channels have one or more sides exposed to the atmosphere, which can provide the benefits for the simplicity of manufacturing, the directness of cleaning, surface modification, and deposition of catalysts. [8] These systems can also overcome the typical problems of traditional microfluidics, such as the risk of channel clogging, bulky equipment, and flow disturbances caused by bubbles. Open channels in reactors can use different geometries to achieve fluid confinement, [9] which are divided into two categories: (i) physical confinement [10] and (ii) wettability contrast confinement. [11] In the first method, a rectangular or triangular (cross-section) channel with an open on one side is used to restrict and manipulate the fluid. A channel can be formed by two opposing vertical walls; or use the narrow sides of a solid surface. In the second case, the wettability contrast constraint means that hydrophilic and hydrophobic regions are made in a specific region. And the fluid can flow in one region restricted by the virtual channel wall composed of the opposite wettability. Hydrophilic and hydrophobic regions can be formed by surface modification or oxidation, such as plasma irradiation. [12] Since the second method for the construction of the open channels is more accessible and applicable for the surface of different materials, it attracts lots of interest among chemists these years. For the successful control of surface wettability, many physical and chemical technologies have been applied, including photolithography, [13] inkjet systems, [14] UV/ozone irradiation, [15] or plasma treatment. [12] However, there are some disadvantages to the above methods. Modification with organic molecules needs lots of solvents. Photolithography is expensive. Surface wettability caused by plasma treatment and ultraviolet irradiation quickly changes over time. Plasma treatment lacks the selectivity of regions. More important, it is difficult to deposit catalysts in the channel at the same time. Therefore, it is still a challenge to develop an efficient, low-toxicity, low-cost, and simple way to realize the region's selective wettability and chemical reactivity for the fluid channel. In this paper, an open flow reactor (OFR) with wetting contrast has been successfully manufactured by direct laser sintering. [16] Laser sintering is an emerging technology for designing patterns on the surface of various materials such as plastic, metal, glass, ceramic, and wood. It is a mask-less process and can write arbitrary geometric figures directly. The laser energy can be accurately transmitted to the required location with good spatial resolution. Only the place irradiated by the laser will change its physical form or chemical composition. Therefore, laser sintering technology has broad application prospects in the design and manufacture of microreactors. [17] The open flow reactor is a kind of reactor with at least one side open to the atmosphere. Open channels have the advantages of simple surface modification, direct cleaning, and relative...