Line Tension and Drop Size Dependence of Contact Angle at the Nanoscale

Waldemar Klauser,Fabian T Von Kleist-Retzow,Sergej Fatikow

doi:10.3390/nano12030369

Waldemar Klauser, Fabian T Von Kleist-Retzow + Show 1 more

Open Access

https://doi.org/10.3390/nano12030369

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Journal: Nanomaterials	Publication Date: Jan 24, 2022
Citations: 5	License type: CC BY 4.0

Affiliation: Carl von Ossietzky University of Oldenburg

Abstract

Despite considerable research efforts, the influence of contact line tension during wetting at the nanoscale and its experimental determination remain challenging tasks. So far, molecular dynamics simulations and atomic force microscope measurements have contributed to the understanding of these phenomena. However, a direct measurement of the size dependence of the contact angle and the magnitude of the apparent line tension has not been realized so far. Here, we show that the contact angle is indeed dependent on the drop size for small drop diameters and determine the magnitude of the apparent line tension via liquid-metal based measurements of advancing and receding contact angle inside a scanning electron microscope. For this purpose, a robotic setup inside an electron microscope chamber and oxide-free Galinstan droplets—produced via an electromigration-based and focused ion beam irradiation-assisted process—are employed. Using the first-order correction of Young’s equation, we find an apparent line tension value of 4.02 × 10−7 J/m for Galinstan© on stainless steel.

Highlights

Understanding wetting phenomena at the micro- and nanoscale is vital for a broad range of applications, including digital microfluidics [1,2], additive manufacturing [3], coating technologies [4], and tribological systems [5]
In contrast to the macroscale, where the liquid–solid contact is governed by the three surface tensions of the solid, the liquid, and the gas phases, at the nanoscale one has to take into account the line tension, i.e., the force of the tensile or compressive strength of the one-dimensional three-phase contact line [6]
In analogy to surface tension, which seeks to reduce the area of the interfaces, the line tension seeks to reduce the free energy of the solid–liquid–gas system by adjusting the length of the contact line of the liquid drop

Summary

Introduction

Understanding wetting phenomena at the micro- and nanoscale is vital for a broad range of applications, including digital microfluidics [1,2], additive manufacturing [3], coating technologies [4], and tribological systems [5]. In this work, we are able to investigate the dependence of the contact angle on drop size via direct measurement of the advancing and receding contact angle of liquid metal droplets with different diameters inside an SEM. From those measurements, we determine the magnitude of the apparent line tension using Equation (1). Our results show that the contact angle is (a) size dependent once the contact area reaches the nanoscale, and (b) the magnitude of the line tension of Galinstan is in the range of 10−7 J/m

Materials and Methods

Cosine

Discussion