Michel Cloitre - Academia.edu (original) (raw)

Papers by Michel Cloitre

Nature Materials, Nov 1, 2011

Soft particle glasses form a broad family of materials made of deformable particles, as diverse a... more Soft particle glasses form a broad family of materials made of deformable particles, as diverse as microgels 1 , emulsion droplets 2 , star polymers 3 , block copolymer micelles and proteins 4 , which are jammed at volume fractions where they are in contact and interact via soft elastic repulsions. Despite a great variety of particle elasticity, soft glasses have many generic features in common. They behave like weak elastic solids at rest but flow very much like liquids above the yield stress. This unique feature is exploited to process high-performance coatings, solid inks, ceramic pastes, textured food and personal care products. Much of the understanding of these materials at volume fractions relevant in applications is empirical, and a theory connecting macroscopic flow behaviour to microstructure and particle properties remains a formidable challenge. Here we propose a micromechanical three-dimensional model that quantitatively predicts the nonlinear rheology of soft particle glasses. The shear stress and the normal stress differences depend on both the dynamic pair distribution function and the solventmediated EHD interactions among the deformed particles. The predictions, which have no adjustable parameters, are successfully validated with experiments on concentrated emulsions and polyelectrolyte microgel pastes, highlighting the universality of the flow properties of soft glasses. These results provide a framework for designing new soft additives with a desired rheological response. Soft particle glasses share common features with hard sphere glasses such as non-ergodicity and caged dynamics. However, whereas hard sphere colloids experience only forces due to excluded volume, soft particles at high volume fraction are compressed against each other by bulk osmotic forces and form flat facets at contact, with the average deformation depending on particle elasticity and volume fraction. The solvent forming the continuous phase is localized in thin films separating the particles (Supplementary Fig. S1). We model a soft particle glass as a suspension of N non-Brownian, elastic spheres (Fig. 1a), which are dispersed at a volume fraction above the random close-packing of hard spheres (φ > φ c ≈ 0.64), in a solvent with viscosity η S. The particles are slightly polydisperse in size, with an average radius R, which prevents them from crystallizing under flow. We characterize the contact between two particles by the overlap distance h αβ and the relative deformation ε αβ = h αβ /R c , where R c is the contact radius (Fig. 1b). The suspension is subject to an imposed shear flow in the plane (x, y) with shear rateγ , particle and fluid inertia being neglected. As two compressed particles move past one another, a flow of solvent develops inside the liquid films separating the facets. This generates a net positive pressure, causing a further elastic

Journal of Rheology, Jul 1, 2022

We explore the rheology during startup flow of well characterized polyelectrolyte microgel suspen... more We explore the rheology during startup flow of well characterized polyelectrolyte microgel suspensions which form soft glasses above the jamming concentration. We present and discuss results measured using different mechanical histories focusing on the variations of the static yield stress and yield strain. The behavior of the shear stress growth function is affected by long-lived residual stresses and strains that imprint a slowly decaying mechanical memory inside the materials. The startup flow response is not reversible upon flow reversal and the amplitude of the static yield stress increases with the time elapsed after rejuvenation. We propose an experimental protocol that minimizes the directional memory and we analyze the effect of aging. The static yield strain γp and the reduced static yield stress σp/σy, where σy is the dynamic yield stress measured from steady flow measurements, are in good agreement with our previous simulations (J. Rheol. 65, 241, 2021). Our results demonstrate the need to consider memory and aging effects in transient measurements on soft particle glasses.

Soft Matter, 2018

We identify the minimal interparticle interactions necessary for a particle dynamics simulation t... more We identify the minimal interparticle interactions necessary for a particle dynamics simulation to predict the structure and flow behaviour of soft particle glasses (SPGs). Generally, two kinds of forces between the particles must be accounted for in simulations of SPGs: viscous or frictional drag forces and elastic contact forces. Far field drag forces are required to dissipate energy in the simulations and capture the effect of the rheology of the suspending fluid. Elastic forces are found to be dominant compared to near-field drag or other forms of friction forces and are the most important component to compute the rheology. The shear stress, the first and second normal stress differences for different interparticle force laws collapse onto universal master curves of the Herschel-Bulkley form by non-dimensionalizing the stress with the yield stress and the shear rate with the viscosity of the suspending fluid divided by the low-frequency shear modulus. The Herschel-Bulkley exponents are close to 0.5 with a slight dependence on the repulsive pairwise elastic forces.

