The impact of contact angle on flow resistance reduction in hydrophobic micro pin fins

• Micro pin fins with different contact angles are prepared.
• Flow resistance in micro pin fins is almost 60% reduced by hydrophobic surfaces.
• The flow separation and transition in wake zones is delayed by hydrophobic surfaces.

The pressure drops and the friction factors are measured when ultra pure water flows through the elliptical, diamond and circular hydrophobic micro pin fins with different contact angles of θ = 99.5°, 119.5° and 151.5°, and the coefficients of pressure drop reduction dp_coe and friction factor reduction df_coe are calculated respectively. It is found that the value of dp_coe becomes large with the increase of θ in elliptical and diamond micro pin fins, but for circular micro pin fins it increases at first and then decreases. With the increase of θ, both values of dp_coe and df_coe are reduced with the increase of the flow rate in elliptical micro pin fins, but the values are reduced at first and then become almost constants in diamond and circular micro pin fins. Besides, the minimal value of df_coe is over 50% in diamond and circular micro pin fins with contact angle of θ = 151.5°. When θ ⩾ 119.5°, the values of dp_coe and df_coe in diamond and circular micro pin fins are both larger than those in elliptical test sections, but for θ = 99.5° the former are slightly smaller than the latter at Re < 600.