Shark sticks

On the one hand, the fda considers one part per million (ppm) of methylmercury in fish to be the limit. Shark, swordfish, king mackerel, and tilefish can contain more than this. On the other hand, the Environmental Protection Association maintains that 0.25 ppm is the upper hmit. If we adhere to this guideline, then white tuna (at 31 ppm) and light tuna (at 16 ppm) pose a problem. The species that canneries use for their light tuna contains less mercury than the albacore they use for white tuna because the fish are smaller. Current advice is that an adult should eat no more than one or two cans of tima a week. A child should eat no more than one tuna sandwich a week, and before the age of five he or she should stick to flounder, haddock, sardines, crab, or shrimp. 

Drag - When swimming, water sticks to your body and forms a boundary layer that causes surface drag. However, researches found that this did not happen to the Mako shark. The shark was able to move quickly through the water because of a V-pattern in its skin. The bodysuits that were developed with this pattern had less resistance than human skin. 

With the increase of flow velocity, polymer melt extruded in the tube will become unstable due to the stick-slip transition near the tube wall, which makes the extrudate shows wave-like, bamboo-Uke, or spiral-like distortions. All these phenomena are known as melt-broken phenomena. In these cases, the shear rate suddenly rises, as illustrated in Fig. 7.17, thus this behavior is also called capillary-jet phenomenon. The string-Uke shark-skin phenomenon upon the extrusion of polyethylene melt can be attributed to the intermittent stick-slip transition near the tube wall of the exit (Wang 1999). 

The shark skin effect can be avoided by using special additives such as fluoropolymers. These coat the machine and the die surfaces the wall slippage rate is increased, and thus the stick-slip effect is counteracted. 

Fig. 2 shows the flow curve for the neat exact 5361 at loot). The instability problem of the metallocene based polymers with narrow molecular distribution is well known. Fig. 3 shows the photographs of the extrudate samples with varying Dechlorane concentration collected during the capillary rheometers measurement. It is interesting to see that while the severe instabilities such as slip-stick and gross-melt fracture were observed at the shear rate from 177.8 s to 3162.2 s for the neat Exact resin, the severe instability appeared at the shear rate between 177.8 s and 562.2 s but disappeared at the shear rate above 1000.1 s for Exact/10% Dechlorane suspension. The shark-skin like instabilities were observed above the Dechlorane concentration of 20% and the shear rate at which the instability started to appear was decreased as the Dechlorane concentration was increased. Since the viscosity of the Dechlorane-filled systems was higher than that of the neat resin at all rates of shear, the instabilities are expected to develop the melt fracture at even lower shear rates. The shear viscosity vs. shear rate relationships measured with plate-plate rheometers and capillary rheometers are shown in Fig. 4. In this figure it is seen that both sets of data are reasonably matched. It is observed that at low shear rate range the viscosity increment due to the increase in the filler concentration is more pronounced than that at high shear rate. Both plate-plate and capillary measurements were carried out with constant shear rate (CSR) mode. While the capillary rheometer could accurately follow the preset shear rate values the plate-plate rheometer couldn t keep up with the preset shear rate values. Above two observations are due to the yield stress developed at low shear rate. At low shear rate particle-particle interaction dominates the flow phenomena and the yield stress was observed. At high shear rate hydrodynamic effect dominates the flow phenomena.

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