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 When the particle was retracted large adhesion occurred,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