Flow in dies with low surface energy

The effect of polymer-wall interactions is an essential parameter that has to be studied in order to understand the conditions under which slip is triggered. To do so. 

In the case of all the polymer-wall pairs considered, analysis of the section of the flow curves corresponding to stable flow shows that flow rate at a given pressure is greater when the fluid flows in fluorinated dies. This can be seen in Fig. 6 for the particular case of the HDPE [11, 25, 36]. This increase, which may reach a factor of 5, demonstrates the combined effect of roughness and the wall material. It may be explained by the occurrence of polymer slip in the fluorinated dies even at low flow rates. 

In the case of regimes corresponding to unstable flow, the flow curves obtained with steel dies and fluorinated dies are virtually the same (Fig. 6). Therefore, they appear to be independent of the cheu-acteristics of the die wall. In fact, these relatively high flow regimes are governed by bulk instability upstream of the contraction [3, 33] volume phenomena appear, therefore, to play a leading part in comparison with interface phenomena. 

These surfaces, characterized by their low energy and referred to as slippery sxirfaces [33], produce a different type of slip from that occurring with a high stress level along the high surface energy and relatively rough walls described in section 

An attempt was made to identify and characterize this type of slip by studying the flow of the PBb in detail. 

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