Jumping into Contact

The skeptical engineer is wary of the idea that molecules leap into contact with their surrounding snrfaces. After all, car parts on an assembly line do not suddenly jump together and adhere strongly to fashion the finished vehicle. However, at the molecular level it is quite obvious that such events occur naturally, and a nanoscale car could be self-assembled by these normal adhesive forces. The problem is that the nano-engine would not work because all its parts would seize togeflier. 

In this example, it is clear that the water molecules are in constant motion, because small particles of pollen within the droplet can be seen dancing with the 

Brownian movement. Thus, the equilibrium of the droplet can be viewed as the rapid wetting and dewetting of the polymer surface by each individual molecule of water. If a force is applied to the drop, by tilting the plate, then the water can detach from the surface quite easily, while making new contact on the other side as the droplet rolls down the surface. Molecular adhesion at equilibrium is clearly only small in this instance, because the water does not strongly adhere to the polymo . 

A third experiment which shows unequivocally that molecules leap into adhesive contact was performed by Johnson, Kendall and Roberts in 1970. This experiment was similar to Newton s original test on glass telescope lenses (Fig. 3.1) but used rubber surfaces because they adhere much more reliably than glass. Roberts had developed a way of moulding rubber in concave glass lenses to produce remarkably smooth elastomeric spherical surfaces as shown in Fig. 3.12. The rubber composition was mixed and then pressed hot into the glass lens. After 

As the two smooth spherical surfaces approached each other, within a few micrometers of contact, the familiar Newton s ring pattern could be seen in the narrow gap between the smooth surfaces. Then, as the rubber lenses were moved still nearer, a sudden jump of the rubber was observed and the black contact spot grew rapidly to a large size as the rubber deformed and spread under the influence of molecular adhesion (Fig. 3.12(c)). The appearance of this was very similar to the liquid drop spreading over a glassy polymer surface. To get the rubber lenses apart, a tensile force had to be applied to overcome this molecular adhesion. It is this cracking apart of adhering surfaces which we consider next. 

---

In the non-contact mode (Fig. 6b), AFM acquires the topographic images from measurements of attractive forces in close proximity of the surface, as the tip does not touch the sample and the cantilever oscillates close to the sample surface [12]. This mode is difficult to work with in ambient conditions due to the interference of the capillary forces. Very stiff cantilevers are needed so that the attraction does not overcome the spring constant of the cantilever. However, the lack of contact with the sample means that this mode should be the least invasive and hence cause the least disruption. The disadvantage of this method is that the tip may jump into contact with the surface due to attractive forces. 

Fig. 7 Force normalized by radius as a function of surface separation. The forces were measured between muscovite mica surfaces across a 0.1 mM KBr solution (unfilled circles) and in the same solution after addition of 20 ppm chitosan-E045 oligomers. The arrow represents the jump into contact to D = 0 due to the action of the attractive van der Waals force...

It is thus seen that a variety of interfacial forces can be investigated by using AFM. hi the noncontact mode, we mainly estimate the van der Waals forces (image resolution -10 ran). On the other hand, the ionic repnlsion forces are measured in the contact mode. These studies thus would provide an understanding of the interaction forces (attractive and repulsive) in much greater detail. It is seen that by AFM we can determine the magnitnde of distance of separation at which two bodies jump into contact. The exponential and power laws have been found to fit the experimental force curves. 

This spreading and equilibrium is an example of the third law of adhesion. The molecules jump into contact with a particular energy, but that energy has to be absorbed into the system by a mechanism which can be quantified. This mechanism is hugely important because it affects the forces of adhesion enormously. For example, if the rubber is not perfectly elastic when flattened, then the contact spot size is different. The same molecules can be in contact, yet a slight change in the mechanism can raise or lower maaoscopic adhesion by orders of magnitude, as we will see later in Chapters 7 and 8. 

After examining the way in which a particle sticks to a surface, as above, it becomes apparent that adhesion is not a single process, but one which we can separate into three different but related actions jumping into contact, achieving a certain black spot size, then cracking apart as a tensile force is applied. 

These three adhesion measures of molecular adhesion, in the contact make-and-break process, also allow us to test the second law of adhesion, that contaminant molecules always decrease the attraction between bodies. Consider immersing the rubber spheres in water and repeating the adhesion experiment. The results show that all three indicators of molecular adhesion (the jumping into contact, the size of the contact spot, and the pull-off force) are diminished by the presence of the water molecules. Adhesives reduce adhesion ... 

