Evaporation of Sessile Water Droplets on Horizontal and Vertical Biphobic Patterned Surfaces.
TL;DR: In this article, a single water droplet of 8 μL volume is placed on a preheated surface and allowed to evaporate in an open laboratory environment, and the influence of substrate orientation (horizontal and vertical) on evaporation dynamics is investigated.
Abstract: This paper presents an experimental study on thermal transport to single water droplets evaporating on heated biphobic surfaces consisting of a superhydrophobic matrix with a circular hydrophobic pattern with strong contact line pinning. A single water droplet of 8 μL volume is placed on a preheated surface and allowed to evaporate in an open laboratory environment. We investigate the influence of substrate orientation (horizontal and vertical) on evaporation dynamics. Using optical and infrared imaging, we report droplet fluid dynamics and heat transfer characteristics of the evaporating droplet. Overall, evaporation is more efficient on the vertical surface, exhibiting higher total heat transfer rates and up to 10% shorter evaporation times. Counterintuitively, on the vertical surface, the substrate-droplet interfacial heat flux was higher near the lower contact line than in the upper region, despite a high contact angle and an expected wedge effect at the bottom. At the same time, the temperature is colder in the lower part of the droplet. We attribute this apparent anomaly to the competition between sensible heating and evaporation, and a modified convective flow signature (both within the droplet and the gas phase) compared to a horizontal surface. We also show that the thermal signature becomes uniform once the contact angles at the upper and lower contact lines become equal toward the end of the evaporation process. Insights from this work can guide the design of spray cooling devices or be used to alter particle deposition patterns during evaporation-based fabrication techniques and ink-jet printing.
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TL;DR: Liao et al. as discussed by the authors demonstrate an optical whispering-gallery mode (WGM) barcode technique involving simultaneous monitoring of the patterns of multiple modes that can provide a direct temperature readout from the spectrum.
Abstract: Temperature is one of the most fundamental physical properties to characterize various physical, chemical, and biological processes. Even a slight change in temperature could have an impact on the status or dynamics of a system. Thus, there is a great need for high-precision and large-dynamic-range temperature measurements. Conventional temperature sensors encounter difficulties in high-precision thermal sensing on the submicron scale. Recently, optical whispering-gallery mode (WGM) sensors have shown promise for many sensing applications, such as thermal sensing, magnetic detection, and biosensing. However, despite their superior sensitivity, the conventional sensing method for WGM resonators relies on tracking the changes in a single mode, which limits the dynamic range constrained by the laser source that has to be fine-tuned in a timely manner to follow the selected mode during the measurement. Moreover, we cannot derive the actual temperature from the spectrum directly but rather derive a relative temperature change. Here, we demonstrate an optical WGM barcode technique involving simultaneous monitoring of the patterns of multiple modes that can provide a direct temperature readout from the spectrum. The measurement relies on the patterns of multiple modes in the WGM spectrum instead of the changes of a particular mode. It can provide us with more information than the single-mode spectrum, such as the precise measurement of actual temperatures. Leveraging the high sensitivity of WGMs and eliminating the need to monitor particular modes, this work lays the foundation for developing a high-performance temperature sensor with not only superior sensitivity but also a broad dynamic range. Extremely precise measurement of temperature using devices known as whispering-gallery mode (WGM) sensors can be greatly improved using a technique that simultaneously monitors different modes, or patterns, of the optical signals. WGM sensors rely on the sustained circulation of light within closed concave microstructures—discs, rings, or spheres—similar to the movement of sound waves in whispering galleries such as the dome of St. Paul’s Cathedral in London. Jie Liao and Lan Yang at Washington University in St. Louis, Missouri, USA, developed procedures to analyse the effect of temperature on the collective patterns of light signals in WGM sensors. The results are converted into optical barcodes which indicate the temperature directly. This multimode system overcomes significant limitations imposed by the restricted range and less direct monitoring methods of existing single-mode sensors.
79 citations
TL;DR: A review of the literature on the factors affecting microscale evaporation, which include the properties and temperature of the solid substrate, vapor transport in the gas domain, microconvection, and engineered surface features, is presented in this paper.
36 citations
TL;DR: In this article, the authors report the dynamic wetting behavior and heat transfer characteristics for impinging droplets on heated bi-phobic surfaces (superhydrophobic matrix with hydrophobic spots).
Abstract: This paper reports the dynamic wetting behavior and heat transfer characteristics for impinging droplets on heated bi-phobic surfaces (superhydrophobic matrix with hydrophobic spots). A non-patterned superhydrophobic and a sticky hydrophobic surface acted as control wettability surfaces. As expected, differences in wetting and heat transfer dynamics were noticeable for all surfaces with the most pronounced variation during the receding phase. During spreading, inertia from the impact dominated the droplet dynamics, and heat transfer was dominated by convection at the contact line and internal flow. As contact line velocities decreased over time, evaporative cooling at the contact line gained importance, especially for the bi-phobic surfaces, where liquid remained trapped on the hydrophobic spots during receding. These satellite droplets increased the contact area and contact line length and assisted heat transfer and substrate cooling after lift-off of the main droplet. Compared with the hydrophobic surface, the contribution of the contact line heat transfer increased by 17%–27% on the bi-phobic surfaces depending on the location of impact relative to the hydrophobic spots. Nonetheless, the bi-phobic surfaces had a lower total thermal energy transfer. However, compared with the plain superhydrophobic surface, heat transfer was enhanced by 33%–46% by patterning the surface. Depending on the application, a trade-off exists between the different surfaces: the sticky hydrophobic surface provides the best cooling efficiency yet is prone to flooding, whereas the superhydrophobic surface repels the liquid but has poor cooling efficiency. The bi-phobic surfaces provide a middle path with reasonable cooling effectiveness and low flooding probability.
