Wettability of titanium thin/tunable liquid/gas diffusion layers (TT-LGDLs) may affect the oxygen bubble dynamics and detachment process, and impact the performance (cell voltage) in proton exchange membrane electrolyzer cells (PEMECs). In this study, a silane monolayer is applied to tune the TT-LGDL wettability for the first time. The ultra-fast and micro-scale oxygen gas bubble dynamics and the two-phase flow in the channel are studied in situ for hydrophobically treated and hydrophilic titanium thin/tunable LGDLs (TT-LGDLs) with a high-speed and micro-scale visualization system (HMVS). The HMVS shows that the micro oxygen bubbles occur only along the CL/TT-LGDL interfaces at the rim of pores on TT-LGDLs. Bubbles more easily coalescence to form a large one in hydrophobic TT-LGDLs. Pore-scale analysis on the single bubble evolution process shows that the detachment diameter and frequency of oxygen bubbles in the hydrophobic TT-LGDLs are much larger than those in the hydrophilic TT-LGDLs. The PEMEC performance with the hydrophobic and hydrophilic TT-LGDLs are very close under 2.0 A/cm(2), which means that the wettability has limited effect on TT-LGDLs mainly due to their thin features and unique structures with straight pores. (C) 2019 Elsevier Ltd. All rights reserved.