Non-wetting surfaces engineered from intrinsically hydrophilic metallic materials are promising for self-cleaning, anti-icing and/or condensation heat transfer applications where the durability of the coating is an issue. In this work, we fabricate and study the wetting behaviour and the condensation performance on two metallic non-wetting surfaces with varying number and size of the roughness tiers without further hydrophobic coating procedure. On one hand, the surface resembling a rose petal exhibits a sticky non-wetting behaviour as drops wet the microscopic roughness features with the consequent enhanced drop adhesion, which leads to filmwise condensation. On the other hand, the surface resembling a lotus leaf provides super- repellent non-wetting behaviour prompting the continuous nucleation, growth and departure of spherical drops in a dropwisecondensation fashion. On a lotus leaf surface, the third nano- scale roughness tier (created by chemical oxidation) combined with ambience exposure prompts the growth of drops in the Cassie state with the benefit of minimal condensate adhesion. The two different condensation behaviours reported are well supported by a drop surface energy analysis, which accounts for the different wetting performance and the surface structure underneath the condensing drops. Further, we coated the above-mentioned surfaces with polydimethylsiloxane surfaces, which resulted in filmwise condensation due to the smoothening of the different roughness tiers. Continuous dropwisecondensation on a hierarchical bioinspired lotus leaf metallic surface without the need for a conformal hydrophobic coating is hence demonstrated, which offers a novel path for the design and manufacture of non-coated metallic super-repellent surfaces for condensation phase change applications, amongst others.
In summary, we show the importance of droplet wetting morphology on condensing growth rate on Cassie stable surfaces via an in situ ESEM study of a coexisting of S and PW morphologies on a superhydrophobic structured surface. The PW wetting mode was shown to have a 6× higher initial growth rate than the S mode due to the increased contact with the substrate. Experiments were validated using an analytical heat transfer model, and are in good agreement. Calculation of the heat transfer ratio of PW to S wetting morphologies for varying droplet coalescence lengths shows the heat transfer of the PW droplets to be 4-6× more effective. To compare the heat transfer enhancement, PW and S droplet heat flux is compared to that of a flat superhydrophobic silane coated surface, showing a 56% enhancement for the PW morphology, and 71% degradation for the S morphology. This study provides insight into the hereto unidentified importance of local wetting morphology on droplet growth rate during superhydrophobic condensation, as well as the importance of designing CB stable surfaces with PW droplet morphologies to achieve enhanced heat during dropwisecondensation.
Dropwisecondensation is the process during which water droplets form from a humid gas (or pure vapor) when its temperature goes below the dew point. 1 This phenomenon can happen when the humid gas is in contact
with a cold substrate with the temperature less than the dew point of the gas. In some applications the size and growth rate of these droplets on the cold substrate can be important because the smaller droplets show higher heat transfer rate. The growth rate of droplets can be controlled by applying different texturing patterns on the substrate. But sometimes due to the similarity between the texture shape and the droplets, the later are not recognizable. In this work, the image processing methods for recognizing the droplets on two substrates (flat and pillared) are presented. The aim of these methods is to binarize the images of the droplets in order to extract the information related to the droplets size and density.
Dropwisecondensation has superior heat transfer efficiency than filmwise condensation; however condensate evacuation from the surface still remains a significant technological challenge. The process of droplets jumping, against adhesive forces, from a solid surface upon coalescence has been studied using both experimental and Computational Fluid Dynamics (CFD) analysis. Both Lattice Boltzmann (LBM) and Volume of Fluid (VOF) methods have been used to evaluate different kinematic conditions of coalescence inducing a jump velocity. In this paper, an optimisation framework for superhydrophobic surface designs is presented which uses experimentally verified high fidelity CFD analyses to identify optimal combinations of design features which maximise desirable characteristics such as the vertical velocity of the merged jumping droplet from the surface and energy efficiency. A Radial Basis Function (RBF)-based surrogate modelling approach using design of experiment (DOE) technique was used to establish near-optimal initial process parameters around which to focus the study. This multidisciplinary approach allows us to evaluate the jumping phenomenon for superhydrophobic surfaces for which several input parameters may be varied, so as to improve the heat transfer exchange rate on the surface during condensation. Reliable conditions were found to occur for droplets within initial radius range of r=20-40 μm and static contact angle θs~160º. Moreover, the jumping phenomenon was observed for droplets with initial radius of up to 500 μm. Lastly, our study also reveals that a critical contact angle for droplets to jump upon coalescence is θ c ~140º.
