Probe normal force

Figure 1 shows the results obtained by Qian et al. [1 ] in a process when AFM probe approaches and then separates from a SiQ2 substrate. The normal force required for separating the probe-substrate contact reads 33 nN. From a thermodynamic point of view, adhesion is in fact a state of the system at the energy minimum when the contact pairs interact with each other through interface, and additional work has to be applied to change the state of the system. 

Surface forces measurement directly determines interaction forces between two surfaces as a function of the surface separation (D) using a simple spring balance. Instruments employed are a surface forces apparatus (SFA), developed by Israelachivili and Tabor [17], and a colloidal probe atomic force microscope introduced by Ducker et al. [18] (Fig. 1). The former utilizes crossed cylinder geometry, and the latter uses the sphere-plate geometry. For both geometries, the measured force (F) normalized by the mean radius (R) of cylinders or a sphere, F/R, is known to be proportional to the interaction energy, Gf, between flat plates (Derjaguin approximation). 

Sagvolden et al. [86] also combined the use of colloids with AFM force sensors to study adhesion. In their case, instead of attaching the colloid to the end of the AFM probe and applying a normal force, they approached the free colloids from side on, with the AFM cantilevered at an angle of approximately 30° to the surface normal. Thus, they applied a predominantly lateral force to the colloid particles. The colloids were coated with protein molecules, and their adhesion was studied against three nonbiological surfaces, consisting... 

In the contact mode, the probe senses forces F acting both perpendicular and parallel to the surface plane, i.e. normal forces and lateral forces, respectively (Fig. 5). Figure 6 shows a typical force-distance curve obtained when the tip is brought into close proximity with the sample surface. The normal force is proportional to the deflection of the tip(Z(), while the distance is given by the position of the cantilever (Zc) relative to the sample surface. Zc=0 corresponds to the straight cantilever (Z(=0) in contact. 

The lateral force microscope (LFM) is a modification of the standard contact mode SFM [87-90]. In addition to the normal forces, the friction forces exerted on the probe are measured via torsion of the cantilever (Fig. 5). This mode is sometimes called friction force microscopy . LFM can be used in combination with topographic imaging as it shows changes in material as well as enhanced contrast on sharp edges (Fig. 9). In addition to morphology, it provides information on the friction and wear properties (Sect. 3.4). 

Various experimental designs have been devised to measure the friction on skin. They focus on measuring friction by pressing a probe onto the skin with a known normal force, and then detecting the skin s frictional resistance to movement of the probe. The designs fall into two categories ... 

An important part of designing a friction measurement apparatus is choosing the probe size, shape, and material. Because friction is an interaction between two surfaces, the probe geometry and material will affect the values calculated for the friction coefficient of the other surface. Also, results will be more accurate when the probe s normal force is maintained at a constant value or continuously monitored previous methods used to maintain the normal force include spring mechanisms or static weights to weigh down the probe. These parameters are revisited critically later in this article. 

Zauscher, S. and Klingenberg, D. J., Normal forces between cellulose surfaces measured with colloidal probe microscopy. J Colloid Interface Sci 2000, 229 (2), 497-510. 

Since the introduction of the STM a number of variations have been devised, such as ATM (atomic force microscope). The basic concept is that piezoelectric actuators move a miniature cantilever arm (with a nm-sized tip) across the sample while a non-contact optical system measures the deflection of the cantilever caused by atomic scale features. The deflection is proportional to the normal force exerted by the sample on the probe tip and images are generated by raster scanning the sample [201]. One application of this technique was to measure the thickness and size distribution of sub-micron clay particles with diameters in the 0.1 to 1 pm size range and thickness from 0,01 to 0.12 pm [202]. 

This work describes a new probe of dynamic processes accompanying the tribological loading of metal/polymer interfaces. We instrument the polymer substrate and a conducting stylus to measure the transient electrical currents generated as the stylus is moved across the substrate under normal load. Simultaneous measurements of the lateral and normal forces on the stylus are also performed. Both sets of measurements are readily made on ms to is time scales. To date, we have measured currents accompanying the abrasion of several insulators. 

Fig. 7.3. Normalized force-versus-piezo displacement plot (retraction) for a polystyrene colloid probe and an ES 404 membrane (NaCI concentration 10" M, pH 8.0). Reprinted from [36]. Copyright 1998, with kind permission from Elsevier...

It is noteworthy that when part of the worn material debris is deposited on the own sliding track, superposed layers of material are generated that contributes to the existence of a lack of uniformity in the contact surface between the pin and the alloy probe, causing discontinuities on the movement and giving rise to instabilities in the friction coefficient and normal force values. Thus, the wear is associated with the removed material quantifying through weight loss of the probes. 

Figure 9.18. (a) Atomic force microscope probe sliding over a smooth surface, (b) Measured friction force- versus normal force fitting Equation (9.16). 

As mentioned in the section imder Normal Forces, van der Waals interactions in different media between SFM probes and apolar polymers have been successfully described using the Lifshitz theory (86). As shown in Figure 22 (left), various polymers can be differentiated clearly in force measurements. For polymers containing polar groups, a correlation of adhesive forces and the cosine of the water contact angle have been predicted and observed (66,85,204). An obvious requirement for reliable force data are flat substrates (205,217,218). 

Fig. 6.16 (A) Potential energy of an atom probe approaching a surface, as a function of separation, and the resulting normal force, the differential of the energy. Point A is the position of equilibrium contact. (B) Enlarged view of the force versus separation curve near the equilibrium contact point. Smaller separations give a repulsive force, and the probe is said to be in contact with the surface. The straight lines correspond to the stiffness of the cantilever, and greater slopes make the probe position unstable. (C) Cantilever deflection as a function of the distance of the cantilever support above the surface. The vertical lines show how the probe will be unstable and jump into and out of contact with the surface. A shffer cantilever would produce the smaller deflection shown as a dotted line, and suppress the instability.

At the beginning, the tip, under each normal force, was moved back and forth for 5 to 10 bi-directional scrapes to make a mar, in order to compare the results with those from crockmeter test, in which a probe, covered by a fresh green 50 x 50mm felt pad, stroked a coated panel back and forth for 5 to 10 times (8-10), Later, we reduced the scrapings to a single one, which is closer to the practical situation. 

Most concentrated structured liquids shown strong viscoelastic effects at small deformations, and their measurement is very useful as a physical probe of the microstructure. However at large deformations such as steady-state flow, the manifestation of viscoelastic effects—even from those systems that show a large linear effects—can be quite different. Polymer melts show strong non-linear viscoelastic effects (see chap. 14), as do concentrated polymer solutions of linear coils, but other liquids ranging from a highly branched polymer such as Carbopol, through to flocculated suspensions, show no overt elastic effects such as normal forces, extrudate swell or an increase in extensional viscosity with extension rate [1]. 

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