Liquid Marbles

Formation, Characterization, and Applications

Andrew T. Tyowua

• Publisher: CRC Press
• Year: 2018
• eBook ISBN: 9781315280233
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Chemistry
• Index: Chemistry

Certain small solid particles are surface-active at fluid interfaces and thus are able to stabilize materials previously considered impossible to stabilize in their absence. Liquid marbles, particle-coated non-sticking liquid droplets, represent one of these materials. Preparation of liquid marbles was described only about 15 years ago and they are now widely studied by many research groups and numerous applications of liquid marbles have been advanced. The book is written for postgraduates and researchers working on the area who are training to become chemists, soft matter physicists, materials scientists, and engineers.

1	Interfaces and the Concept of Surface Tension

1.1 DEFINITION AND TYPES OF INTERFACES

There exists a two-dimensional plane, frontier or boundary, with no thickness, when two surfaces meet known as an “interface”. A “surface”, an “interphase”, or “interfacial region” is a three-dimensional region, of finite thickness (few Å), where two homogeneous bulk phases meet. The properties of this region are entirely different from those of the two bulk phases. Interfaces are very common and are everywhere, e.g. they occur in the food we eat, bodies of living organisms, and natural and chemical environments. There are several types of interfaces, and they have been classified based on the nature of the surfaces involved in their formation. This classification yields five types of interfaces under two broad themes, namely

• Fluid interfaces: (1) liquid-air or gas and (2) liquid-liquid interfaces,

• Non-fluid or solid interfaces: (1) solid-air or gas, (2) solid-liquid and (3) solid-solid interfaces.

Because gases are miscible with one another, there are no gas-gas interfaces. Sometimes three surfaces meet in a line to form an interface. This line is called a triple interface.

1.2 SURFACE TENSION AND CURVED INTERFACES

1.2.1 SURFACE TENSION

Surface (or interfacial) tension is an important concept when studying fluid interfaces. Surface tension is force per unit length acting, perpendicularly, on an imaginary line drawn in the interface. Its SI units are Newton per metre (N m⁻¹), but it is sometimes reported in Joule per metre square (J m⁻²) on the basis that 1 J is equivalent to 1 N m. Other units, e.g. dyne per centimetre, are also used. Note that 1 dyne cm⁻¹ is equivalent to 1 mN m⁻¹. The surface tension (25°C) of some liquids is given in Table 1.1. A liquid surface is basically a liquid-air interface. The cohesive forces between the molecules of the liquids are greater than the adhesive forces between the air and the liquid molecules. The result is a force imbalance with a net inward force in the liquid bulk phase. This causes the liquid surface to stretch as though it were covered with an elastic membrane. This is the origin of surface tension in liquids. Surface tension is responsible for many observed physical phenomena. For example, due to surface tension, liquid drops minimize their surface area by taking a spherical shape (geometry of least surface area). The rising of liquids in thin capillaries once submerged in them is also a consequence of surface tension. It is also because of surface tension that insects are able to walk on the surface of water.

TABLE 1.1
Surface Tension of Some Liquids at 25°C

Liquid	γla/mN m^−1 at 25°C
Hexane	17.89
Ethanol	21.97
Methanol	22.07
Cyclohexane	24.65
Acetone	24.02
Chloroform	26.67
Acetic acid	27.10
Toluene	27.93
Benzene	28.22
Hexadecane	27.05
Formamide	57.02
Water	71.99

Source: Jasper, J.J., J. Phys. Chem. Ref. Data 1, 841–100...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Dedication
• Table of Contents
• Foreword
• Preface
• Acknowledgments
• About the Author
• Base Units, Derived Units, Prefixes, and Conversions
• Chapter 1 Interfaces and the Concept of Surface Tension
• Chapter 2 Nature of Solid Surfaces
• Chapter 3 Sticking and Non-sticking Drops
• Chapter 4 Principles and Properties of Liquid Marbles
• Chapter 5 Applications of Liquid Marbles
• Answers to Numerical Questions
• Index