We describe the co-current flow pattern of gas and liquid through micro-fabricated beds of solid and pillars under variable (i) capillary number, (ii) contact angle or wettability and (iii) pillar arrangement, i.e. modifying the distance between pillars or their size and comparing regular with more chaotic systems. Laser-induced fluorescent microscopy and image analysis are used to study the hydrodynamic interactions in terms of dynamics, liquid hold-up, and gas–liquid interfacial area per reactor volume. Those parameters provide insights into the multiphase flow patterns in these systems, how to control them, maximize mass transfer rate and unlock the potential of microreactors to reveal further intrinsic information.