Resistant Energy Analysis of Dropwise Condensation on Superhydrophobic Surfaces With Hierarchical Roughness

2017 ◽  
Author(s):  
Jiangtao Cheng
Keyword(s):  
Contact Line ◽  
Growth Dynamics ◽  
Adhesion Energy ◽  
Superhydrophobic Surfaces ◽  
Dropwise Condensation ◽  
Textured Surfaces ◽  
Biomimetic Surfaces ◽  
Molecular Kinetic Theory ◽  
Submicron Scale ◽  
Second Tier

Recently there have appeared multiscale lotus-leaf-like superhydrophobic surfaces that can enhance dropwise condensation in well-tailored supersaturation conditions. However, designs of most biomimetic surfaces were not driven by the understanding of underlying physical mechanisms. We report energy-based analysis of growth dynamics of condensates from surface cavities. As observed in condensation experiments, these textured surfaces with two tier roughness are superior to flat or solely nanotextured surfaces in spatial control of condensate droplets. To understand the role of condensate state transition in enhancing condensation heat transfer, we considered adhesion energy, viscous dissipation and contact line dissipation as the main portion of resistant energy that needs to be overcome by the condensates formed in surface cavities. By minimizing the energy barrier associated with the self-pulling process, we optimized first tier roughness on the hierarchically textured surfaces allowing condensates to grow preferentially in the out-of-plane direction. The nano-roughness of the second tier plays an important role in abating the adhesion energy in the cavities and contact line pinning. From the perspective of molecular kinetic theory, the dual scale engineered surface is beneficial to remarkably mitigating contact line dissipation. This study indicates that scaling down surface roughness to submicron scale can facilitate self-propelled condensate removal.

Contact Line Dynamics of Water Droplets Spreading on Nano-Structured Teflon Surfaces in Dropwise Condensation

2017 ◽  
Author(s):  
Lei Zhao ◽  
Jiangtao Cheng
Keyword(s):  
Contact Line ◽  
Condensation Process ◽  
Dropwise Condensation ◽  
Water Droplets ◽  
Molecular Kinetic Theory ◽  
Comprehensive Knowledge ◽  
Order Of Magnitude ◽  
Contact Line Friction ◽  
Solid Liquid ◽  
Contact Line Dynamics

In dropwise condensation process, superhydrophobicity is usually achieved by introducing micro/nano-roughness to hydrophobic materials. The analysis of droplets growing and moving and the optimization of the surface structures entails a comprehensive knowledge of the contact line dissipation. However, it in many cases is neglected due to the insufficient understanding, particularly regarding its magnitude and characteristics. In this study, we report a study on the contact line dynamics of water droplets spreading on nano-structured Teflon surfaces. The Teflon surfaces are modeled on Gromacs 5.1.2 and based on the OPLSAA force field. The Teflon model is then validated by examining the glass transition temperature and thermal expansion coefficient. Patterned pillars are created by a confined layer method. The contact line dynamics of water on as-formed surfaces with different solid fraction is then analyzed using the molecular kinetic theory modified by incorporating both viscous damping and solid-liquid retarding. The unit displacement length of contact line is demonstrated to be a constant value of 0.605 nm on both flat and pillar-arrayed surfaces. The contact line friction coefficient is calculated to be on the same order of magnitude with the dynamic viscosity of water, and can be significantly decreased on superhydrophobic surfaces as a result of reduced liquid-solid contact, although contact line experiences stronger resistance on a single pillar.

scholarly journals Drops bouncing off macro-textured superhydrophobic surfaces

2017 ◽  
Vol 824 ◽  
pp. 866-885 ◽  
Author(s):  
Ali Mazloomi Moqaddam ◽  
Shyam S. Chikatamarla ◽  
Iliya V. Karlin
Keyword(s):  
Contact Time ◽  
Numerical Study ◽  
Three Dimensional ◽  
Superhydrophobic Surfaces ◽  
Nonlinear Behaviour ◽  
Textured Surfaces ◽  
Texture Density ◽  
Wide Range ◽  
Quantitative Manner ◽  
The Impact

