Study of the Relation Between the Reynolds Number and the Formation of Au and Ag Nanostructures by Flow-Driven Surface Modification in Microfluidic Reactors

Microfluidics enables spatially controlled nanostructure synthesis by coupling confined flows with surface reactions. In this work, we study how geometry-induced laminar mi-cro-environments govern the in-situ formation of Au and Ag nanostructures inside 3D-printed microfluidic reactors. Proof-of-concept fish-scale valves were fabricated by masked stereolithography in three architectures designed to define three recurring zones in the microreactor, inside the scales (zone 1), between the scales (zone 2), and along the rows of scales (zone 3). A Cu thin film was deposited on the inner walls of the channel to serve as the sacrificial surface for galvanic replacement using AgNO3 or HAuCl4. Distinct 0D, 1D, and 2D nanostructures were simultaneously obtained in a zone-dependent man-ner across the valves, including nanoparticle and nanopore-rich regions, nanowires, nanoflakes and clustered 2D features. COMSOL simulations were used to solve the Na-vier-Stokes equation and extract specific-zone flow descriptors, including Reynolds num-ber, velocity, and wall shear stress, and relate them to the nanostructure morphologies observed by SEM. The flow throughout the devices is strongly laminar, with local Reyn-olds numbers up to 0.04, exhibiting systematic spatial gradients imposed by the valve geometry. These results provide a design-guided route to tune nanostructure morphology through microchannel architecture under constant global operating conditions.