From Sticky to Slippery Droplets: Dynamics of Contact Line Depinning on Superhydrophobic Surfaces

Wei Xu and Chang-Hwan Choi

Phys. Rev. Lett. 109, 024504 – Published 13 July 2012

Abstract

This study explores how surface morphology affects the dynamics of contact line depinning of an evaporating sessile droplet on micropillared superhydrophobic surfaces. The result shows that neither a liquid-solid contact area nor an apparent contact line is a critical physical parameter to determine the depinning force. The configuration of a contact line on a superhydrophobic surface is multimodal, composed of both two phases (liquid and air) and three phases (liquid, solid, and air). The multimodal state is dynamically altered when a droplet recedes. The maximal three-phase contact line attainable along the actual droplet boundary is found to be a direct and linear parameter that decides the depinning force on the superhydrophobic surface.

Received 1 April 2012

DOI:https://doi.org/10.1103/PhysRevLett.109.024504

Authors & Affiliations

Wei Xu and Chang-Hwan Choi^*

Department of Mechanical Engineering, Stevens Institute of Technology, Hoboken, New Jersey 07030, USA

^*Corresponding author. Chang-Hwan.Choi@stevens.edu

Article Text (Subscription Required)

Click to Expand

Supplemental Material (Subscription Required)

Click to Expand

References (Subscription Required)

Click to Expand

Issue

Vol. 109, Iss. 2 — 13 July 2012

Reuse & Permissions

Access Options

Article Available via CHORUS

Download Accepted Manuscript

Author publication services for translation and copyediting assistance advertisement

Images

Figure 1
The pinning and depinning phenomena of an evaporating droplet on a patterned superhydrophobic surface in a partial wetting (Cassie) state [28]. A pinning mode transits to a depinning mode during the evaporation process under the effect of a depinning force ( $F_{d}$ ). $θ_{e}$ is an initial quasiequilibrium contact angle, $θ_{r}$ is a receding contact angle, and $R$ is the contact radius of the droplet.Reuse & Permissions
Figure 2
Scanning electron microscopy images (45° tilted view) of the micropillared superhydrophobic surfaces. The liquid-solid contact area fraction in a partial wetting (Cassie) state varies from (a) 0.20, (b) 0.09, (c) 0.03, to (d) 0.01. Scale bars are $30 μ m$ .Reuse & Permissions
Figure 3
The initial quasiequilibrium contact angles ( $θ_{e}$ ), the receding contact angles ( $θ_{r}$ ), and the depinning forces ( $F_{d}$ ) on the patterned superhydrophobic surfaces ( $Φ 0.20$ , $Φ 0.09$ , $Φ 0.03$ , and $Φ 0.01$ ) and a planar hydrophobic surface ( $Φ 1.00$ ).Reuse & Permissions
Figure 4
The multimodal droplet boundary on a pillar-patterned superhydrophobic surface. The droplet boundary is observed from the backside of a transparent superhydrophobic substrate by using reflection interference contrast microscopy (RICM). The light reflected from the liquid-air interface and the substrate forms the interference fringe, which indicates the profile of liquid-air interface and locates the actual droplet boundary. This actual droplet boundary is different from the apparent boundary (drawn with the alternated long and short dash line), and is constituted by the three-phase (liquid-solid-air) contact line (shown by the solid line) and the two-phase (liquid-air) contact line (shown by the dash line). Scale bar is $20 μ m$ .Reuse & Permissions
Figure 5
The evolution of the actual boundary of an evaporating droplet on a micropillared superhydrophobic surface. The RICM images (a)–(j) show the sequential depinning progress from pillars $A$ (a)–(d), $B$ (e)–(g), and $C$ (h)–(j), indicated by the arrows. The three-phase (liquid-solid-air) and the two-phase (liquid-air) contact lines on the droplet boundary are shown by the solid and dashed lines, respectively. (k) shows time history analysis of the normalized length ratio of the three-phase contact line to the circular pillar perimeter. It indicates that the depinning from each pillar initiates when the three-phase contact line reaches the maximum.Reuse & Permissions
Figure 6
The linear correlation between the depinning force and the normalized maximal three-phase contact line ( $δ$ ) at the droplet boundary. Linearly depending on the surface parameter $δ$ , the superhydrophobic surfaces behave as sticky, slippery, or superslippery surfaces, compared to a planar hydrophobic surface. The result from Öner et al. [23] has been analyzed [33] with the new parameter $δ$ for comparison.Reuse & Permissions

Physical Review Letters