Collision and Attachment Interactions of Single Air Bubbles with Flat Surfaces in Aqueous Solutions - Drops and Bubbles in Contact with Solid Surfaces - page 212

3.CrossoversofScales ....................................... 234

G.SummaryandConclusions....................................... 235

H.Acknowledgements........................................... 236

I.References................................................ 236

A. Introduction
Interaction between a gas bubble and a solid surface in a liquid has been the fo-
cus of scientific interest and industrial applications for many years [10, 17, 22,
33]. It plays a vital role in many applications, ranging from industrial multiphase
applications such as bubble columns, chemical and biological reactors, mass and
heat exchangers to medical applications such as echography and novel applications
such as microfluidics devices and drag reductions of swimming clothes and ships
using injected microbubbles. One of the important industrial applications of gas
bubbles is the flotation separation of hydrophobic particles from a suspension of
hydrophobic and hydrophilic particles [47]. Flotation is a well-known process that
was originally developed around the turn of last century for recovering fine valu-
able mineral particles from the rock [53]. Today, more than ten of billion tons of
ores are reportedly concentrated by flotation globally each year. At the heart of
flotation is the bubble-particle interaction which has intensively been investigated
for many years. Many important steps involved in the bubble interaction with the
particle surface in flotation has been identified and quantified, and can analogically
be applied to the collision and attachment of bubbles with a flat surface in water
and surfactant solutions. However, by no means is the interaction of air bubbles
with solid surfaces a simple process. On the contrary, it is a complicated process,
which can be divided into a number steps as illustrated in Fig. 1. These steps can
be grouped into the bubble rise before interacting with the surface and the bubble-
surface interactions. The free rise of bubbles before making contact with the surface
is considered when the bubble is far away from the surface and is strongly affected
by the so-called long-range hydrodynamics. When the bubble and surface are at
close proximity, the contact interaction is considered and is controlled by interfacial
physics, e.g., interfacial hydrodynamics of the intervening liquid film, capillarity of
bubble surface deformation, de-wetting dynamics, and colloidal interactions gov-
erned by intermolecular forces.
This chapter aims to review the steps involved in the interaction between an air
bubble and flat solid surface in water and surfactant solutions. Specifically, the chap-
ter focuses on single air bubbles rising in an aqueous solution towards a large planar
solid which is placed horizontally to the direction of gravity, i.e., perpendicularly to
the direction of the bubble rise.
This chapter is organised as follows. In Section B, physiochemical hydrody-
namics and interfacial rheology of rising bubbles in an aqueous solution is briefly
reviewed. Since this topic is reviewed in topics, only the key aspects of the bub-
ble rise and its velocity are described here for reference in the subsequent sections.
On approaching the solid surface, the rising bubble first loses its momentum by