1,720,975 research outputs found
Effect of extreme wetting scenarios on pool boiling conditions in a quiescent medium
No full textThis study focuses on the detailed description of the heat transfer and bubble dynamics processes occurring for the boiling of water on surfaces with extreme wetting regimes, namely hydrophilicity and superhydrophobicity. The wettability is changed by modifying the surface chemistry and without significant variations in the mean surface roughness. Under these conditions and for the range studied here the effect of the extreme wetting regimes was dominant, thus the influence of surface topography was not addressed. A particular trend is observed for the boiling curve obtained with the superhydrophobic surfaces, as the heat flux increases almost linearly with the superheat, although with a much lower slope than the hydrophilic surfaces. This occurs due to the formation of a large stable vapour film over the entire surface just at around 1 K superheat, because of the almost immediate coalescence of the bubbles generated on the surface. This behaviour agrees with the so-called “quasi-Leidenfrost” regime recently reported in the literature and with a theoretical prediction of the heat flux that is presented in this study. Furthermore, a comprehensive analysis of bubble dynamics, useful for comparison with numerical simulations is given. Such analysis is based on the temporal evolution of the bubble diameter together with bubble contact angle and with the velocity of the contact line. The results suggest that the existing models and correlations can predict the trends of the bubble growth using a modified contact angle value, called the bubble contact angle (or its supplemental value), for the hydrophilic surfaces, even if they cannot accurately predict bubble sizes. Approximating the modified contact angle with the quasi-static contact angle, obtained during surface characterization is practical for a qualitative evaluation, but the results obtained here do not support for its use when estimating the bubble departure diameter. On superhydrophobic surfaces, the effect of the vapour film must be considered, since although this is not the starting point of the boiling process, it represents the actual working conditions when using this kind of surfaces
Effect of gamma irradiation on poly(butylene naphthalate) based polyesters
No full textThe present work investigates the effect of gamma radiation on the properties of three naphthalate-based polyesters, i.e. poly(butylene naphthalate) (PBN), poly(diethylene naphthalate) (PDEN) and poly(thiodiethylene naphthalate) (PTDEN). In addition, the analogous terephthalate-based polymers of PDEN and PTDEN, i.e. poly(diethylene terephthalate) (PDET) and poly(thiodiethylene terephthalate) (PTDET), are also investigated, in order to check the effect of a lower number of aromatic rings. All the polymers, irradiated in air at different absorbed doses, were characterized by several techniques. The data obtained indicate that all the polymers, except PBN, show a decrease of molecular weight with the dose increase. The thermal behavior and the morphology confirm the previous results and show that the higher the crystallinity degree and number of aromatic rings, the higher the radiation resistance. The introduction of heteroatoms decreases the ability of a polymer to crystallize due to a reduction of polymer chain symmetry, thus worsening their radiation resistance
Impact of Different Liquid Drops on Micro/Nano-Structured Superhydrophobic Surfaces
No full textOne of the most fascinating natural phenomena is the ability of some surfaces to repel water drops. Since the discovery of lotus leaf surface structure, several attempts of artificial, biomimetic superhydrophobic surfaces (SHS) have been made. Nowadays, we know that superhydrophobicity arises from a combination of surface morphology and chemical composition. However, different surfaces can display analogous wetting properties in static conditions (e.g. similar contact angles) but radically divergent drop impact output. Thus, drop impact studies provide enhanced insight on surface wetting properties in dynamic conditions.
We fabricated SHS with different morphology and/or chemical composition. Namely, surface S had a flower-like alumina structure with nanoscale cavities (as observed with FESEM), chemically modified with fluorosilanes; surfaces LAU and FAS had a terrace-like alumina structure with micro-cavities, modified with lauric acid or fluorosilanes, respectively. Then, we assessed their quasi-static wetting properties (e.g. advancing contact angle θA, receding contact angle θR and contact angle hysteresis Δθ) and drop impact behavior in a range of Weber number 1135°, Δθ120° with hexadecane, LAU was oleophilic (θR≈0).
In water drop impacts, S surfaces always produced a rebound, indicating a stable Cassie-Baxter wetting state. Meanwhile, for LAU and FAS a Cassie-to-Wenzel transition (CWT) was observed at high We, with partial rebound as an output. Such behavior is consistent with results in literature: nano-cavities on S surface cause high capillary pressure PC against wetting, while micro-cavities on LAU and FAS were penetrated by drops when wetting pressures (i.e. effective water hammer pressure PEWH and gas layer pressure PGL) exceeded PC. Significantly, CWT occurred at higher We for LAU than for FAS, notwithstanding their identical surface structure and water contact angles. This result hints at a role of surface chemistry in drop impact behavior, a phenomenon that has never been reported before and certainly deserves further studies.
On the other hand, hexadecane drops never rebounded, even on S and FAS surfaces. Antonini et al. (Langmuir 2013) defined θR>100° as a criterion for water drop rebound. However, this does not hold for hexadecane: PC is lower when s is smaller, thus causing CWT even if the surface is oleophobic in static conditions.
In conclusion, the results highlight that it is not possible to easily correlate contact angles and drop impact dynamics of low- and high-surface-tension liquids on different surfaces, as CWT can be observed even on statically repellent surfaces. Thus, an accurate design of surface properties must be pursued in the future research towards dynamically amphiphobic, biomimetic surfaces
Assessing durability of superhydrophobic surfaces
No full textSuperhydrophobic surfaces (SHS) show remarkable water repellency properties, and their use may have a tremendous impact 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
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Is a Knowledge of Surface Topology and Contact Angles Enough to Define the Drop Impact Outcome?
Full text linkIt is well known that a superhydrophobic surface may not be able to repel impacting droplets because of the so-called 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 hydrophobic surfaces. For the same Weber numbers, a Cassie-to-Wenzel state transition occurs after the impact as a result of the easier penetration of low-surface-tension fluids in the surface structure. Hence, a criterion for the 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 enhanced static repellence against oils and organics, generally the realization of synthetic materials with self-cleaning and antisticking abilities in dynamic phenomena, such as spray impact, remains an unsolved task. Moreover, it is demonstrated that the chemistry of the surface, the physicochemical interactions with the liquid drops, and the possible wettability gradient of the surface asperity also play important roles in determining the critical Weber number above which impalement occurs. Therefore, the classical numerical simulations of drop impact on dry surfaces are definitively not able to capture the final outcomes of the impact for all 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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