Friction and adhesion

Detachment of Particles. The force acting on an adherent particle in the direction perpendicular to the dust-covered surface determines the magnitude of adhesive interaction. If this force is directed tangential to the surface, we are measuring the static friction in the detachment of particles. Under real conditions, the detaching force may be directed at an angle to the dust-covered surface [30-32]. 

The conditions under which detachment of particles is possible are expressed by the following inequality  

If the detaching force is directed perpendicular to the surface, 3 = 1 and Fdet ad the force is directed tangential to the surface, /3 = 0 and F et Fff. The friction of adherent particles in the absence of any external load is caused by adhesion Fg. = /xFgd (where [x is the coefficient of friction). Then, with p = 0,we find that F et Fad - 

The relationship between adhesion numbers determined by tangential and normal detaching forces is expressed by empirical formulas obtained through treatment of experimental data [14]  

Equations (1.44) and (1.45) were obtained from data with 180° a 0° for the adhesion of spherical glass particles to steel surfaces with these equations, we can use adhesion numbers for a normal or tangential detaching force to determine the adhesion numbers (ja/n and ya/t) the case in which the detaching force is directed at an angle a(pc 0 and a 90°) to the dust-covered surface [14]. 

The conditions under which the detachment of the particle is possible are expressed in the following manner  

For a force of detachment directed perpendicularly to the surface p = 1 and F j F d if the force is directed tangentially. 

Equations (1.18) and (1.19) are obtained with angles of 180° O 0° for the adhesion of spherical glass particles to steel surfaces [13] by using these equations and the adhesion numbers for the normal or tangential force of detachment, we may determine the adhesion numbers Ta/N Ta/T case in which the de- 

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Luengo G ef al 1997 Measurement of the adhesion and friction of smooth C-60 surfaces Chem. Mater. 9 1166-71... 

Dutrowski [5] in 1969, and Johnson and coworkers [6] in 1971, independently, observed that relatively small particles, when in contact with each other or with a flat surface, deform, and these deformations are larger than those predicted by the Hertz theory. Johnson and coworkers [6] recognized that the excess deformation was due to the interfacial attractive forces, and modified the original Hertz theory to account for these interfacial forces. This led to the development of a new theory of contact mechanics, widely referred to as the JKR theory. Over the past two decades or so, the contact mechanics principles and the JKR theory have been employed extensively to study the adhesion and friction behavior of a variety of materials. 

Carpick, R.W., The study of contact, adhesion, and friction at the atomic scale by atomic force microscopy. University of Califomia-Berkeley, Berkeley, CA, 1997. 

L ger, L., Raphael, E. and Hervet, H., Surface-anchored polymer chains their role in adhesion and friction. Adv. Polym. Sci., 138, 185-225 (1999). 

The dynamical adhesion process described in this section is also referred to as adhesion hysteresis. We will come back to this subject in later sections when discussing the correlation between adhesion and friction. 

Adhesion and friction result from the interactions between the surfaces in contact or relative motion. It is therefore ex-... 

L4ger, L, Raphael, E, Hervet, H. Surface-Anchored Polymer Chains Their Role in Adhesion and Friction. VoL 138 pp. 185-226. 

Several types of interaction can be probed with AFM (i) Van der Waals forces and ionic repulsion, (ii) magnetic and electrostatic forces, (iii) adhesion and frictional forces and (iv) the elastic and plastic properties of the surface. In terms of the interactions relevant to electrochemistry, only those interactions typified in (i) will be considered. 

Effects on the Adhesion and Friction between Alkanethiol Monolayers Molecular Dynamics Simulations. 

The concentration of a small molecule reactant inside the polymer coils can be lower than outside when one uses a poor solvent for the polymer. This results in lower local and overall reaction rates. In the extreme, a poor solvent results in reaction occurring only on the surfaces of a polymer. Surface reactions are advantageous for applications requiring modification of surface properties without affecting the bulk physical properties of a polymer, such as modification of surface dyeability, biocompatibility, adhesive and frictional behavior, and coatability [Ward and McCarthy, 1989]. 

Fluoropolymers. These form one of our oldest and most spectacular families of engineering plastics. Polytetrafluoroethylene was developed by DuPont over two decades ago, and more recently by Allied Chemical, Hoechst, ICI, Pennwalt, and other manufacturers as well. It combines unusually low adhesion and friction, high temperature and flame resistance, excellent electrical properties, and extreme chemical inertness. Its high melting point and melt viscosity make thermoplastic processing extremely difficult, so that many... 

The adhesion and friction characteristics of the plastic material determine the intensity of the flow. In the case of Newtonian fluids this is half of the theoretical conveying (at constant pressure) and even less with counterpressure (extrusion), even down to zero. In the latter case, the product rotates with the shaft and throughput ceases. 

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