Teo(dor) Burghelea, Director of Research, CNRS, France

Transport phenomena in yield stress materials: the "Slow" and the "Curious"

A broad class of materials exhibit a dual response when subjected to an external stress. For low applied stresses they behave as solids (loosely speaking they may deform but they do not flow) but, if the stress exceeds a critical threshold generally referred to as the "yield stress”, they behave as fluids (typically non-Newtonian) and a macroscopic flow is observed. This distinct class of materials has been termed as"yield stress materials” and, during the past several decades it attracted a constantly increasing level of interest from both theoreticians and experimentalists. The motivation behind this issue is two-fold. From a practical standpoint, such materials have found a significant number of applications in several industries (which include food, cosmetical, pharmaceutical, oil field engineering, etc.) and they are encountered in daily life in various forms such as food pastes, hair gels and emulsions, cement, mud etc..

We approach this topic by table top and microfluidics experiments, simple physical modelling and, most recently, by numerical simulations using the Gerris flow solver.

Our research on viscoplasticity is reinforced by a number of international collaborations which greatly enrich our perspective by bringing aboard scientific skills missing in our group.

The solid-fluid transition in a yield stress material: from the "yield stress myth" to the "viscoplastic Holy Grail"

People: Eliane Younes, Cathy Castelain, Teo Burghelea

Collaborations: Volfango Bertola (University of Liverpool, UK), Miguel Moyers-Gonzalez (University of Canterbury, NZ), Raazesh Sainudiin (Uppsala University, Sweden)

FIG. 1: Macroscopic rheological tests performed with three chemically distinct yield stress materials: (a) commercial shaving foam (b) mustard (c) hair gel (Carbopol).

Confined flows of a yield stress fluid in the presence of wall slip

People: Eliane Younes, Cathy Castelain, Teo Burghelea

Collaborations: Volfango Bertola (University of Liverpool, UK), Miguel Moyers-Gonzalez (University of Canterbury, NZ),

Related publication: "Slippery flows of a Carbopol gel in a micro-channel", Phys. Rev. Fluids 2020.

FIG. 2: (Top-left): Microstructure of a Carbopol gel visualised by a custom designed flouresxcent staining of the Carbopol back bone with Rhodamine. (Top-Bottom) Experimentally measured scaling of the wall velocity gradients with the wall shear stresses. (Right) Macrosocpic flow curve of the Carbopol highlighting three distinct flow regimes: (S) - solid, (S+F) - solid/fluid coexistence regime, (F) - fluid regime. The full line is a Herschel-Bulley fit.

The microscopic structure of a physical gel such as Carbopol flowing in a confined micro-channel is strongly heterogeneous over space (top-left image). Thus, tackling the wall slip behavior using the classical assumption of a presence of a thin layer of constant width of Newtonian solvent near the channel’s walls is unjustified. To address the wall slip scaling behavior, we propose a novel method of directly relating macroscopic rheological measurements (right plot) to the measured wall velocity gradients. The scaling behavior we obtain via this method differs substantially from the previously published results (bottom-left plot).

Active mixing of yield stress fluids by laminar chaotic advection (project Naymis coordinated by Yves Le Guer)

People: Eliane Younes, Cathy Castelain (thesis Director), Kamal El Omari (thesis Co-director), Teo Burghelea (some sort of advisor I guess), Yves Le Guer (collaborator)

The natural mechanism of mixing two fluid streams is the molecular diffusion. In the case of highly viscous fluids the diffusion coefficients are small and the characteristic mixing times become extremely large which is highly inconevenient for a number of practical settings relevant to some industries including (but not restricted to) cosmetic, food and polymer processing industries.

Fig. 3 Schematic representation of an online miwer with arc-rotating walls. Each of the theree arcs roates oscillatory with the same aplitude but different phase shifts. See Fig. 4 bellow for different rotation protocols.

Fig. 4: Different forcing protocols (rotation protocols of the arcs) and the corresponding flow topology.

Fig. 5: Dependence of the apparent visocosity averaged during 4000 s on the temperature. The error bars are defined by the rms deviation of each apparent viscosity time series.

YieldStressStrongStirring.mov

Yielding to heat: Chaos in a Melting Pot (part of the project Exprodil coordinated by Cathy Castelain)

People: Rawad Himo, Cathy Castelain (thesis Director),, Teo Burghelea (co-Director of the thesis)

We typically think about yielding in terms of stress. Yet, a broad class of materials coined as ``phase change materials" yield to heat rather than stress. Thus, at temperature lower than a critical temperature called the melting temperature Tm they behave as solids whereas beyond this temeprature they behave as fluids. The literature on phase change materials is abundant. Yet, a very basic question has not yet been addressed:

What happens to the hydrodynamic stability of such materials being gradually coooled while beeing sheared at a constant rate?

To address this question we focus on a classical low Reynolds number rheometric flow driven in a cone-plate geometry schematically illsutarted in the figure below.

Fig. 6: Schematic representation of the rheometer setup (not in scale): (C) - cone, (GP) - glass plate, (S) - sample, (WLS) - white light source, (CL) - collimating lens, (M1) - semitransparent mirror, (M2) - plane mirror, (P) - polariser, (MO) - microscope objective, (CCD) - charged-coupled device, (EP) - eye piece, (A) - analizer.

Fig. 7: Dependence of the time averaged apparent viscosity on the temperature mesured for a rate of shear of 10s-1. The insert presents the coeficient of variation characterising the fluctuations of the apparent viscosity.

Fig. 8: Time series of the apparent viscosity measured at T:=57.8 C during 4000 s.

Google Sites

Report abuse