Soft Matter, 2016

We explore the influence of particle softness on the state diagram of well characterized polylect... more We explore the influence of particle softness on the state diagram of well characterized polylectrolyte microgel suspensions using dynamic light scattering and rheology. Upon increasing the polymer concentration, we cross successively the well defined glass and jamming transitions which delimit four different states: dilute colloidal suspension, entropic glass, jammed glass, and dense glass. Each state has a specific dynamical fingerprint dictated by two key ingredients related to particle softness: elastic contact interactions, and osmotic or steric deswelling. Soft interactions control yielding and flow of the jammed glasses. The shrinkage of the microgels makes the glass transition look smoother than in hard sphere suspensions. We quantify the relation between the polymer concentration and the volume fraction, and show that the glass transition behaviour of soft microgels can be mapped onto that of hard sphere glasses once the volume fraction is used as the control parameter.

HAL (Le Centre pour la Communication Scientifique Directe), Dec 27, 2013

The long time persistence of mechanical stresses is a generic property of glassy materials. Here ... more The long time persistence of mechanical stresses is a generic property of glassy materials. Here we identify the microscopic mechanisms that control internal stresses in highly concentrated suspensions of soft particles brought to rest from steady flow. The persistence of the asymmetric angular distortions which characterize the pair distribution function during flow is at the origin of the internal stresses. Their long time evolution is driven by in-cage rearrangements of the elastic contacts between particles. The trapped macroscopic stress is related to the solvent viscosity, particle elasticity and volume fraction through a universal scaling derived from simulations and experiments.

Journal of Rheology, Mar 1, 2021

The rheology and microstructure of soft particle glasses during startup flow are studied using th... more The rheology and microstructure of soft particle glasses during startup flow are studied using three-dimensional particle dynamics simulations at different particle volume fractions and shear rates. The behavior of transient stress depends on the applied shear rate. At high shear rates, soft particle glasses exhibit a static yield stress signaled by a stress overshoot followed by a relaxation to a steady-state value. The buildup of the stress is driven by an interplay between structural anisotropy due to an accumulation of particles along the compression axis and a depletion along the extension axis and a compression of particles that are soft and deformable. At low shear rates, the stress increase is monotonic and without any stress overshoot. The time scale at which structural anisotropy and the stress are maximum is correlated to the nonaffine dynamics of SPGs through the persistence time of shear-induced particle collisions and to the residence time of particles inside their transient cages. The static yield strain γ p and the reduced static yield stress σ p / σ y, where σ y is the dynamic yield stress deduced from steady flow measurements, follow universal behaviors when correlated with the dimensional shear rate η s γ ˙ / G 0, with η s being the suspending fluid viscosity and G 0 the storage modulus, which expresses the competition between elastic restoring forces and viscous dissipation. Dense suspensions of thermosensitive core–shell colloids, star-like micelles, and poly(ethylene oxide)-protected silica particles follow the same universal curves, suggesting the generality of our results.

Journal of Rheology, Sep 1, 2014

ABSTRACT

Physical Review Letters, May 11, 2004

Concentrated dispersions of soft particles are shown to exhibit a generic slip behavior near smoo... more Concentrated dispersions of soft particles are shown to exhibit a generic slip behavior near smooth surfaces. Slip results from a balance between osmotic forces and noncontact elastohydrodynamic interaction between the squeezed particles and the wall. A model is presented that predicts the slip properties and provides insight into the behavior of the bulk paste.

Journal of Rheology, Mar 1, 2020

The transport properties of soft particle glasses, such as dynamic viscosity, normal stress coeff... more The transport properties of soft particle glasses, such as dynamic viscosity, normal stress coefficients, and shear-induced diffusivity of its particles, are determined by the microstructure of the suspension under flow. A thermodynamic measure of the microstructure is the excess entropy, which we show here accurately correlates the transport properties of soft particle glasses onto master curves across a wide range of volume fractions, suspending fluid viscosities, particle moduli and shear rates. The excess entropy for soft particle glasses is approximated with the twobody excess entropy computed from the pair distribution function extracted from dynamic simulations. The shear viscosity and normal stress functions diverge and the diffusivity vanishes at a critical excess entropy, corresponding to the yield stress of the suspension. An effective temperature is computed and is found to vary linearly with the shear stress and the elastic energy of the sheared soft particle glass. From this an equation of state is derived relating the excess entropy to the shear stress. Consequently, three of the four transport properties are determined from the measurement of just one. Finally, a single master curve of particle diffusivity versus excess entropy is presented that unifies observations for both equilibrium and non-equilibrium suspensions.

