Microfluidic assembly of silica nanoparticles within emulsion droplets: Considering the rheological behavior of the dispersed phase

Microfluidic assembly of nanoparticles (NPs) is advantageous in various fields, such as drug delivery and synthesis of photonic crystals and catalysts. The present study uses a flow-focusing microfluidic device to encapsulate silica NPs as water-in-oil (W/O) emulsions. Hence, water-based nanofluids with silica NPs are prepared, and nanofluid droplets are generated at different capillary numbers, flow rate ratios, and nanoparticle concentrations. Although most previous studies neglect the effect of the rheology of nanofluids on droplet formation, this study reveals its significance for determining droplet formation regimes as well as the size and production rates of droplets. As the nanofluid concentration increases, smaller droplets are generated in general with higher production rates. Also, droplet formation regimes are affected by nanoparticle concentrations such that an unusual mode of droplet formation, the binary dripping regime, appears at high concentrations of silica NPs. With the privilege of a computational fluid dynamics (CFD) analysis, it is revealed that the rheological behavior of the employed nanofluids is responsible for these observations. The viscosity of the employed nanofluids significantly affects the duration of the growth stage during droplet formation. As the nanoparticle concentration increases, viscous dissipation increases, which leads to generating smaller droplets with higher production rates. Compared to the interfacial tension, the rheological behavior of the employed nanofluids is more prevailing for determining droplet formation characteristics. This study highlights the rheology effects on the microfluidic generation of nanofluid droplets for the first time and has the potential to push the frontiers of knowledge in colloid science.