Full length articleCondensation of water vapor underneath an inclined hydrophobic textured surface machined by laser and electric discharge
Graphical abstract
Introduction
Manju and Sagar [1] describe a serious concern for the world in the next decade about the scarcity of clean drinking water. According to the United States Geological Survey, approximately 97% of water is present in the oceans in the form of saline water, while only 3% of the total water available on earth is potable. Surface water and groundwater take the form of icecaps and glaciers in the Arctic and Antarctic regions. A total of 87% of surface water is in lakes and rivers. According to the authors, this figure represents only 1% of the world's drinkable water. Tiwari and Tiwari [2] elaborate an approach towards cleaning impure or saline water via solar still, wherein water evaporated via solar energy is condensed and drained from an inclined surface. The authors have presented a detailed mathematical model for desalination using a solar still. Typically, the surface over which water is condensing plays an important role not only in removing heat of condensation, but also in determining the condensation pattern. In several solar stills, a metallic sheet is used as a condensing surface in a separate chamber instead of glass. Dimri et al. [3] conducted an experimental study to investigate the effect of the condensing cover material on water production. Copper and polyvinyl chloride were used as a condensing material while copper was found to give the maximum yield. Various efforts are presently being explored to improve the efficiency of water collection from metallic surface. Khandekar and Muralidhar [4] describe the dropwise condensation process via experiments and simulation for inclined chemically textured surfaces. Sikarwar et al. [5] have stated that an untextured condensing surface is typically hydrophilic and water vapor undergoes filmwise condensation. It is possible to create textured surfaces that are hydrophobic (contact angle > 90°) to facilitate dropwise condensation and such surfaces yield greater wall heat fluxes and effective heat transfer coefficient. Author studied dropwise condensation on an inclined chemically textured surface via experiments and simulation. The condensation cycle was shown to involve four stages: formation of initial droplet nuclei, growth due to direct condensation of vapor, growth due to coalescence of adjacent drops, and gravitational instability and motion over the substrate.
An approach towards creating such hydrophobic surfaces is by texturing the surface via micro-pillars. Boreyko and Chen [6] studied self-propelled dropwise condensation on a super hydrophobic micro-pillared surface. As shown by the authors, surface energy released upon drop coalescence results in an out of plane “jumping” motion of the drop and spontaneous drop removal. However, the work lacks the detailed discussion on the different inclination angle and this gap has been addressed here in current study. Miwa et al. [7] investigated the effect of surface inclination angle on the apparent contact angle for various surface textures. The critical angle at which sliding is initiated is seen to be smaller for a hydrophobic surface when compared to a plain surface. The authors correlated the critical angle with contact angle as well as the pattern of roughness using a regression model. Marmur [8] developed a mathematical model that covers the extremes of homogeneous wetting (Wenzel) and heterogeneous wetting (Cassie-Baxter). The model, stated in the form of an algorithm highlights the importance of Gibbs free energy under homogeneous and heterogeneous wetting conditions.
Zhu et al. [9] fabricated a physically textured silicon surface using micromachining combined with self-assembled monolayers with small pillars of dimensions 9.45 μm × 9.45 μm × 16 μm, which showed spontaneous movement of the drop on the textured surface due to roughness gradient. While such small sized pillars are possible on a silicon surface, these small pillars cannot be fabricated on a copper surface using the laser ablation process. Patankar [10] shows that a liquid drop on a physically textured surface may attain either the Cassie-Baxter state, wherein the drop is stable on top of the pillar or the Wenzel state, wherein the drop penetrates into the inter-pillar void space (Fig. 1). Hence, Cassie-Baxter is effectively hydrophobic while the Wenzel state is akin towards being hydrophilic.
A detailed description of the Cassie-Baxter to Wenzel transition has been reported by Bormashenko et al. [11]. The authors show that drops placed on a rough surface undergo Cassie to Wenzel transition under dynamic conditions, such as surface vibrations. The spreading is by 1D progression of a liquid front around the triple line in the inter-pillar gap and not by spreading over the surface intrusions.
In the present study, the exact moment of transition, namely, the critical set of parameters driving the cross-over is not examined. These emerge naturally from the simulations. However, one may expect the changeover to take place with increasing drop volume, increasing surface inclination, and increasing hydrophobicity.
