Forces in atomic force microscopy in air and water

Abstract

A new atomic force microscope, which combines a microfabricated cantilever with an optical lever detection system, now makes it possible to measure the absolute force applied by a tip on a surface. This absolute force has been measured as a function of distance (=position of the surface) in air and water over a range of 600 nm. In the absolute force versus distance curves there are two transitions from touching the surface to a total release in air caused by van der Waals interaction and surface tension. One transition is due to lifting off the surface; the other is due to lifting out of an adsorbed layer on the surface. In water there is just one transition due to lifting off the surface. There is also a transition in air and water when the totally released tip is pulled down to touch the surface as the surface and tip are brought together. Based on the force versus distance curves, we propose a procedure to set the lowest possible imaging force. It can now be as low as 10−9 N or less in water and 10−7 N in air.

Force measurements with the atomic force microscope: Technique, interpretation and applications

“…Under ambient conditions, reduction of forces is limited by the existence of the meniscus force that arises from capillary condensation around the contact sites between tip and surface. One way to avoid the meniscus force is to do imaging in water [6].…”

“…In addition, microscopists tried to understand the contrast mechanism of the AFM to interpret images correctly. One prominent example was the observation of meniscus forces under ambient conditions by Weisenhorn et al [6] in 1989. Weisenhorn et al did not only detect meniscus forces but they also realized that imaging in liquid could significantly reduce the interaction between tip and sample in AFM imaging and thus increase the resolution.…”

Force-controlled spatial manipulation of viable mammalian cells and micro-organisms by means of FluidFM technology

“…Already 20 years ago, Ashkin et al pioneered the development of the optical tweezers as a mean to manipulate biological objects. [2][3][4][5][6][7] Exploiting the trapping forces due to the radiation pressure through intense and collimated lasers 8,9 viruses, bacteria as well as cellular organelles could be displaced in a controlled way. However, optical tweezers might damage cells, [10][11][12] and are not appropriate for detaching adherent, spread cells from surfaces.…”

Characterization of atomic force microscope probes at low temperatures

“…The adhesion force is characterized as the force required separating the tip from the surface. 15 In our measurements, both freshly cleaved mica and graphite were used. Mica is known to carry some negative surface charges after cleavage.…”