Soft Matter, 2012

We use fluorescence microscopy and particle tracking velocimetry to image the motion of concentra... more We use fluorescence microscopy and particle tracking velocimetry to image the motion of concentrated emulsions and microgel suspensions near solid surfaces. The local deformation involves a combination of slip and bulk flow, which are found to be controlled by surface forces. With smooth surfaces, two slip mechanisms are identified depending on whether particle-wall interactions are repulsive or weakly attractive. In the former case, the materials yield uniformly and the local rheology can be mapped on the macroscopic flow curve. In the latter case, yielding is non-uniform which reveals a continuous distribution of states from the immediate vicinity of the smooth surface to the bulk of the material. The effect of the surface is long-ranged and decays exponentially with the distance, which can be described by a non-local fluidity model. Our results establish a link between surface forces, lubrication and yielding in soft glassy or jammed materials and open new routes to manipulate their flow through the surface chemistry of the confining boundaries.

Physical review fluids, Sep 1, 2017

The microstructure and shear rheology of highly concentrated, jammed suspensions of soft particle... more The microstructure and shear rheology of highly concentrated, jammed suspensions of soft particles are shown to depend on polydispersity and shear rate from computational simulations. Rich behavior is observed depending on the degree of polydispersity and the shear rate. Glassy suspensions with a low degree of polydispersity evolve to face-centered cubic (FCC) and hexagonal close-packed (HCP) structures at low and high shear rates, respectively. The structural rearrangement occurs over several units of strain and reduces the shear stress and elastic energy. Suspensions with a higher degree of polydispersity exhibit a microstructural transition from a glass to a layered-like structure for sufficiently high shear rates. In this case the soft particles first rearrange themselves in the flow-vorticity plane during an induction time (or strain) before layers parallel to the flow-vorticity plane are formed. The induction strain decays exponentially with shear rate revealing that the devitrification of monodisperse and polydisperse suspensions is a shear-activated process. Finally a generic dynamical state diagram is found that depends on the polydispersity and the ratio of viscous to elastic forces due to shear.

Journal of Rheology, Mar 1, 2020

Soft particle glasses are amorphous materials made of soft and deformable particles that are jamm... more Soft particle glasses are amorphous materials made of soft and deformable particles that are jammed above close-packing. They behave like weak solids at rest but they yield and flow under external mechanical constraints. Although soft particle glasses are widely used in applications, little is known about how the particle softness and microscopic dynamics determine the macroscopic rheology. Here we use three-dimensional particle dynamic simulations to analyze the dynamical properties of soft particle glasses at different scales. We demonstrate how the dynamics is determined by the persistence time and the magnitude of the fluctuating elastic forces that develop at contact in the flow. The shear-induced diffusion coefficient, the local structural relaxation times, the shear stress, and the normal stress differences are interconnected through simple relationships that allow the prediction of the macroscopic rheology from the microscopic dynamics.

APS, Nov 1, 2003

Concentrated suspensions of soft, elastic particles display several interesting rheological prope... more Concentrated suspensions of soft, elastic particles display several interesting rheological properties such as a yield stress, wall slip, shear-thinning, aging and memory. These effects appear to arise from elasto-hydrodynamic particle-particle and particle-wall interactions. A micromechanical model of elasto-hydrodynamic lift between a sphere and wall will be presented and solved numerically and asymptotically. These results will be used to explain wall-slip

Journal of Rheology, Nov 1, 2004

Microgel pastes and concentrated emulsions are shown to exhibit a generic slip behavior at low st... more Microgel pastes and concentrated emulsions are shown to exhibit a generic slip behavior at low stresses when sheared near smooth surfaces. The magnitude of slip depends on the applied stress. Well above the yield stress, slip is negligible compared to the bulk flow. Just above the yield stress, slip becomes significant and the total deformation results from a combination of bulk flow and slip. At and below the yield stress, the bulk flow is negligible and the apparent motion is entirely due to wall slip. By directly imaging the deformation of pastes and from rheological measurements, we show that slip is characterized by universal scaling properties, which depend on solvent viscosity, bulk shear modulus, and particle size. A model based on elastohydrodynamic lubrication between the squeezed particles and the shearing surface explains these properties quantitatively.

