Adhesion molecular forces

Consider now an experiment which demonstrates molecular adhesion simply and unequivocally. Take a sample of very fine, pure solid powder, say 4 g of 0.2 pm diameter grains of titanium dioxide. This powder can be poured into a hard steel pelleting die, as shown in Fig. 2.9(a). Coulombic forces can be prevented by the presence of a radioactive source to ionise the air and leak away any stray electrons. 

By testing the pellet in bending or tension, it is easy to find that the compacted pellet has the properties of a porous titania eeramie. It is strong, elastic, and brittle, but not so strong as dense transparent titania which has no pores to weaken it. Typically, the pellet contains 50% by volume of air cavities. 

la) Loose powder loaded into stoel die (b) powder compressed by die (c) pellet alter 

William Hyde Wollaston first described this type of adhesion experiment in 1829. He was interested in making dense and strong wires from platinum and other rare metals, such as palladium and osmium, which he had just diseovered. Platinum is so hard and refractory that it is extremely difficult to work by ordinary melting and casting techniques. Wollaston prepared the platinum in fine particle form by precipitating the metal from an acid solution which had been used to remove impurities. This produced a muddy mixture of water and particles which were cleaned by washing, then dispersed by milling in a wooden mortar and pestle. Wood was used to limit contamination since it would burn out later. 

Heating of the pellet with burning charcoal was sufficient to remove moisture and organic lubricant. Then the pellet was raised to white heat in a Staffordshire coke furnace. This caused the pellet to contract as the particles sinteed together. Pounding the hot pellet with a hammer produced a mataial which was 99% dense and which produced platinum wire of the highest tenacity.  

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When the water film is squeezed out, the thick water layer is removed and the surfaces are separated by lubricant film of only molecular dimensions. Under these conditions, which are referred to as BL conditions, the very thin film of water is bonded to the substrate by very strong molecular adhesion forces and it has obviously lost its bulk fluid properties. The bulk viscosity of the water plays little or no part in the frictional behavior, which is influenced by the nature of the underlying surface. By comparing with the friction force of an elastomer sliding on a rigid surface in a dry state, Moore was able to conclude that for an elastomer sliding on a rigid surface under BL conditions, one can expect ... 

Adhesion The adhesion of two different materials caused by atomic or molecular adhesive forces. 

However, molecular adhesion is very different. This falls off in a very short distance of separation. As a consequence, these molecular adhesion forces cannot be measured with a meter ruler, but need a nanometer scale. The adhesion force may be high when the molecules are touching, but even a separation of one nanometer causes the force to drop almost to nothing. Thus the surfaces snap apart in a brittle fashion, totally different from the other types of adhesion force. The area under the curve is very small, hi other words, the energy of molecular adhesion may be negligible. Taking this energy for one square metre of joint, we define the work of adhesion ITin Joules per square metre. 

This definition raises a number of questions which will be addressed in the following chapters. The obvious question relates to the origins and laws of molecular adhesion. How can one measure and interpret such phenomena Clearly, molecular adhesion forces have the same origins as the forces of cohesion which hold solids and hquids together. These can be understood in terms of the heats of melting or evaporation, the elastic stiffnesses, or the chemical reactivities of materials, as described in Chapter 5. 

Molecular adhesion forces are of such short range that various mechanisms can have large effects. Examples of such mechanisms are surface roughness. Brownian motion, cracking, viscous deformation, etc. These mechanisms lead to a rich variety of adhesion phenomena which may cause macroscopic adhesion to vary, even though the molecular adhesion remains the same. 

We normally observe adhesion at the macroscopic level, say by peeling a film from a surface as in Fig. 3.9. In order to understand the force which is necessary to pull the fihn off, we have to connect this mechanical picture with the molecular adhesion forces which we know to be universal at the nanometer level. 

