Rheology of yield stress fluids: a multiscale approach

Project Information

RheoYield

Grant agreement ID: 885146

DOI

10.3030/885146

EC signature date 23 April 2020

Start date 1 October 2020

End date 30 September 2027

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 2 374 753,75

EU contribution

€ 2 374 753,75

Coordinated by

UNIVERSITY OF DURHAM
United Kingdom

Periodic Reporting for period 3 - RheoYield (Rheology of yield stress fluids: a multiscale approach)

Reporting period: 2023-10-01 to 2025-03-31

Summary of the context and overall objectives of the project

Complex fluids include colloids, emulsions, polymers, liquid crystals, surfactants, and their biological counterparts. Internally they contain microscopic substructures: colloidal particles, emulsion droplets, chainlike polymer molecules, etc. Due to their large size and sluggish thermal jostling dynamics, compared to simple molecules, these substructures are readily reorganized by an imposed flow, often taking the system far from equilibrium. This reorganisation in turn feeds back on the flow, leading to strongly nonlinear flow response at the macroscopic scale. These nonlinear flows are widespread in industrial applications such as pharmaceuticals, display technologies, paints, coatings, lubricants, etc., whether during processing or directly in use. Understanding this remarkable flow behaviour is thus of great practical importance.

Conventionally materials are categorised into solids, which retain their own shape, and liquids, which assume the shape of their container. Many complex fluids defy this categorisation, instead behaving as so called yield stress fluids. Examples include dense colloids, emulsions, foams, microgels, pastes and slurries. At imposed loads below a critical yield stress, such materials show solid-like response, often because their constituent substructures are too densely packed to rearrange. At larger stresses, they yield and flow like a liquid. This leads to important applications in foods, pharmaceuticals, adhesives, construction, fire-fighting, etc.

For any complex fluid, a key challenge is to understand how its macroscopic deformation and flow properties emerge out of the underlying collective dynamics of the microscopic substructures. For yield stress fluids this is a particularly difficult challenge, because the elasticity of a jammed particle packing makes long-ranged spatial cooperativity between the constituent microstructures important.

RheoYield's overall objectives are:

* To understand how the macroscopic rheological (deformation and flow) properties of yield stress fluids emerge out of the collective microscopic dynamics of their constituent substructures.

* To pioneer new, microscopically aware computational studies for the prediction of the rheology of yield stress fluids.

* To develop basic new science underpinning strategies to enable us to control the rheology of yield stress materials.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

A major focus has been the basic process via which an amorphous material yields from an initially solid-like state to a finally fluid-like or catastrophically fractured one. In this arena, we have demonstrated that the yielding of an athermal amorphous material subject to slow shearing should always be brittle for large enough sample sizes; predicted a novel phenomenon of ultra-delayed material failure after the imposition of a strain; elucidated the processes involved in the slow fatigue of amorphous materials in cyclic shear, leading to eventual catastrophic material failure; studied the slow creep and final failure of disordered soft materials under an imposed shear load, with both thermal and athermal dynamics; developed a novel understanding of the way in which so-called auxetic materials fail; cross fertilised concepts of elastic instability to study the long-standing problem of the highly delayed gravitational collapse of soft gels; established a theoretical framework to understand the significantly increased failure toughness of double network materials; developed a new understanding of the constitutive rheology of frictional granular materials in fixed volume vs fixed pressure ensembles; and provided a new understanding of power fluctuations in sheared amorphous materials.

We have also studied long term memory effects in the deformation and flow properties of amorphous materials. In particular, we have modelled experiments showing slow, non-monotonic stress relaxation after the cessation of steady shear. We have furthermore uncovered two different possible physical mechanisms that may explain experimental observations of significant recoverable creep.

More broadly, we have cross-fertilised concepts of yielding in soft materials to provide a possible new understanding of the long-standing problem of friction between co-sliding rough surfaces, developing new models for the way in which plasticity and stress propagation within a material influence its surface mechanics.

We have similarly cross-fertilised concepts of yielding in soft materials to address problems in geophysical fluid flows.

We have furthermore cross-fertilised concepts to help better understand the deformation and flow properties of biological tissues, which are core to important processes such as wound healing, cancer metastasis and embryo development. Specifically, we have predicted a strain hardening transition in the vertex model of biological tissues; developed a continuum model for the deformation properties of biological tissues and shown its predictions to be in excellent agreement with simulations of the cell based vertex model; and predicted a novel discontinuous shear thickening transition in biological tissues.

Finally, we have developed new approaches to understanding mechanical cloaking in complex materials.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

Our work in elucidating the dynamical process of yielding in amorphous materials goes beyond the state of the art in several key respects.

For example, our prediction that a material can fail suddenly and dramatically with an ultra-long delay since it was last deformed is counter-intuitive to normal assumptions. The prediction that yielding must always be brittle in the limit of large enough system sizes in slowly sheared athermal amorphous materials is also novel.

Our theoretical understanding of the increased failure toughness of double network materials and also of auxetic materials explains for the first time two large bodies of experimental data.

Our work on the rheology of biological tissues has predicted two important mechanical phenomena that are novel in this context: strain stiffening and discontinuous shear thickening.

Our work on friction between co-sliding surfaces provides a novel theoretical explanation for a long standing major outstanding puzzle in tribology: that the ratio of tangential to normal forces tends to a non-zero constant in the limit of zero relative sliding velocity.

Between now and the end of the project we expect further significant progress in:

* Elucidating the yielding behaviour of the major subclasses of disordered soft materials.

* Identifying the initial precursors of yielding.

* Understanding the similarities and differences between yielding in different mechanical protocols (imposed strain vs imposed stress, for example).

* Continuing to cross fertilise ideas of yielding to understand other important physics, notably in the arena of friction, granular matter, geophysical fluids and biological tissues.

* Understanding how the long term memory properties of soft materials can be used to develop control strategies to optimise their performance

* Understanding the major implications of a material having a distribution of internal local stresses on its macroscopic rheological behaviour

* Understanding power law viscoelasticity and non-affinity in important classes of soft materials

* Developing constitutive models for the rheology of yielding.

* Understanding mechanical cloaking

Simulation of a yield stress material