In the present work, to predict the state of the drop over (sessile) and below (pendant) a pillared surface we first simulate the stable drop geometry. It is demonstrated that the Cassie Baxter state can be retained for deeper pillars and transition to Wenzel state is seen for shallow pillars. The degree of hydrophobicity as a function of the pillar geometry is studied and validated against the literature. The geometric parameter is the aspect ratio defined as
The computational results are further explored experimentally, by generating a hydrophobic surface via laser texturing for lower aspect ratio and by the wire-electric discharge machining (EDM) method for a higher aspect ratio on metal substrates such as copper. The resulting physically textured surfaces are often a combination of pillars and micro channels. Laser texturing produced shallow pillars up to 10 μm depth for various combinations of laser power, speed and frequency. Wire EDM was capable of deeper pillars as per requirements of a hydrophobic surface. We have fabricated deeper pillars via wire EDM with depth up to 300 μm. This yields a drop shape of greater stability without liquid penetrating the gap between pillars, as compared to shallow pillars. The maximum equilibrium contact angle obtained with laser machining for 10 μm pillar is 135° (±2) and that with the wire EDM for 300 μm is 145° (±2). Further, water production with untextured and physically-textured copper surfaces in an experimental apparatus is reported. Drop movement underneath a physically-textured surface is experimentally investigated by measuring the time required for sliding of the first drop on surfaces with inclinations of 15°, 35°, and 55° with 35° inclination being found to be optimal. Water production for laser and wire EDM textured surface is measured at different inclination using the distillation apparatus.
Rykaczewski et al. [12] present qualitative design of a superhydrophobic surface, especially for condensation applications. The limiting conditions for creating nanorough surfaces are investigated. Condensation in the base area due to confinement causes spherical drops to form and leads to dropwise condensation. The condensation mechanism is dependent on the height of the liquid film in pillars and volume of the drop. There is strong relationship between the base diameter and the nanoscale roughness. Condensation on a superhydrophobic surface in the form of drops has been modelled by Miljkovic et al. [13]. The authors investigate the effect of a promoter layer on a pillared surface from the viewpoint of encouraging dropwise condensation. The structured surface sustains dropwise condensation and maximizes wall heat transfer rates.
From the manufacturing perspective, depth and pillar spacing are correlated and cannot be independent varied. Hence, it is quite common to discuss the hydrophobicity of the surface in terms of depth alone.
Guo et al. [14] investigated the effect of interpillar spacing as well as height of the pillar on condensation rates. For very small spacing, the critical nucleus is expected to be in the Cassie-Baxter state, irrespective of the pillar height. With increase in pillar spacing, its height plays an important role for in fixing the drop configuration as the Cassie state or the Wenzel state. Starostin el al. [15] conducted experiments on super-hydrophobic and super-oleophobic condensing surface made of aluminium using an E-SEM. Results showed filmwise condensation on a super-hydrophobic surface and dropwise condensation on super-oleophobic surface. Higher energy barriers of the latter were seen to sustain dropwise condensation. Thus, energy barrier was seen to play a significant role in determining the mode of condensation.
Section snippets
Simulations of sessile and pendant droplets on textured surface
To model drop transition on the pillared surface from the Cassie-Baxter state to the Wenzel state, simulations were carried out using the multiphysics finite element-based solver, Comsol© 5, Ismail [16] has described a method to investigate drop movement using the level set and phase field methods in a multi-phase flow framework. Here the problem was formulated in terms of Navier-Stokes equations using the level set method with two phase flow physics modules using an axisymmetric coordinate
Results from simulation and experiments
Vapor condensation data from experiments and simulation are compared in the following sections. The substrates for condensation studied are a plain bare copper surface, laser-textured surface with shallow pillars and a wire-cut EDM surface with deeper pillars.
Discussion
The extent of hydrophobicity attainable from a pillared surface for sessile and pendant drops was studied using Comsol© simulation. Simulations revealed that the Cassie-Baxter state was preferred for deep pillars while the drop spread in the Wenzel configuration for shallow pillars. A pillared hydrophobic copper surface was fabricated using the laser machining technique for use as a water vapor condensing substrate in a distillation apparatus. While each pillar was of square cross-section,
Conclusions
Physically textured copper surfaces with 10, 100, and 300 μm deep pillars have been studied in the context of their application as a condensing surface in a water distillation system. Texturing has been carried out using laser-ablation and wire-cut EDM processes. Pillared surfaces have been compared with a plain copper surface. Both, simulations and experiments have been carried out. Simulations showed that the Cassie-Baxter state is preferred by a droplet for deep pillars (>100 μm) while a
Nomenclature
- a
pillar dimension (square pillar width), μm
- A
area ratio of liquid contacting with droplet of rough surface to the projected surface
- b
spacing between the two pillars (channels), μm
Bond number
- g
acceleration due to gravity, m/s2
- H
height of the pillar, μm
- qc
contact angle of drop for Cassie Baxter stage, deg
- qw
contact angle of drop for Wenzel stage, deg
- γ
liquid vapor surface tension coefficient, N/m
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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