Surfactants in Tribology, Volume 6

Girma Biresaw, K.L. Mittal

• Publisher: CRC Press
• Year: 2019
• Print ISBN: 9781138584570
• eBook ISBN: 9781000022445
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Chemistry
• Index: Chemistry

Surfactants play a critical role in Tribology controlling friction, wear, and lubricant properties such as emulsification, demulsification, bioresistance, oxidation resistance, rust prevention and corrosion resistance. This is a critical topic for new materials and devices particularly those built at the nanoscale. This newest volume will address important advances, methods, and the use of novel materials to reduce friction and wear. Scientists from industrial research and development (R&D) organizations and academic research teams in Asia, Europe, the Middle East and North America will participate in the work.

Section II

Adhesion, Wetting, and Their Relevance to Tribology

5	Moving Contact Line of Droplets on Structured Surfaces Some Problems Relevant to Tribology Ya-Pu Zhao and Zhanlong Wang

CONTENTS

5.1 Introduction

5.2 Theoretical Background

5.2.1 Bridging Length and Time Scales

5.2.2 Wetting on Rough Surfaces

5.2.3 Rigid Wetting and Elastowetting

5.2.4 Wetting on Anisotropic Surfaces

5.3 Experiments and Molecular Dynamics Simulations

5.3.1 Wetting Dynamics on Grooved Surfaces

5.3.1.1 Anisotropic Factor

5.3.1.2 Static Contact Angle

5.3.1.3 Scaling Law of Droplet Spreading on Corrugated Surfaces

5.3.2 Electrowetting (EW) on Structured Surfaces

5.3.2.1 Extended EW Equations

5.3.2.2 EW on a Pillar-Arrayed Surface

5.3.2.3 EW on a Corrugated Substrate

5.3.2.4 Extended EW Equation on Corrugated Substrate

5.3.3 Droplet Wetting on Pillar-Arrayed Surfaces

5.3.3.1 The Wetting Shapes of Droplets

5.3.3.2 Details of Droplet Wetting on Pillar-Arrayed Surfaces

5.3.3.3 Scaling Law of Droplet Wetting on Pillar-Arrayed Surfaces

5.4 Summary and Conclusions

Acknowledgments

References

Symbols

5.1 INTRODUCTION

In recent years, droplet wetting and spreading have received significant interest from the scientific community due to their broad applications in, for example, self-assembly [1,2], droplet movement controlling [3,4], water directional transport [5,6], water harvesting [7,8], self-clean surfaces [9,10], printing, and anti-icing [11,12]. For these droplets, the roughness of a surface can significantly alter the behavior of fluids moving over that surface [13], for example, the leaves of plants where micrometer-scale bumps on the leaves lead to super-hydrophobic behavior [14], desert beetles who use hydrophilic patches on their back to collect dew [8], and butterfly wings that are patterned anisotropically to promote directional runoff [15]. On some special surfaces like nepenthes, droplets are easily moved and gnats are prone to slip off. These specially structured surfaces provide effective methods to control the droplet movement and help to fabricate ultrasmooth surfaces [6]. Based on these phenomena, surfaces with regular arrays of chemical patches [16,17,18] and posts [19,20,21,22] are fabricated to provide special functions. Previous works have shown the possibilities of manufacturing multiscale surface patterns [23–25].

On patterned surfaces, the motion of drops is generally complex due to the contact line pinning and contact angle hysteresis. Surfaces with pillars often generate larger contact angles for partially wetting liquids and faster velocities for totally wetting liquids. On anisotropically patterned surfaces, the droplet is elongated in the direction parallel to texture, and it has an elliptic drop shape and different spreading behaviors [26,27]. On these surfaces, drop volume generally affects the drop anisotropy and contact angle hysteresis to some extent [21]. On chemically [16,28,29] nanostructured surfaces, there is an anisotropy in the average value of the contact angle and contact angle hysteresis. Elongated drop shapes were also obtained by Chen et al. [30] and Chung et al. [31] for hydrophobic and hydrophilic grooved surfaces, respectively.

Drops condensing on grooved surfaces also show similar elongated drop shapes during growth when the surface is hydrophilic [32]. At the nanometer scale, the anisotropy still persists [33]. The anisotropic behavior depends on the size of surface structure, and has a nonlinear relation with the surface morphology. For the dynamics of drops sliding on chemically striped and hydrophobic grooved surfaces, the sliding angles are considerably larger for drops moving perpendicular to the stripes [17,22]. In nature, the use of these specific wettability patterned surfaces helps many animals and plants in arid environments to collect humidity more efficiently to obtain water. In this review, we summarize the latest developments in wetting and electrowetting (EW) on structured surfaces, including pillar-arrayed and corrugated surfaces. The apparent contact angle, spreading scaling law, and EW state transformation are explained in detail.

5.2 THEORETICAL BACKGROUND

Wetting is a natural phenomenon, depicting the contact situation between a liquid and a solid. Droplet wetting situation can be characterized by contact angle. When the contact angle is 0°, the liquid wets the solid surface completely. When the contact angle is larger than 0°, less than 180°, the liquid partially wets the solid surface. When the contact angle equals 180°, the liquid does not wet the solid surface. For the cases of partial wetting, the solid surfaces are hydrophilic when the contact angle is less than 90°, and hydrophobic when the contact angle is larger than 90° (Figure 5.1).

FIGURE 5.1 The cases of droplets wetting a solid surface. Droplets wet the solid surface completely, partially (hydrophilic), partially (hydrophobic), and not at all in the figures from left to right.

For a droplet deposited on a solid surface, the equilibrium is determined by the adhesion between droplet and solid. And the adhesion can...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface
• Editors
• Contributors
• SECTION I Ionic Liquids, Hydrogels, and Biosurfactants: Applications in Tribological Phenomenon
• SECTION II Adhesion, Wetting, and Their Relevance to Tribology
• SECTION III Green, Nano-, and Biotribology
• Index