Session H59: Rheology of Gels II

Viscoelasticity of gels with dynamic bonds: molecular kinetics and macroscopic mechanics

Incorporation of dynamic bonds into the polymer network of soft gels has been exploited as a strategy to enhance fracture toughness and to enable self-healing. Gels with dynamic bonds often exhibit macroscopic viscoelasticity which can be traced back to the kinetics of dynamic bonds undergoing dissociation and reformation. I will present our recent efforts to connect the molecular-level chemical kinetics to the continuum-level viscoelasticity. As a model system, I will focus on a hydrogel with two types of crosslinks: dynamic physical crosslinks and permanent chemical crosslinks, and describe two different approaches for modeling the relation between molecular kinetics and viscoelasticity. These two approaches are shown to capture experimental data of tensile tests with various strain histories and oscillatory torsion tests. I will also discuss how our modeling approach can potentially lead to a general theoretical framework of nonlinear viscoelasticity.   

Structural and Mechanical Self-healing of Physically Cross-linked Polymer Hydrogels

Telechelic polymers with hydrophobic, ‘sticky’ end groups form micelles with polymers bridging the hydrophobic cores, resulting in a transient network gel. These gels have interesting properties such as tunable flow dynamics and ability to self-heal after mechanical disruption by molecular exchange between the micelle cores [1,2]. Studies using rheology and in combination with Small-Angle Neutron Scattering (SANS) are presented, addressing the structure of the polymer chains and the crystalline structure of the micelle cores in the gel, using selective deuteration. The gel is disrupted by oscillatory strain, and self-heals on a time scale from seconds to tens of minutes. Different recovery patterns are observed for a gel that is liqueified by the strain vs. a gel that remains solid under strain. The results show how the mechanical and the structural recovery are linked and highly dependent on the processing history of the gel.

1. Zinn T, Willner L, Lund R, Pipich V, Richter D, Soft Matter 2012; 8: 623-626.
2. Zinn T, Willner L, Lund R, ACS Macro Lett 2016; 5(12): 1353-1356.   

In-situ photopolymerization and gelation of ionic liquids

Ionic liquids pose a novel and tunable medium for bulk polymerization of vinyl monomers. 1-vinylimidazole (VIm) displays intriguing viscosity growth trends when mixed with lithium bistriflimide (LiTf₂N) to form an ionic liquid (IL) and then photopolymerized via a free-radical mechanism. Previous work has shown that increasing [LiTf₂N] results in faster chemical conversion. We used rheology to monitor the in-situ synthesis of poly(VIm) from ILs containing varying ratios of VIm:LiTf₂N. Three distinct regimes were observed: (1) at low [LiTf₂N], samples increased in viscosity faster with [LiTf₂N] and behaved as solutions; (2) at intermediate [LiTf₂N], viscosity growth was maximized and samples underwent sol-to-gel transitions; (3) at high [LiTf₂N], viscosity growth slowed with [LiTf₂N] and samples exhibited viscoelastic behavior. We attribute gelation to Li+ complexing with imidazole pendant groups to form physical crosslinks. Our samples also exhibited "dark curing", the extent of which increased with [LiTf₂N]. EPR spectroscopy demonstrates that Li+ can stabilize propagating radicals, leading to enhanced curing following UV exposure. Taken together, these results demonstrate that the rheological behavior of photopolymerized ILs can be tuned by varying the composition and UV dosage.   

Effect of morphology on ion transport in presence of surface conduction pathways, and rheology of model colloidal gels

Electrolytes containing colloidal gels are have been proposed as a possible alternative to liquid electrolytes in batteries due to their thermal stability and mechanical strength. In this work, we specifically considered the rheology and conductivity characteristics of open, equilibrium gels designed from an inverse design strategy on the interaction potentials. Using small amplitude oscillatory shear (SAOS) simulations, rheology of these suspensions is probed from a coarse-grained framework. Ion transport through these gel was studied using kinetic Monte Carlo simulations. We show that for certain parametric conditions, some gel structures exhibit simultaneously optimal ion diffusivity and storage modulus. These results suggest that such open network colloidal gels may provide a means to overcome the the mechanical strength and ionic conductivity trade-off in electrolytes.   

Selectively enhanced diffusion through polymer gels

Designing selective hydrogels that passively filter particles based on their interactions is a fundamental goal of nanotechnology. While it is conceptually straightforward for a particle’s interactions to facilitate its own retention (specific binding domains on the gel trap particles with complementary recognition sites), it is much more difficult for a particle to facilitate its own diffusion. This requires the binding interaction to locally reorganize the internal structure of the gel. We present a mechanism for this behavior and describe a theoretical model of the diffusion of interacting and non-interaction particles that is backed by numerical results. We also discuss how our results can lead to the design of artificial selective gels inspired by the Nuclear Pore Complex.   

