Extensional rheometry through hyperbolic contraction
Doktorsavhandling, 2015

To understand fluid rheology, properties such as consistency and viscosity are determined in both shear and extensional flow. Many commonly-used materials and formulations exhibit complex rheological properties and in order to determine the behaviour as well as predict and govern their properties, effective and reliable characterisation of these fluids is important. Rheological properties are used as quality control parameters, since they have an impact at all stages of material processing, across multiple disciplines, from formulation development, stability during processing to final product performance and stability. Furthermore, rheological properties can vary depending upon external conditions, and can be correlated with the product microstructure. Various techniques are available for characterising viscoelastic liquids in shear, but extensional rheological properties are experimentally more difficult to determine and thus not many techniques are available commercially. In this work, a measuring technique for extensional rheology based on hyperbolic contraction flow is developed and evaluated through numerical and experimental studies. The evaluation was performed using numerical simulations through a hybrid finite element/finite volume scheme to evaluate the behaviour of the flow through a hyperbolic contraction nozzle. Several configurations, differing in contraction angle, were considered, and contraction-expansion configurations were also evaluated. The hyperbolical shape was found to give an almost constant strain rate throughout the measuring regime in contrast to the other configurations, and the influence of shear from the wall was minimised. The use of a corrected pressure drop, called the excess pressure drop, as a measure of extensional properties has been evaluated and proved to strongly depend on extensional viscosity. By changing the configuration to a contraction-expansion configuration, the shear contribution to the excess pressure drop could be neglected. This was also verified experimentally. The numerical results were compared with experimental measurements of the pressure drop using a set of model test fluids with various rheological properties, a Newtonian, a Boger and a shear-thinning fluid. By matching the shear viscosity of the model fluids, it was possible to address the influence of elasticity and shear thinning. Both numerical and experimental results demonstrated increasing excess pressure drops over the Newtonian reference line with increasing deformation rates for both the hyperbolic contraction configuration and the contraction-expansion configuration. This opposed to previously reported results on the sharp 4:1:4 configurations, which show pressure drops under the Newtonian reference line. Finally, the hyperbolic contraction device was utilised to develop edible model fluids differing in rheological properties with the aim of determining the rheological influence on swallowing. Fluid elasticity was overall found to be beneficial for safe and easy swallowing.

extensional rheology

dysphagia

Boger fluid

axisymmetric contraction–expansion

hyperbolic contraction flow

numerical simulation

Viscoelastic fluid

pressure drop

VDL-salen, Rännvägen 2A, CHALMERS
Opponent: Prof. Bettina Wolf, Associate Professor in Biomaterials Science, Faculty of Science, Nottingham, UK

Författare

Magda Nyström

Chalmers, Material- och tillverkningsteknik, Polymera material och kompositer

SuMo Biomaterials

Simulation of extensional flow through contractions towards a measuring system for extensional viscosity

Annual Transactions the Nordic Rheology Society,; (2011)p. 195-199

Paper i proceeding

Det finns en mängd olika situationer omkring oss där vätskor deformeras, t.ex. när vi rör om i en färg, sväljer mat, eller vid tillverkning av produkter som pumpas genom olika rör. Så kallade icke Newtonska vätskor har en viskositet (flyttröghet) som ändras och påverkas av bland annat flödesgeometri, temperatur och deformationshastighet och det är därför viktigt att kunna bestämma deras viskositeter för att förutse deras beteende. En vätskas så kallade töjviskositet kan påverka t.ex. beläggningsprocessen, den sensoriska upplevelsen av mat eller bakning (ju töjbarare deg desto större luftbubblor). Det finns många vedertagna mätmetoder för att bestämma skjuvviskositeten men att bestämma töjviskositeten är svårare. I det här doktorandarbetet har en hyperbolisk kontraktion undersökts som mätmetod för att bestämma olika vätskors töjviskositet. För att få full förståelse för vad som händer inne i kontraktionen och hur det påverkar mätresultatet har flödet simulerats. Den här mätmetoden användes sedan för att undersöka om man kan utnyttja en vätskas töjegenskaper för att underlätta sväljning hos patienter som lider av dysfagi (sväljsvårigheter). De huvudsakliga resultaten från denna avhandling visar att de antagna matematiska modellerna och den numeriska algoritmen återspeglar samma resultat som experimenten uppvisar. Dessutom hittades vissa indikationer på att vätskor med hög töjviskositet har en positiv påverkan på det orala sväljningsstadiet hos patienter med dysfagi.

Ämneskategorier

Livsmedelsteknik

Annan materialteknik

Styrkeområden

Materialvetenskap

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 3926

VDL-salen, Rännvägen 2A, CHALMERS

Opponent: Prof. Bettina Wolf, Associate Professor in Biomaterials Science, Faculty of Science, Nottingham, UK