Journal of Rheology, Sep 1, 2008

Concentrated suspensions of soft deformable particles, e.g., polymer microgel pastes and compress... more Concentrated suspensions of soft deformable particles, e.g., polymer microgel pastes and compressed emulsions, display a generic slip behavior [Meeker et al., J. Rheol. 92, 18302 (2004a); Meeker et al., J. Rheol. 48, 1295–1320 (2004b)]. When sheared with smooth surfaces, they exhibit apparent motion due to slip at the wall. Wall-slip stops at a sliding yield stress the value of which is much lower than the bulk yield stress. The physical mechanism of slip at low stresses and the origin of the sliding yield stress have so far been unresolved issues. We propose that the paste-wall interactions control the wall-slip behavior and determine the occurrence of the sliding yield point. We present experiments performed with different shearing surfaces. Two distinct slip behaviors are identified: depending on whether the interaction between the microgel particles and the wall is attractive or repulsive, wall-slip can be either suppressed or promoted. We provide an extension to the elastohydrodynamic slip model of Meeker and co-workers by incorporating attractive or repulsive interactions between the slipping paste particle and the wall. The interplay of various short range forces due to van der Waals, hydrophobic/hydrophilic, and/or electrostatic interactions and elastohydrodynamics is used to explain the influence of the shearing surface on wall-slip. The model encompasses the different slip regimes observed in our experiments and can predict the slip behavior accurately for well characterized surfaces.

Journal of Rheology, 2020

The transport properties of soft particle glasses, such as dynamic viscosity, normal stress coeff... more The transport properties of soft particle glasses, such as dynamic viscosity, normal stress coefficients, and shear-induced diffusivity of its particles, are determined by the microstructure of the suspension under flow. A thermodynamic measure of the microstructure is the excess entropy, which we show here accurately correlates the transport properties of soft particle glasses onto master curves across a wide range of volume fractions, suspending fluid viscosities, particle moduli and shear rates. The excess entropy for soft particle glasses is approximated with the twobody excess entropy computed from the pair distribution function extracted from dynamic simulations. The shear viscosity and normal stress functions diverge and the diffusivity vanishes at a critical excess entropy, corresponding to the yield stress of the suspension. An effective temperature is computed and is found to vary linearly with the shear stress and the elastic energy of the sheared soft particle glass. From this an equation of state is derived relating the excess entropy to the shear stress. Consequently, three of the four transport properties are determined from the measurement of just one. Finally, a single master curve of particle diffusivity versus excess entropy is presented that unifies observations for both equilibrium and non-equilibrium suspensions.

Journal of Rheology, 2020

Soft particle glasses are amorphous materials made of soft and deformable particles that are jamm... more Soft particle glasses are amorphous materials made of soft and deformable particles that are jammed above close-packing. They behave like weak solids at rest but they yield and flow under external mechanical constraints. Although soft particle glasses are widely used in applications, little is known about how the particle softness and microscopic dynamics determine the macroscopic rheology. Here we use three-dimensional particle dynamic simulations to analyze the dynamical properties of soft particle glasses at different scales. We demonstrate how the dynamics is determined by the persistence time and the magnitude of the fluctuating elastic forces that develop at contact in the flow. The shear-induced diffusion coefficient, the local structural relaxation times, the shear stress, and the normal stress differences are interconnected through simple relationships that allow the prediction of the macroscopic rheology from the microscopic dynamics.

Soft Matter, 2018

Particle dynamic simulations are used to investigate the structural and flow properties of jammed... more Particle dynamic simulations are used to investigate the structural and flow properties of jammed suspensions for different soft interaction potentials and lubrication forces.

Langmuir, 2017

We present a multipurpose technology to encapsulate hydrophobic substances in micron-size emulsio... more We present a multipurpose technology to encapsulate hydrophobic substances in micron-size emulsion droplets and capsules. The encapsulating agent is a comb-like stimuli-responsive copolymer comprising side chain surfactants attached to a methacrylic acid/ethylacrylate polyelectrolyte backbone. The composition and structure of the hydrophobic moieties of the side chains are customized in order to tune the particle morphology and the processing conditions. The technology exploits the synergy of properties provided by the copolymer: interfacial activity, pH responsiveness, and viscoelasticity. A one-pot process produces emulsion gels or capsule dispersions consisting of a hydrophobic liquid core surrounded by a polymer shell. The dispersions resist to high ionic strengths and exhibit long-term stability. The versatility of the method is demonstrated by encapsulating various hydrophobic substances covering a broad range of viscosities and polaritiesconventional and technical oils, perfumes, alkyd paints-with a high degree of morphological and rheological control. MATERIALS AND METHODS Chemicals. Chemicals were purchased from Sigma-Aldrich (Silicon oil DC200), Fluka (fluorescent dyes Nile Red and Rhodamine B), and VWR (silicon oil Rhodorsil 47V20). Alkyd resin, Esterol A, and Citronellal were provided by Coatex (Arkema group). Olive oil was bought from groceries.

Slip in soft particle pastes is dominant at low shear rates. By directly imaging the flow of past... more Slip in soft particle pastes is dominant at low shear rates. By directly imaging the flow of pastes using videomicroscopy, we show that slip is characterized by universal properties, which depend on solvent viscosity, bulk elasticity and particle size. A generic slip model based on elasto-hydrodynamic lubrication between the squeezed particles and the wall explains these properties quantitatively.

