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.

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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)

Two modes of cluster dynamics govern the viscoelasticity of colloidal gels

Colloidal gels formed by strongly attractive particles at low particle volume fractions are composed of space-spanning networks of uniformly sized clusters. We study the thermal fluctuations of the clusters using differential dynamic microscopy by decomposing them into two modes of dynamics, and link them to the macroscopic viscoelasticity via rheometry. The first mode, dominant at early times, represents the localized, elastic fluctuations of individual clusters. The second mode, pronounced at late times, reflects the collective, viscoelastic dynamics facilitated by the connectivity of the clusters. By mixing two types of particles of distinct attraction strengths in different proportions, we control the transition time at which the collective mode starts to dominate, and hence tune the frequency dependence of the linear viscoelastic moduli of the binary gels.

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.

Lasting effects of discontinuous shear thickening in cornstarch suspensions upon flow cessation
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Dense suspensions that exhibit discontinuous shear thickening (DST) undergo complex stress relaxation when the flow abruptly stops. Using rotational rheometry, we study the two-step relaxation of aqueous cornstarch suspensions out of the DST state upon flow cessation and show that the DST fluid retains the memory of its shear-thickening state until the shear stress reaches a constant value at late times. We find that this residual stress at the end of the relaxation increases with the steady-state viscosity before the cessation. Furthermore, the timescales that characterize the two-step exponential decay of the shear stress exhibit near linear dependence on the steady-state viscosity. Within the current framework that ascribes DST to the breakdown of hydrodynamic lubrication layers leading to interparticle frictional contacts, the lasting effects of the steady-state viscosity suggest that the memory of frictional contacts persists until the end of the relaxation, despite the presence of repulsive forces between the particles. These results indicate that complete, spontaneous relaxation of the system out of DST is stalled by the partial retention of the frictional force chains, which may be caused by the stationary boundaries and the adhesion between cornstarch particles.

Lasting effects of discontinuous shear thickening in cornstarch suspensions upon flow cessation

J. H. Cho, A. H. Griese, I. R. Peters, I. Bischofberger, under review (2022)

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

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.

Co-nonsolvency of PNiPAM at the transition between solvation mechanisms

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 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)