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

Abstract

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 non-uniform 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. PACS numbers:An evaporating liquid drop, either single or multicomponent, containing solutes or particulates leaves a deposit whose form is determined by various parameters, for instance internal flow fields [1][2][3], liquid compositions [4][5][6][7][8][9][10], and interactions between suspended particles and a solid substrate [11][12][13][14], which are crucial for coating processes. In particular, control of the deposit uniformity and thickness can be important in surface patterning [15][16][17], ink-jet [4,18,19] and 3D printing technologies [20]. These processes are complex because of physicochemical dynamics that arise from Marangoni effects [2, 5-10, 12, 21, 22] and particle deposition mechanisms [11,12,14,23]. In fact, although a binary mixture is used quite often to achieve uniform particle deposition from droplets smaller than 100 µm [4,18,19], to our best knowledge such coatings have not been achieved for larger droplets. Furthermore, while the wetting and dewetting behaviors of binary mixture drops have been investigated [24,25], the relation between the deposition pattern and the evaporatively driven flow field in a binary mixture droplet is incomplete (Table S1, Supporting Information (SI)) [26].In this Letter, to achieve a uniform coating, we identify key characteristics of a multicomponent solution, which consists of a binary mixture, surface-active surfactant, and surface-adsorbed polymer. We were motivated to pursue the ideas here from examining a whisky droplet after drying on an ordinary glass where it creates a relatively uniform particle deposit (see Fig. 1), which is in contrast to the well-known 'coffee-ring stain' [1]. Based on our understanding of the drying and coating mechanisms of binary liquid droplets, whisky droplets, and more complex solution droplets, we design a model liquid that yields nearly uniform deposits by taking the approach that whisky is an ethanol-water mixture containing diverse dissolved molecules, which contribute to the * Electronic address: hastone@princeton.edu complexity of the system, the flows, and the final particle deposits.We begin with a few remarks about whisky, since it serves as a model complex mixture, where nearly uniform particle deposits are observed after drying. Whisky is an alcoholic liquid (ethanol:water, 35:65 % b...

Dynamics and universal scaling law in geometrically-controlled sessile drop evaporation

“…In the final stage, the outward flow was identical to that found in pure water drops. Qualitatively similar continuous mixing by distinct Marangoni flows has been recently observed in evaporating drops of whiskey15. It should be noted that the evaporation of the binary-mixture drops features a solutal-thermocapillary flow, that is, σ = σ 0 − γ T ( T − T 0 )− γ c ( c − c 0 ) where γ T =−∂ σ /∂ T , γ c =−∂ σ /∂ c , and c here is the ethanol concentration in the liquid; normally, .…”

“…This is in an effort to ameliorate the complexity of the evaporation process related to a delicate interaction between the bulk flow45678, evaporation kinetics91011, thermocapillary stability1213 and binary-mixture dynamics141516. The flow is complicated further by the presence of moving contact lines as well as regions of high curvature, both of which influence the evaporation.…”

Droplet Drying Patterns on Solid Substrates: From Hydrophilic to Superhydrophobic Contact to Levitating Drops

“…On the other hand, in some cases, the whole solid is deposited near the center of the drop [13,23,109]. This is more prominent when the angle of contact is large and is named the Marangoni effect [54,[110][111][112]. It arises due to a gradient in surface tension.…”

Altering the coffee-ring effect by adding a surfactant-like viscous polymer solution

“…Recently, Kim et al . 18 reported that multiple sequential Marangoni flows and particle-surface interactions are key parameters to achieve uniform particle coatings in a droplet and suggested a small concentration of surfactant and surface-adsorbed polymer via physisorption for a uniform deposit in a binary mixture.…”