Stick value

It is evident that boundary lubrication is considerably dependent on the state of the monolayer. Frewing [48] found that, on heating, the value of fi rose sharply near the melting point sometimes accompanied by a change from smooth to stick-slip sliding. Very likely these points of change correspond to the transition between an expanded film and a condensed film in analogy with... 

Here, if Z is expressed in moles of collisions per square centimeter per second, r is in moles per square centimeter. We assume the condensation coefficient to be unity, that is, that all molecules that hit the surface stick to it. At very low Q values, F as given by Eq. XVII-3 is of the order expected just on the basis that the gas phase continues uniformly up to the surface so that the net surface concentration (e.g., F2 in Eq. XI-24) is essentially zero. This is the situation... 

A physical value for 7 for each particle can be chosen according to Stokes law (with stick boundary conditions) ... 

Imposition of no-slip velocity conditions at solid walls is based on the assumption that the shear stress at these surfaces always remains below a critical value to allow a complete welting of the wall by the fluid. This iraplie.s that the fluid is constantly sticking to the wall and is moving with a velocity exactly equal to the wall velocity. It is well known that in polymer flow processes the shear stress at the domain walls frequently surpasses the critical threshold and fluid slippage at the solid surfaces occurs. Wall-slip phenomenon is described by Navier s slip condition, which is a relationship between the tangential component of the momentum flux at the wall and the local slip velocity (Sillrman and Scriven, 1980). In a two-dimensional domain this relationship is expressed as... 

G is a multiplier which is zero at locations where slip condition does not apply and is a sufficiently large number at the nodes where slip may occur. It is important to note that, when the shear stress at a wall exceeds the threshold of slip and the fluid slides over the solid surface, this may reduce the shearing to below the critical value resulting in a renewed stick. Therefore imposition of wall slip introduces a form of non-linearity into the flow model which should be handled via an iterative loop. The slip coefficient (i.e. /I in the Navier s slip condition given as Equation (3.59) is defined as... 

The filaments of all plant fibers consist of several cells. These cells form crystalline microfibrils (cellulose), which are connected together into a complete layer by amorphous lignin and hemi-cellulose. Multiple layers stick together to form multiple layer composites, filaments. A single cell is subdivided into several concentric layers, one primary and three secondary layers. Figure 5 shows a jute cell. The cell walls differ in their composition and in the orientation of the cellulose microfibrils whereby the characteristic values change from one natural fiber to another. 

The use of hydrostatic slideways - in which pressurized oil or air is employed - completely eliminates stick-slip and reduces friction to very low values but there are disadvantages in the form of higher costs and greater complication. 

The solidihed layer yields and returns to the liquid phase if the shear stress excesses the critical value, which initiates the sliding. When the stress is relaxed as a result of slip, the solid phase resumes again. The periodic transition between the solid and liquid states has been interpreted in the literature as a major cause of the stick-slip motion in lubricated sliding. Understanding the stick-slip and static friction in terms of solid-liquid transitions in thin films makes a re-... 

The example demonstrates that the instability and consequent energy dissipation, similar to those in the Tomlinson model, do exist in a real molecule system. Keep in mind, however, that it is observed only in a commensurate system in which the lattice constants of two monolayers are in a ratio of rational value. For incommensurate sliding, the situation is totally different. Results shown in Fig. 21(b) were obtained under the same conditions as those in Fig. 21 (a), but from an incommensurate system. The lateral force and tilt angle in Fig. 21(b) fluctuate randomly and no stick-slip motion is observed. In addition, the average lateral force is found much smaller, about one-fifth of the commensurate one. 

At the beginning of sliding, the system is accelerated because the driven force must excess the resistance from lubricating film. For this reason, the system actually jumps from A to the point B, instead of B, to gain a shear stress lower than the critical value This phenomenon, so called velocity-weakening has been regarded widely in the literatures as the cause for instability and stick-slip motion in lubricated systems. 

Contamination is another pitfall that can influence the measurement of a sticking coefficient, particularly when its value is small. For example, if the sticking coefficient of the gas of interest is of the order of 10 , an impurity gas present at the ppm... 

List some adsorption systems with extremely low and very high sticking coefficients. Can you rationalize these values with transition state theory (Chapter 3) ... 

Determine the absolute values of the sticking coefficients at the various temperatures. 

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