In this work, we report the investigation of various experimental conditions and their influence on polymorphism of 5-methyl-2-[(2-nitrophenyl)amino]-3-thiophen carbonitrile, commonly known as ROY. These conditions include an in-house developed microfluidic chip with controlled mixing of parallel flows. ROY is known for its ability to form a large variety of polymorphs, including at least ten forming under standard conditions. Nucleation is triggered by adding water as antisolvent to ROY dissolved in acetone. We observe that different ROY concentrations and different solvent to antisolvent ratios naturally favor different polymorphs. Nonetheless, identical samples prepared with different mixing methods, such as shaking and magnetic stirring, consistently lead to the formation of different polymorphs. In addition, different mixing rates that can be linked to shear stress, strongly influence the crystallization. A fourth parameter, namely confinement of the sample is also found to be critical. Untangling all those parameters and their influences on polymorphism call for an experimental setup allowing all four to be controlled accurately. To that end, we developed a novel customized microfluidic setup allowing reproducible and controlled mixing conditions. Two parallel flows of antisolvent and ROY dissolved in solvent are injected into a transparent microchannel. Next, slow and progressive mixing can be obtained by molecular diffusion. Additionally, the microfluidic chip is equipped with a piezoceramic element, allowing the implementation of various mixing rates by acoustic mixing. With this device, we demonstrate the importance of parameters other than concentration involved in polymorphism of ROY.