Droplet coalescence and freezing on hydrophilic, hydrophobic, and biphilic surfaces

Abstract

Frost and ice formation can have severe negative consequences, such as aircraft safety and reliability. At atmospheric pressure, water heterogeneously condenses and then freezes at low temperatures. To alter this freezing process, this research examines the effects of biphilic surfaces (surfaces which combine hydrophilic and hydrophobic regions) on heterogeneous water nucleation, growth, and freezing. Silicon wafers were coated with a self-assembled monolayer and patterned to create biphilic surfaces. Samples were placed on a freezing stage in an environmental chamber at atmospheric pressure, at a temperature of 295 K, and relative humidities of 30%, 60%, and 75%. Biphilic surfaces had a significant effect on droplet dynamics and freezing behavior. The addition of biphilic patterns decreased the temperature required for freezing by 6 K. Biphilic surfaces also changed the size and number of droplets on a surface at freezing and delayed the time required for a surface to freeze. The main mechanism affecting freezing characteristics was the coalescence behavior.Keywords: Biphilic, droplet coalescence, freezing, frost On a cooled surface at atmospheric pressure, water first nucleates heterogeneously and subsequently, droplets may grow, coalesce, and freeze depending on the level of supercooling. Researchers have examined superhydrophobic [1-10], nano-engineered [7,11,12], and oil-impregnated surfaces [13,14] to prevent frost or freezing by delaying nucleation, increasing droplet mobility, and reducing droplet contact area with the surface. Most research reports the total surface freezing time delay. Previous research that investigated direct condensation followed by freezing was performed at surface temperatures varying from 253 K -268 K and relative humidities of 30 -60% [2-4, 6, 9, 10]. This research examines the effects that biphilic surfaces (surfaces which combine hydrophilic and hydrophobic regions) have on heterogeneous water nucleation, droplet growth, and freezing behavior.Biphilic surfaces [15][16][17] were fabricated on silicon wafers with a thermally grown oxide layer. Photolithography was used to mask the hydrophilic spots during a deposition of a self-assembled monolayer of Octadecyltrichlorosilane (OTS). The contact angle of the hydrophobic coating was measured to be 105°. Hydrophilic (Philic), hydrophobic (Phobic), and three types of biphilic surfaces (200IL, 200S, 25IL) were fabricated. The biphilic surfaces consisted of 25-μm-or 200-μm-hydrophilic circles on a hydrophobic background as shown in Figure 1. Before each experiment, the sample was cleaned with isopropyl alcohol and dried with nitrogen. After cleaning, the sample was placed on a freezing stage in an environmental chamber under quiescent flow conditions with a controlled air temperature (295 K) and relative humidity (RH). Each surface was tested at three relative humidities: 30%, 60%, and 75%. For each humidity and surface, the maximum freezing temperature was determined by incrementally decreasing the temperature of the freezing...

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“…To promote the DWC, micro/nano surface structures (Shang et al, 2018 andZarei et al, 2018), as well as inorganic (gold) coatings Wilkins et al, 1973, hydrophobic porous membranes (Hu and N. Chung, 2018) and hydrophobic organic (polymers (Lu et al, 2015), self-assembled monolayers (Modak et al, 2019)), wick structure in pipe (Yunus and S. Alsoufi, 2019) have been used. To further enhance the heat transfer coefficient, bilphilic (patterned hydrophobic and hydrophilic) surfaces have been created with onedimensional (1-D) (Peng et al, 2015) and two-dimensional (2-D) (Van Dyke et al, 2015) patterns.…”