The interpretation o f A F M results between a particle and a non rigid surface is always complicated by the understanding o f the interface mobility. The ability o f measuring the bubble surface position was the motivation behind the modifications made by Berg [110] who added an interferometer unit to Paulsen’s A F M [109]. The layout o f the
A F M at the University o f Maine is shown in Figure 5-22. Unfortunately, the interferometric system did not show the same measuring capability for the oil-water interface, as w ill be detailed in the following section.
The system supplied by Zygo Corporation (Middlefield, C T) used a non-contact probe interferometer to focus the laser light beam onto the bubble surface. Resolution o f the interferometer was approximately 2.48 nm with a focused spot size on the bubble o f 60- 90 pm. This particular system used a single stabilized He-Ne laser source (1 m W ) to emit a beam with two frequencies separated by exactly 20 M H z. One frequency was used as the measurement beam and one as the reference. A fiber optic pickup was employed to carry the fringe pattern (described below) to a detector and accumulator contained in the control unit. Software was supplied by Zygo to measure the mobility o f the surface focused under the laser beam. The original measurement technique used to record bubble surface position is shown schematically in Figure 5-20. The non-contact probe split the two distinct frequencies, using one for the reference signal (shown as black in Figure 5-20), which passed straight through the beam splitter and was reflected o ff the retro reflector, maintaining a fixed beam length to be eventually collected on the fiber optic pick-up. The second frequency was used as the measurement beam (shown as magenta in Figure 5-20), which was turned 90 degrees upward and sent to the bubble surface. The reflected signal from the bubble (shown as yellow Figure 5-20) then re entered the probe and after final deflection toward the fiber optic pick-up was combined with the reference beam. From the fiber optic pick-up the recombined beam is conveyed to a detector inside the Zygo unit via a fiber optic cable and processed in order to evaluate any movement o f the interface.
The alignment o f the interferometer in order to work with the air bubble interface was made by Berg according to the manual o f the instrument, but the procedure was, unfortunately, not reported in his thesis [110]. As recommended by Zygo, the distance between the upper lens o f the interferometer beam splitter (non contact probe) and the focused target must be 44 mm.
As the path length o f the measurement beam changes, an interference pattern develops between the two frequencies. The fringe bands are then counted by a photo-detector and accumulator located in the control unit o f the Zygo interferometer.
In order to measure the movement o f the bubble surface by means o f the interferometric system, Berg modified the original cell used by Paulsen. He substituted the original bottom o f the cell with a stainless steel plate that had a 2 mm hole drilled in it as a
reservoir for the bubble The bottom o f the reservoir was composed o f an optical window with M gp2 anti-reflective coating, through which the interferometer beam
passed to reach the interior surface o f the bubble. The new cell assembly used by Berg is shown in Figure 5-21.
bubble
w indow
laser source
fiber optic pick-up
non-contact probe
reference measurement reflected
Figure 5-20 Schematic showing the interferometer measurement technique. Legend designates different portions of laser beam. The distance between the upper lens of the interferometer beam splitter and the focused spot must be 44 mm.
cantilever beam position detector particle bubble cantilever ■TïïTPnivB Figure 5-21 interferometer beam
View inside the bubble cell, illustrating the bubble and particle measurement technique as modified by Berg.
A centre aperture in the TS-300 Burleigh z-axis stage allowed the upper lens o f the interferometer access to the bottom o f the water cell, as shown in Figure 5-22. The cell was also provided with a Plexiglas skirt screwed on the base o f the Burleigh TS 300 z- axis stage, in order to gain an appropriate height to ease the focusing o f the apex o f the bubble before an experiment was carried out. Through fine adjustments with the Burleigh TS 300 z-axis stage the apex o f the bubble was focused at ~44 mm above the upper lens o f the beam splitter. To ascertain that the apex was correctly focused, Berg used the Vis Sim software to drive the Burleigh motor at very low speed (0.5 pm/s) whilst the output voltage on the Zygo equipment resulting from the interferometric laser patterns was controlled. This voltage rises from 3.7 to 4.15V as a target is approached and at last focused. The reading on the Zygo interferometer is not direct but passes through a multimeter connected to a voltage pick-up point on the Zygo front panel.
particle positioner h
□
cantilever measurement bubble positioner camera system laser sourcenon-contact probe interferometer
Figure 5-22 Schematic of the force measurement apparatus including interferometer addition by Berg. The figure also shows also the video system described in section 5.2.4.
Berg also introduced a cantilever x-y-z-axes fine positioning system (indicated as particle positioner in Figure 5-22) in order to align the particle precisely over the bubble already aligned to the interferometer measurement beam. In the previous design the cantilever could only be moved vertically.