Exceptional Anti-Icing Performance of Self-Impregnating Slippery Surfaces

A heat exchange interface at subzero temperature in a water vapor environment, exhibits high probability of frost formation due to freezing condensation, a factor that markedly decreases the heat transfer efficacy due to the considerable thermal resistance of ice. Here we report a novel strategy to delay ice nucleation on these types of solid-water vapor interfaces. With a process-driven mechanism, a self-generated liquid intervening layer immiscible to water, is deposited on a textured superhydrophobic surface and acts as a barrier between the water vapor and the solid substrate. This liquid layer imparts remarkable slippery conditions resulting in high mobility of condensing water droplets. A large increase of the ensuing ice coverage time is shown compared to the cases of standard smooth hydrophilic or textured superhydrophobic surfaces. During deicing of these self-impregnating surfaces we show an impressive tendency of ice fragments to skate expediting defrosting. Robustness of such surfaces is also demonstrated by operating them under subcooling for at least 490hr without a marked degradation. This is attributed to the presence of the liquid intervening layer, which protects the substrate from hydrolyzation enhancing longevity and sustaining heat transfer efficiency.Comment: This document is the Accepted Manuscript version of a Published Work that appeared in final form in ACS Applied Materials & Interfaces, copyright (c) American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see pubs.acs.org/doi/abs/10.1021/acsami.7b0018

Thermoeconomic design optimization of a thermo-electric energy storage system based on transcritical CO2 cycles

The conceptual design of a thermo-electric energy storage (TEES) system for large scale electricity storage is discussed in this work by showing the results of the thermoeconomic optimization of three different system configurations that were identified in previous works. The system is based on transcritical CO2 cycles, water storage and salt-water ice storage and is designed for a capacity of 2 h discharge and equal charge and discharge power of 50 MW. A two-step optimization procedure is used. The system intensive design parameters are optimized at the master level through a genetic algorithm. The optimal cycle mass flow rates are calculated in a nested linear programming step where the heat integration between the cycles is optimized subject to the heat transfer feasibility imposed through Pinch Analysis cascade calculations. The synthesis of the heat exchanger network and of the storage tank systems was solved through a set of heuristic rules. Equipment purchasing costs were estimated by means of cost functions that were built upon vendors quotations. The results are discussed by showing the Pareto fronts of the three optimization cases and the trends of the decision variables along each optimal front. Nine solutions are discussed in more detail by showing the values of the design parameters and the process flow diagrams including storage and heat exchanger layouts. Design guidelines are then formulated which can be used in future works for detail plant design. The topological features that are found to maximize the system performance at the minimum costs are: superheating before the CO2 heat pump, two independent systems of hot water storage tanks above and below the ambient temperature, and air cooling at the heat pump side. The design parameters that affect significantly the costs and performances are the cycle pressures. These are in fact directly associated with temperature differences both at the cold and hot storage sides which should be carefully optimized to obtain the best trade-off between exergy losses and costs for heat exchangers. Due to the change in specific heat of the supercritical CO2 along the temperature range of hot water storage, a system of multiple storage tanks was used. Intermediate storage tanks help reduce significantly the temperature differences at the hot storage side and therefore their number represent another critical design parameter that must be optimized to achieve the best trade-off between costs and performances

Effect of temperature on the dielectric properties of hydrofluoroethers and fluorinated ketone

The AC and DC dielectric properties of hydrofluoroethers (HFE) [C3F7 OCH3 ] and fluorinated ketone (FK) [C2F5 C(O)CF(CF3)2] have been characterised by dielectric spectroscopy and DC conductivity at different temperatures. Results show that DC conductivity and imaginary permittivity of both fluids are positively correlated with increasing temperature. However, the real permittivity decreases with increasing temperature. The breakdown voltages of HFE and FK at 295 K are∼10 kV mm−1 . Reducing temperature is an effective method to increase the breakdown voltage of the FK coolant, but the breakdown voltage of HFE is less temperature dependent. Finally, as expected repeated breakdown had no significant effect on the AC dielectric strength of both liquids. </p

On the shedding of impaled droplets: The role of transient intervening layers.

Maintaining the non-wetting property of textured hydrophobic surfaces is directly related to the preservation of an intervening fluid layer (gaseous or immiscible liquid) between the droplet and substrate; once displaced, it cannot be recovered spontaneously as the fully penetrated Wenzel wetting state is energetically favorable. Here, we identify pathways for the lifting of droplets from the surface texture, enabling a complete Wenzel-to-Cassie-Baxter wetting state transition. This is accomplished by the hemiwicking of a transient (limited lifetime due to evaporation) low surface tension (LST) liquid, which is capable of self-assembling as an intervening underlayer, lifting the droplet from its impaled state and facilitating a skating-like behavior. In the skating phase, a critical substrate tilting angle is identified, up to which underlayer and droplet remain coupled exhibiting a pseudo-Cassie-Baxter state. For greater titling angles, the droplet, driven by inertia, detaches itself from the liquid intervening layer and transitions to a traditional Cassie-Baxter wetting state, thereby accelerating and leaving the underlayer behind. A model is also presented that elucidates the mechanism of mobility recovery. Ultimately, this work provides a better understanding of multiphase mass transfer of immiscible LST liquid-water mixtures with respect to establishing facile methods towards retaining intervening layers

On the shedding of impaled droplets: The role of transient intervening layers

Maintaining the non-wetting property of textured hydrophobic surfaces is directly related to the preservation of an intervening fluid layer (gaseous or immiscible liquid) between the droplet and substrate; once displaced, it cannot be recovered spontaneously as the fully penetrated Wenzel wetting state is energetically favorable. Here, we identify pathways for the “lifting” of droplets from the surface texture, enabling a complete Wenzel-to-Cassie-Baxter wetting state transition. This is accomplished by the hemiwicking of a transient (limited lifetime due to evaporation) low surface tension (LST) liquid, which is capable of self-assembling as an intervening underlayer, lifting the droplet from its impaled state and facilitating a skating-like behavior. In the skating phase, a critical substrate tilting angle is identified, up to which underlayer and droplet remain coupled exhibiting a pseudo-Cassie-Baxter state. For greater titling angles, the droplet, driven by inertia, detaches itself from the liquid intervening layer and transitions to a traditional Cassie-Baxter wetting state, thereby accelerating and leaving the underlayer behind. A model is also presented that elucidates the mechanism of mobility recovery. Ultimately, this work provides a better understanding of multiphase mass transfer of immiscible LST liquid-water mixtures with respect to establishing facile methods towards retaining intervening layers.ISSN:2045-232