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