[Spm] Tapping in Liquid - Phase vs. Amplitude images

[Don Chernoff at ASM]

My questions today concern whether phase imaging in liquid is useful and how to set it up.

1) Is phase imaging in liquid a useful technique, as it is in air?  Does it have significant advantages over amplitude imaging?


As background, consider this:
A 1997 paper from Helen Hansma and colleagues reports an impressive set of observations using tapping mode phase imaging to visualize DNA molecules in liquid.  [see 
http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=147043&blobtype=pdf]



They reported:

Phase imaging is optimal at lower imaging forces than height

imaging (Fig. 3). This is a significant advantage for phase

imaging over height imaging, since there is less damage to

biological samples at lower imaging forces. In addition, phase

images remained clear at a 3- to 6-fold faster scan rate than height

images, 27.5 Hz in these experiments (Fig. 2), corresponding to

a tip speed of 110 mm/s. With faster scan rates images can be

captured more quickly for observing processes in the AFM.

...

 We have not experimented

systematically with the amplitude images of moving DNA, but

those who can do amplitude imaging ... might find that 

amplitude imaging is also useful for imaging moving

DNA at faster scan speeds and lower forces.

===

With the perspective of almost 11 years since that publication, what is the situation of phase vs amplitude imaging in liquid?



2) When doing phase imaging in air, it is easy to find the fundamental frequency of the cantilever and (we think) it is important to set the drive frequency a little bit lower than the resonant frequency.  In liquid, it can be hard to choose the correct resonance.  Assuming you can choose it, is there a benefit to adjusting the drive frequency slightly in order to make the phase response more reproducible?  Are there any other hints the experienced users can give?




regards,
Don
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Don Chernoff, Ph.D., President
Advanced Surface Microscopy, Inc.    E-Mail:  donc at asmicro.com
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