Recent experiments with droplets impacting macro-textured superhydrophobic surfaces revealed new regimes of bouncing with a remarkable reduction of the contact time. Here we present a comprehensive numerical study that reveals the physics behind these new bouncing regimes and quantifies the roles played by various external and internal forces. For the first time, accurate three-dimensional simulations involving realistic macro-textured surfaces are performed. After demonstrating that simulations reproduce experiments in a quantitative manner, the study is focused on analysing the flow situations beyond current experiments. We show that the experimentally observed reduction of contact time extends to higher Weber numbers, and analyse the role played by the texture density. Moreover, we report a nonlinear behaviour of the contact time with the increase of the Weber number for imperfectly coated textures, and study the impact on tilted surfaces in a wide range of Weber numbers. Finally, we present novel energy analysis techniques that elaborate and quantify the interplay between the kinetic and surface energy, and the role played by the dissipation for various Weber numbers.

scholarly journals Binarization of the gray scale images of droplets during dropwise condensation on textured surfaces

2019 ◽  
Author(s):  
Solmaz Boroomandi Barati ◽  
Hélène Martin ◽  
Jean-Charles Pinoli ◽  
Stéphane Valette ◽  
Yann Gavet
Keyword(s):  
Dropwise Condensation ◽  
Textured Surfaces ◽  
Gray Scale

Effects of Free Surface Evaporation on Water Nano-Droplet Wetting Kinetics: A Molecular Dynamics Study

2013 ◽  
Author(s):  
Gui Lu ◽  
Yuan-Yuan Duan ◽  
Xiao-Dong Wang
Keyword(s):  
Molecular Dynamics ◽  
Free Surface ◽  
Contact Line ◽  
Dynamics Simulation ◽  
Water Molecules ◽  
Dynamic Wetting ◽  
Surface Evaporation ◽  
Molecular Kinetic Theory ◽  
Line Region ◽  
Wetting Kinetics

The dynamic wetting of water nano-droplet with evaporation on the heated gold substrate was examined using molecular dynamics simulation. Various substrate and droplet pre-heated temperatures were calculated to obtained different evaporating rates. Water molecules attachment-detachment details were traced near the contact line region to show the microscopic details and evidences for the spreading-evaporating droplet. The increasing substrate temperature greatly affected the dynamic wetting process, while the initial temperature of water droplet had very limited effects. The effects of free surface evaporation on wetting kinetics for both hydrophobic and hydrophilic substrates were examined. The radius versus time curves agree well with the Molecular kinetic theory (MKT) for spreading without evaporation and deviate from the MKT for the spreading with evaporation. The enhancement on wetting kinetics due to evaporation can be attributed to the reducing of liquid-vapor surface tension and the strengthening in water molecules transport in contact line region and bulk droplet.

Three dimensional aspects of droplet coalescence during dropwise condensation on superhydrophobic surfaces

Soft Matter ◽  
2011 ◽  
Vol 7 (19) ◽  
pp. 8749 ◽  
Author(s):  
Konrad Rykaczewski ◽  
John Henry J. Scott ◽  
Sukumar Rajauria ◽  
Jeff Chinn ◽  
Amy M. Chinn ◽  
...  
Keyword(s):  
Three Dimensional ◽  
Superhydrophobic Surfaces ◽  
Dropwise Condensation ◽  
Droplet Coalescence

Electrowetting-Based Coalescence of Droplets During Dropwise Condensation of Humid Air

2019 ◽  
Author(s):  
Enakshi Wikramanayake ◽  
Vaibhav Bahadur
Keyword(s):  
Heat Transfer ◽  
Electric Field ◽  
Electric Fields ◽  
Applied Electric Field ◽  
Growth Dynamics ◽  
Condensation Heat Transfer ◽  
Droplet Growth ◽  
Dropwise Condensation ◽  
Condensation Rate ◽  
Humid Air

Abstract Dropwise condensation yields higher heat transfer coefficients by avoiding the thermal resistance of the condensate film, seen during filmwise condensation. This work explores further enhancement of dropwise condensation heat transfer through the use of electrowetting to achieve faster droplet growth via coalescence of the condensed droplets. Electrowetting is a well understood microfluidic technique to actuate and control droplets. This work shows that AC electric fields can significantly enhance droplet growth dynamics. This enhancement is a result of coalescence triggered by various types of droplet motion (translation of droplets, oscillations of three phase line), which in turn depends on the frequency of the applied AC waveform. The applied electric field modifies droplet condensation patterns as well as the roll-off dynamics on the surface. Experiments are conducted to study early-stage droplet growth dynamics, as well as steady state condensation rates under the influence of electric fields. It is noted that this study deals with condensation of humid air, and not pure steam. Results show that increasing the voltage magnitude and frequency increases droplet growth rate and overall condensation rate. Overall, this study reports more than a 30 % enhancement in condensation rate resulting from the applied electric field, which highlights the potential of this concept for condensation heat transfer enhancement.

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