Nature Materials, Nov 1, 2011

Soft particle glasses form a broad family of materials made of deformable particles, as diverse a... more Soft particle glasses form a broad family of materials made of deformable particles, as diverse as microgels 1 , emulsion droplets 2 , star polymers 3 , block copolymer micelles and proteins 4 , which are jammed at volume fractions where they are in contact and interact via soft elastic repulsions. Despite a great variety of particle elasticity, soft glasses have many generic features in common. They behave like weak elastic solids at rest but flow very much like liquids above the yield stress. This unique feature is exploited to process high-performance coatings, solid inks, ceramic pastes, textured food and personal care products. Much of the understanding of these materials at volume fractions relevant in applications is empirical, and a theory connecting macroscopic flow behaviour to microstructure and particle properties remains a formidable challenge. Here we propose a micromechanical three-dimensional model that quantitatively predicts the nonlinear rheology of soft particle glasses. The shear stress and the normal stress differences depend on both the dynamic pair distribution function and the solventmediated EHD interactions among the deformed particles. The predictions, which have no adjustable parameters, are successfully validated with experiments on concentrated emulsions and polyelectrolyte microgel pastes, highlighting the universality of the flow properties of soft glasses. These results provide a framework for designing new soft additives with a desired rheological response. Soft particle glasses share common features with hard sphere glasses such as non-ergodicity and caged dynamics. However, whereas hard sphere colloids experience only forces due to excluded volume, soft particles at high volume fraction are compressed against each other by bulk osmotic forces and form flat facets at contact, with the average deformation depending on particle elasticity and volume fraction. The solvent forming the continuous phase is localized in thin films separating the particles (Supplementary Fig. S1). We model a soft particle glass as a suspension of N non-Brownian, elastic spheres (Fig. 1a), which are dispersed at a volume fraction above the random close-packing of hard spheres (φ > φ c ≈ 0.64), in a solvent with viscosity η S. The particles are slightly polydisperse in size, with an average radius R, which prevents them from crystallizing under flow. We characterize the contact between two particles by the overlap distance h αβ and the relative deformation ε αβ = h αβ /R c , where R c is the contact radius (Fig. 1b). The suspension is subject to an imposed shear flow in the plane (x, y) with shear rateγ , particle and fluid inertia being neglected. As two compressed particles move past one another, a flow of solvent develops inside the liquid films separating the facets. This generates a net positive pressure, causing a further elastic

Journal of Rheology, Jul 1, 2022

We explore the rheology during startup flow of well characterized polyelectrolyte microgel suspen... more We explore the rheology during startup flow of well characterized polyelectrolyte microgel suspensions which form soft glasses above the jamming concentration. We present and discuss results measured using different mechanical histories focusing on the variations of the static yield stress and yield strain. The behavior of the shear stress growth function is affected by long-lived residual stresses and strains that imprint a slowly decaying mechanical memory inside the materials. The startup flow response is not reversible upon flow reversal and the amplitude of the static yield stress increases with the time elapsed after rejuvenation. We propose an experimental protocol that minimizes the directional memory and we analyze the effect of aging. The static yield strain γp and the reduced static yield stress σp/σy, where σy is the dynamic yield stress measured from steady flow measurements, are in good agreement with our previous simulations (J. Rheol. 65, 241, 2021). Our results demonstrate the need to consider memory and aging effects in transient measurements on soft particle glasses.

Soft Matter, 2018

We identify the minimal interparticle interactions necessary for a particle dynamics simulation t... more We identify the minimal interparticle interactions necessary for a particle dynamics simulation to predict the structure and flow behaviour of soft particle glasses (SPGs). Generally, two kinds of forces between the particles must be accounted for in simulations of SPGs: viscous or frictional drag forces and elastic contact forces. Far field drag forces are required to dissipate energy in the simulations and capture the effect of the rheology of the suspending fluid. Elastic forces are found to be dominant compared to near-field drag or other forms of friction forces and are the most important component to compute the rheology. The shear stress, the first and second normal stress differences for different interparticle force laws collapse onto universal master curves of the Herschel-Bulkley form by non-dimensionalizing the stress with the yield stress and the shear rate with the viscosity of the suspending fluid divided by the low-frequency shear modulus. The Herschel-Bulkley exponents are close to 0.5 with a slight dependence on the repulsive pairwise elastic forces.