Rheology of aluminosilicate hydrogels

Thanks to their environmental acceptability and their adaptability over a wide range of applications, alkali solutions of aluminosilicates are increasingly used in nuclear or building and construction industry. Although such solutions are increasingly used in the industry, there remain outstanding questions regarding the gelation process, that are driven by the composition of the solution, their stabilities and their aging. It is therefore crucial to provide a clear and realistic description of such fluids in order to tune and taylor macroscopic properties.
The aim of this work is to study the linear viscoelasticity properties of the gels throughout a variety of rheological tests, combining with time resolved Small Angle X-ray Scattering (SAXS) from liquid to gelling state. The aluminosilicate gels show a power law behavior that is well described by the Fractional Maxwell Model (FFM). The derived parameters of the FFM are well correlated to the fractal dimension that can be directly extracted from the SAXS measurements. Aging of these gels will also be discussed in relation to the results obtained at mesoscale by SAXS   

Rheology of Gels

Gels, nonfluid networks of particles or polymers that are pervaded by fluid, appear ubiquitously within soft matter in practical applications as well as in living biological systems. The mechanical properties of gels are intermediate between those of fluids and solids, and depend sensitively on the structure of the gel constituents across multiple length scales. This focus session invites experimental, theoretical, and computational studies of the rheological properties of gels, including chemical and physical gels, hydrogels, colloidal gels, and biological gels, with particular interest and emphasis on connecting structural properties to flow properties. Contributions examining the effect of non-equilibrium activity (driven by molecular motors or by active particles) on gel mechanics are encouraged.   

Phase behavior of equilibrium linker gels

Gels made from networks of colloids linked by physical bonds are an important class of soft materials. A standard route to produce a gel is to rapidly cool a suspension of isotropically attractive colloids, with gelation resulting from kinetic arrest during spinodal decomposition. However, such gels are inherently nonequilibrium, and the gel’s properties inevitably change as it ages. Alternatively, “equilibrium” gels with open, homogeneous structures that are resilient to aging can be created by restricting the number of bonds that form between particles. Here, we report on equilibrium gelation controlled by the addition of a secondary “linker” macromolecule that mediates bonding between colloids. The phase diagrams of such mixtures were predicted using Wertheim’s thermodynamic perturbation theory (TPT) and compared to molecular dynamics simulations. Good agreement was obtained between the predictions and simulations, with the spinodal region depressed to lower colloid densities by increased linker length at fixed linker-to-colloid number ratio. However, the presence of looping in the colloid–linker networks inhibited percolation compared to TPT predictions at low linker concentrations.   

Effect of Surface Stiffness on the Interfacial Dynamics of Dense Microgel Liquids

Many biological and engineering processes involve highly soft and deformable surfaces, ranging from biolubrication between synovial joints to aquaplaning of rubbery tires. In this talk, I will present our recent study of the effect of surface stiffness on the glassy dynamics of dense microgel liquids. Specifically, we investigate the interfacial dynamics of poly(N-isopropylacrylamide) (PNIPAM) microgels, whose particle stiffness can be tuned by polymer crosslinking degree upon polymerization, at a solid surface coated with PNIPAM microgels of matched or mismatched stiffness. By using confocal laser scanning microscopy, we analyze the mean-squared-displacement (MSD) of PNIPAM particles in the first 1-2 confined layers near the coated surface. The MSD shows strong dependence on surface stiffness and particles tend to approach the Brownian motion at the softest surface even at the microgel volume fraction approaching the one for glass transition. The correlation between dynamic heterogeneity and friction of confined dense PNIPAM suspensions is further examined with varied particle volume fraction and particle-to-surface stiffness ratio.
  

Non-equilibrium deformation and relaxation of giant floppy vesicles in a precisely controlled extensional flow

In this work, we study the non-equilibrium dynamics of single floppy vesicles under large strain rates (~15 s^-1) using a Stokes trap, which is a new technique developed in our lab for controlling the center-of-mass position of multiple particles or single molecules in a free solution. In this way, we directly observe the vesicle shape and conformations as a function of reduced volume,which is a measure of a vesicle’s equilibrium shape departure from sphericity. We observe the formation of asymmetric dumbbell shapes, symmetric dumbbell, pearling, and wrinkling and buckling instabilities for vesicles depending upon the nature of flow and amount of membrane floppiness. We report the precise stability boundary of the flow-based phase diagram for vesicles in Capillary number (Ca)-reduced volume space, where Ca is the ratio of the bending time scale to the of flow time-scale. We further probe the stability boundary at two different viscosity ratios to understand how the onset of instability in vesicles depends on viscosity ratio. We also present results on the long-time relaxation dynamics of vesicles from high deformation back to their equilibrium shapes after the cessation of flow.   

Dependence of Hydrogel-Glass Energy of Adhesion and Kinetics of Delamination on Hydrogel Concentration

Pseudomonas aeruginosa bacteria move collectively at viscous interfaces via twitching motility¹, which plays an important role in biofilm formation and subsequent infection. This phenomenon is studied by forcing the bacteria to twitch at the interface of a glass microscope slide and a hydrogel such as agar. The expanding bacterial colony needs to break the adhesive bond between the glass and the agar. Although the pulling force generated by the type-IV-pilus² and the stiffness of the agar hydrogel³ have been measured, the adhesive interaction between the glass and the agar is not well characterized. We have used micropipette deflection experiments to measure the increase in the adhesive shear strength with increasing agar concentration: 120 ± 60 N/m² for 1.0% w/v agar, and 260 ± 110 N/m² for 1.5% w/v agar. To more accurately mimic the breaking of the adhesive bond by the bacterial cells, we have developed a confined blister test that measures the energy of adhesion and the kinetics of delamination of agar-glass interfaces. I will describe the results of our measurements performed for different agar concentrations.
1 Semmler, Whitchurch, Mattick (1999) Microbiology 145:2863
2 Maier (2013) Soft Matter 9:5667
3 Sharma, Bhattacharya (2014) Journal of Food Engineering 141:93