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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9.6 MESOSCALE ExPERIMENTAL STUdIES 265<br />
a ‘bare’ cantilever <strong>and</strong> a reflective silica substrate, the reflected image is<br />
apparent in the lower half of the figure. When probeless cantilevers are<br />
brought into contact with liquid layers or drops, the cantilever is often<br />
enveloped by the fluid. In this respect the use of a colloid probe can help<br />
prevent wetting of the cantilever beam <strong>and</strong> the associated degradation of<br />
the laser signal.<br />
Figure 9.11(a) shows a colloid probe attached to the tip of a V-shaped<br />
cantilever supported underneath the dimension scanner. In this example,<br />
no optical filters are used as careful alignment of both the optical microscope<br />
<strong>and</strong> the focal point of the laser reduces the transmission of diffuse<br />
laser light to the camera; although diffuse reflections are apparent (on the<br />
right h<strong>and</strong> side of the image), they are not excessive. If the observation<br />
angle is varied, such that the cantilever is viewed slightly from above,<br />
then either (i) optical filters are used or (ii) the field of view of the microscope<br />
is adjusted to omit all but the tip of the cantilever (under the same<br />
illumination conditions, omission of the optical filters permits higher<br />
photographic recording rates). This method is not employed using a light<br />
intesified system as the intensifier could be irreparably damaged. Figure<br />
9.11(b) shows the interaction between a 15-�m diameter colloid probe<br />
<strong>and</strong> a ‘large’ droplet of silicon oil, which is spread upon a reflective silica<br />
substrate. When first brought into contact, the silicon oil immediately<br />
enveloped the sphere <strong>and</strong> also adhered to the underside of the cantilever<br />
even though the cantilever was far above the surface of the drop; this<br />
effect was not readily apparent from above (as observed via the integrated<br />
AFM optics). The image shows the non-ideal liquid geometry during the<br />
(a) (b)<br />
FIguRE 9.11 (a) A colloid probe attached to the end of a V-shaped cantilever as<br />
observed using the optical system shown in Figure 9.9. The cantilever is observed directly<br />
from the side, hence the V-shape is not apparent. (b) The partial envelopment of a 15-�m<br />
diameter colloid probe by an excess of silicon oil (� � 12 Pa s).
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