As the sample is pushed nearer to the probe, the attractive force increases rapidly and the probe then jumps into contact with the sample. This appears as a sudden deflection of the cantilever. Then, once the probe is stuck to the sample surface, the movement of sample and cantilever are equal, as shown by the linear contact deflection in Fig. 3.18. 

The next improvement came from the stria control of the separation of the strrfaces, to show the jumping into contact in the most dramatic way, as demonstrated by Tabor and Winterton. 

In practice, this idealized experiment is impossible because, when the surfaces get close together, the adhesion foree increases very rapidly and an instability occurs such that the surfaces jump into contact. Essentially, the adhesion force is so strong that it overcomes the elastic resistance of the materials. So the situation shown in Fig. 7.8(b) cannot exist instead, the system goes to the position shown in Fig. 7.8(c), however much we try to control the positions of the surfaces. This is the crack geometry. Molecular contact is made over part of the surface, and there is no contact over the rest of the bodies. 

Pashley and Israelachvili measured the force as the gap was varied. Fig. 10.7(b), and compared the results with the sum of the van der Waals attractive force and the repulsion given by Equation (10.3). They obtained good agreement, but when the gap was reduced to about 5 nm, the surfaces jumped into contact, because the van der Waals force exceeded the electrical double layer repulsion. 

Fig. 2. Idealized representation of a force-distance profile resulting from an SFA measurement. Items to be noted include the fact that distances are measured in nanometers or less, force is measured in mN. A jump into contact is shown. In some cases, more detailed information about molecular level forces can be obtained by examining data near to the jump into contact...

All surfaces will jump into contact at a certain distance when the derivative of the potential energy curve for the materials exceeds the spring constant of the cantilever beam. The JKR theory predicts the shape of the bodies in contact just outside the contact zone and it also predicts that the surfaces will jump out of contact at a specific non-zero contact area. One result obtainable from the SFA combined with the JKR theory comes from the force measured at the jump out of contact ... 

The AFM force-distance plots are similar, but not identical to the force-distance profiles obtained by the surface force apparatus. The bending of the cantilever can follow the force exerted on it by the sample as long as the spring constant of the cantilever exceeds the force gradient of the exerted force. Otherwise, the cantilever jumps into contact with the surface in an attractive regime and in the repulsive regime, is merely pushed a distance equal to the distance the sample is mov, thus... 

Figure 2a. A schematic of a typical AFM force-distance plot using unmodified tips. The arrows show how the force plot is generated as the sample is advanced and retracted. At (a) the tip and sample are far apart. When the tip gets close enough to experience the attractive van der Waals force, the cantilever starts to bend (b). When the force gradient exceeds the cantilever spring constant, the tip jumps into contact with the surface (c). Once in contact, the tip and sample move the same amount as shown by the linear portion (d). Upon the retraction (e), the cantilever relaxes a distance equal to the amount the sample has been retracted. If there is an adhesive force, then there is hysteresis in the loop (f). The inset depict the state of cantilever bending. Positions 1 and 2 show how to calibrate the force scale. The distance the cantilever has moved, ax, multiplied by the spring constant, k, yields the force difference, aF, between the two positions.

Figure 3. Plot A depicts force-distance curves recorded using a plasma cleaned tip and freshly cleaved mica in the presence of 0.1 N KNO3. These curves display a jump into contact upon approach and an adhesive force upon retraction of the tip. Plot B shows force-distance curves recorded using a PEG 2 IdDa treated tip and freshly cleaved mica in the presence of 0.1 N KNO3. This plot does not show a monotonically increasing repulsive force upon approach, but does show an adhesive force upon retraction and is similar to the force-distance plot of a tip plasma cleaned only. Plot C shows force-distance curves recorded using a PEG 9(X) kDa treated tip and freshly cleaved mica in the presence of 0.1 N KNO3. This plot shows no jump into contact upon approach and an adhesive force upon retraction. The cantilever spring constant was 0.064 nN/nm and the frequency of oscillation was 25 Hz in all cases.

Fig. 4. Measurement of force-distance curves (schematic representation). The sample is approaching the tip (1) at some distance the gradient of the force overcomes the cantilever spring constant and the tip jumps into contact (2) further movement up causes a deflection of the cantilever (3), during the retraction the tip sticks usually much longer (4) and snaps off, when the spring constant overcomes the force gradient (5). Adapted with permission from Ref 21.

Fig. 5 Force normalized by radius between mercaptohexadecane-modifled gold surfaces (a flat and a colloidal probe) as a function of separation. The armw indicates a jump into contact due to the action of attractive surface forces...

---