Non-contact mode prevents tip and sample degradation from contact between the two, and is therefore good for soft samples such as biological specimens and thin organic films. If a very thin fluid layer is covering a rigid sample, in non-contact mode the tip may be oscillating above the fluid and so the liquid layer and the underlying surface can be imaged.
Under ambient conditons though, often a thin layer of water inherently forms on the tip and the sample surface. In non-contact mode, a 'bridge' of liquid may form between the tip and the sample surface due to capillary action, and this causes the tip to in effect 'contact' the surface. Therefore, accurate imaging using non-contact mode under ambient conditions is difficult. Non-contact mode AFM works best under ultra-high vacuum conditions and then atomic scale resolution is possible..
In non-conact mode the attractive forces are weak (see Figure 8 previous page), and the technique is sensitive to external vibration.