Visualization experiments on the performance of mesh-wick heat pipes with differing wick wettability

•Visualization experiments performed for copper mesh heat pipes with various surface wettabilities.•Untreated wick exhibits best performances in both total thermal resistance and Qmax.•In a hydrophobic condenser, water level is lower and large drops grow and rupture on exposed area.•Large round condensing drops on a hydrophobic condenser act as heavy thermal barriers to raise Rc.•Hydrophilic treatment on evaporator fails to enlarge Qmax due to increased liquid flow hindrance.