Soft Matter, 2016

We explore the influence of particle softness on the state diagram of well characterized polylect... more We explore the influence of particle softness on the state diagram of well characterized polylectrolyte microgel suspensions using dynamic light scattering and rheology. Upon increasing the polymer concentration, we cross successively the well defined glass and jamming transitions which delimit four different states: dilute colloidal suspension, entropic glass, jammed glass, and dense glass. Each state has a specific dynamical fingerprint dictated by two key ingredients related to particle softness: elastic contact interactions, and osmotic or steric deswelling. Soft interactions control yielding and flow of the jammed glasses. The shrinkage of the microgels makes the glass transition look smoother than in hard sphere suspensions. We quantify the relation between the polymer concentration and the volume fraction, and show that the glass transition behaviour of soft microgels can be mapped onto that of hard sphere glasses once the volume fraction is used as the control parameter.

HAL (Le Centre pour la Communication Scientifique Directe), Dec 27, 2013

The long time persistence of mechanical stresses is a generic property of glassy materials. Here ... more The long time persistence of mechanical stresses is a generic property of glassy materials. Here we identify the microscopic mechanisms that control internal stresses in highly concentrated suspensions of soft particles brought to rest from steady flow. The persistence of the asymmetric angular distortions which characterize the pair distribution function during flow is at the origin of the internal stresses. Their long time evolution is driven by in-cage rearrangements of the elastic contacts between particles. The trapped macroscopic stress is related to the solvent viscosity, particle elasticity and volume fraction through a universal scaling derived from simulations and experiments.

Journal of Rheology, Mar 1, 2021

The rheology and microstructure of soft particle glasses during startup flow are studied using th... more The rheology and microstructure of soft particle glasses during startup flow are studied using three-dimensional particle dynamics simulations at different particle volume fractions and shear rates. The behavior of transient stress depends on the applied shear rate. At high shear rates, soft particle glasses exhibit a static yield stress signaled by a stress overshoot followed by a relaxation to a steady-state value. The buildup of the stress is driven by an interplay between structural anisotropy due to an accumulation of particles along the compression axis and a depletion along the extension axis and a compression of particles that are soft and deformable. At low shear rates, the stress increase is monotonic and without any stress overshoot. The time scale at which structural anisotropy and the stress are maximum is correlated to the nonaffine dynamics of SPGs through the persistence time of shear-induced particle collisions and to the residence time of particles inside their transient cages. The static yield strain γ p and the reduced static yield stress σ p / σ y, where σ y is the dynamic yield stress deduced from steady flow measurements, follow universal behaviors when correlated with the dimensional shear rate η s γ ˙ / G 0, with η s being the suspending fluid viscosity and G 0 the storage modulus, which expresses the competition between elastic restoring forces and viscous dissipation. Dense suspensions of thermosensitive core–shell colloids, star-like micelles, and poly(ethylene oxide)-protected silica particles follow the same universal curves, suggesting the generality of our results.

Journal of Rheology, Sep 1, 2014

ABSTRACT

Physical Review Letters, May 11, 2004

Concentrated dispersions of soft particles are shown to exhibit a generic slip behavior near smoo... more Concentrated dispersions of soft particles are shown to exhibit a generic slip behavior near smooth surfaces. Slip results from a balance between osmotic forces and noncontact elastohydrodynamic interaction between the squeezed particles and the wall. A model is presented that predicts the slip properties and provides insight into the behavior of the bulk paste.

Journal of Rheology, Mar 1, 2020

The transport properties of soft particle glasses, such as dynamic viscosity, normal stress coeff... more The transport properties of soft particle glasses, such as dynamic viscosity, normal stress coefficients, and shear-induced diffusivity of its particles, are determined by the microstructure of the suspension under flow. A thermodynamic measure of the microstructure is the excess entropy, which we show here accurately correlates the transport properties of soft particle glasses onto master curves across a wide range of volume fractions, suspending fluid viscosities, particle moduli and shear rates. The excess entropy for soft particle glasses is approximated with the twobody excess entropy computed from the pair distribution function extracted from dynamic simulations. The shear viscosity and normal stress functions diverge and the diffusivity vanishes at a critical excess entropy, corresponding to the yield stress of the suspension. An effective temperature is computed and is found to vary linearly with the shear stress and the elastic energy of the sheared soft particle glass. From this an equation of state is derived relating the excess entropy to the shear stress. Consequently, three of the four transport properties are determined from the measurement of just one. Finally, a single master curve of particle diffusivity versus excess entropy is presented that unifies observations for both equilibrium and non-equilibrium suspensions.

