Contact interactions surface forces

The derived force versus tip separation curve is shown in Fig. 3.27B. Why is this not the same as the curves in Fig. 3.25 and 3.26 On the scale used, the forces in the noncontact regime before jump-in are very small, and since the surface is ideally rigid, the repulsive force curve is vertical and not merely steep. With a real specimen, the surface forces may be large enough to lead to deformation during the jump-in. At smaller separations, if the contact pressure in the repulsive regime exceeds the yield strength of the surface, then plastic deformation will occur. This is one of several possible types of force curves. They depend on many factors such as the nature of the interaction, surface forces, the deformability of the surface, and the medium between the tip and surface (air, liquid or vacuum) [116,117]. Force ciu es can be used to study and verify fundamental theories of contact mechanics and adhesion on polymer surfaces [118]. 

Loads that are produced by external forces acting directly on the surface of the body are transmitted throughout the body by the interactions of the constituent molecules any element of the body exerts a force on neighbouring parts. The forces exerted on the surface of any element of the body by the material surrounding it are known as contact or surface forces, and are proportional to the surface area of the element. That the interaction of one part of a body with an adjacent part can be regarded as a contact or surface force across the common boundary is, of course, a consequence of the short-range nature of the intermolecular forces. 

Binnig et al. [48] invented the atomic force microscope in 1985. Their original model of the AFM consisted of a diamond shard attached to a strip of gold foil. The diamond tip contacted the surface directly, with the inter-atomic van der Waals forces providing the interaction mechanism. Detection of the cantilever s vertical movement was done with a second tip—an STM placed above the cantilever. Today, most AFMs use a laser beam deflection system, introduced by Meyer and Amer [49], where a laser is reflected from the back of the reflective AFM lever and onto a position-sensitive detector. 

Studies based on the Frenkel-Kontorova model reveal that static friction depends on the strength of interactions and structural commensurability between the surfaces in contact. For surfaces in incommensurate contact, there is a critical strength, b, below which the depinning force becomes zero and static friction disappears, i.e., the chain starts to slide if an infinitely small force F is applied (cf. Section 3). This is understandable from the energetic point of view that the interfacial atoms in an incommensurate system can hardly settle in any potential minimum, or the energy barrier, which prevents the object from moving, can be almost zero. 

Figure 7.13 Left interaction potential and force between an atom at the apex of the tip and an atom in the surface. Tip-surface interactions can be described by a summation of these potentials over all combinations of atoms from the tip and the surface. Right interaction potential between the tip, approximated as a sphere, and a plane surface, valid in the non-contact mode of force microscopy. To stress the long-range character of the non-contact potential, the Lennard-Jones interaction potential between two atoms has been included as well (dotted line).

The surface molecules are under a different force field from the molecules in the bulk phase or the gas phase. These forces are called surface forces. A liquid surface behaves like a stretched elastic membrane in that it tends to contract. This action arises from the observation that, when one empties a beaker with a liquid, the liquid breaks up into spherical drops. This phenomenon indicates that drops are being created under some forces that must be present at the surface of the newly formed interface. These surface forces become even more important when a liquid is in contact with a solid (such as ground-water oil reservoir). The flow of liquid (e.g., water or oil) through small pores underground is mainly governed by capillary forces. Capillary forces are found to play a very dominant role in many systems, which will be described later. Thus, the interaction between liquid and any solid will form curved surface that, being different from a planar fluid surface, initiates the capillary forces. 

Fig. 2. A. The force between silica surfaces at cg= 1M and pH=9, determined experimentally in Ref. [22] (circles) the continuous line represents an unsuccessful fit with Eq. (43a). The origin of the x axis corresponds to the smallest separation distance between the two surfaces, attainable by AFM. The coordinate of the true point of contact between the surfaces cannot be obtained directly from experiment. The inset shows the region of the secondary minimum, where the magnitude of the interaction is comparable with instrumental resolution of 0.01 mN/m, B. The fit of the experimental data (cE=- 1M, pH=9) with an exponential repulsion and a van der Waals attraction (Eq. (43b)). As in Fig. 2A, the origin of the t axis corresponds to the smallest separation attained in the experiment. The true point of contact between surfaces, obtained from fit, is located at a distance which is by 2 =15 A larger than the smallest separation recorded by AFM.

The interferometric SFA has served as an invaluable tool in studying the hydrophobic attraction among other things due to the fact that it is the only technique available today that enables direct observation of occurrence of cavitation. For instance, recently Lin et al. [89] employed a dynamic surface forces measurement method to study interactions between DODAB LB coated surfaces. High-speed camera images of FECO revealed that there are no bubbles on the surfaces prior to contact. However, short-lived cavities, typically lasting 3 ps before disappearing, have been observed to form upon separation (Fig. 6). 

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