Droplet motion on contrasting striated surfaces

Abstract

Liquid droplets move readily under the influence of surface tension gradients on their substrates. Substrates decorated with parallel microgrooves, or striations, presenting the advantage of homogeneous chemical properties yet varying the topological characteristics on either side of a straight-line boundary are considered in this study. The basic type of geometry consists of hydrophobic micro-striations/rails perpendicular to the boundary, with the systematic variation of the width to spacing ratio, thus changing the solid-liquid contact fraction and inducing a well-defined wettability contrast across the boundary. Droplets in the Cassie-Baxter state, straddling the boundary, move along the wettability contrast in order to reduce the overall surface free energy. Results show the importance of average solid fraction and contrasting fraction in a wide range for given geometries across the boundary on droplet motion. A unified criterion for contrasting striated surfaces, which describes the displacement and the velocity of the droplets, is suggested, providing guidelines for droplet manipulation on microstriated/railed surfaces.

Droplet motion and oscillation on contrasting micro-striated surfaces

“…An oscillation with a larger amplitude or an elongated shape leads to a bigger deviation between the theoretical and experimental results, mainly in the values of the amplitude, whereas the oscillation of the period are rather well captured. In our earlier work (Zhao et al 2020), the effect of the amplitude on the horizontal migration/displacement by releasing the droplet from certain heights was found to increase with increasing the releasing height, i.e. Weber number, within a certain range.…”

“…Compared with the experimental results for maximum displacement, the best fits for the coefficients, B * and C (in (4.18), (4.19) and (4.26)), are proposed as B * ≈ 18.07 N m −2 • r φ/γφ, C ≈ 0.13. In our previous work (Zhao et al 2020), the average velocity of the droplet, v, is a function of φ/φ,v ≈ γ (cos θ Y + 1) φ/(2ξφ), where ξ is a friction coefficient. Thus, here, B ∝v.…”

“…solid-liquid, γ SL , liquid-vapour, γ , and solid-vapour, γ SV , are paramount. One of the common methodologies exploited for droplet manipulation is to create a single sharp contrast (He & Lee 2003;Kita et al 2018;Zhao et al 2020) or a continuous/stepwise gradient (Shastry, Case & Böhringer 2005; Yang et al 2006;Reyssat et al 2009;Launay et al 2020) on the solid by adjusting the surface roughness on an intrinsically hydrophobic background, either imposed by the material itself or upon a subsequent coating. In the last century, the influence of roughness on static wetting has been investigated and has been shown to obey the theories of Wenzel (1936) or Cassie & Baxter (1944) depending on the intrinsic wettability, the structure of the solid surface and the nature of the liquid.…”

“…2018; Zhao et al. 2020) or a continuous/stepwise gradient (Shastry, Case & Böhringer 2005, 2006; Yang et al. 2006; Reyssat et al.…”

“…More recently, Zhao et al. (2020) proposed a unified criterion where it is described that the droplet motion, as displacement, and average velocity are solely functions of the structural parameters of the micro-striated surfaces, . However, on a superhydrophobic, micro-structured surface, droplets typically do not experience steady motion, especially in the initial stages when they first contact or spread on the surface (Bartolo, Josserand & Bonn 2005; Yang et al.…”