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...
Rough or patterned surfaces infused with a lubricating liquid display many of the same useful properties as conventional gas-cushioned superhydrophobic surfaces. However, liquid-infused surfaces exhibit a new failure mode: the infused liquid film may drain due to an external shear flow, causing the surface to lose its advantageous properties. We examine shear-driven drainage of liquid-infused surfaces with the goal of understanding and thereby mitigating this failure mode. On patterned surfaces exposed to a known shear stress, we find that a finite length of the surface remains wetted indefinitely, despite the fact that no physical barriers prevent drainage. We develop an analytical model to explain our experimental results, and find that the steady-state retention results from the ability of patterned surfaces to wick wetting liquids, and is thus analogous to capillary rise. We establish the geometric surface parameters governing fluid retention and show how these parameters can describe even random substrate patterns.
We review recent progress, based on the approach introduced by McKeon and Sharma [J. Fluid Mech. 658, 336-382 (2010)], in understanding and controlling wall turbulence. The origins of this analysis partly lie in nonlinear robust control theory, but a differentiating feature is the connection with, and prediction of, state-of-the-art understanding of velocity statistics and coherent structures observed in real, high Reynolds number flows. A key component of this line of work is an experimental demonstration of the excitation of velocity response modes predicted by the theory using non-ideal, but practical, actuation at the wall. Limitations of the approach and promising directions for future development are outlined. C 2013 American Institute of Physics. [http://dx
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