Force, zero creep point

This condition is the zero creep point, when the surface forces represented by the terms on the right hand side of equation (4) are exactly balanced by the force due to the load imparted to the sample (Fq). Physically, there would be neither contraction nor elongation when this situation occurs. For this reason, the force at or approaching the zero creep point is difficult to measure directly. Therefore, the force at the zero creep point is determined by measuring the strains of several loaded samples and plotting these strains against the stress which caused them. The force at the zero creep point is determined by interpolation. 

Even with an adequate description of molecular velocities near the particle surface, it is not possible to completely establish all variables influencing thermal force. This is because there also exists a so-called thermal slip flow or creep flow at the particle surface. Reynolds (see Niven, 1965) and others have pointed out that as a consequence of kinetic theory, a gas must slide along the surface of a solid from the colder to the hotter portions. However, if there is a flow of gas at the surface of the particle up the temperature gradient, then the force causing this flow must be countered by an opposite force acting on the particle, so that the particle itself moves in an opposite direction down the temperature gradient. This is indeed the case, known as thermal creep. Since the velocity appears to go from zero to some finite value right at the particle surface, this phenomenon is often described as a velocity jump. A temperature jump also exists at the particle surface. 

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