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The flow of plastic disperse systems in the presence of the wall effect

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Summary

The results of studies of the wall effect in the flow of lubricating greases of different types are presented. It has been shown that the wall effect substantially influences the flow curves obtained for smooth capillaries, especially at low shear rates; the magnitude of this effect is estimated as a function of the capillary radius and shear rate at the capillary wall. It has been proved experimentally that the use of grooved surfaces eliminates the wall effect completely.

The properties of a wall layer were assessed on the basis of the concept of the activation nature of the viscous flow of disperse systems. The following parameters of the wall layer have been determined from the variations of the free activation energy in the wall layer and in the bulk of the grease: thickness, disperse phase concentration and viscosity characteristics. The mechanism of flow generation and development in the wall layer is explained on the basis of the data obtained.

Good agreement of theory with experiment is shown.

Zusammenfassung

Es werden die Ergebnisse von Untersuchungen des Wandeffekts beim Fließen von Schmierfetten unterschiedlichen Ursprungs mitgeteilt. Dabei wird gezeigt, daß der Wandeffekt die Fließkurven in glatten Kapillaren wesentlich beeinflußt, besonders bei niedrigen Schergeschwindigkeiten. Die Größe dieses Effekts wird in Abhängigkeit vom Kapillarradius und der Schubspannung an der Kapillarwand ermittelt. Experimentell wird nachgewiesen, daß bei Verwendung gerillter Oberflächen der Wandeffekt vollkommen verschwindet.

Die Eigenschaften der Wandschicht werden unter Zugrundelegung der Vorstellungen von der Aktivierungsnatur der Prozesse des viskosen Fließens der Dispersionssysteme beurteilt. Aus der Veränderung der freien Aktivierungsenergie in der Wandschicht und im Volumen werden die Parameter der Wandschicht ermittelt: Dicke, Konzentration der Dispersionsphase und Viskositätskennlinie. Aufgrund dieser Kennwerte wird der Mechanismus der Bildung und der Entwicklung der Strömung in der Wandschicht erörtert.

Es wird gezeigt, daß eine gute Übereinstimmung zwischen Theorie und Experiment besteht.

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Vinogradov, G.V., Froishteter, G.B., Trilisky, K.K. et al. The flow of plastic disperse systems in the presence of the wall effect. Rheol Acta 14, 765–775 (1975). https://doi.org/10.1007/BF01521405

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