Supra-particle formation by evaporation of aerosol droplets containing binary mixtures of colloidal particles: Controlling the final morphology

Hypothesis
Supra-particle formation by evaporation of an aqueous aerosol droplet containing nano-colloidal particles is challenging to investigate but has significant applications. We hypothesise that the Peclet number, Pe, which compares the effectiveness of evaporation-induced advection to that of colloidal diffusion, is critical in determining supra-particle morphology and can be used to predict the dried morphology for droplet containing polydisperse nanoparticles.

Experiments
Sterically-stabilized diblock copolymer nanoparticles were prepared via polymerization-induced self-assembly (PISA). The systematic study was performed for evaporation rates by levitating an aqueous aerosol droplet and collecting dry supra-particles using electrodynamic balance (EDB) and falling droplet column (FDC), respectively for single-size particles and binary mixtures particles. The supra-particle morphology was characterized using scanning electron microscopy (SEM).

Findings
We validate the hypothesis of a higher Pe increases the degree of buckling for both unimodal and bimodal nanoparticle size distributions by employing a higher evaporation rate (K) to increase Pe. However, if Pe is increased by lowering the mean diffusion coefficient (Davg) at a fixed K, the degree of buckling is reduced. For the binary mixture of nanoparticles of differing size, this can be achieved by reducing the concentration of smaller nanoparticles relative to that of larger nanoparticles. Hence consideration of Pe alone is insufficient to reliably predict the final supra-particle morphology.