In summary, we studied the condensation modes of fluids with surface tension in the range of 12–30 mN/m on oleophobic, super- omniphobic, and lubricant-impregnated surfaces. We demonstrated that smooth oleophobic and stable lubricant-impregnated surfaces promote continual droplet formation and shedding of a majority of the studied fluids, whereas the studied unimpregnated superomni- phobic textures become flooded due to nucleation within their re-entrant texture. By promoting dropwisecondensation, we dem- onstrate a four to eight-fold increase in heat transfer coefficient as compared to filmwise condensation. This enhancement in heat transfer coefficient demonstrated for pentane can significantly improve efficiency of organic Rankine cycles used for heat recovery from low temperature sources such as biomass combustion, indus- trial waste, or geothermal heat sources. Further increases in heat transfer coefficients on lubricant-impregnated surfaces could be rea- lized by using higher thermal conductivity lubricant, minimizing the solid pinned fraction or using a lubricant-solid pair that have positive spreading coefficient in the presence of the condensate, which may also reduce lubricant drainage. Although the results for most of the studied fluids are promising, this study demonstrates the significant difficulty in promoting dropwisecondensation of ultra-low-surface tension liquids, specifically fluorocarbon liquids. Future work should be directed toward the development of durable ultra-low-surface energy modifiers, and also to lubricants that are not non-wetting to fluorinated refrigerants.
Dropwisecondensation has superior heat transfer efficiency than filmwise condensation; however condensate evacuation from the surface still remains a significant technological challenge. The process of droplets jumping, against adhesive forces, from a solid surface upon coalescence has been studied using both experimental and Computational Fluid Dynamics (CFD) analysis. Both Lattice Boltzmann (LBM) and Volume of Fluid (VOF) methods have been used to evaluate different kinematic conditions of coalescence inducing a jump velocity. In this paper, an optimisation framework for superhydrophobic surface designs is presented which uses experimentally verified high fidelity CFD analyses to identify optimal combinations of design features which maximise desirable characteristics such as the vertical velocity of the merged jumping droplet from the surface and energy efficiency. A Radial Basis Function (RBF)-based surrogate modelling approach using design of experiment (DOE) technique was used to establish near-optimal initial process parameters around which to focus the study. This multidisciplinary approach allows us to evaluate the jumping phenomenon for superhydrophobic surfaces for which several input parameters may be varied, so as to improve the heat transfer exchange rate on the surface during condensation. Reliable conditions were found to occur for droplets within initial radius range of r=20-
The so called dropwisecondensation when liquid condense on a solid surface in a form of droplets has been studied for several decades and there has been a huge effort to enhance the heat transfer from vapour to liquid state using dropwisecondensation which shows much higher heat transfer rate than filmwise condensation ( Lee2013) when surface is covered by a liquid film. In dropwisecondensation crucial task is to efficiently remove condensate from the surface and create place for new droplets nucleation. This is where the droplets coalescence and self propelled (’jumping’) phenomenon can enhance this process.
To study the scale dependency of discrete droplet formation and stability of dropwisecondensation, we quantified the droplet distri- bution for nano- to macroscale droplets during condensation on a vertical flat plate. The experimental setup is described elsewhere (26). Briefly, the sample is cooled to 0.05° ± 0.1°C. Saturated water vapor was sparged from a second water bath of deionized (DI) water at 73° ± 3°C. The resulting condensation area spanned roughly 2.5 cm horizontally and 1 cm vertically and represented the topmost area of a vertical pate. To capture droplet data during all stages of a sweeping period and to ensure statistical significance, we captured one image every 20 s. Figure 3 shows the self-similar nature for both hydrophilic PEGylated and hydrophobic surfaces. On both hydro- phobic (Fig. 3, A and B) and hydrophilic (Fig. 3, C and D, and movie S1) surfaces, all droplets remained as spherical caps and dropwise con- densation ensued without film formation.
Experimental research was performed to develop a Cu-GNP micro-textured coating to promote dropwisecondensation through optimization of the coating procedures. To investigate the effect of the addition of a graphene nanoplatelets on the microstructure and stability of the coating, several samples with two different thermal spraying methods (APS and HVOF) and parameters were developed. The spraying strategy used was to minimize the operational temperature of the process to maintain graphene nanoplatelets properties. The first set of samples was sprayed with copper as a feedstock to determine the window of operation in the APS process. The hydrogen flow rate was the variable parameter to optimize the process; consequently, the plasma power was chosen accordingly. After finding the lowest temperature to form a coating, GNP was added to the feedstock to determine its impact on the microstructure and properties of the final coating. To reduce the surface energy, samples were treated by a stearic acid solution. The second set was sprayed with the HVOF process using the same strategy to optimize the process for maintaining the GNP in the coatings and developing further desired microstructures.
One of the main challenges of visual detection of condensation on a glass or a camera lens is the complexity of modelling, simulating or even reproducing the same exact phenomena. When the light that is reflected by objects in a scene travels toward the camera sensor and passes through glass with partial or total condensation, it undergoes many alterations giving the captured image a blurry or hazy look. A simple blur detection approach to recognize the moisture build-up is inadequate because there are multiple levels of degradations. When light traverses a water droplet some of it will be reflected causing information loss and some of it will be refracted; thus, some may land in the wrong spot of the image whereas some will be transmitted as normal. The glass holding the suspended water droplets is located between the camera sensor and the viewing scene which causes the image to be out of focus. To further compound the problem, the droplets are distributed in a non-uniform fashion making it impossible to model the phenomena using an invariant blur kernel. In addition, the image sampling process at the camera sensor level produce a down-sampled image where each pixel contains information for a region of the viewing scene as well as one or many water droplets depending on the size of the droplet and the distance between the glass and the camera sensor.
