W. Richard Bowen and Nidal Hilal 4

3.5 EFFECT OF sURFACE PREPARATION ON PARTICLE–BUBBLE INTERACTIONs 95
Fthr/R/mN/m
8
6
4
2
0
SDS
DTAB
0 2 4 6 8 10 12
Surf.conc./mM
FIGurE 3.5 Effect of SDS and DTAB concentration on the (normalised) threshold force
(F/R) needed to achieve a TPC with an air bubble in aqueous solution. Reprinted with permission
from [36]. Copyright 1998 American Chemical Society.
more stable. To make a TPC contact, the loading force would have to be
sufficient to cause penetration of the surfactant film, characterised by the
threshold force, which would increase with thicker surfactant films.
Recently, force interaction measurements have been carried out
between silica glass spheres and colloidal gas aphrons (CGAs) [49].
CGAs are a form of very small (�100 �m) surfactant-stabilized bubbles
created by high shear rate flows, first described by Sebba in 1971 as
micro-foams [50]. The use of aphrons has been proposed for a number
of potentially useful applications. These include the flotation of fine
mineral particles [51, 52], the treatment of particulates and contaminant
chemicals including solvents and heavy metals from wastewater [53–56],
treatment of soil contamination [53, 57, 58], the harvesting of microbial
cells [59] and extraction of protein products [58, 60, 61], and the production
of tissue engineering scaffolds [62]. Sebba proposed a multi-walled
structure for CGAs consisting of an outer surfactant bilayer, followed by
an internal layer of surfactant solution, in turn separated by a surfactant
monolayer from a central gas core. Although this putative structure has
not been confirmed conclusively, there are a number of indirect experimental
observations which suggest that the internal structure of CGAs
are different from those observed with conventional foams, including xray
diffraction, and transmission electron microscopy (TEM) [58, 63].