Wetting Transition at a Threshold Surfactant Concentration of Evaporating Sessile Droplets on a Patterned Surface

Abstract

Wetting transitions induced by varying the components in a solution of a drying droplet can lead to its evolving shape on a textured surface. It can provide new insights on liquid pattern control through manipulating droplet solutions. We show the pronounced transitions of wetting for surfactant solution droplets drying on a micropyramid-patterned surface. At low initial surfactant concentrations, the droplet maintains an octagonal shape until the end of drying. At intermediate initial surfactant concentrations, the early octagon spreads to a square, which later evolves to a stretched rectangle. At high initial surfactant concentrations, the droplet mainly exhibits the "octagon-to-square" transition, and the square shape is maintained until the end. The octagon-to-square transition occurs at similar temporal volumeaveraged surfactant concentrations for the various initial surfactant concentrations. It results from the dependence of the surface energy change of spread over the micropyramid structure on the temporal volume-averaged surfactant concentration. At high initial surfactant concentrations, the accumulation of the surfactant near the contact line driven by outward flows could raise the local viscosity and enhance the pinning effect, leading to the great suppression of the "square-to-rectangle" transition.

Evaporation‐driven octagonal particle deposition on the patterned substrate

“…The droplet volume change is computed by referring to the calculation method discussed in Zhong et al 40 The formula is obtained by integrating an infinitesimal angle over the octagonal wetting area (see Figure 2a) and expressed as, $$$V={\int }_{0}^{\frac{{\rm{\pi }}}{8}}{\rm{d}}V=\frac{1}{4}hr\tan \theta +\frac{1}{12}{h}^{3}\theta ,$$$ where $$r$$ is the base radius, $$h$$ equates to the height of the droplet, and $$\theta $$ represents the angle of each segment such as the angle at one volume, $$V$$, of 16 sections. In the computation method shown in Equation (), each of 16 sections of the droplet volume is calculated by the maximum inscribe and minimum circumscribe radius to obtain the boundary that covered the actual sessile droplet volume.…”

“…Courbin et al 38 investigated the effect of a droplet contact angle on the patterned substrates and formed different wetting patterns such as octagon, square, and hexagon using various liquids (ethanol, isopropanol, hexane, and silicone oil) on the substrates patterned with the different types of micropillars. The wetting geometry transitions from octagon to the square were observed in the evaporating droplets of potassium chloride aqueous solutions, 39 water-surfactant solutions, 40,41 and water-ethanol solutions 42 on the polymethyl methacrylate (PMMA) micropyramid surfaces. The previous studies of the sessile droplets on the structured substrates mainly focused on controlling the noncircular wetting geometries by adjusting the liquid surface tension or micropillar design.…”

Geometrical Deposits on Microstructured Surfaces

“…Interestingly, compared with the study of ethanol droplets done by Feng et al, both KCl and ethanol produced the same effect on the shape of droplets, despite the different effect that these solutions have on surface tension. The study of droplet profile as a function of surfactant concentration on textured surfaces showed different wetting transitions of evaporating droplets, corresponding to the initial surfactant concentration [31,32] . The transition of droplet profile from octagon to square, and from square to rectangle, was observed during the drying process.…”

“…The transition of droplet profile from octagon to square, and from square to rectangle, was observed during the drying process. Additionally, the ability to maintain a uniform droplet profile throughout the whole evaporation process was achieved for octagonal droplets [31,32] . The findings unanimously demonstrate that surface chemistry and topography, as well as the intrinsic contact angle of the fluid, govern the wetting dynamics and droplet shape.…”

Effect of Particle Concentration on Surfactant-Induced Alteration of the Contact Line Deposition in Evaporating Sessile Droplets

“…By using FSO surfactant and PAM polymer, Zhu et al 32 were able to significantly enhance the gelation behavior of the droplet and modulate the morphology in inkjet printing. Zhong et al 33 recently investigated the effect of different surfactant concentrations on the wetting transition of an evaporative sessile droplet and could achieve exiting shapes of deposition. Kim et al, 34 by using surfactants in bisolvent gelatin drops, could induce strong Marangoni eddies, which in coexistence with a stagnation zone near the contact line changed the volcano-like depositions to dome-like depositions.…”