10294 | Soft Matter, 2021, 17, 10294–10300 This journal is © The Royal Society of Chemistry 2021 Cite this: Soft Matter, 2021, 17, 10294 DC field coupled evaporation of a sessile gold nanofluid droplet† A. W. Zaibudeen and Ranjini Bandyopadhyay * 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. The tunability of nanomaterial properties, by tailoring the sizes and shapes of the constituent nanoparticles or even by applying external forces, has tremendous technological implications. 1–3 The alignment of anisotropic molecules in response to electric fields has practical applications in smartphones and displays. Controlling the assembly and orientation of anisotropic nano- particles such as nanorods (NRs) is highly desirable as the shape anisotropy leads to exotic optical, magnetic, and elec- trical properties, with the resultant superstructures being employed in biomedical applications, 4 sensors, 5–7 detection of food contamination, 8 etc. The use of an electric field to induce directed assembly of NRs has emerged as a promising approach for the manipulation of nanoparticles that are poten- tially applicable in technologies involving the tailoring of interactions between colloidal particles. Distinct hybrid assem- blies of polymer-coated Au-NRs were reported by coupling an electric field and a 2D confined channel. 9 The evaporation of sessile droplets containing solute particles leaves behind coffee rings/stains 10,11 that depend on various parameters such as particle concentration, size and morphology, substrate wett- ability, and physio-chemical conditions. By coupling the eva- poration of a colloidal dispersion droplet with an electric field, the assembly of solute particles can be controlled by tuning the relative contributions of the interfacial energies and the exter- nal field. 12,13 Ordered structures composed of gold particles can be pro- duced using various strategies. 14 The present study reports the coupling of evaporation induced self-assembly (EISA) of a sessile gold nanofluid droplet (containing CTAB capped Au- NRs) on a hydrophilic silicon substrate to DC electric fields. The contributions of the evaporation induced and the electric field induced forces in the evaporating Au-NR dispersion droplet are studied under applied electric fields of different magnitudes. The evaporation of the sessile gold colloidal dis- persion droplet results in coffee rings with distinct regions. 15 The assembly pattern of Au-NRs is different in each region of the coffee ring. In the absence of an applied electric field, homogeneous 2D ordered structures of Au-NRs are noted at the coffee ring edge. We show here, for the first time to the best of our knowledge, that despite the presence of very strong eva- poration induced forces, the applied electric force dominates and flips the Au-NRs at the coffee ring edge perpendicular to the substrate in the direction of the applied electric field. However, the orientation of the Au-NRs in regions other than the coffee ring edge is found to be homogeneous both in the absence and presence of applied electric fields. A well-established seed-mediated protocol is followed to synthe- size a fairly monodisperse CTAB capped Au-NR dispersion, 16 as Soft Condensed Matter Group, Raman Research Institute, Bangalore – 560 080, India. E-mail: ranjini@rri.res.in † Electronic supplementary information (ESI) available. See DOI: 10.1039/d1sm00820j Received 2nd June 2021, Accepted 7th November 2021 DOI: 10.1039/d1sm00820j rsc.li/soft-matter-journal Soft Matter COMMUNICATION Published on 08 November 2021. Downloaded by RAMAN RESEARCH INSTITUTE on 11/25/2021 3:32:30 AM. View Article Online View Journal | View Issue