W. Richard Bowen and Nidal Hilal 4

3.5 EFFECT OF sURFACE PREPARATION ON PARTICLE–BUBBLE INTERACTIONs 93
In addition the authors compared the contact angles obtained from the
AFM measurements with those obtained conventionally on flat surfaces.
It was found that the � r values obtained by the two methods differed,
depending upon the contact angles measured. At low contact angles (i.e.
more hydrophilic surfaces), contact angles measured with the colloid
probe were higher than those measured on planar surfaces. At contact
angles �60° the values obtained with the colloid probes were lower than
those measured on flat surfaces, with measurements at intermediate contact
angles in close agreement. These differences were explained by the
authors as being possibly due to the differences in line tension between
the different experimental set-ups [39].
The interactions of ZnS spheres with air bubbles at a range of solution
pH values was examined by Gillies et al. [46, 47]. On approach, repulsion
between the spheres and the bubbles was observed due to DLVO forces (the
micro-spheres have a negatively charged surface in solution under the conditions
of the experiments) and confinement of hydration layers, prior to a
jump-in event. The magnitude of the repulsive force prior to jump-in was
found not to vary upon alteration of solution pH to a significant effect at
pH values of 8.5 or less. On increasing the pH above this level, the repulsion
was increased by an order of magnitude, due to a much greater concentration
of negative charges on the micro-sphere surface. Contact angles measured
at the same time appeared to alter, based upon the number of times
the micro-sphere had previously interacted with the bubble surface, leading
to speculation that the ZnS surface was changing, either by being cleaned
or coated by the interaction. In addition a slight increase in the receding
contact angle was observed concomitant with a rise in pH. Leaving the
micro-spheres in zinc solutions for extended periods of time caused the
particle surface to change from a predominantly zinc hydroxide to a predominantly
zinc oxide surface, resulting in a greater degree of hydrophobicity.
This resulted in contact angles measured in these aged micro-spheres
which were approximately 10° greater than those of the none-aged spheres.
Wangsa-Wirawan et al. [13] measured the adhesion forces due to the
interactions between protein inclusion bodies and air bubbles. Both pH
and ionic strength of the surrounding solution was altered and the effects
measured. A maxima in the adhesion forces was reached at pH 5, with a
decrease at higher pH values, although adhesion was still observed. The
inclusion bodies were expected to carry a net overall negative charge at
pH values �5, leading to electrostatic repulsion between the bodies and
the negatively charged air–water interface. It was concluded that whilst
the electrostatic forces modulated the adhesion, hydrophobic interactions
between the inclusion bodies and the air–bubble played a more dominant
role in the adhesion. Additionally it was reported that the ionic strength
had an effect on the measured adhesion values in a way that could not be
explained purely by DLVO interactions.