Scaling Laws in Directional Spreading of Droplets on Wettability-Confined Diverging Tracks

Abstract

Spontaneous pumpless transport of droplets on wettability-confined tracks is important for various applications, such as rapid transport and mixing of fluid droplets, enhanced dropwise condensation, biomedical devices, and so forth. Recent studies have shown that on an open surface, a superhydrophilic track of diverging width, laid on a superhydrophobic background, facilitates the transport of water from the narrower end to the wider end at unprecedented rates (up to 40 cm/s) without external actuation. The spreading behavior on such surfaces, however, has only been characterized for water. Keeping in mind that such designs play a key role for a diverse range of applications, such as handling organic liquids and in point-of-care devices, the importance of characterizing the spreading behavior of viscous liquids on such surfaces cannot be overemphasized. In the present work, the spreading behavior on the aforementioned wettability-patterned diverging tracks was observed for fluids of different viscosities. Two dimensionless variables were identified, and a comprehensive relationship was obtained. Three distinct temporal regimes of droplet spreading were established: I), a Washburn-type slow spreading, II) a much faster Laplace pressure-driven spreading, and III), a sluggish density-augmented Tanner-type film spreading. The results offer design guidance for tracks that can pumplessly manage fluids of various viscosities and surface tensions.

Orthogonal liquid-jet impingement on wettability-patterned impermeable substrates

“…A facile and scalable approach to produce superhydrophobic and superhydrophilic spatial domains on square aluminum samples of dimensions 100 mm × 100 mm (mirror-finish 6061 aluminum sheets, 2 mm thick, McMaster-Carr) was employed; the method (Fig. 1) is the one employed by Koukoravas et al [61] and Sen et al [62]. The metal plate samples were initially treated with a 5 M HCl solution (ACS reagent 37%, Sigma-Aldrich) for 4 min (Fig.…”

All-graphene-based open fluidics for pumpless, small-scale fluid transport via laser-controlled wettability patterning

“…location, t is time, γ and μ denote the fluid surface tension and viscosity, respectively. 37,60 It was found that the spreading front of the water progresses along the track as the square root of time,…”

“…This agrees with a previous report of pumpless water spreading on hydrophilic tracks. 60 A fitting with D values of 5.5 and 7.3 for 2 mm and 1.5 mm initial channel width; respectively, are plotted in dashed lines (Figure 4b). The fitting of 1.5-mm and 2-mm channel width (Figure 4b) displayed good agreement with the experimental results while fitting with results garnered from a 1-mm channel did not ( Figure S1).…”

High-efficiency bubble transportation in an aqueous environment on a serial wedge-shaped wettability pattern

“…Since the moving path of the laser can be designed and controlled by computer, the serial-wedge-shaped wettability pattern with adjustable size composed of the exterior surrounding superhydrophilic region with water contact angle of 0° and inside interior superhydrophobic region with water contact angle of 159.0±2.3° was obtained. The laser etching processes were inspired by Hwang et al 34 and Megaridis et al 35,36…”