DC field coupled evaporation of a sessile gold nanofluid droplet.

The coffee stain formed when a sessile nanofluid colloidal droplet dries on a substrate displays distinct nanoparticle aggregation regimes. We employ scanning electron microscopy to study the coffee stain morphologies when DC electric fields are applied to drying aqueous suspension droplets of CTAB capped gold nanorods (Au-NRs) on a hydrophilic substrate. We observe a typical coffee ring edge with several Au-NR domains due to outward capillary flow both in the absence and presence of the electric field. The Au-NRs at the coffee ring edge assemble in a smectic-like phase with homogeneous alignment in a zero DC field. Despite the presence of strong evaporation-induced flows, application of a DC electric field perpendicular to the substrate results in homeotropic alignment of the Au-NRs at the coffee ring edge. Clusters of Au-NRs with short-range order form at the inner coffee ring edge which we attribute to Marangoni eddies. Moving towards the centre of the coffee stain, we note a depletion region lacking particles, followed by non-uniform deposition of Au-NRs. Au-NR arrays are also found to deposit outside the coffee ring, presumably due to depinning of the evaporating droplet during the initial stages of droplet drying. In contrast to the outer coffee ring edge, we note no change in Au-NR orientation in other regions of the stain due to the extremely low particle concentrations. We believe that our results are applicable to assemblies of a variety of surfactant capped metal nanorods.