Therefore, Binnig, Quate and Gerber devised the atomic force microscope (AFM) shown schematically in Fig. 3.17. The probe was now much smaller and lighter so as to detect the molecular adhesion forces. These forces caused a slight movement of the probe which could be observed by several different sensor methods. However, laser deteetion, as shown in Fig. 3.17, turned out to be most convenient. Small silicon cantilever probes were made by etching a silicon wafer. Laser light was reflected from the top of the silicon cantilever probe, and entered a detector where any deflection could be registered. 

At a critical voltage, the small polymer beads jumped across to the other electrode in the cell. This occurred because the electrostatic force applied to each sphere overcame the van der Waals adhesion force. The theory shows that there are three forces acting on the particles in this experiment first, there is the molecular adhesion force due to van der Waals force, inWDjA, where D is the diameter of the small spheres and W is the work of adhesion second, there is the electrostatic charge on the particle which pulls it onto the surface of the plastic film, giving a force Jta O /4e , where a is the charge density on the particles and () is the permittivity of free space and third, there is the applied electric field V which acts to make the particles jump across the gap of thickness Di, These three forces are drawn schematically in Fig. 13.16 and balanced in the equation below... 

From the results as the voltage was gradually raised. Fig. 13.16(b), it was evident that the particles all seemed to jump at the same voltage, within about 20%. Different diameters of particles were then tested. Fig. 13.16(c), and it was clear that the molecular adhesion force dominated for small particles, with the electrostatic force becoming significant above I00(un. The work of adhesion... 

Once the interface crack has initiated inside a composite material, then traveled some distance along the interface, there may be a tendency for the crack to heal, if the crack surfaces come into close proximity, so that the molecular adhesion forces can pull the smooth crack faces together again. The simplest situation in which this interface crack healing arises is shown in Fig. 16.17. This schematic diagram illustrates the observations made on a model composite laminate made from four strips of smooth, transparent rubber. 

There is little doubt that the theory of adhesion will improve rapidly as computer calculations become more rapid and sophisticated. These improvements will take place in three areas as shown schematically in Fig. 17.13 first is the enhanced understanding of molecular adhesion forces at the atomic level second is the modeling of the statistical behavior of Brownian adhesive systems finally, the analysis of adhesion in continuum mechanics terms will increase as specific adhesion computer packages become available. 

The net adhesive force between the particles in a contact formed in a nonwetting liquid, p i, is given by the sum of the molecular adhesive force between the particles, p, acting in the gas phase and the capillary attractive force, Ap=p + ph... 

Strictly speaking, the force is the molecular adhesive force in an atmosphere of saturated vapor. However, because the adsorption of vapors of a nonwetting liquid on solid surfaces is insignificant and does not influence adhesion between particles, one can nearly always assume that the value of the force Pss is ih same as that in air. 

It is thus evident that, within a first approximation. Equation 1.30 is valid for both wetting and nonwetting liquids, with a meniscus either present or absent. It is, however, worth pointing out that for macroscopic particles, this is valid only in the case of molecularly smooth surfaces. In this case, the equations for the molecular adhesive forces and for the capillary contraction force both contain the same macroscopic value of R. The situation is different for rough surfaces. Namely, the value of R in the expression for the molecular forces may be determined by the radii of microheterogeneities between which the contact is formed, while the value of R in the expression for the capillary adhesion force may be determined by the macroscopic radii of the particles. Consequently, particles with a microscopically... 

The point of separation of the mica foil from the block, that is the crack line, could not be seen directly because this kind of interference experiment only detected gaps down to about 50 nm. But the shape of the bent mica foil could be accurately measured and was shown to be cubic. In other words, the strip was behaving as a simple cantilever, just like Galileo s beam, and its shape was not affected by the molecular adhesion forces. This was an important observation because it proved that the molecular forces were only acting across the very small gap near the line of separation. Thus the molecular forces could be neglected in terms of the large-scale behaviour of the system. A one-parameter model of adhesion can be made to work in such circumstances. 

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