In many industrial applications, ranging from chemical reactions to transport of suspensions, colloids can be produced and deposited within the processing equipment leading to significant fouling and clogging issues. Here, a continuous-flow reactor for the production of pharmaceutically relevant arylamines through the Buchwald-Hartwig synthesis is coupled to a microfluidic channel to investigate the complex interplay among fluid dynamics, crystallization, reaction and surface chemistry. The process of wall deposition and crystal formation under flow is studied in situ by fragmenting to the micron-scale the particles coming from the reactor and using glass microchannels. Tuning the flow rate, a dramatic change in crystal morphology and composition is found, from dendritic needle-like potassium tert-pentoxide structures to irregular colloidal clusters mainly made of potassium bromide. The results are characterized in terms of dimensionless parameters and provide novel insights for an improved understanding and control of fouling and crystal growth under flow. (C) 2019 Elsevier Ltd. All rights reserved.