Soft materials
Emergence of multiscale dynamics in colloidal gels

Many of the squishy materials we consume every day, like lotion, hair gel, mayonnaise and yogurt, can be categorized as colloidal gels. Composed of a space-spanning, elastic network of submicron-sized particles immersed in a viscous fluid, a colloidal gel exhibits both solid-like and fluid-like behaviors, depending on the magnitude and the timescale of the stress applied. We study the structure, dynamics, and mechanics of different types of colloidal gels, via rheometry and microscopy, aiming to develop a microscopic understanding of their macroscopic properties.

To gain insight into the kinetics of colloidal gel evolution at low particle volume fractions φ, we utilize differential dynamic microscopy to investigate particle aggregation, geometric percolation, and the subsequent transition to nonergodic dynamics. We report the emergence of unexpectedly rich multiscale dynamics upon the onset of nonergodicity, which separates the wave vectors q into three different regimes.In the high-q domain, the gel exhibits φ-independent internal vibrations of fractal clusters. The intermediate-q domain is dominated by density fluctuations at the length scale of the clusters, as evidenced by the q independence of the relaxation time τ. In the low-q domain, the scaling of τ as q^-3 suggests that the network appears homogeneous. The transitions between these three regimes introduce two characteristic length scales, distinct from the cluster size.

Emergence of Multiscale Dynamics in Colloidal Gels

J. H. Cho, R. Cerbino, I. Bischofberger, Phys. Rev. Lett., 124, 088005 (2020)

Hydrogel reinforcement

We investigate the mechanical properties and microstructure of composite materials consisting of nanoparticles embedded in a polymer matrix. Such composite materials have long been used to enhance the mechanical properties of rubber. Car tires, for example, are composed of rubber and carbon black particles; the addition of the particles leads to a strong increase in the tire toughness and modulus. We probe the potential of reinforcing materials by adding particles to a different class of materials, hydrogels, soft materials composed of a polymer network in water. We use rheological measurements and confocal microscopy to link the macroscopic behavior of these particle-filled polymer gels to their microscopic properties and find a surprisingly general reinforcement mechanism.

Relaxation behavior of dense suspensions

Stabilized dense suspensions, solid particles dispersed at high concentration in a liquid matrix, are characterized by complex rheological properties: they are yield-stress materials – flowing only at stresses above a critical stress – and exhibit both discontinuous shear-thickening and shear-jamming behavior as a response to an applied stress. We are studying the relaxation behavior of a shear-thickening dense suspension, aiming to understand relaxation timescales and possible memory effects at different mass fractions of the dense suspension. To access the relaxation we get a sample of a cornstarch and water suspension in an excited, stressed state using a permanent magnet shaker, and monitor the relaxation as the shaker is turned off.

With Ivo Peters (University of Southampton)

 

New aspects in the phase behaviour of poly-N-isopropyl acrylamide:
systematic temperature dependent shrinking of PNiPAM assemblies well beyond the LCST

with Veronique Trappe (University of Fribourg)

 

We investigate the phase behaviour of aqueous dispersions of poly-N-isopropyl acrylamide (PNiPAM) microgels above their lower critical solution temperature (LCST) and find that beyond a well-defined concentration the systems exhibit a peculiar behaviour: the microgels assemble into space-spanning gels that shrink in time while maintaining the shape of the container in which they have been formed. Over a wide range of concentrations this shrinking behaviour is independent of PNiPAM concentration, but systematically depends on temperature in a temperature range significantly exceeding the LCST. The overall shrinking characteristics are consistent with those expected for scaffolds made of materials that exhibit thermal contraction. However, for the PNiPAM assemblies contraction is irreversible and can be as large as 90%. Such characteristics disclose complex interactions between fully collapsed PNiPAM and water well beyond the LCST, the origin of which has yet to be elucidated.

