arizona, examining 58 full-sun days. Deviations without correcting for asymmetrical surface heating-found to be the main cause of errors-reached ± 15-20 °C MRT. By accounting for asymmetric heating through [Formula: see text] calculations, new corrective algorithms were derived for the low-cost sensor models. Results show significant improvements in the estimated MRT error for each sensor (i.e., ∆MRT

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.

The balanced design of extreme heat adaptation strategies that engineer the radiative properties of infrastructure or human clothing requires realistic methods of spectrally and spatially resolving irradiation from a complex built environment on the human body. Current irradiation measurement methods oversimplify the human body as points, spheres, or cylinders. In contrast, computational thermal manikins often used in convective simulations resolve the 3D body shape but have predominantly been used to model radiation exchange only with a large isothermal surrounding. To bridge this gap, we formulated six anisotropic radiation source models for thermal manikin simulations based on net-radiometer 6-directional, acrylic globe, and cylindrical radiation thermometer measurements. In some cases, we augmented the models with solar zenith angle and theoretical global solar irradiation. The models predict similar diffuse longwave (LW) as well as direct and diffuse shortwave (SW) radiation for measurements taken during a representative hot, sunny day with an unobstructed sky view. For three times of the day, we simulated irradiation distributions on an average male thermal manikin. Comparing the resulting irradiation maps showed that the direct SW component should be based on solar direction instead of the 6-directional "box" approach. Results also highlight the importance of the diffuse irradiation implementation method. However, experimental benchmarking (e.g., using physical thermal manikins in real-world settings) is required to determine whether the volumetric, box, or spherical diffuse source specification is the most realistic approach. The introduced techniques can be applied to realistically simulate irradiation from real-world built environments on a wide range of individuals.

Highly absorbent polymers capable of trapping large volumes of organic liquids are attractive materials for cleaning up toxic chemical spills. Here we report development of a homopolymer network using N-butyl-N-phenylacrylamide with poly(ethylene glycol) dimethacrylate as the cross-linker, that absorbs a variety of organic liquids by swelling. The nonelectrolytic basis of this polymer allows it to be used in areas where organic liquids and water coexist in the form of an emulsion. We demonstrate that this new polymer swells up to 20× in nonpolar organic liquids and up to 33× in more polar organic liquids but has negligible swelling in water. Consequently, this novel selectively superabsorbent polymer has potential use in a broad range of applications to capture and remove hazardous materials.

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.

“Deadly heat waves: it’s only getting worse”, a headline from this summer, succinctly summarizes recent weather and predictions on its future patterns. According to World Metrological Organization,...

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.

The formation of frost and ice can have negative impacts on travel and a variety of industrial processes and is typically addressed by dispensing antifreeze substances such as salts and glycols. Despite the popularity of this anti-icing approach, some of the intricate underlying physical mechanisms are just being unraveled. For example, recent studies have shown that in addition to suppressing ice formation within its own volume, an individual salt saturated water microdroplet forms a region of inhibited condensation and condensation frosting (RIC) in its surrounding area. This occurs because salt saturated water, like most antifreeze substances, is hygroscopic and has water vapor pressure at its surface lower than water saturation pressure at the substrate. Here, we demonstrate that for macroscopic drops of propylene glycol and salt saturated water, the absolute RIC size can remain essentially unchanged for several hours. Utilizing this observation, we demonstrate that frost formation can be completely inhibited in-between microscopic and macroscopic arrays of propylene glycol and salt saturated water drops with spacing (S) smaller than twice the radius of the RIC (δ). Furthermore, by characterizing condensation frosting dynamics around various hygroscopic drop arrays, we demonstrate that they can delay complete frosting over of the samples 1.6 to 10 times longer than films of the liquids with equivalent volume. The significant delay in onset of ice nucleation achieved by dispensing propylene glycol in drops rather than in films is likely due to uniform dilution of the drops driven by thermocapillary flow. This transport mode is absent in the films, leading to faster dilution, and with that facilitated homogeneous nucleation, near the liquid-air interface.

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.

Thermally conductive soft composites are in high demand, and aligning the fill material is a potential method of enhancing their thermal performance. In particular, magnetic alignment of nickel particles has previously been demonstrated as an easy and effective way to improve directional thermal conductivity of such composites. However, the effect of compression on the thermal performance of these materials has not yet been investigated. This work investigates the thermal performance of magnetically aligned nickel fibers in a soft polymer matrix under compression. The fibers orient themselves in the direction of the applied magnetic field and align into columns, resulting in a 3× increase in directional thermal conductivity over unaligned composites at a volume fraction of 0.15. Nevertheless, these aligned fiber columns buckle under strain resulting in an increase in the composite thermal resistance. These results highlight potential pitfalls of magnetic filler alignment when designing soft composites for applications where strain is expected such as thermal management of electronics.

