Michel Cloitre

A micromechanical model to predict the flow of soft particle glasses

Soft particle glasses form a broad family of materials made of deformable particles, as diverse as microgels, emulsion droplets, star polymers, block copolymer micelles and proteins, 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 solvent-mediated 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.

Ageing and yield behaviour in model soft colloidal glasses

We use multi-arm star polymers as model soft colloids with tuneable interactions and explore their behaviour in the glassy state. In particular, we perform a systematic rheological study with a well-defined protocol and address aspects of ageing and shear melting of star glasses. Ageing proceeds in two distinct steps: a fast step of O(103u2009s) and a slow step of O(104u2009s). We focus on creep and recovery tests, which reveal a rich, albeit complex response. Although the waiting time, the time between pre-shear (rejuvenation) of the glassy sample and measurement, affects the material’s response, it does not play the same role as in other soft glasses. For stresses below the yield value, the creep curve is divided into three regimes with increasing time: viscoplastic, intermediate steady flow (associated with the first ageing step) and long-time evolving elastic solid. This behaviour reflects the interplay between ageing and shear rejuvenation. The yield behaviour, as investigated with the stress-dependent recoverable strain, indicates a highly nonlinear elastic response intermediate between a low-stress Hookean solid and a high-stress viscoelastic liquid, and exemplifies the distinct characteristics of this class of hairy colloids. It appears that a phenomenological classification of different colloidal glasses based on yielding performance may be possible.

Dynamics and rheology of colloidal star polymers

We attempt to elucidate the dynamics of multiarm star polymer solutions, which are representative of a large class of soft hairy colloids, over a wide range of concentrations. In addition to the usual β-relaxation (in-cage rattling) and α-relaxation (terminal cage escape), the relaxation of stars in the glassy state involves a mode associated with arm interpenetration. From linear and nonlinear rheological measurements, we establish a set of criteria for identifying the colloidal glass transition, including aging, yielding and elastic properties. Linear viscoelasticity and steady-shear measurements merge at low frequencies, indicating that terminal behavior is accessible, albeit at very long times. The transition from linear to nonlinear response is controlled by star elasticity and a balance between Brownian diffusion and flow advection. In the nonlinear regime, the flow curves collapse on a universal flow curve using a scaling that expresses a competition between solvent-mediated interactions and elastic forces. While applied to stars, our framework appears to be a generic tool for fingerprinting liquid-solid transitions in colloidal suspensions.

Micromechanics of Soft Particle Glasses

Soft glasses encompass a broad class of materials at the boundaries between polymers, granular dispersions, and colloidal glasses. Although they display a huge diversity of compositions and architectures, soft glasses share a common structure as well as generic static and flow properties. In this chapter, we show that the dense amorphous microstructure of soft glasses, combined with the existence of repulsive elastohydrodynamic interactions mediated by the solvent, lie at the heart of their behavior. These two basic ingredients are incorporated into a micromechanical model and a dynamic molecular-like simulation. Our theory successfully predicts near-equilibrium quantities such as the pair distribution function and shear moduli, the slip properties that are observed when soft glasses are sheared along solid surfaces, as well as the bulk shear rheology. These results, which connect properties at the particle scale to macroscopic behavior, provide predictive tools for the design of materials with a desired rheological response.

Probing glassy states in binary mixtures of soft interpenetrable colloids

We present experimental evidence confirming the recently established rich dynamic state diagram of asymmetric binary mixtures of soft colloidal spheres. These mixtures consist of glassy suspensions of large star polymers to which different small stars are added at varying concentrations. Using rheology and dynamic light scattering measurements along with a simple phenomenological analysis, we show the existence of re-entrance and multiple glassy states, which exhibit distinct features. Cooperative diffusion, as a probe for star arm interpenetration, is proven to be sensitive to the formation of the liquid pockets which signal the melting of the large-star-glass upon addition of small stars. These results provide ample opportunities for tailoring the properties of soft colloidal glasses.

Monitoring mesoglobules formation in PNIPAm solutions using Nile Red solvatochromism

Nile Red solvatochromism is used to monitor phase separation in concentrated poly(N-isopropyl acrylamide) (PNIPAm) aqueous solutions. Below the lower critical solution temperature (LCST), Nile Red molecules are in a polar environment and thus exhibit negligible fluorescence. Above the LCST, the aggregation of the PNIPAm chains into hydrophobic mesoglobules provides a nonpolar environment, causing a strong increase of fluorescence. The spectra show two emission bands, which can be related to the partitioning of Nile Red molecules between the core and the surface of the mesoglobules. More generally, the technique appears as a new and promising tool to probe microheterogeneities in polymer solutions or mixtures.