Boiling heat transfer on superhydrophilic, superhydrophobic, and superbiphilic surfaces

TLDR: In this article, the authors characterized pool boiling on surfaces with wettabilities varied from superhydrophobic to super-hydrophilic, and provided nucleation measurements, and developed an analytical model that describes how biphilic surfaces effectively manage the vapor and liquid transport, delaying critical heat flux and maximizing the heat transfer coefficient.

About: This article is published in International Journal of Heat and Mass Transfer.The article was published on 2013-02-01 and is currently open access. It has received 428 citations till now. The article focuses on the topics: Enhanced heat transfer & Critical heat flux.

Figures

Figure 1. Six types of surfaces are considered in this study. The first four types (top row, from left to right) have spatially uniform wettability: an oxidized silicon hydrophilic surface (7-30º wetting angle, as shown by the imaged water drop), a fluoropolymer-coated hydrophobic silicon surface (110-120º), a SHPi surface (0º), and a SHPo surface (150-165º). The fifth type is a biphilic surface, bottom left, which juxtaposes hydrophilic and hydrophobic regions, as indicated by the arrows. The sixth type is a superbiphilic (SBPi) surface, bottom right, which juxtaposes SHPi and SHPo regions. That SEM picture appears grainy because of the surface nanostructuring, and a magnified view is in the inset.

Figure 6. Boiling curves comparing hydrophilic, biphilic, SHPi and SBPi surfaces, (a) is HTC vs. superheat and (b) shows the heat flux vs. superheat. All surfaces biphilic have the same topography, 50 µm spots with d/p =0.5. The wetting contrast of the biphilic surface is (20°/120°).

Figure 3. Measured density of active nucleation sites for a hydrophilic, hydrophobic, SHPi and SHPo surface, as a function of the superheat. A power law fit is provided for modeling purposes, and

Figure 5. Boiling curves of biphilic surfaces with wettability contrast as in the legend, with solid curves denoting modeling and dotted curves, experiments: (a) Effect of the wettability contrast of biphilic surfaces (b) Effect of the spot size of the biphilic surface, for surfaces with wettability contrast of (20°/120°). All biphilic surfaces have a spot diameter/pitch ratio d/p = 0.5. For comparison purposes, (a) also shows modeling results for surfaces with uniform wettability, with corresponding experimental

Figure 4. (a) Boiling measurements (points) for a hydrophilic, hydrophobic, SHPi and SHPo surface, compared with our theoretical model (lines). Insets (b) and (c) show the variation in surface topology for two different SHPi surfaces.

Figure 2: Model of bubble growth and departure based on wetting and buoyancy forces. The departure diameter dd of a vapor bubble and its maximum contact diameter dcon depend on the wetting angle for a surface of uniform wettability (a), and also on the diameter of the hydrophobic spot on a biphilic surface (b). The cartoons above the plots illustrate the growth and departure of the bubble.

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "Boiling heat transfer on superhydrophilic, superhydrophobic, and superbiphilic surfaces" ?

In this paper, the authors show how to construct a pool-boiling surface with super-hydrophobic ( SHP ) properties, where the apparent contact angle of water on the surface in air is close to zero. 

Q2. What future works have the authors mentioned in the paper "Boiling heat transfer on superhydrophilic, superhydrophobic, and superbiphilic surfaces" ?

Future work will aim at a better understanding and control of multiphase flow on biphilic surfaces by means of, e. g., parametric studies on the surface topography. The long-term stability of these surfaces will also be characterized towards the development of technical applications. 

Q3. Why do they have the drawback of reaching CHF at low heat fluxes?

the nucleation enhancement on hydrophobic and SHPo surfaces comes with the drawback that they reach CHF at low heat fluxes, in the range of 30 W/cm2; this is due to their strong tendency to form an insulating vapor film, a phenomenon called the Leidenfrost effect. 

Q4. What is the interesting result of their measurements?

The most interesting result of their measurements is that the largest heat transfer coefficients are reached not on surfaces with spatially uniform wettability, but on biphilic surfaces, which juxtapose hydrophilic and hydrophobic regions. 

Q5. What is the effect of the wetting angle on the hydrophilic surface?

At lower values of superheat, the HTC is independent on the hydrophilic wetting angle, probably because most nucleation and boiling occur on the hydrophobic regions. 

Q6. How does the first strategy enhance the performance of pool boiling?

The first strategy enhances the performance at low heat fluxes, in the isolated bubble regime, by promoting nucleation and enhancing HTC [16]. 

Q7. What is the process used to create the SBPi surfaces?

Thin film heaters made of indium tin oxide (ITO) are directly deposited on the reverse side of the silicon wafer used to create the SBPi surfaces. 

Q8. How many copper electrodes were thermally deposited onto the heater?

Copper electrodes of 1 cm x 1 cm were thermally deposited onto each end of the ITO heater, also using a polycarbonate shadow mask, leaving a 1cm x 1 cm square of ITO exposed. 

Q9. What is the way to measure nucleation on a SHPo surface?

measurements of nucleation on SHPo surfaces call for more accurate temperature measurement methods, such as resistive temperature devices [42, 43] or arrays of thin film thermocouples [44]. 

Q10. How does the departure diameter of the bubbles change?

While the maximum contact diameter increases monotonically, the departure diameter reaches its maximum around a wetting angle of 110º. 

Q11. Why is the performance of the SBPi surfaces higher than predicted?

Note that the measured performance of the SBPi surfaces is higher than predicted by the analytical model, possibly because the model only accounts for effects of wettability contrast and not for capillary transport enhancement caused by the surface nanostructuring. 

Q12. What is the effect of the wetting angle on the hydrophobic surface?

Note that the authors assume that the wetting line of the bubble advances until pinning occurs at the edge of the hydrophobic spot; also the number of active nucleation sites of the hydrophobic regions cannot exceed the number of hydrophobic spots. 

Q13. What is the significance of the noise on the SHPi surfaces?

Significant HTC noise is also visible on the SHPi surfaces, which might be attributed to the random nature of the nanostructuring process used, such as peak density (0.8-3.8 peaks/µm2), peak height (0.7-1.98 µm) and peak width at the base of the structure (0.3-1 µm), as visible in the two samples of SHPi surfaces in Figure 4b-c. 

Q14. What is the significance of biphilic surfaces?

Few biphilic surfaces have been fabricated [25, 28, 36], but they all have been shown to significantly enhance boiling heat transfer. 

Q15. How much better is the HTC on a smooth hydrophilic surface?

Compared to a smooth hydrophilic surface (SiO2, contact angle 7º), the improvement in HTC in pool boiling is larger than one order of magnitude at low superheat (best shown from 5K to 10K) and about 300% for larger values of superheat. 

Q16. How much higher is the heat transfer coefficient on SBPi surfaces?

Heat transfer coefficients measured on SBPi surfaces are up to three times higher than on state-of-the-art nanostructured surfaces.