Delayed elastic contributions to the viscoelastic response of foams.

We show that the slow viscoelastic response of a foam is that of a power-law fluid with a terminal relaxation. Investigations of the foam mechanics in creep and recovery tests reveal that the power-law contribution is fully reversible, indicative of a delayed elastic response. We demonstrate how this contribution fully accounts for the non-Maxwellian features observed in all tests, probing the linear mechanical response function. The associated power-law spectrum is consistent with soft glassy rheology of systems with mechanical noise temperatures just above the glass transition [Fielding et al., J. Rheol. 44, 323 (2000)] and originates from a combination of superdiffusive bubble dynamics and stress diffusion, as recently evidenced in simulations of coarsening foam [Hwang et al., Nat. Mater. 15, 1031 (2016)].

Aligned and Oriented Collagen Nanocomposite Fibers as Substrates to Activate Fibroblasts.

Purified collagen possesses weak mechanical properties, hindering its broad application in tissue engineering. Strategies based on manipulating the hydrogel to induce fiber formation or incorporate nanomaterials have been proposed to overcome this issue. Herein, we use a microfluidic device to fabricate, for the first time, collagen hydrogels with aligned and oriented fibers doped with gold nanoparticles and carbon nanotubes. Results based on rheology, atomic force microscopy, and scanning electron microscopy reveal the formation of aligned and oriented collagen fibers possessing greater rigidity and stiffness on the doped hydrogels in comparison with native collagen. The mechanical properties of the hydrogels increased with the nanomaterial loading percentage and the stiffest formulations were those prepared in the presence of carbon nanotubes. We further evaluate the in vitro response of NIH-3T3 fibroblasts to the change in stiffness. The cells were found to be viable on all substrates with directional cell growth observed for the carbon nanotube-doped collagen fibers. No significant differences in the cell area, aspect ratio, and intensification of focal adhesions driven by the increase in stiffness were noted. Nonetheless, fibroblast proliferation and secretion of TGF-ß1 were greater on the hydrogels doped with carbon nanotubes. This nanomaterial-collagen composite provides unique features for cell and tissue substrate applications.

Multiple dynamic regimes in a coarsening foam.

Intermittent dynamics driven by internal stress imbalances in disordered systems is a fascinating yet poorly understood phenomenon. Here, we study it for a coarsening foam. By exploiting differential dynamic microscopy and particle tracking we determine the dynamical characteristics of the foam at different ages in reciprocal and direct space, respectively. At all wavevectors q investigated, the intermediate scattering function exhibits a compressed exponential decay. However, the access to unprecedentedly small values of q highlights the existence of two distinct regimes for the q-dependence of the foam relaxation rate Γ(q). At high q, Γ(q) ∼ q consistent with directionally-persistent and intermittent bubble displacements. At low q, we find the surprising scaling Γ(q) ∼ q δ , with δ = 1.6 ± 0.2. The analysis of the bubble displacement distribution in real space reveals the existence of a displacement cut-off of the order of the bubble diameter. Introducing such cut-off length in an existing model, describing stress-driven dynamics in disordered systems, fully accounts for the observed behavior in direct and reciprocal space.

Colloidal gelation, a means to study elasto-capillarity effects in foam.

We explore the evolution of the mechanical properties of a coarsening foam containing colloidal particles that undergo a sol-gel transition in the continuous phase. This enables us to investigate the impact of elasto-capillarity on foam mechanics over a wide range of elasto-capillary numbers. Right after initiating aggregation the foam mechanics is predominantly determined by the elasticity of the bubbles, while the contributions of the continuous phase become dominant as the colloidal particles form a gel. Taking into account the confined configuration of the foam skeleton for the formation of a space spanning gel, we find that for elasto-capillary numbers exceeding unity the foam mechanics can be described as a simple linear combination of the contributions due to respectively the bubble elasticity and the elastic skeleton. Surprisingly, the contributions of the elastic skeleton to the overall foam mechanics are larger for smaller elasto-capillary numbers, scaling as the inverse of the capillary number.

Glass transition of soft colloids.

We explore the glassy dynamics of soft colloids using microgels and charged particles interacting by steric and screened Coulomb interactions, respectively. In the supercooled regime, the structural relaxation time τ_{α} of both systems grows steeply with volume fraction, reminiscent of the behavior of colloidal hard spheres. Computer simulations confirm that the growth of τ_{α} on approaching the glass transition is independent of particle softness. By contrast, softness becomes relevant at very large packing fractions when the system falls out of equilibrium. In this nonequilibrium regime, τ_{α} depends surprisingly weakly on packing fraction, and time correlation functions exhibit a compressed exponential decay consistent with stress-driven relaxation. The transition to this novel regime coincides with the onset of an anomalous decrease in local order with increasing density typical of ultrasoft systems. We propose that these peculiar dynamics results from the combination of the nonequilibrium aging dynamics expected in the glassy state and the tendency of colloids interacting through soft potentials to refluidize at high packing fractions.

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.

Viscoelasticity of nematic liquid crystals at a glance.

Polarised microscopy is shown to be a powerful alternative to light scattering for the determination of the viscoelasticity of aligned nematic liquid crystals. We perform experiments in a wide range of temperatures by using an adapted version of the recently introduced differential dynamic microscopy technique, which enables us to extract scattering information directly from the microscope images. A dynamic analysis of the images acquired in different geometries provides the splay, twist and bend viscoelastic ratios. A static analysis allows a successful determination of the bend elastic constant. All our results are in excellent agreement with those obtained with the far more time-consuming depolarised light scattering techniques. Remarkably, a noteworthy extension of the investigated temperature-range is observed, owing to the lower sensitivity of microscopy to multiple scattered light. Moreover, we show that the unique space-resolving capacities of our method enable us to investigate nematics in the presence of spatial disorder, where traditional light scattering fails. Our findings demonstrate that the proposed scattering-with-images approach provides a space-resolved probe of the local sample properties, applicable also to other optically anisotropic soft materials.

