Session: Basic Research in Colloids, Surfactants and Interfaces:

For protocols of colloidal particle lithography, a film of monodisperse spheres is used as a surface mask to form a template for nanopatterning. When a drop of an aqueous suspension of monodisperse latex or silicon spheres is dried on a flat substrate, in certain cases, a crystalline arrangement with hexagonal packing is formed spontaneously after water has evaporated. However, at other times a disordered arrangement is generated, which we have found to be directly related to the wettability of the substrate. To evaluate how the surface wettability affects the crystalline assembly of monodisperse particles, we prepared samples with hydrophilic (mica) and hydrophobic (silicon) substrates. In a further step, we exposed silicon substrates to ozone to render the surfaces to be hydrophilic. After drying particle suspensions, tapping-mode AFM was used to investigate the differences for the surface packing arrangements. On freshly cleaved mica, which is hydrophilic, the particles assembled to generate a well-ordered, crystalline arrangement of closely packed spheres with long range order spanning tens of microns. A disordered arrangement of the particles was produced when suspensions were dried on hydrophobic silicon substrates. However, treatment of the silicon pieces with ozone or UV irradiation generated a hydrophilic interface which produced crystalline arrangements of spheres through evaporative lithography. Further investigations are in progress to understand the mechanism of self-ordering, as it relates to differences in the convective flow of water during evaporation. Surface treatments which influence the wettability of substrates can be useful for generating fewer defects for emerging manufacturing processes that are based on evaporative colloidal lithography.

Quynh Do

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