The influence of nanoparticle shape on the drying of colloidal suspensions

Patterns formed by 1 ppb dried laponite particles on glass at the drop edge. [Display omitted] ► Our work has demonstrated that the shape of the nanoparticle is important in the final structure of the dry deposit of nanoparticles. ► For spherical nanoparticles, the deposit structure is found to change steadily with concentration. ► For non-spherical, disk-like nanoparticles the deposit structure builds up steadily until a critical particle concentration is reached, above which a smoother deposit is formed. Dried deposits of spherical Ludox silica and disk-like laponite clay nanoparticles have been examined by dark-field optical microscopy and atomic force microscopy (AFM) to investigate the effects of nanoparticle shape on the deposit structure. Dark-field optical images indicated that a higher concentration of Ludox nanoparticles was required, compared to laponite, for an optically visible deposit to be formed. Compared with the relatively simple ring-like features observed at the edges of Ludox deposits, the laponite deposits were more complex, with dendritic features appearing below 10 ppm that disappeared at higher laponite concentrations. AFM images revealed that whilst the Ludox rim deposit structure gradually increased in height and width with increasing nanoparticle concentration, the laponite rim deposits increased steadily in height up to 1 ppm, above which the rim height suddenly decreased and the deposit structure became smoother. The widths of the rim deposits were observed to increase in a similar manner for both nanoparticle types. Nanoparticle shape is suggested as the main reason for differences in the structural features of the rim for each nanoparticle type. The disk-like laponite forms tall thin rim profiles at low concentrations, before creating a more uniform rim profile at higher concentrations. We suggest that a critical laponite rim height is reached before partial collapse of the nanoparticle stack at the rim takes place as the particle concentration is further increased. This produces much thinner and smoother films of laponite at high particle concentrations than is found for similar concentrations of Ludox. Our work suggests that both the shape and the concentration of the nanoparticles themselves are crucial in determining the structure of the final dried nanoparticle deposit.