The atmospheric air mainly consists of five substances; nitrogen, oxygen, argon, carbon dioxide and water vapour. The relative humidity can be expressed as the amount of water vapour that can be obtained by air. And is controlled by the by temperature and pressure of the air, therefore the maximum relative humidity varies with two parameters. Condensation is constitute as small water droplets and can be seen as fog. Buoyancy force is play an important role in the condensation; due to the buoyancy force it will push the lighter fluid upward replaced by heavier fluid. The dimension of the buoyancy force is equal to the weight of the fluid displaced by the body. That is,
In conclusion, the attempt to force an information condensation be conform to a complex node representation with de facto spokes, does not seem to be a satisfying strategy. Hence, the nucleus-spoke structure is the preferable candidate for additive QoS measures, while the spanning tree structure is more suited for a single min(max) QoS measure. However, the extension to multiple min(max)-QoS measures requires a spanning tree algorithm for multiple min(max) QoS measures which is very likely to be NP-complete. Moreover, the basic property of a maximum weight spanning tree, derived in the single min(max)-QoS case, does not extend to multiple min(max)-QoS because it is unclear whether there always exist a spanning tree that is a good reflection of the full mesh between ingress-egress pairs.
continuous flow of hot air from the bottom to the top of the box. A scaled-up version of the two-stage-ESP developed by Bedmutha et al. (10) is used in this study. The ESP is made of a stainless steel pipe, 0.6 m long and 0.06 m in diameter. Throughout the experimental study, the ESP is operated with an applied voltage of 14 kVDC. The temperature of the C-ESP can be varied by changing the hot air temperature entering the C-ESP enclosed chamber. An in-line air process heater (Omega AHP-7561), controlled using a Watlow series C on-off temperature controller, is used for the heating of the air. In the experiments conducted to optimize the fractional condensation train, the temperature of C-ESP was maintained at 30, 50 or 70 °C. While the C-ESP temperature was maintained at 30, 50 or 70 °C, the average temperature of the stream exiting the C- ESP was 38, 49 or 56 °C, respectively.
From the boundary sigma model point of view, one starts with a conformal theory with d Neumann boundary conditions in the UV and adds relevant (tachyon) perturbations driving the theory to an IR fixed point that corresponds to a new (stable or unstable) vacuum with (d − 1 − p) Dirichlet boundary conditions. The IR fixed point is then interpreted as a closed string vacuum with a Dp-brane. Given that the tension of a Dp-brane is correctly reproduced [23, 24], a further crucial test is to find the spectrum and the effective action for light modes on a Dp-brane obtained as a result of tachyon condensation.
is now consistent with the observed hemispheric pattern. By the end of the simulation period on Day 383 ( Figure 6 ( h )) , a large fraction of the sinistral skew in the north and dextral in the south has been removed. Nevertheless, some pockets of minority chirality remain. It is interesting to note that even with the high value of vorticity, which is over ﬁ ve times greater than the peak value in the gradient of differential rotation, some minority chirality is still produced in each hemisphere. In addition, signi ﬁ cant lengths of the PILs have weak skew. The variation of the coronal ﬁ eld that leads to this pattern is considered through a speci ﬁ c example in Section 5.4. There is a complex relationship between the local effects of differential rotation and helicity condensation that inject helicity and the nonlocal effects of surface diffusion that act to concentrate the helicity along the PIL. These processes were quanti ﬁ ed through case studies in Mackay et al. ( 2014 ) . Within our simulations, the skew along any one PIL changes continually as these combined effects act. In addition, abrupt changes in chirality and loss of helicity occur as ﬂ ux ropes are ejected. These ejections can partially or fully remove the shear above the PILs, leading to reductions in the absolute amount of helicity.
These FFLO states, which have been sought in a wide range of systems, may be the ground states where there is an im- balance in the populations of the two pairing species. How- ever, they involve increasing the kinetic energy in order to gain pairing energy and in practice this restricts them to small regions of parameter space. Here, however, the state achieved is determined by the Cooper equation and a gain- loss criterion with the energetics playing a subsidiary role. Thus, as indicated by Fig. 3, condensation at a finite momen- tum may be achieved without fine tuning of parameters.
One consequence of this nonequilibrium nature is that, whereas in equilibrium only the lowest energy single-particle state can be macroscopically occupied, for polaritons large occupations can build up in other orbitals. Furthermore, the condensation can occur in several orbitals simultaneously, enabling the study of interacting macroscopic quantum states. The presence of several highly occupied states of a trapping potential can be seen directly in the emission spectra [8,9] and inferred from the presence of Josephson oscillations [10,11]. Such multimode condensation can also occur in spatially extended states, in particular in the Bloch states of one  and two  dimensional lattices. The in-plane potentials that control these condensates can arise from growth-induced disorder in the Bragg mirrors [5,10,14], metal-film patterning of the mirror surfaces [12,13], and interaction effects , as well as from the use of nonplanar structures such as micropillars and photonic molecules .