Controlled Uniform Coating from the Interplay of Marangoni Flows and Surface-Adsorbed Macromolecules

Surface coatings and patterning technologies are essential for various physicochemical applications. In this Letter, we describe key parameters to achieve uniform particle coatings from binary solutions. First, multiple sequential Marangoni flows, set by solute and surfactant simultaneously, prevent nonuniform particle distributions and continuously mix suspended materials during droplet evaporation. Second, we show the importance of particle-surface interactions that can be established by surface-adsorbed macromolecules. To achieve a uniform deposit in a binary mixture, a small concentration of surfactant and surface-adsorbed polymer (0.05 wt% each) is sufficient, which offers a new physicochemical avenue for control of coatings.

Figure 1

Left: A dried mark of a whisky droplet (Macallan, UK) on a normal glass. The image is obtained using an orange color flashlight. Right: A dried deposit pattern of a Glenlivet whisky (UK) with fluorescent polystyrene particles.

Figure 2

(a) Sketch of a liquid drop on a solid substrate. (b) Flow fields (vectors) and wall-normal vorticity $\omega$ fields (color contours) of a Glenlivet whisky. The flow field was measured near the substrate. The total drying time was about 470 s. Below each flow field plot a schematic of the side view of the evaporating droplet is provided. The red arrows represent the flow pattern. There are different flow regimes, multiple vortices ($I$), two circulatory flows ($I_1$ and $I_2$), and radial outward flow ($II$). At the stage $II$, from the outward radial flow, we estimate that the ethanol is almost evaporated and there is no significant surfactant effect along the droplet interface.

Figure 3

(a) Flow fields (vectors) and wall-normal vorticity $\omega$ fields (color contours) of an ethanol-water ($35:65$ wt%) mixture with 0.05 wt% SDS. Below each flow field plot a schematic of the side view of the evaporating droplet is provided. The red arrows represent the flow pattern. The total drying time was about 400 s. At the stage $II$, from the outward radial flow, we estimate that the ethanol is almost evaporated and there is no significant surfactant effect along the droplet interface. Schematic of (b) solutal and surfactant-driven Marangoni effects and (c) the surfactant-driven Marangoni effect and the evaporatively driven flow effect along the drop interface. The gray, dark blue, and light purple arrows indicate the surfactant, solutal, and evaporative flux effects, respectively. (d) The final deposition pattern of the binary mixture drop with SDS on the cover glass where the particle concentration is $8\times {10}^{-4}\mathrm{vol}\%$.

Figure 4

A diagram of effects of surfactant driven Marangoni flows and surface-adsorbed materials in the binary mixture on the final deposit. The concentration of ethanol 35% in DI water. PEO ($4\times {10}^6\mathrm{Da}$) and SDS concentration are 0.05 wt%, respectively.

Figure 5

(a) Comparison of the final deposition patterns on top of the cover glass (VWR, USA): (left) whisky (McCllenland, UK), (center) water, and (right) a model liquid [ethanol:water $(35\%:65\%)+\mathrm{SDS}(0.05\%)+\mathrm{PEO}(0.05\%)$ by weight] containing $1\mu \mathrm{m}$ polystyrene particles ($5\times {10}^{-3}\mathrm{vol}\%$). $2R$ is the diameter of the final area. (b) Deposit profiles along $r$ are plotted for each of the images in (a). The intensity profile $\mathfrak{T}(r)$ is normalized with the maximum of $\mathfrak{T}(r)$ where $\mathfrak{T}(r)=(1/2\pi ){\int }_0^{2\pi }i(r,\theta )d\theta$ and $i(r,\theta )$ is the local light intensity.