Packing of Bidisperse Nanospheres at a Fluid Interface

Two-dimensional assembly of Brownian bidisperse nanosphere mixtures on a planar liquid surface was investigated by in situ scanning electron microscopy (SEM). The PEGylated silica nanospheres of different number and size ratios adsorbed strongly to an ionic liquid surface, spontaneously forming a mixed monolayer that organized according to the ratios selected. Several structural parameters for the monolayers were calculated from the SEM images, including translational order parameter T*, orientational order parameter <ψ_6,k>, pair correlation function, correlation length, mixed triangular loop fraction, and probability of nearest neighbor number with respect to size for small, large, and all particles. A home-built liquid cell that allows variation of interfacial area caused particle jamming and unjamming. Our results show how the degree of order/disorder in spontaneously assembled bidisperse nanosphere monolayers can be controlled.