Vapor condensation is a well-known
phase-change phenomenon observed
in nature as well as in different industrial applications. Superhydrophobic
surfaces (SHSs) with low hysteresis can efficiently drain off the
condensate and rejuvenate the nucleation sites further. In this work,
three distinct SHSs were fabricated by nanocoating three hydrophobic
agents, viz., perfluoro-octyl-triethoxy-silane (PFOTS), perfluoro-octanoic-acid
(PFOA), and commercial Glaco solution on a hierarchical aluminum surface.
The surface morphology of all surfaces was investigated, and its effects
on the wetting, droplet departure, and overall heat-transfer coefficient
(HTC) during condensation phenomena in the humid air (>95% noncondensable
gases) were analyzed. The contact angle hysteresis of all three surfaces
was very low (∼5°); however, different wetting behaviors
were observed during the condensation, depending on the adhesion of
the condensate drop with nanoscale textures in the microcavities.
Dropwise condensation (DWC) was observed in silane and Glaco-coated
surfaces. A gravity-assisted sweeping mechanism removed the condensate
from the silane-coated surface. In contrast, the condensate was ejected
out of the plane of the Glaco-coated surface by droplet jumping. The
PFOA-coated surface has shown DWC initially and floods in the later
stages due to highly pinned condensed droplets. This study reports
an enhancement of ∼35 to ∼110% in the HTC for the SHS-exhibiting
gravity-assisted sweeping mechanism compared to the droplet-jumping
mechanism. The present work will provide substantial insights into
the fabrication of efficient hierarchical interfaces for water-energy
nexus applications.
Wettability patterning of a surface is a passive method to manipulate the flow and heat transport mechanism in many physical processes and industrial applications. This paper proposes a rational wettability pattern comprised of multiple superhydrophilic wedges on a superhydrophobic background, which can continuously remove the impacted spray droplets from the horizontal surface. We observed that the spray droplets falling on the superhydrophilic wedge region spread and form a thin liquid film, which is passively transported away from the surface. However, most of the droplets falling on the superhydrophobic region move towards the wedge without any flooding. The physics of the passive transport of the liquid film on a wedge is also delved into using numerical modelling. In particular, we elucidate the different modes of droplet transport in the superhydrophobic region and the interaction of multiple droplets. The observed droplet dynamics could have profound implications in spray cooling systems and passive removal of liquid from a horizontal surface. This study’s findings will be beneficial for the optimization of efficient wettability patterned surfaces for spray cooling application.
Condensing atmospheric water vapor on surfaces is a sustainable approach to addressing the potable water crisis. However, despite extensive research, a key question remains: what is the optimal combination of the mode and mechanism of condensation as well as the surface wettability for the best possible water harvesting efficacy? Here, we show how various modes of condensation fare differently in a humid air environment. During condensation from humid air, it is important to note that the thermal resistance across the condensate is nondominant, and the energy transfer is controlled by vapor diffusion across the boundary layer and condensate drainage from the condenser surface. This implies that, unlike condensation from pure steam, filmwise condensation from humid air would exhibit the highest water collection efficiency on superhydrophilic surfaces. To demonstrate this, we measured the condensation rates on different sets of superhydrophilic and superhydrophobic surfaces that were cooled below the dew points using a Peltier cooler. Experiments were performed over a wide range of degrees of subcooling (10−26 °C) and humidity-ratio differences (5−45 g/kg of dry air). Depending upon the thermodynamic parameters, the condensation rate is found to be 57−333% higher on the superhydrophilic surfaces compared to the superhydrophobic ones. The findings of the study dispel ambiguity about the preferred mode of vapor condensation from humid air on wettability-engineered surfaces and lead to the design of efficient atmospheric water harvesting systems.
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