Origin of apparent viscosity in yield stress fluids below yielding

Abstract

Origin of apparent viscosity in yield stress fluids below yieldingMøller, P.C.F.; Fall, A.; Bonn, D. General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. Abstract -For more than 20 years it has been debated if yield stress fluids are solid below the yield stress or actually flow; whether true yield stress fluids exist or not. Advocates of the true yield stress picture have demonstrated that the effective viscosity increases very rapidly as the stress is decreased towards the yield stress. Opponents have shown that this viscosity increase levels off, and that the material behaves as a Newtonian fluid of very high viscosity below the yield stress.In this paper, we demonstrate experimentally (on four different materials, using three different rheometers, five different geometries, and two different measurement methods) that the low-stress Newtonian viscosity is an artifact that arises in non-steady-state experiments. For measurements as long as 10 4 seconds we find that the value of the "Newtonian viscosity" increases indefinitely. This proves that the yield stress exists and marks a sharp transition between flowing states and states where the steady-state viscosity is infinite -a solid!

Issues in the flow of yield-stress liquids

“…Møller et al (2009a) recently repeated these experiments and reached a different conclusion; their data on a 0.2% Carbopol, which is one of the materials cited by Barnes, are shown in Fig. 4.…”

“…(Note that this is not true of all Carbopols; see Divoux et al (2010) for an example of complex time-dependent behavior during startup of shear in a Carbopol system.) One conclusion appears to be that (Møller et al 2009a) the yield stress is a material property that separates the mobile liquid from the solid at equilibrium. Several questions also immediately arise: First, what is the meaning of an infinite viscosity?…”

“Everything flows?”: elastic effects on startup flows of yield-stress fluids

“…Møller et al (2009a) subsequently showed that these high viscosities do not correspond to a steady state, and thus should be discarded. For the system studied here, the apparent viscosity is shown in Fig.…”

An attempt to categorize yield stress fluid behaviour

“…(ii) Simple yield stress fluids: emulsions and foams (Bertola et al 2003;Moller et al 2009), hair gel (Moller et al 2009), carbopol (Moller et al 2009). …”