Microfluidic-induced growth and shape-up of three-dimensional extended arrays of densely packed nanoparticles.

We use evaporation within a microfluidic device to extract the solvent of a (possibly very dilute) dispersion of nanoparticles and concentrate the dispersion until a solid made of densely packed nanoparticles grows and totally invades the microfluidic geometry. The growth process can be rationalized as an interplay between evaporation-induced flow and kinetic and thermodynamic coefficients which are system-dependent; this yields limitations to the growth process illustrated here on two main cases: evaporation- and transport-limited growth. Importantly, we also quantify how colloidal stability may hinder the growth and show that care must be taken as to the composition of the initial dispersion, especially regarding traces of ionic species that can destabilize the suspension upon concentration. We define a stability chart, which, when fulfilled, permits us to grow and shape-up solids, including superlattices and extended and thick arrays of nanoparticles made of unary and binary dispersions, composites, and heterojunctions between distinct types of nanoparticles. In all cases, the geometry of the final solid is imparted by that of the microfluidic device.