  Dewetting of ultrathin (< 100 nm) polymer films, by heating above the glass
transition, produces droplets of sizes of the order of microns and mean
separations between droplets of the order of tens of microns. These relatively
large length scales are because of the weak destabilizing van der Waals forces
and the high surface energy penalty required for deformations on small scales.
We show a simple, one-step versatile method to fabricate sub-micron (>~100 nm)
droplets and their ordered arrays by room temperature dewetting of ultrathin
polystyrene (PS) films by minimizing these limitations. This is achieved by
controlled room temperature dewetting under an optimal mixture of water,
acetone and methyl-ethyl ketone (MEK). Diffusion of organic solvents in the
film greatly reduces its glass transition temperature and the interfacial
tension, but enhances the destabilizing field by introduction of electrostatic
force. The latter is reflected in a change in the exponent, n of the
instability length scale, {\lambda} ~h^n, where h is the film thickness and n =
1.51 \pm 0.06 in the case of water-solvent mix, as opposed to its value of 2.19
\pm 0.07 for dewetting in air. The net outcome is more than one order of
magnitude reduction in the droplet size as well as their mean separation and
also a much faster dynamics of dewetting. We also demonstrate the use of this
technique for controlled dewetting on topographically patterned substrates with
submicrometer features where dewetting in air is either arrested, incomplete or
unable to produce ordered patterns.