Soft Matter, 2012

We use fluorescence microscopy and particle tracking velocimetry to image the motion of concentra... more We use fluorescence microscopy and particle tracking velocimetry to image the motion of concentrated emulsions and microgel suspensions near solid surfaces. The local deformation involves a combination of slip and bulk flow, which are found to be controlled by surface forces. With smooth surfaces, two slip mechanisms are identified depending on whether particle-wall interactions are repulsive or weakly attractive. In the former case, the materials yield uniformly and the local rheology can be mapped on the macroscopic flow curve. In the latter case, yielding is non-uniform which reveals a continuous distribution of states from the immediate vicinity of the smooth surface to the bulk of the material. The effect of the surface is long-ranged and decays exponentially with the distance, which can be described by a non-local fluidity model. Our results establish a link between surface forces, lubrication and yielding in soft glassy or jammed materials and open new routes to manipulate their flow through the surface chemistry of the confining boundaries.

Physical review fluids, Sep 1, 2017

The microstructure and shear rheology of highly concentrated, jammed suspensions of soft particle... more The microstructure and shear rheology of highly concentrated, jammed suspensions of soft particles are shown to depend on polydispersity and shear rate from computational simulations. Rich behavior is observed depending on the degree of polydispersity and the shear rate. Glassy suspensions with a low degree of polydispersity evolve to face-centered cubic (FCC) and hexagonal close-packed (HCP) structures at low and high shear rates, respectively. The structural rearrangement occurs over several units of strain and reduces the shear stress and elastic energy. Suspensions with a higher degree of polydispersity exhibit a microstructural transition from a glass to a layered-like structure for sufficiently high shear rates. In this case the soft particles first rearrange themselves in the flow-vorticity plane during an induction time (or strain) before layers parallel to the flow-vorticity plane are formed. The induction strain decays exponentially with shear rate revealing that the devitrification of monodisperse and polydisperse suspensions is a shear-activated process. Finally a generic dynamical state diagram is found that depends on the polydispersity and the ratio of viscous to elastic forces due to shear.

Journal of Rheology, Mar 1, 2020

Soft particle glasses are amorphous materials made of soft and deformable particles that are jamm... more Soft particle glasses are amorphous materials made of soft and deformable particles that are jammed above close-packing. They behave like weak solids at rest but they yield and flow under external mechanical constraints. Although soft particle glasses are widely used in applications, little is known about how the particle softness and microscopic dynamics determine the macroscopic rheology. Here we use three-dimensional particle dynamic simulations to analyze the dynamical properties of soft particle glasses at different scales. We demonstrate how the dynamics is determined by the persistence time and the magnitude of the fluctuating elastic forces that develop at contact in the flow. The shear-induced diffusion coefficient, the local structural relaxation times, the shear stress, and the normal stress differences are interconnected through simple relationships that allow the prediction of the macroscopic rheology from the microscopic dynamics.

APS, Nov 1, 2003

Concentrated suspensions of soft, elastic particles display several interesting rheological prope... more Concentrated suspensions of soft, elastic particles display several interesting rheological properties such as a yield stress, wall slip, shear-thinning, aging and memory. These effects appear to arise from elasto-hydrodynamic particle-particle and particle-wall interactions. A micromechanical model of elasto-hydrodynamic lift between a sphere and wall will be presented and solved numerically and asymptotically. These results will be used to explain wall-slip

Journal of Rheology, Nov 1, 2004

Microgel pastes and concentrated emulsions are shown to exhibit a generic slip behavior at low st... more Microgel pastes and concentrated emulsions are shown to exhibit a generic slip behavior at low stresses when sheared near smooth surfaces. The magnitude of slip depends on the applied stress. Well above the yield stress, slip is negligible compared to the bulk flow. Just above the yield stress, slip becomes significant and the total deformation results from a combination of bulk flow and slip. At and below the yield stress, the bulk flow is negligible and the apparent motion is entirely due to wall slip. By directly imaging the deformation of pastes and from rheological measurements, we show that slip is characterized by universal scaling properties, which depend on solvent viscosity, bulk shear modulus, and particle size. A model based on elastohydrodynamic lubrication between the squeezed particles and the shearing surface explains these properties quantitatively.

Journal of Rheology, Sep 1, 2008

Concentrated suspensions of soft deformable particles, e.g., polymer microgel pastes and compress... more Concentrated suspensions of soft deformable particles, e.g., polymer microgel pastes and compressed emulsions, display a generic slip behavior [Meeker et al., J. Rheol. 92, 18302 (2004a); Meeker et al., J. Rheol. 48, 1295–1320 (2004b)]. When sheared with smooth surfaces, they exhibit apparent motion due to slip at the wall. Wall-slip stops at a sliding yield stress the value of which is much lower than the bulk yield stress. The physical mechanism of slip at low stresses and the origin of the sliding yield stress have so far been unresolved issues. We propose that the paste-wall interactions control the wall-slip behavior and determine the occurrence of the sliding yield point. We present experiments performed with different shearing surfaces. Two distinct slip behaviors are identified: depending on whether the interaction between the microgel particles and the wall is attractive or repulsive, wall-slip can be either suppressed or promoted. We provide an extension to the elastohydrodynamic slip model of Meeker and co-workers by incorporating attractive or repulsive interactions between the slipping paste particle and the wall. The interplay of various short range forces due to van der Waals, hydrophobic/hydrophilic, and/or electrostatic interactions and elastohydrodynamics is used to explain the influence of the shearing surface on wall-slip. The model encompasses the different slip regimes observed in our experiments and can predict the slip behavior accurately for well characterized surfaces.

