Size-discriminative self-assembly of nanospheres in evaporating drops

Evaporation of liquid drops containing nanospheres resulted in circular deposition patterns. The circularity of the patterns depended on the uniformity of the surface tension on the substrate. By employing binary suspensions, containing two differently sized nanospheres, it was possible to modulate the fine structure of such rings. Slow evaporation on mirror-polished substrates resulted in well-ordered distributions, where larger particles self-assembled in dense hexagonal packages, forming apparently an external ring, deposited around the massive inner ring. Deposition started at the air/liquid/solid-contact line. Results could inspire principles for the fabrication of optical devices and may be fruitfully used to design biomaterials with cell-selective properties. A simple model is employed to predict the radial arrangement of nanospheres in rings. Deviations from a standard order (predicted by the model) may be useful to detect biologically active nanoparticles.