Explaining Evaporation-Triggered Wetting Transition Using Local Force Balance Model and Contact Line-Fraction

Rama Kishore Annavarapu,Sanha Kim,Minghui Wang,A John Hart,Hossein Sojoudi

doi:10.1038/s41598-018-37093-6

Rama Kishore Annavarapu, Sanha Kim + Show 3 more

Open Access

https://doi.org/10.1038/s41598-018-37093-6

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Abstract

Understanding wettability and mechanisms of wetting transition are important for design and engineering of superhydrophobic surfaces. There have been numerous studies on the design and fabrication of superhydrophobic and omniphobic surfaces and on the wetting transition mechanisms triggered by liquid evaporation. However, there is a lack of a universal method to examine wetting transition on rough surfaces. Here, we introduce force zones across the droplet base and use a local force balance model to explain wetting transition on engineered nanoporous microstructures, utilizing a critical force per unit length (FPL) value. For the first time, we provide a universal scale using the concept of the critical FPL value which enables comparison of various superhydrophobic surfaces in terms of preventing wetting transition during liquid evaporation. In addition, we establish the concept of contact line-fraction theoretically and experimentally by relating it to area-fraction, which clarifies various arguments about the validity of the Cassie-Baxter equation. We use the contact line-fraction model to explain the droplet contact angles, liquid evaporation modes, and depinning mechanism during liquid evaporation. Finally, we develop a model relating a droplet curvature to conventional beam deflection, providing a framework for engineering pressure stable superhydrophobic surfaces.

Highlights

In adhesion is due to an increase in the contact line-fraction along the three phase contact line (TPCL), whereas for the latter the higher adhesion is due to the tip geometry[36,37] and nanopores[38] on the microstructure
The proposed model is verified by performing droplet evaporation studies on patterned cylindrical and line-shaped microstructures made of poly-perfluorodecylacrylate coated vertically aligned carbon nanotubes (VA-CNTs) via initiated-chemical vapor deposition[47,48,49,50,51,52,53,54]
We focused on the micropillar itself, determining its critical force per unit length (FPL) value

Summary

Introduction

In adhesion is due to an increase in the contact line-fraction along the three phase contact line (TPCL), whereas for the latter the higher adhesion is due to the tip geometry[36,37] and nanopores[38] on the microstructure. We propose a new model for explaining the evaporation-triggered wetting transition using a critical force per unit length (FPL) value. This model allows the educated selection and design of the superhydrophobic surfaces. We avoid the capillary-bridging effect and discuss the lone impact of contact line-fraction on receding contact angles and droplet evaporation modes. This understanding leads to an efficient and experiment-free way of designing and fabricating pressure-stable superhydrophobic surfaces. The spreading of ink on the substrate and its detachment from the stamp can be engineered to increase the speed and the resolution of the flexography printing[47,51,55]

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