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Finned heat exchangers used in energy applications regularly undergo condensation frosting. Understanding the effects of fin spacing on frost growth with surfaces having differing wettability is important for the design of next-generation heat exchangers. This study presents an experimental investigation using high-resolution optical imaging of frost growth between two parallel vertical plain aluminum fins as a function of surface wettability (hydrophilic, superhydrophilic, superhydrophobic), fin surface temperature (-5 to -15 °C), fin spacing (2 to 8 mm), and relative humidity (≈ 50 and 70%). We observe two distinct regimes of frost growth for all test conditions. The effect of surface wettability on frost growth rate becomes significant for fin spacing ≤ 6 mm at fin surface temperatures below -10 °C. The fin surface temperature plays a significant role for fin spacing > 4 mm. Furthermore, for hydrophilic surfaces, the effect of …