Unexpected decoupling of stretching and bending modes in protein gels.

We show that gels formed by arrested spinodal decomposition of protein solutions exhibit elastic properties in two distinct frequency domains, both elastic moduli exhibiting a remarkably strong dependence on volume fraction. Considering the large difference between the protein size and the characteristic length of the network we model the gels as porous media and show that the high and low frequency elastic moduli can be respectively attributed to stretching and bending modes. The unexpected decoupling of the two modes in the frequency domain is attributed to the length scale involved: while stretching mainly relates to the relative displacement of two particles, bending involves the deformation of a strand with a thickness of the order of a thousand particle diameters.

Generic pathways to stability in concentrated protein mixtures.

We present a series of experimental results that disclose the crucial role of ionic strength and partial volume fractions in the control of the phase behaviour of binary protein mixtures. Our findings can be understood as that the ionic strength determines the relative contribution of the entropy of the protein counter-ions to the overall thermodynamics of the system. Associative phase separation and crystallization observed at, respectively, low and high ionic strength are suppressed at intermediate salt concentrations, where the entropy gain upon releasing the counter-ions from the double layer of the proteins is negligible and the entropy loss upon confining the counter-ions within the protein crystal phase significant. Moreover, we find that the partial volume fraction of the protein prone to crystallize determines the crystallization boundary and that the presence of other proteins strongly delays crystallization, leading to temporarily stable mixtures. These findings suggest that stability in more complex protein mixtures, such as the cytosol, relates to the ionic strength and protein composition rather than to protein specific properties.

Multiple dynamic regimes in concentrated microgel systems.

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 characterized by a fluid-solid transition, where the system properties switch from a viscous to an elastic low-frequency behaviour. Approaching this transition from below, we find that the range xi of spatial correlations in the dynamics increases. Beyond this transition, xi 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 xi on volume fraction is reminiscent of the behaviour 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 switches from purely thermally driven to stress driven.

Scattering information obtained by optical microscopy: differential dynamic microscopy and beyond.

We describe the use of a bright-field microscope for dynamic light scattering experiments on weakly scattering samples. The method is based on collecting a time sequence of microscope images and analyzing them in the Fourier space to extract the characteristic time constants as a function of the scattering wave vector. We derive a theoretical model for microscope imaging that accounts for (a) the three-dimensional nature of the sample, (b) the arbitrary coherence properties of the light source, and (c) the effect of the finite numerical aperture of the microscope objective. The model is tested successfully against experiments performed on a colloidal dispersion of small spheres in water, by means of the recently introduced differential dynamic microscopy technique [R. Cerbino and V. Trappe, Phys. Rev. Lett. 100, 188102 (2008)]. Finally, we extend our model to the class of microscopy techniques that can be described by a linear space-invariant imaging of the density of the scattering centers, which includes, for example, dynamic fluorescence microscopy.

Differential dynamic microscopy: probing wave vector dependent dynamics with a microscope.

We demonstrate the use of an ordinary white-light microscope for the study of the q-dependent dynamics of colloidal dispersions. Time series of digital video images are acquired in bright field with a fast camera, and image differences are Fourier analyzed as a function of the time delay between them. This allows for the characterization of the particle dynamics independent of whether or not they can be resolved individually. The characteristic times are measured in a wide range of wave vectors and the results are found to be in good agreement with the theoretically expected values for Brownian motion in a viscous medium.

Time-resolved-correlation measurements of temporally heterogeneous dynamics.

Time resolved correlation (TRC) is a recently introduced light scattering technique that allows one to detect and quantify dynamic heterogeneities. The technique is based on the analysis of the temporal evolution of the speckle pattern generated by the light scattered by a sample, which is quantified by cI(t, tau), the degree of correlation between speckle images recorded at time t and t + tau. Heterogeneous dynamics results in significant fluctuations of cI(t,tau) with time t. We describe how to optimize TRC measurements and how to detect and avoid possible artifacts. The statistical properties of the fluctuations of cI are analyzed by studying their variance, probability distribution function, and time autocorrelation function. We show that these quantities are affected by a noise contribution due to the finite number N of detected speckles. We propose and demonstrate a method to correct for the noise contribution, based on a N--> infinity extrapolation scheme. Examples from both homogeneous and heterogeneous dynamics are provided. Connections with recent numerical and analytical works on heterogeneous glassy dynamics are briefly discussed.

Effect of temperature on carbon-black agglomeration in hydrocarbon liquid with adsorbed dispersant.

Suspensions of carbon black in oil, stabilized with adsorbed polyisobutylene succinimide (PIBSI) dispersant, are commonly used as model systems for investigating the soot-handling characteristics of motor oils. The structure of the carbon-black agglomerates changes dramatically with temperature; this results in a concomitant change in the suspension rheology. Linear and nonlinear rheological experiments indicate a large increase of the interparticle attractions as the temperature is raised. To elucidate the origin of this behavior, we investigate the effect of temperature on the stabilizing effect of the dispersant. Measurements of adsorption isotherms of the dispersant on carbon black indicate that there is little variation of the binding energy with temperature. Intrinsic viscosity measurements of PIBSI dispersants in solution clearly exhibit an inverse dependence of the dispersant chain dimension with temperature. These results suggest that the temperature-dependent changes in the chain conformation of the PIBSI dispersant are primarily responsible for the changes in the dispersion rheology, and we propose a simple model to account for these data.