Surface force meniscus attractive forces

As the tip is brought towards the surface, there are several forces acting on it. Firstly, there is the spring force due to die cantilever, F, which is given by = -Icz. Secondly, there are the sample forces, which, in the case of AFM, may comprise any number of interactions including (generally attractive) van der Waals forces, chemical bonding interactions, meniscus forces or Bom ( hard-sphere ) repulsion forces. The total force... 

It is well known that a vapour is apt to condense in a narrow gap, capillary or between two solid surfaces in close proximity. Such a condensed meniscus between two particles results in a strong attractive force between these particles due to surface tension. 

Attractive forces exist between the water in a pore and the solid surfaces when the liquid evaporates, the tension in the meniscus is transferred to the walls, and the pore tends to shrink. The shrinkage is not necessarily reversed when all the liquid has gone, because the pore may collapse. This effect could be important between about 90% and 45% RH. Above 90% it is unlikely to be important because the pores that arc being emptied arc wide, and the resulting stresses are small, and below about 45%, a stable meniscus cannot form. 

Capillary action When water is placed in a narrow container such as a graduated cylinder, you can see that the surface of the water is not straight. The surface forms a concave meniscus that is, the surface dips in the center. Figure 13-15 models what is happening to the water at the molecular level. There are two types of forces at work cohesion and adhesion. Cohesion describes the force of attraction between identical molecules. Adhesion describes the force of attraction between molecules that are different. Because the adhesive forces between water molecules and the sihcon dioxide in glass are greater than the cohesive forces between water molecules, the water rises along the inner walls of the cylinder. 

Water forms a concave meniscus, whereas mercury produces a convex meniscus. In the case of mercury, there is essentially no attractive force between the mercury atoms and the silicon dioxide to compete with the attractive forces between the mercury atoms themselves. The mercury forms a convex (high-centered) meniscus because the only force Is the Interparticle attractive force between mercury atoms, which produces surface tension. 

An interesting phenomenon based on capillarity is the appearance of a capillary attractive force between particles of moistened solids. As a result of wetting, a meniscus is formed upon the particle contact (Fig. 1-14). This meniscus between two contacting particles of radii r0 has a shape of surface of rotation, and can be characterized at each point by the two curvature radii r, and r2 (in Fig. 1-14, a these radii are of opposite sign, i.e. r,>0 and r2<0), which are related to each other as 1/r, + /r2 = const. If r, r0, both rx and r2 may be considered to be constant. 

The origin of the attractive forces is twofold the force exerted by the pressure differential across the meniscus that results from its curvature and the component of the liquid/vapor surface energy normal to the two surfaces. To better appreciate these two forces, consider the thought experiment illustrated in Fig. 10.23. Here a solid cylinder of radius X is placed between two plates, and the system is heated so as to melt the solid. Consider what happens in the following three cases. 

Since the AFM is commonly used under ambient conditions, it must be borne in mind that the sample is likely to be covered with multilayers of condensed water. Consequently, as the tip approaches the surface, a meniscus forms between tip and surface, introducing an additional attractive capillary force. Depending on the tip radius, the magnitude of this force can be equal to or greater than that of the van der Waals forces and is observed clearly in the approach curve [98], In fact, this effect has been exploited for the characterization of thin liquid lubricant films on surfaces [95], The capillary forces may be eliminated by operation in ultrahigh vacuimi, provided both tip and sample are baked, or, most simply, by carrying out the experiment under a contamination-free liquid environment, using a liquid cell [99],... 

The capillary condensation is a well known phenomenon in nature and is in most cases associated with the condensation of water in pores and cracks with hydi ophilic surfaces. A curved meniscus is formed due to the surface tension of the water-vapour interface. As a result of the change of pressure across this meniscus, a strong attractive force acts between the two surfaces. The phenomenon is known for quite a long time and has been explained by Lord Kelvin back in 1871 [15] with his famous equation [16]. 

Surface tension is the net pull inward on molecules at the surface of a liquid. Surface tension is related to cohesion, the attractive forces between molecules within a substance, ia A adhesion, the attractive forces between molecules in a substance and their container. The balance between cohesion and adhesion determines whether a liquid meniscus is concave or convex. It also gives rise to capillary action, in which liquid is drawn upward into a narrow tube against gravity. 

FIG U RE 1.21 Estimation of the capillary attractive force between two spherical particles in the presence of a liquid meniscus. (Redrawn from Shchukin, E.D. et ah, Colloid and Surface Chemistry, Elsevier, Amsterdam, the Netherlands, 2001.)... 

Factors other than capillary pressure may operate during drying of gels, and some authors (e.g., [17,18]) consider them to be of primary importance. Adsorption forces bind solvent into an ordered layer a few molecules thick near a solid surface the attractive force may drive flow of a film over an exposed solid surface [19] and influence the curvature of the meniscus (Fig. 8a). However, if there were no capillary force (j lv - 0), no compressive... 

Figure 1.10 Stiction that can occur during the sacrificial release etch, (a) Before sacrificial etching, the sacrificial oxide is below the mechanical layer, (b) After the chip is removed from the etch bath it begins to dry and the remaining fluid forms a bridge between the substrate and the mechanical layer, (c) Capillary forces from the meniscus of the fluid exert a downward force on the cantilever and cause it to come into contact with the substrate, (d) The surface forces, such as Van der Waals attraction, that dominate at the microscale cause the cantilever to become stuck to the substrate. (Reprinted with permission from lOP Publishing Ltd.) [15].

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