Journal of Rheology, 2020

The transport properties of soft particle glasses, such as dynamic viscosity, normal stress coeff... more The transport properties of soft particle glasses, such as dynamic viscosity, normal stress coefficients, and shear-induced diffusivity of its particles, are determined by the microstructure of the suspension under flow. A thermodynamic measure of the microstructure is the excess entropy, which we show here accurately correlates the transport properties of soft particle glasses onto master curves across a wide range of volume fractions, suspending fluid viscosities, particle moduli and shear rates. The excess entropy for soft particle glasses is approximated with the twobody excess entropy computed from the pair distribution function extracted from dynamic simulations. The shear viscosity and normal stress functions diverge and the diffusivity vanishes at a critical excess entropy, corresponding to the yield stress of the suspension. An effective temperature is computed and is found to vary linearly with the shear stress and the elastic energy of the sheared soft particle glass. From this an equation of state is derived relating the excess entropy to the shear stress. Consequently, three of the four transport properties are determined from the measurement of just one. Finally, a single master curve of particle diffusivity versus excess entropy is presented that unifies observations for both equilibrium and non-equilibrium suspensions.

Journal of Rheology, 2020

Soft particle glasses are amorphous materials made of soft and deformable particles that are jamm... more Soft particle glasses are amorphous materials made of soft and deformable particles that are jammed above close-packing. They behave like weak solids at rest but they yield and flow under external mechanical constraints. Although soft particle glasses are widely used in applications, little is known about how the particle softness and microscopic dynamics determine the macroscopic rheology. Here we use three-dimensional particle dynamic simulations to analyze the dynamical properties of soft particle glasses at different scales. We demonstrate how the dynamics is determined by the persistence time and the magnitude of the fluctuating elastic forces that develop at contact in the flow. The shear-induced diffusion coefficient, the local structural relaxation times, the shear stress, and the normal stress differences are interconnected through simple relationships that allow the prediction of the macroscopic rheology from the microscopic dynamics.

Soft Matter, 2018

Particle dynamic simulations are used to investigate the structural and flow properties of jammed... more Particle dynamic simulations are used to investigate the structural and flow properties of jammed suspensions for different soft interaction potentials and lubrication forces.

Langmuir, 2017

We present a multipurpose technology to encapsulate hydrophobic substances in micron-size emulsio... more We present a multipurpose technology to encapsulate hydrophobic substances in micron-size emulsion droplets and capsules. The encapsulating agent is a comb-like stimuli-responsive copolymer comprising side chain surfactants attached to a methacrylic acid/ethylacrylate polyelectrolyte backbone. The composition and structure of the hydrophobic moieties of the side chains are customized in order to tune the particle morphology and the processing conditions. The technology exploits the synergy of properties provided by the copolymer: interfacial activity, pH responsiveness, and viscoelasticity. A one-pot process produces emulsion gels or capsule dispersions consisting of a hydrophobic liquid core surrounded by a polymer shell. The dispersions resist to high ionic strengths and exhibit long-term stability. The versatility of the method is demonstrated by encapsulating various hydrophobic substances covering a broad range of viscosities and polaritiesconventional and technical oils, perfumes, alkyd paints-with a high degree of morphological and rheological control. MATERIALS AND METHODS Chemicals. Chemicals were purchased from Sigma-Aldrich (Silicon oil DC200), Fluka (fluorescent dyes Nile Red and Rhodamine B), and VWR (silicon oil Rhodorsil 47V20). Alkyd resin, Esterol A, and Citronellal were provided by Coatex (Arkema group). Olive oil was bought from groceries.

Slip in soft particle pastes is dominant at low shear rates. By directly imaging the flow of past... more Slip in soft particle pastes is dominant at low shear rates. By directly imaging the flow of pastes using videomicroscopy, we show that slip is characterized by universal properties, which depend on solvent viscosity, bulk elasticity and particle size. A generic slip model based on elasto-hydrodynamic lubrication between the squeezed particles and the wall explains these properties quantitatively.