28 citations
TL;DR: In this article, the authors report the dynamic wetting behavior and heat transfer characteristics for impinging droplets on heated bi-phobic surfaces (superhydrophobic matrix with hydrophobic spots).
Abstract: This paper reports the dynamic wetting behavior and heat transfer characteristics for impinging droplets on heated bi-phobic surfaces (superhydrophobic matrix with hydrophobic spots). A non-patterned superhydrophobic and a sticky hydrophobic surface acted as control wettability surfaces. As expected, differences in wetting and heat transfer dynamics were noticeable for all surfaces, with the most pronounced variation during the receding phase. During spreading, inertia from the impact dominated the droplet dynamics and heat transfer was dominated by convection at the contact line and internal flow. As contact line velocities decreased over time, evaporative cooling at the contact line gained importance, especially for the bi-phobic surfaces, where liquid remained trapped on the hydrophobic spots during receding. These satellite droplets increased the contact area and contact line length, and assisted heat transfer and substrate cooling after lift-off of the main droplet. Compared with the hydrophobic surface, the contribution of the contact line heat transfer increased by 17 to 27% on the bi-phobic surfaces, depending on the location of impact relative to the hydrophobic spots. Nonetheless, the bi-phobic surfaces had a lower total thermal energy transfer. However, compared with the plain superhydrophobic surface, heat transfer was enhanced by 33% to 46% by patterning the surface. Depending on the application, a trade-off exists between the different surfaces: the sticky hydrophobic surface provides the best cooling efficiency, yet is prone to flooding, whereas the superhydrophobic surface repels the liquid, but has poor cooling efficiency. The bi-phobic surfaces provide a middle path with reasonable cooling effectiveness and low flooding probability.
25 citations
TL;DR: Surfactant droplet evaporation experiments were performed on two commercial crop species, wheat and capsicum, along with two synthetic surfaces, up to a 90° incline, and mathematical models were developed to simulate the experiments.
24 citations
References
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TL;DR: The Surface Evolver is a computer program that minimizes the energy of a surface subject to constraints that is represented as a simplicial complex.
Abstract: The Surface Evolver is a computer program that minimizes the energy of a surface subject to constraints. The surface is represented as a simplicial complex. The energy can include surface tension, gravity and other forms. Constraints can be geometrical constraints on vertex positions or constraints on integrated quantities such as body volumes. The minimization is done by evolving the surface down the energy gradient. This paper describes the mathematical model used and the operations available to interactively modify the surface.
2,241 citations
TL;DR: In this article, the evaporation of a sessile droplet with a pinned contact line was investigated experimentally, by analytic theory and by computation using the finite element method (FEM).
Abstract: The evaporation of a sessile droplet with a pinned contact line is investigated experimentally, by analytic theory and by computation using the finite element method (FEM). Because of the low value of R2/Dtf = cv(1 − H)/ρ = 1.4 × 10-5, where R is the contact-line radius, D is the water vapor diffusivity, cv is the saturated water vapor concentration, H is the relative humidity, and ρ is the liquid water density, the evaporation can be considered as a quasi-steady-state process. Hence, the vapor concentration distribution above the droplet satisfies the Laplace equation but with a time-varying droplet surface. It is found both theoretically and experimentally that the net evaporation rate from the droplet remains almost constant with time for a small initial contact angle (θ < 40°), even though the evaporation flux becomes more strongly singular at the edge of the droplet as the contact angle decreases during evaporation. We also measured the critical contact angle at which the contact line starts to reced...
1,302 citations
TL;DR: It is found that surfactant contamination, at a surface concentration as small as 300 molecules/microm(2), can almost entirely suppress the Marangoni flow in the evaporating droplet.
Abstract: We study the effects of Marangoni stresses on the flow in an evaporating sessile droplet, by extending a lubrication analysis and a finite element solution of the flow field in a drying droplet, developed earlier.1 The temperature distribution within the droplet is obtained from a solution of Laplace's equation, where quasi-steadiness and neglect of convection terms in the heat equation can be justified for small, slowly evaporating droplets. The evaporation flux and temperature profiles along the droplet surface are approximated by simple analytical forms and used as boundary conditions to obtain an axisymmetric analytical flow field from the lubrication theory for relatively flat droplets. A finite element algorithm is also developed to solve simultaneously the vapor concentration, and the thermal and flow fields in the droplet, which shows that the lubrication solution with the Marangoni stress is accurate for contact angles as high as 40°. From our analysis, we find that surfactant contamination, at a...
803 citations
TL;DR: In this article, the authors provide an introduction to spray cooling for electronic cooling applications, review some proposed spray cooling heat transfer mechanisms, and summarizes the data regarding the effects of non-condensable gas, surface enhancement, spray inclination, and gravity.
751 citations
TL;DR: A new method based on B-spline snakes (active contours) for measuring high-accuracy contact angles with good accuracy and applicability to a variety of images thanks to the high-quality image-interpolation model and an advanced image-energy term.
736 citations