Braquel Burnett and Faculty Mentor: Julie Crockett, Mechanical Engineering

Superhydrophobic surfaces offer unique characteristics such as extreme water repellency, drag reduction and enhanced condensation rates. These effects are possible due to a combination of micro/nano-texturing and a hydrophobic chemical coating. With the advent of micro/nano-fabrication, research and development on superhydrophobic surfaces has sky-rocketed due to the potential advantages across multiple industries including lab-on-a-chip technology, micro-electronic cooling and anti-icing applications.

Thus far, researchers have explored the hydrodynamics of droplets rolling on superhydrophobic surfaces at room temperature ¹. However, heat removal by droplets on heated superhydrophobic surfaces has not received much attention. A quantitative study of this thermal process is of paramount importance if integration of superhydrophobic surfaces in thermal management applications is to be realized. While research has been conducted for droplets bouncing on heated superhydrophobic surfaces ², rolling droplets, the expected behavior inside certain industrial instruments such as large-scale condensers, have not yet been considered. In order to bridge the gap between laboratory research and a more accurate representation of real-world machinery, this study sought to quantify the bulk heat transfer of a constant array of droplets rolling down a heated superhydrophobic incline.

The four superhydrophobic surfaces used in this work exhibited center-to-center pillar spacing of 8, 16, and 32 microns. Pillar height and diameter were held constant across all surfaces at 15 microns, an example of which is shown in Figure 1. This variation meant that the only control variable was the cavity fraction, or the percentage of pillar area to cavity area. The surfaces were placed on a heated aluminum plate tilted at a 45 degree angle and embedded with cartridge heaters. Two thermocouples were incorporated within the plate to monitor a representative temperature of the surfaces. The temperatures at which the surfaces were tested were 50˚C, 75˚C, and 100˚C. These temperatures were chosen in order to observe a maximum amount of heat transfer without inducing boiling. Water droplets (~3 mm in diameter) were released at room temperature at the top of the surface and rolled due to gravity. The mechanism for releasing the droplets consisted of a series of syringes connected by a series of tubing originating from a single water reservoir. A flow meter monitored volumetric flow rates of experiments and to keep them constant. A thermal high speed camera filmed the events to measure the temperature of the surface with time. A humidity and ambient temperature sensor were used as part of the calibration process for using the thermal camera. Five videos were taken for each scenario of varying surface type and varying temperature so that 90 total experiments were accomplished. Associated thermal camera software was used to process temperature data. The data collected consisted of the change in temperature over a time interval of 90 seconds in order to give an idea of occurring heat flux.

It was found that higher temperatures and larger flow rates produced more significant results. A temperature of 50 degrees Celsius provided a heat flux that did not display a significant difference in temperature change with the varying flow rates for the 16P and 32P surfaces in these experiments (See Figure 2). Significant results were found, however, in that the smallest pitch surface showed the greatest heat transfer rate for all scenarios. This is as expected since the hydrophobicity of the surfaces causes droplets to remain on top of the pillars. The smaller pitch surface means that the contact area between the surface and the droplets was greater than the larger pitch surfaces. These results show that surface to water heat conduction was the most contributing heat transfer factor in these scenarios.

With these results it is shown that smaller cavity fraction surfaces, or smaller pitch distances for the types of surfaces used in this study, should be used in applications where heat transfer or cooling of surfaces or droplets is desirable. However, this principle may not be a necessary application at lower temperatures. At what temperature the cavity fraction is significant will depend upon the particular application and a more detailed interpretation of the data will be necessary. Further, a more extensive study should be done to create a more all-inclusive map of conduction rate of inclined surfaces including a larger variety of surfaces so that it may be even more useful to all real world applications of superhydrophobic surfaces.

1. Hao, Lv & Yao, 2013; Liu, Fu, Rode & Craig, 2011; Miwa, Nakajim, Fujishima, Hashimoto & Watanabe, 2000
2. Bertola, 2015; Tran, Staat, Prosperetti, Sun & Lohse, 2012; Tran et al., 2013