Superhydrophobic surfaces (SHS) show remarkable water repellency properties, and their use may have a tremendousimpact for a plethora of applications, where liquid water accumulation needs to be controlled or minimised. However, the durability of SHS in operational conditions is a severe issue that currently represents a bottleneck for the technology transfer from laboratory to industrial applications. In the present work, we try to fill in the gap caused by the absence of a standard for evaluation for SHS durability, by developing a protocol for testing surface durability.The proposed protocol includes nine tests as follows: water immersion, acidic environment, alkaline environment, ionic solution, mechanical erosion, ultraviolet exposure, resistance to heating, alcohol immersion and hydrocarbon immersion. The protocol can serve to give an indication of surface robustness in a variety of potentially harmful environments, by providing a global figure of merit and ranking for different SHS and thereby allowing for identifying those surfaces fulfilling requirements for a specific application. To illustrate the protocol, we tested a SHS developed in-house by grafting of lauric acid molecules on an aluminum substrate. This article contains supporting information that is available online.

Pool boiling experiments were performed with degassed water on stainless steel substrates with different surface topography and wettability. Boiling curves were determined and high-speed visualizations of the boiling processes were performed. The onset of nucleate boiling (ONB) has been measured and the influence of roughness and wettability has been quantified. Boiling curve shape is different between hydrophilic and superhydrophobic case ( Fig. 1): on superhydrophobic surfaces the ONB is reached at lower superheat and boiling presents a quasiLeidenfrost regime without showing the typical "S" shape of the boiling curve, i.e. without passing through a CHF point. Bubbles are easier to form on superhydrophobic surfaces, therefore the nucleation temperature is smaller, and bubbles are larger and stable. The ONB appears after less than 5°C of superheat on superhydrophobic surfaces, while on hydrophilic surfaces with the same surface roughness the required superheat is above 8°C. Furthermore, superhydrophobic samples, although presenting different roughness, present the same boiling curve, meaning that the wettability has a predominant role on the surface roughness when the contact angle exceeds a certain value. quasi-Leidenfrost regime

Loop heat pipes (LHP) and other two-phase passive thermal devices, such as heat pipe loops (HPL), represent a very attractive solution for the energy management of systems characterized by a distributed presence of heating and cooling zones and by the needs of fast start-up, reliability, low cost and lightness. Even if the usual application for these devices is in the space sector, there could be a potential significant application for the automotive industry, for the development of embedded thermal networks for full electric vehicles (FEV), in order for example to recover the waste heat for cabin heating and cooling or to improve the aerodynamic efficiency. In the present investigation, the possibility to implement a new thermal control for an electric vehicle comprising from heat pumps (HP) and LHP, is here evaluated. In more detail, a 1-D lumped parameter model (LPM) that is able to predict the transient behaviour of a LHP in response of varying boundary and initial conditions, is developed and validated against literature experimental data. A novel methodology for treating numerically the condenser is proposed and validated for three different working fluids. An extensive parametric analysis is also conducted, showing the robustness of the thermal solution for different conditions and proving the possibility of using the proposed numerical code both for feasibility studies and for optimization purposes. A feasibility study utilizing the proposed model is also conducted and the results indicate that an array of LHPs can effectively transport heat from the motor section of the vehicle to the underbody, reducing significantly the aerodynamic losses.

ManuscriptClick here to download Manuscript: Teodori et al_ETFS_12ev.docx Click here to view linked References  Detailed pool boiling bubble dynamics on extreme wetting regimes is performed  Under extreme wetting regimes, wettability plays a dominant role  "Quasi-Leidenfrost" regime is supported by comparison with theoretical predictions  The macro-contact angle can be used in correlations predicting bubble size  Apparent angles should not be approximated to quasi-static values for predictions

ABSTRACT:It is well known that a superhydrophobic surface may not be able to repel impacting droplets due to the socalled Cassie-to-Wenzel transition. It has been proven that a critical value of the receding contact angle (θR) exists for the complete rebound of water, recently experimentally measured to be 100° for a large range of impact velocities. On the contrary, in the present work, no rebound was observed when low surface tension liquids such as hexadecane (σ = 27.5 mN/m at 25°C) are concerned, even for very low impact velocities and very high values of θR and low contact angle hysteresis. Therefore, the critical threshold of θR ≈ 100° does not sound acceptable for all liquids and for all the hydrophobic surfaces. For the same Weber numbers a Cassie-to-Wenzel state transition occurs after the impact due to the easier penetration of low surface tension fluids in the surface structure. Hence a criterion for drop rebound of low surface tension liquids must consider not only the contact angle values with surfaces, but also their surface tension and viscosity. This suggests that, even if it is possible to produce surfaces with an enhanced static repellence against oils and organics, generally the realization of synthetic materials with remaining self-cleaning and anti-sticking abilities in dynamic phenomena, such as spray impact for example, still remains an unsolved task. Moreover, it is demonstrated that also the chemistry of the surface and the physico-chemical interactions with the liquid drops and the possible wettability gradient of the surface asperity play an important role in determining the critical Weber number above which impalement occurs. Therefore the classical numerical simulations of drop impacts onto dry surfaces are definitively not able to capture the final outcomes of the impact for all the possible fluids, if the surface topology and chemistry and/or the wettability gradient in the surface structure are not properly reflected.

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