Sticking phenomenon

The frictional behaviour of polymers differs somewhat from that of perhaps more familiar materials. The frictional force tends to be proportional not to load (as in the classical case) but to speed. The coefficient of friction is very dependent on the nature of the two surfaces in contact, but is generally low, when suitable pairs are selected. This means that plastics gears can usually be run without external lubricants. Often the static friction coefficient is lower than the dynamic, which helps to explain the absence of slip-stick phenomenon exhibited by some plastics systems in motion this is especially marked with PTFE, which has an exceptionally low coefficient (around 0.02). The non-classical response of plastics materials results from their much lower modulus. Their frictional response is characterized by adhesion and deformation. 

Harrison et al. also indicated that a uniform lubricating layer at the die wall interface must occur to eliminate the slip-stick phenomenon responsible for forced flow. Development of a lubricating layer was dependent on the length of the die (a minimum length required), wall shear stress and upstream pressure loss. They represent the frictional forces at the die wall interface and the estimated pressure loss at zero die length in the barrel of the ram extruder. The method for deriving these values is described in Ref. 27. These parameters allow for a quantitative comparison between formulations and process however, no specific values can be targeted since they vary with materials. 

To get the quantity discount, DO may order 10,000 bottles over the last week even though it expects to sell only 3,000. hi this case, cycle inventory in the supply chain goes up in spite of the fact that there is no lot-size-based quantity discount. The situation in which orders peak toward the end of a financial horizon is referred to as the hockey stick phenomenon because demand increases dramatically toward the end of a period, similar to the way a hockey stick bends upward toward its end This phenomenon has been observed in many industries. One possible solution is to base the volume discounts on a rolhng horizon. For example, each week the manufacturer may offer DO the volume discount based on sales over the past 12 weeks. Such a rolling horizon dampens the hockey stick phenomenon by making each week the last week in some 12-week horizon. 

Information distortion is also observed in threshold incentives offered by companies to their sales staff. Under these incentives, staff is offered rewards for crossing sales thresholds during a specified period of time (e.g., a quarter). The problem observed is that sales effort and orders peak during the last few weeks of the quarter, as salespeople try to cross the threshold. This pattern, in which sales peak close to the end of the evaluation period, is referred to as the hockey stick phenomenon. This information distortion arises because the incentive is offered over a fixed time period, making the last few weeks of each quarter a period of intense activity for all sales staff. 

On the other hand, amount of CaF also affects the wear rate of the sintering nickel material. Generally, density of the sintering nickel matrix material decrease with increase of CaFj, and when CaFj is beyond 20%, wear rate of the sintering material dramatically increases. Fig.3 is the test results of wear rate vs. amount of CaF at room temperature and 600 C. It is obvious the higher wear rate occurs at 20% CaF In the experiment, it is also observed that serious stick occurs at 4% CaF. The stick phenomenon gradually disappeared and surface film presents on the worn surface after CaF beyond 8%. 

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... 

In addition to elastic turbulence (characterised by helical deformation) another phenomenon known as sharkskin may be observed. This consists of a number of ridges transverse to the extrusion direction which are often just barely discernible to the naked eye. These often appear at lower shear rates than the critical shear rate for elastic turbulence and seem more related to the linear extrudate output rate, suggesting that the phenomenon may be due to some form of slip-stick at the die exit. It appears to be temperature dependent (in a complex manner) and is worse with polymers of narrow molecular weight distribution. 

Adhesion The phenomenon of particles sticking to the fiber surfaces in a filter. 

Stick-slip motion is another issue that has been explored using SFA. It is found that the occurrence of stick-slip depends on the sliding velocity and the stiffness of the system, and the mechanism of the phenomenon can be interpreted in terms of periodic transition between liquid and solid states of the conhned lubricant [40],... 

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. 

The term bioadhesion ean be defined as the ability of a material (synthetic or natural) to stick (adhere) to a biological tissue for extended periods of time [27]. The phenomenon of bioadhesion can be visualized as a two-step process. The first step involves the initial contaet between polymer and the biological tissue. The second step is the formation of seeondary bonds due to noncovalent interactions. The strength of bioadhesion (expressed as the foree of detachment) for a novel oligosaccharide gum Hakea Gibbosa) contained in a buccal tablet developed by Alur et al. [28,29]... 

Yeast flocculation mechanism can be described as a phenomenon of adhesion to certain surfaces. The ability to adhere to surfaces and to form biofilm is the basis of the pathogenicity of Candida species. Pathogens adhere to mucous membranes and wounds, they stick to medical instruments and prosthesis, and thus contaminate surfaces in food processing facilities. The high mortality rate in disseminated fungal infections caused an increase in the amount of research on the molecular basis of the adhesive phenomena in Candida. This research discovered a considerable overlap in the molecular regulation of all forms of adhesive behavior. ... 

Thermoplasticity of lactose. Unless certain precautions are taken during the drying of whey or other solutions containing high concentrations of lactose, the hot, semi-dry powder may adhere to the metal surfaces of the dryer, forming deposits. This phenomenon is referred to as thermoplasticity. The principal factors influencing the temperature at which thermoplasticity occurs ( sticking temperature ) are the concentrations of lactic acid, amorphous lactose and moisture in the whey powder. 

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