Hans M. Wyss

Soft colloids make strong glasses

Glass formation in colloidal suspensions has many of the hallmarks of glass formation in molecular materials. For hard-sphere colloids, which interact only as a result of excluded volume, phase behaviour is controlled by volume fraction, φ; an increase in φ drives the system towards its glassy state, analogously to a decrease in temperature, T, in molecular systems. When φ increases above φ* ≈ 0.53, the viscosity starts to increase significantly, and the system eventually moves out of equilibrium at the glass transition, φg ≈ 0.58, where particle crowding greatly restricts structural relaxation. The large particle size makes it possible to study both structure and dynamics with light scattering and imaging; colloidal suspensions have therefore provided considerable insight into the glass transition. However, hard-sphere colloidal suspensions do not exhibit the same diversity of behaviour as molecular glasses. This is highlighted by the wide variation in behaviour observed for the viscosity or structural relaxation time, τα, when the glassy state is approached in supercooled molecular liquids. This variation is characterized by the unifying concept of fragility, which has spurred the search for a ‘universal’ description of dynamic arrest in glass-forming liquids. For ‘fragile’ liquids, τα is highly sensitive to changes in T, whereas non-fragile, or ‘strong’, liquids show a much lower T sensitivity. In contrast, hard-sphere colloidal suspensions are restricted to fragile behaviour, as determined by their φ dependence, ultimately limiting their utility in the study of the glass transition. Here we show that deformable colloidal particles, when studied through their concentration dependence at fixed temperature, do exhibit the same variation in fragility as that observed in the T dependence of molecular liquids at fixed volume. Their fragility is dictated by elastic properties on the scale of individual colloidal particles. Furthermore, we find an equivalent effect in molecular systems, where elasticity directly reflects fragility. Colloidal suspensions may thus provide new insight into glass formation in molecular systems.

Tough stimuli-responsive supramolecular hydrogels with hydrogen-bonding network junctions.

Hydrogels were prepared with physical cross-links comprising 2-ureido-4[1H]-pyrimidinone (UPy) hydrogen-bonding units within the backbone of segmented amphiphilic macromolecules having hydrophilic poly(ethylene glycol) (PEG). The bulk materials adopt nanoscopic physical cross-links composed of UPy-UPy dimers embedded in segregated hydrophobic domains dispersed within the PEG matrix as comfirmed by cryo-electron microscopy. The amphiphilic network was swollen with high weight fractions of water (w(H2O) ≈ 0.8) owing to the high PEG weight fraction within the pristine polymers (w(PEG) ≈ 0.9). Two different PEG chain lengths were investigated and illustrate the corresponding consequences of cross-link density on mechanical properties. The resulting hydrogels exhibited high strength and resilience upon deformation, consistent with a microphase separated network, in which the UPy-UPy interactions were adequately shielded within hydrophobic nanoscale pockets that maintain the network despite extensive water content. The cumulative result is a series of tough hydrogels with tunable mechanical properties and tractable synthetic preparation and processing. Furthermore, the melting transition of PEG in the dry polymer was shown to be an effective stimulus for shape memory behavior.

Strain-Rate Frequency Superposition: A Rheological Probe of Structural Relaxation in Soft Materials

The rheological properties of soft materials often exhibit surprisingly universal linear and nonlinear features. Here we show that these properties can be unified by considering the effect of the strain-rate amplitude on the structural relaxation of the material. We present a new form of oscillatory rheology, strain-rate frequency superposition (SRFS), where the strain-rate amplitude is fixed as the frequency is varied. We show that SRFS can isolate the response due to structural relaxation, even when it occurs at frequencies too low to be accessible with standard techniques.

Nonlinear viscoelasticity of metastable complex fluids

Many metastable complex fluids such as colloidal glasses and gels show distinct nonlinear viscoelasticity with increasing oscillatory-strain amplitude; the storage modulus decreases monotonically as the strain amplitude increases whereas the loss modulus has a distinct peak before it decreases at larger strains. We present a qualitative argument to explain this ubiquitous behavior and use mode-coupling theory (MCT) to confirm it. We compare theoretical predictions to the measured nonlinear viscoelasticity in a dense hard-sphere colloidal suspension; reasonable agreement is obtained. The argument given here can be used to obtain new information about linear viscoelasticity of metastable complex fluids from nonlinear strain measurements.

Microgel suspensions : fundamentals and applications

Providing a vital link between chemistry and physics on the nanoscale, this book offers concise coverage of the entire topic in five major sections, beginning with synthesis of microgel particles and continuing with their physical properties. The phase behavior and dynamics of resulting microgel suspensions feature in the third section, followed by their mechanical properties. It concludes with detailed accounts of numerous industrial, commercial and medical applications. Edited by David Weitz, Professor at Harvard and one of the worlds pre-eminent experts in the field.

