Overview of the microchannel flow-loop apparatus and TDTR measurement methodology: (a) syringe pump-based microchannel flow loop, (b) optical bench schematic of our two-tint TDTR measurement setup, (c) expanded, construction view of the microchannel sample stage (also depicted in (a) and (b)), and (d) schematic illustrations of both hydrodynamic BL growth (⁠$δh(x)$⁠) in a microchannel of height (⁠$H≈$ 400 μm) and thermal BL growth (⁠$δth(x)$⁠) from a hot-spot in the metal-coated glass wall by the TDTR pump-probe lasers