Physics of Wetting

Edward Yu. Bormashenko

• Publisher: De Gruyter
• Year: 2017
• ISBN:
  9783110437164
• Edition: 1
• Topic: Sciences physiques
• Subtopic: Mécanique des fluides

Motivated by a plethora of phenomena from nature, this textbook introduces into the physics of wetting of surfaces. After a brief discussion of the foundations of surface tension, its implementation for floating objects, capillary waves, bouncing droplets, walking of water striders, etc. is discussed. Furthermore, Marangoni flows, surface tension inspired instabilities, condensation and evaporation of droplets, liquid marbles, superhydrophobicity and superoleophobicity (lotus effect) are introduced. All relevant concepts are illustrated by the numerous qualitative and quantitative exercises.

Contents
What is surface tension?
Wetting of surfaces: the contact angle
Surface tension-assisted floating of heavy and light objects and walking of water striders
Capillary interactions between particles. Particles placed on liquid surfaces. Elasticity of liquid surfaces, covered by colloidal particles
Capillary waves
Oscillation of droplets
Marangoni flow and surface instabilities
Evaporation of droplets. The Kelvin and the coffee-stain effects
Condensation, growth and coalescence of droplets and the breath-figure self-assembly
Dynamics of wetting: bouncing, spreading and rolling of droplets (water hammer effect – water entry and drag-out problems)Superhydrophobicity and superoleophobicity: the Wenzel and Cassie wetting regimes
The Leidenfrost effect. Liquid marbles: self-propulsion
Physics, geometry, life and death of soap films and bubbles

1What is surface tension?

1.1Surface tension and its definition

Surface tension is one of the most fundamental properties of liquid and solid phases. Surface tension governs a diversity of natural or technological effects, including floating of a steel needle, capillary rise, walking of water striders on the water surface, washing and painting. It governs many phenomena in climate formation, plant biology and medicine. Surface tension is exactly what it says: the tension in a surface and the reality of its existence are demonstrated in Fig. 1.1, which shows a metallic needle supported by water surface. The locomotion of water striders to be addressed in detail in Section 3.7 is another well-known manifestation of the surface tension-inspired phenomenon.

Fig. 1.1: Manifestation of surface tension: steel needle floating on the water surface.

Imagine a rectangular metallic frame closed by a mobile piece of wire as depicted in Fig. 1.2. If one deposits a soap film within the rectangle, the film will want to diminish its surface area. Thus, it acts perpendicularly and uniformly on the mobile wire as shown in Fig. 1.2. Surface tension $\overrightarrow{\gamma }$

could be defined as a force per unit length of the wire.

The surface tension defined in this way is a tensor that acts perpendicularly to a line in the surface. Surface tension is often identified with the specific surface free energy. Indeed, when the mobile rod l in Fig. 1.2 moves by a distance dx, the work 2γldx is done (factor 2 reflects the presence of upper and lower interfaces). Thus, the surface tension γ could be identified with the energy supplied to increase the surface area by one unit. This identification may give rise to misinterpretations: the surface tension defined as force per unit length of a line in the surface is a tensor, whereas specific surface free energy is a scalar thermodynamic property of an area of the surface without directional attributes [1]. However, for liquids at a constant temperature and pressure and in equilibrium, the surface tension is numerically equal and physically equivalent to the speci...

Table of contents

• Cover
• Title Page
• Copyright
• Dedication
• Preface
• Contents
• Symbol index
• 1 What is surface tension?
• 2 Wetting of surfaces: the contact angle
• 3 Surface tension-assisted floating of heavy and light objects and walking of water striders
• 4 Capillary interactions between particles. Particles placed on liquid surfaces. Elasticity of liquid surfaces, covered by colloidal particles
• 5 Capillary waves
• 6 Oscillation of droplets
• 7 Marangoni flow and surface instabilities
• 8 Evaporation of droplets. The Kelvin and the coffee-stain effects
• 9 Condensation, growth and coalescence of droplets and the breath-figure self-assembly
• 10 Dynamics of wetting: bouncing, spreading and rolling of droplets (water hammer effect – water entry and drag-out problems)
• 11 Superhydrophobicity and superoleophobicity: the Wenzel and Cassie wetting regimes
• 12 The Leidenfrost effect. Liquid marbles: self-propulsion
• 13 Physics, geometry, life and death of soap films and bubbles
• Index