Glasslike arrest in spinodal decomposition as a route to colloidal gelation

Colloid-polymer mixtures can undergo spinodal decomposition into colloid-rich and colloid-poor regions. Gelation results when interconnected colloid-rich regions solidify. We show that this occurs when these regions undergo a glass transition, leading to dynamic arrest of the spinodal decomposition. The characteristic length scale of the gel decreases with increasing quench depth, and the nonergodicity parameter exhibits a pronounced dependence on scattering vector. Mode coupling theory gives a good description of the dynamics, provided we use the full static structure as input.

Biophysical properties of normal and diseased renal glomeruli

The mechanical properties of tissues and cells including renal glomeruli are important determinants of their differentiated state, function, and responses to injury but are not well characterized or understood. Understanding glomerular mechanics is important for understanding renal diseases attributable to abnormal expression or assembly of structural proteins and abnormal hemodynamics. We use atomic force microscopy (AFM) and a new technique, capillary micromechanics, to measure the elastic properties of rat glomeruli. The Youngs modulus of glomeruli was 2,500 Pa, and it was reduced to 1,100 Pa by cytochalasin and latunculin, and to 1,400 Pa by blebbistatin. Cytochalasin or latrunculin reduced the F/G actin ratios of glomeruli but did not disrupt their architecture. To assess glomerular biomechanics in disease, we measured the Youngs moduli of glomeruli from two mouse models of primary glomerular disease, Col4a3(-/-) mice (Alport model) and Tg26(HIV/nl) mice (HIV-associated nephropathy model), at stages where glomerular injury was minimal by histopathology. Col4a3(-/-) mice express abnormal glomerular basement membrane proteins, and Tg26(HIV/nl) mouse podocytes have multiple abnormalities in morphology, adhesion, and cytoskeletal structure. In both models, the Youngs modulus of the glomeruli was reduced by 30%. We find that glomeruli have specific and quantifiable biomechanical properties that are dependent on the state of the actin cytoskeleton and nonmuscle myosins. These properties may be altered early in disease and represent an important early component of disease. This increased deformability of glomeruli could directly contribute to disease by permitting increased distension with hemodynamic force or represent a mechanically inhospitable environment for glomerular cells.

Temperature‐Controlled Transitions Between Glass, Liquid, and Gel States in Dense p‐NIPA Suspensions

In this article we demonstrate that concentrated p-NIPA suspensions in a low temperature glassy state can liquefy and then solidify again as the temperature is raised across the LCST. Our system exhibits all the typical behavior of disordered colloidal suspensions, but the behavior is controlled by temperature. Below the LCST it shows the behavior typical of a colloidal glass, near the LCST it behaves like a liquid, while above the LCST it exhibits the properties typical of a colloidal gel. Moreover, we show that the elasticity of these suspensions exhibits critical-like behavior as a function of temperature both above and below the LCST, with a critical temperature that corresponds to the LCST. Our results thereby suggest interesting analogies between the glass and gel phases of these thermosensitive microgel particles.

Arrested fluid-fluid phase separation in depletion systems: Implications of the characteristic length on gel formation and rheology

We investigate the structural, dynamical, and rheological properties of colloid-polymer mixtures in a volume fraction range of ϕ=0.15–0.35. Our systems are density-matched, residual charges are screened, and the polymer-colloid size ratio is ∼0.37. For these systems, the transition to kinetically arrested states, including disconnected clusters and gels, coincides with the fluid-fluid phase separation boundary. Structural investigations reveal that the characteristic length, L, of the networks is a strong function of the quench depth: for shallow quenches, L is significantly larger than that obtained for deep quenches. By contrast, L is for a given quench depth almost independent of ϕ; this indicates that the strand thickness increases with ϕ. The strand thickness determines the linear rheology: the final relaxation time exhibits a strong dependence on ϕ, whereas the high frequency modulus does not. We present a simple model based on estimates of the strand breaking time and shear modulus that semiquantit...

Injectable Hydrogels from Segmented PEG-Bisurea Copolymers

We describe the preparation of an injectable, biocompatible, and elastic segmented copolymer hydrogel for biomedical applications, with segmented hydrophobic bisurea hard segments and hydrophilic PEG segments. The segmented copolymers were obtained by the step growth polymerization of amino-terminated PEG and aliphatic diisocyanate. Due to their capacity for multiple hydrogen bonding within the hydrophobic segments, these copolymers can form highly stable gels in water at low concentrations. Moreover, the gels show shear thinning by a factor of 40 at large strain, which allows injection through narrow gauge needles. Hydrogel moduli are highly tunable via the physical cross-link density and the length of the hydrophilic segments. In particular, the mechanical properties can be optimized to match the properties of biological host tissues such as muscle tissue and the extracellular matrix.