## 1 Introduction  
Effective mixing in a small channel and microfluidic devices are crucial in various scientific fields, like biomedical diagnostics, drug development, and various industries, due to their small flow channels that enhance the surface-to-volume ratios [1; 2; 3; 4; 5]. In chemical reactions, it is important to thoroughly mix the sample and reagent. If the mixing takes longer than the required reaction time, it hampers the performance of the reaction [6; 7; 5; 8]. Therefore, in lab-on-chip or organ-on-chip devices, mixing is needed. The purpose of micromixing is to achieve a thorough, rapid, and efficient mix process in microscale device, despite the difficulties in the laminar behavior of the fluids in microdevices [1; 2]. Active microdevices are systems incorporating sensors, actuators, or electronic components, designed to actively interact with their environment and perform specific functions. Devices with this technology include enhanced control and adaptability, with challenges such as increased complexity and potential issues in fabrication and integration [9; 10; 11; 12]. On the other hand, passive micromisers have a low cost, a reduced power requirement, high portability, and more straightforward integration with other planar biomedical chips [13; 14; 15]. However, developing an accurate passive micromizer in a short manufacturing time would be challenging. The efficient mixing of two fluids in a microchannel largely depends on the velocity of the fluids and the diffusion coefficient of the species. The longer the fluids remain in contact with each other, the greater the degree of mixing. However, it is also essential to consider that an excessively long microchannel can increase the pressure loss and design complexity [13; 14; 15; 16; 17]. Therefore, generally, passive micromixers using conventional soft lithography involve long channels or complex soft lithography processes with via holes. SLA additive manufacturing, also known as stereolithography, employs a laser to solidify a liquid resin, building layers to form 3D objects with high precision and detail. The possibility to use the three dimensions in the current SLA manufacturing methods allows an increase in the contact area between the mixing species by the design configurations of the microchannels [18; 19]. However, recent examples show that good performance can be achieved, albeit using external pumping sources; see Table 1.  
According to the literature review in the chart above and complementary information, researchers propose different mixing methods for 3D-printed microdevices, including active methods that require long-length channels (25-600 um), a wide range of mixing times (0.6-146 s), or high flow rates (10 to 2000 um/min) [18; 23; 24; 25; 26; 27; 28; 29; 30; 31]. All the devices listed in the table above require an active method for their operation. Since they rely on an external component, the required speeds and pressures are high. The minimum channel length for proper fluid mixing execution is 25 mm. The novel propose of this study is to achieve results obtained with active devices using passive methods such as capillarity. The low Reynolds number in microchannels results in limited turbulent mixing, making species' mixing primarily dependent on slow diffusion. This poses a challenge for efficient mixing within short channels, crucial for maintaining the miniature nature of microfluidic devices. Developing effective mixing schemes becomes essential not only for thorough species mixing but also for increased throughput in microfluidic systems and to achieve the concept of micro-total-analysis systems and lab-on-a-chip devices [1; 2]. As a result, enhancing the fabrication method to achieve complex and novel geometries is a crucial aspect of the current research. This improvement would allow for the management of laminar fluids in small devices and an efficient mixing process.