Biomedical Engineering Reference
In-Depth Information
Fig. 2.34. Examples of different types of vibration isolation. Left: an active vibration isolation platform
(reproduced with permission from Herzan). Centre: a compressed air-based isolation table. This type of
passive vibration isolation is particularly appropriate for large instruments. Reproduced with permission
from Kinetic Systems, Inc. Right: an image of an AFM mounted on a bungee cord suspended platform.
The suspended platform is enclosed in a cabinet designed to further shield the AFM from acoustic noise.
Image used with permission fromAgilent. Metallic springs are also suitable for suspension of platforms.
Passive vibration solutions basically consist of some sort of spring, a weight, and a damping
device. Usually, these will take the form of an air table or a suspended platform. The suspended
platform is a very simple device, and consists of placing the AFM on a heavy stage (which
forms the weight) that is suspended by springs (either strong elastic bungee cords, or metal
springs). With long springs and a large weight, these devices have excellent isolation charac-
teristics, but use a lot of space. The air table is marginally more compact and consists of a
suspended platform, which is cushioned by air. In general, any of the solutions mentioned here
work extremely well, and attenuate floor vibrations frommost locations to an acceptable level
for high-resolution AFM work. Some typical devices are shown in Figure 2.34.
2.7 Scanning environment
Typically AFM is carried out under ambient conditions, but a major advantage of AFM is
the ability to image the sample in almost any environment, from vacuum, to gas, to liquid.
The most common application is to scan in liquid, which is particularly important in
biological applications in order to measure the samples in their native state, or for electro-
chemical measurements of metallic surfaces. For more examples of liquid scanning appli-
cations see Chapter 7. In addition, scanning in liquid has some advantages over scanning in
air, such as reproducibility of the scanning environment, and notably, the tip-sample
interactions in liquid are much weaker. One of the major problems of scanning in ambient
air is that typically a meniscus layer of water will coat the sample and/or tip. Interaction of
the AFM with this layer of water will dominate the behaviour of the tip-sample interaction
[85], making it difficult to apply a small force to the sample when scanning (especially in
contact mode) [86]. Scanning in water removes these forces altogether.
Practically, most AFMs require the use of a specific liquid cell in order to scan in
fluid. The leaking of water in an AFM experiment can cause problems for the instru-
ment. Apart from the normal incompatibilities between electronics and water, piezo-
electric scanners are highly sensitive to moisture and must be well sealed to prevent
 
 
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