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

The risk of failure of electronic components due to tin (Sn) whiskers growth has become an issue with the current regulations limiting the use of lead in Sn solders. New strategies using engineered coatings for mitigating Sn whiskers are being developed. Typically, these coatings are evaluated by an aging process where whiskers are allowed to grow naturally. Unfortunately, this process can produce unreliable growth results and can take several years. Thus, faster, more reliable methods are needed. In this study, a simple, rapid (3–10 days), and cost-effective method was developed for testing the efficacy of nano-engineered coatings for mitigating the growth of Sn whiskers. This method consisted of a micro-indentation process using a ball-bearing adhered to a few hundred gram weight, which are placed in a stabilizing printed holder. For uncoated samples, Sn whiskers and hillocks were abundant near the indentation area, while only hillocks were found further outside the area (i.e., >0.2 mm). For samples coated with nano-engineered ceramic or polymeric coatings, the indentation method was observed to damage coatings only at the point of contact (e.g., no delamination), while still allowing Sn whiskers and hillocks to grow outside the indentation area.

The use of personal protective gear made from omniphobic materials that easily shed drops of all sizes could provide enhanced protection from direct exposure to most liquid-phase biological and chemical hazards and facilitate the postexposure decontamination of the gear. In recent literature, lubricated nanostructured fabrics are seen as attractive candidates for personal protective gear due to their omniphobic and self-healing characteristics. However, the ability of these lubricated fabrics to shed low surface tension liquids after physical contact with other objects in the surrounding, which is critical in demanding healthcare and military field operations, has not been investigated. In this work, we investigate the depletion of oil from lubricated fabrics in contact with highly absorbing porous media and the resulting changes in the wetting characteristics of the fabrics by representative low and high surface tension liquids. In particular, we quantify the loss of the lubricant and the dynamic contact angles of water and ethanol on lubricated fabrics upon repeated pressurized contact with highly absorbent cellulose-fiber wipes at different time intervals. We demonstrate that, in contrast to hydrophobic nanoparticle coated microfibers, fabrics encapsulated within a polymer that swells with the lubricant retain the majority of the oil and are capable of repelling high as well as low surface tension liquids even upon multiple contacts with the highly absorbing wipes. The fabric supported lubricant-swollen polymeric films introduced here, therefore, could provide durable and easy to decontaminate protection against hazardous biological and chemical liquids.

This article reviews recent developments and applications of two beam systems (focused ion beam [FIB] and scanning electron microscope [SEM]) for in situ characterization and manipulation of material at the micro- and nanoscale. In these applications, the sample may be manipulated, ion milled, mechanically or electrically excited, and its temperature varied from above room temperature to cryogenic levels. FIB-SEM instruments offer new opportunities for in situ characterization by enabling localized exposure of surface layers within the high vacuum microscope chamber environment (especially in conjunction with cryogenic cooling of the bulk sample), through experiments that require either highly accurate material removal or localized material addition through beam-induced gas deposition, and by using micro- and nano-manipulation technologies for probing or positioning. This article describes the current state of the art of this experimental methodology and provides case studies in the areas of cryogenic, electrical, and mechanical characterization.

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.

Ice accretion is an important problem and passive approaches for reducing ice-adhesion are of great interest in various systems such as aircrafts, power lines, wind turbines, and oil platforms. Here, we study the ice-adhesion properties of lubricant-impregnated textured surfaces. Force measurements show ice adhesion strength on textured surfaces impregnated with thermodynamically stable lubricant films to be higher than that on surfaces with excess lubricant. Systematic ice-adhesion measurements indicate that the ice-adhesion strength is dependent on texture and decreases with increasing texture density. Direct cryogenic SEM imaging of the fractured ice surface and the interface between ice and lubricant-impregnated textured surface reveal stress concentrators and crack initiation sites that can increase with texture density and result in lowering adhesion strength. Thus, lubricant-impregnated surfaces have to be optimized to outperform state-of-the-art icephobic treatments.

Frost formation is a major problem affecting a variety of industries including transportation, power generation, construction, and agriculture. Currently used active chemical, thermal, and mechanical techniques of ice removal are time-consuming and costly. The use of nanotextured coatings infused with perfluorinated oil has recently been proposed as a simple passive antifrosting and anti-icing method. However, we demonstrate that the process of freezing subcooled condensate and frost formation on such lubricant-impregnated surfaces is accompanied by the migration of the lubricant from the wetting ridge and from within the textured substrate to the surface of frozen droplets. For practical applications, this mechanism can comprise the self-healing and frost-repelling characteristics of lubricant impregnated-surfaces, regardless of the underlying substrate's topography. Thus, further research is necessary to develop liquid-texture pairs that will provide a sustainable frost suppression method.

Metal-assisted Chemical Etching of silicon has recently emerged as a powerful technique to fabricate 1D, 2D, and 3D nanostructures in silicon with high feature fidelity. This work demonstrates that out-of-plane rotational catalysts utilizing polymer pinning structures can be designed with excellent control over rotation angle. A plastic deformation model was developed establishing that the catalyst is driven into the silicon substrate with a minimum pressure differential across the catalyst thickness of 0.4-0.6 MPa. Force-displacement curves were gathered between an Au tip and Si or SiO(2) substrates under acidic conditions to show that Derjaguin and Landau, Verwey and Overbeek (DLVO) based forces are capable of providing restorative forces on the order of 0.2-0.3 nN with a calculated 11-18 MPa pressure differential across the catalyst. This work illustrates that out-of-plane rotational structures can be designed with controllable rotation and also suggests a new model for the driving force for catalyst motion based on DLVO theory. This process enables the facile fabrication of vertically aligned thin-film metallic structures and scalloped nanostructures in silicon for applications in 3D micro/nano-electromechanical systems, photonic devices, nanofluidics, etc.

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.