W. Richard Bowen and Nidal Hilal 4

92 3. QUANTIFICATION OF PARTICLE–BUBBLE INTERACTIONs
sign under the influence of the approaching silica particle. A similar argument
was used to explain the adhesion forces observed (�4 mN m �1 ),
measured during retraction of the hydrophilic silica particles by Fielden
et al. It has been noted elsewhere that if there is a sufficient difference
in the magnitude of the surface potentials of two approaching bodies,
then even if the bodies have potentials of the same sign at sufficiently
small separations, an attractive force may occur [45]. It is worth bearing
in mind that due to the very small surface areas of the interactions, these
types of experiments are very sensitive to any contamination, even if great
care is taken to prevent this. Deviations from the expected behaviour
could easily be caused as a result of such contamination. In the literature,
papers describing colloid probe-based particle–bubble interactions
describe stringent techniques to minimise the risk of contamination. The
potential problems due to probe contamination are a serious concern for
all SPM techniques. See Chapter 1 for more information on this subject.
Glass particles used by Butt et al. [10] were made hydrophobic by
silanizing them in an atmosphere of dichloromethane. When these particles
were allowed to approach an air bubble, their behaviour was found to
have altered from that of hydrophilic particles. This time jump-in events
occurred on approach to the bubble surface with no repulsive interaction
prior to contact. When the particle was retracted large adhesion occurred,
with the cantilever deflecting by a greater amount than could be detected
by the instrument being used [10]. In the study by Fielden et al. [37] the
silica particles were hydrophobized by either reacting with octadecyltrichlorosilane,
or by dehydroxylation to create a surface with a more moderate
degree of hydrophobicity. For the two types of hydrophobic surface,
interactions between the particles and the bubbles were attractive at small
separation distances. Furthermore, the attraction was dependent upon
the degree of hydrophobicity of the surface. Frequently the particle was
engulfed by the air bubble and much larger adhesion forces were measured
than with the hydrophilic particle. It was concluded that the jumpin
events observed were a result of hydrophobic attraction. These two
studies illustrate the importance of hydrophobicity in particle–bubble
attachment and for the degree of stability of particle–bubble aggregates.
Preuss and Butt [39] measured the interactions of bubbles with silica
particles coated with different surface concentrations of alkyl-thiol
molecules and the resultant receding contact angles both with the colloid
probe technique and by conventional means. They noticed that as
well as more hydrophilic particles having shorter jump-in distances
and less adhesion than hydrophobic particles, both the � r and adhesion
forces increased as the mole fraction of alkyl-thiols was increased.
As an increase in contact angle is conventionally related to an increase
in hydrophobicity of a surface [27], the relationship between hydrophobicity
and the stability of particle bubble attachment is again reiterated.