​​Multiple dynamic regimes in concentrated microgel systems

with David Sessoms, Veronique Trappe (University of Fribourg),

and Luca Cipelletti (University of Montpellier, France)

We investigate dynamical heterogeneities in the collective relaxation of a concentrated microgel system, for which the packing fraction can be conveniently varied by changing the temperature. The packing fraction dependent mechanical properties are characterised by a fluid-solid transition, where the system properties switch from a viscous to an elastic low-frequency behavior. Approaching this transition from below, we find that the range of spatial correlations in the dynamics increases. Beyond this transition, the spatial correlation range reaches a maximum, extending over the entire observable system size of approximately 5 mm. Increasing the packing fraction even further leads to a second transition, which is characterized by the development of large zones of lower and higher dynamical activity that are well separated from each other; the range of correlation decreases at this point. This striking non-monotonic dependence of the spatial correlation length on volume fraction is reminiscent of the behavior recently observed at the jamming/rigidity transition in granular systems. We identify this second transition as the transition to 'squeezed' states, where the constituents of the system start to exert direct contact forces on each other, such that the dynamics becomes increasingly determined by imbalanced stresses. Evidence of this transition is also found in the frequency dependence of the storage and loss moduli, which become increasingly coupled as direct friction between the particles starts to contribute to the dissipative losses within the system. To our knowledge, our data provide the first observation of a qualitative change in dynamical heterogeneity as the dynamics switch from purely thermally-driven to stress-driven.

Multiple dynamic regimes in concentrated microgel systems

D. A. Sessoms, I. Bischofberger, L. Cipelletti, V. Trappe, Phil. Trans. Royal Society A 367, 5013 (2009)

 

Co-nonsolvency of PNiPAM at the transition between solvation mechanisms

with Davide Calzolari and Veronique Trappe (University of Fribourg)

We investigate the co-nonsolvency of poly-N-isopropyl acrylamide (PNiPAM) in different water–alcohol mixtures and show that this phenomenon is due to two distinct solvation contributions governing the phase behavior of PNiPAM in the water-rich and alcohol-rich regime respectively. While hydrophobic hydration is the predominant contribution governing the phase behavior of PNiPAM

 in the water-rich regime, the mixing contributions governing the phase behavior of classical polymer solutions determine the phase behavior of PNiPAM in the alcohol-rich regime. This is evidenced by distinct scaling relations denoting the energetic state of the aqueous medium as a key parameter for the phase behavior of PNiPAM in the water-rich regime, while the volume fractions of respectively water, alcohol and PNiPAM become relevant parameters in the alcohol-rich regime. Adding alcohol to water decreases the energetics of the aqueous medium, which gradually suppresses hydrophobic hydration, while adding water to alcohol decreases the solvent quality. Consequently, PNiPAM is insoluble in the intermediate range of solvent composition, where neither hydrophobic hydration nor the mixing contributions prevail. This accounts for the co-nonsolvency phenomenon observed for PNiPAM in water–alcohol mixtures.

Co-nonsolvency of PNiPAM at the transition between solvation mechanisms

I. Bischofberger, D. C. E. Calzolari, V. Trappe, Soft Matter, 10, 8288-8295 (2014)

 

​​Hydrophobic hydration of PNiPAM ​: a matter of the mean energetics of water

with Veronique Trappe, Davide Calzolari (University of Fribourg), Paolo de los Rios (EPL Lausanne) and Ilian Jelezarov (University of Zurich)

The enthalpically favored hydration of hydrophobic entities, termed hydrophobic hydration, impacts the phase behaviour of numerous amphiphiles in water. Here, we show experimental evidence that hydrophobic hydration is strongly determined by the mean energetics of the aqueous medium. We investigate the aggregation and collapse of an amphiphilic polymer, poly-N-isopropyl acrylamide (PNiPAM), in aqueous solutions containing small amounts of alcohol and find that the thermodynamic characteristics defining the phase transitions of PNiPAM evolve relative to the solvent composition at which the excess mixing enthalpy of the water/alcohol mixtures becomes minimal. Such correlation between solvent energetics and solution thermodynamics extends to other mixtures containing neutral organic solutes that are considered as kosmotropes to induce a strengthening of the hydrogen bonded water network. This denotes the energetics of water as a key parameter controlling the phase behaviour of PNiPAM and identifies the excess mixing enthalpy of water/kosmotrope mixtures as a gauge of the kosmotropic effect on hydrophobic assemblies.

Hydrophobic hydration of poly-N-isopropyl acrylamide: a matter of the mean energetics of water

I. Bischofberger, D. C. E. Calzolari, P. De Los Rios, I. Jelezarov, V. Trappe, Sci. Rep. 4, 4377 (2014)