Nanoparticle Positioning on Liquid and Polymerized Faceted Droplets

Nanoparticle (NP)-decorated emulsion droplets are widely employed in science and technology. Yet, no method for a precise positional control of individual NPs on the droplet's surface has been hitherto demonstrated, thus restricting the nanotechnological application of such systems. Here, we present such a method based on the surface-freezing effect, which forms a crystalline monolayer at the surface of oil-in-water emulsion droplets, and, upon further cooling, renders the liquid droplets faceted. We demonstrate experimentally that by tuning the temperature (T) below the faceting temperature, T_d, the NPs spontaneously self-position at the vertices of the faceted droplets because this reduces the system's elastic energy. The precise vertex positions of the NPs are revealed by scanning electron microscopy of faceted droplets that were solidified by polymerization at T < T_d. The details of the energetics underlying the NPs' self-positioning dynamics are elucidated by Brownian dynamics simulations, and the low limit on the NP sizes exhibiting self-positioning is estimated.