Tetsuharu Narita

Soft colloidal matter: A phenomenological comparison of the aging and mechanical responses with those of molecular glasses

Highly concentrated colloidal suspensions are often considered to exhibit behavior similar to that of glass-forming systems. While there is considerable rheological information in the literature concerning the flow behavior of such systems, there is little that has examined the mechanical response in a fashion that makes explicit comparisons with the relaxation behavior of molecular or polymeric glasses. On the other hand there is a significant literature that looks at “shear melting” and subsequent aging of such glasslike or “pasty” liquids. Here we report results for a polymer latex particle system at different concentrations near to the glassy or pasty regimes. Stress relaxation experiments and aging after shear melting experiments were performed. Single step stress relaxation results presented as isochrones of stress vs strain show behavior similar to that of polymers at the lower concentrations (50% and 53%). That is, there is a linear regime of behavior (generally less than a deformation of 1%) foll...

Advancing liquid contact line on visco-elastic gel substrates: stick-slip vs. continuous motions

We studied the dynamics of water sessile droplets advancing on hydrophobic and visco-elastic poly(styrene-butadiene-styrene)(SBS)–paraffin gel substrates at various inflation rates. During the advancing process, the droplet contact line exhibits three different regimes of motions. When the contact line advances at a high velocity, it moves continuously with a constant contact angle. As the contact line slows down, it starts a stick-slip motion: the contact line is pinned at a certain position and then suddenly slips forward. With further decrease of the velocity, the contact line stops the stick-slip motion and continuously advances again. The observed threshold values for the transitions of the contact line motions (continuous–stick-slip–continuous) indicate that the rheology of the gel drastically affects the dynamics of liquid on its surface. We suggest that on visco-elastic gels, the moving contact line exhibits both aspects of wetting on elastic solids and wetting on viscous liquids depending on the characteristic frequency of the gel surface deformation. At an intermediate regime, the stick-slip motion of the contact line appears. We also propose a simple geometrical model in the stick-slip regime which allows us to relate the jumps of the droplet radius to the jumps of the apparent contact angle.

Contact-line recession leaving a macroscopic polymer film in the drying droplets of water-poly(N,N-dimethylacrylamide) (PDMA) solution.

We found that the drying process of the droplet of water-poly(N,N-dimethylacrylamide) (PDMA) solution on a glass substrate shows a somewhat unusual behavior. In this system, the contact line starts to recede at an early stage of drying, and as it recedes, it leaves a macroscopic polymer film behind. The resulting film has a volcano-like profile, but the peak is not located at the edge of the film but in the middle of the film. We studied the drying process changing the polymer concentration and the wetting property of the substrate. We found that the onset of the contact-line recession depends upon the initial contact angle greatly, while the receding contact angle does not depend upon the initial contact angle. We conjecture that this phenomenon is caused by the Marangoni force, which acts to bring the surface of the solution inward because of the negative dependence of the surface tension on the polymer concentration.

Fracture of dual crosslink gels with permanent and transient crosslinks

We have carried out systematic fracture experiments in a single edge notch geometry over a range of stretch rates on dual crosslink hydrogels made from polyvinyl alcohol chains chemically crosslinked with glutaraldehyde and physically crosslinked with borate ions. If the energy release rate necessary for crack propagation was calculated conventionally, by using the work done to deform the sample to the critical value of stretch c where the crack propagates, we found that the fracture energy  peaks around ~ 0.001 s-1 before decreasing sharply with increasing stretch rate, in contradiction with the measurements of crack velocity. Combining simulations and experimental observations, we propose therefore here a general method to separate the energy dissipated during loading before crack propagation, from that which is dissipated during crack propagation. For fast loading rates (with a characteristic strain rate only slightly lower than the inverse of the typical breaking time of physical bonds), this improved method to estimate a local energy release rate glocal at the onset of crack propagation, gives a value of the local fracture energy  local which is constant, consistent with the constant value of the crack propagation velocity measured experimentally. Using this improved method we also obtain the very interesting result that the dual crosslink gels have a much higher value of fracture energy at low loading rates than at high loading rates, contrary to the situation in classical chemically crosslinked elastic networks.

Dynamics of the contact line in wetting and diffusing processes of water droplets on hydrogel (PAMPS–PAAM) substrates

We studied the dynamics of the wetting and diffusing processes of water droplets on hydrogel (Poly(2-acrylamido-2-methyl-propane-sulfonic acid -co- acrylamide) (PAMPS–PAAM)) substrates. The profiles of the droplet and substrate were measured simultaneously using a grid projection method. We observed that as the water droplet diffuses into the gel, the contact line of the droplet exhibits successively two different behaviors: pinned and receding, and the transition between these two behaviors is closely related to the local deformation of the gel substrate. The contact line is pinned at an early stage. As the water diffusion proceeds, the contact angle of the droplet decreases while the angle of the local slope of the gel surface near the contact line increases. At the moment where these two angles almost correspond to each other, the contact line starts to recede. Our results indicate that due to the water diffusion, a locally swollen region is formed in the vicinity of the droplet–gel interface, and whether the contact line is pinned or recedes is determined by the surface property of this swollen region.

Rheology of a dual crosslink self-healing gel: Theory and measurement using parallel-plate torsional rheometry

Tough hydrogels can be synthesized by incorporating self-healing physical crosslinks in a chemically crosslinked gel network. Due to the breaking and reattachment of these physical crosslinks, these gels exhibit a rate-dependent behavior that can be different from a classical linear viscoelastic solid. In this work, we develop a theory to describe the linear mechanical response of a dual-crosslink gel in a parallel-plate torsional rheometer. Our theory is based on a newly developed finite strain constitutive model. We show that some of the parameters in the constitutive model can be determined by carrying oscillatory torsional experiments. For consistency, we also show that the torsion data in an oscillatory test can be predicted using our theory with parameters obtained from tension tests. Our theory provides a basis for interpreting and understanding the test data of these gels obtained from rheometry.

Advancing-drying droplets of polymer solutions: Local increase of the viscosity at the contact line

We present experimental results concerning the advancing motion of drops of polymer solutions in the presence of controlled evaporation. We find that at high advancing velocities the classical Cox-Voinov law is verified, i.e. the advancing contact angle varies linearly with the capillary number. Below a critical velocity the contact angle increases as the advancing velocity is reduced. These results can be explained by taking into account the divergence of the rate of evaporation close to the contact line leading to an accumulation of polymer close to the edge of the drop. The induced local increase of the viscosity explains the increase of the contact angle. We show that the accumulation of polymer over a few nanometers is sufficient to slow down the contact line.

Wrinkling of a nanometric glassy skin/crust induced by drying in poly(vinyl alcohol) gels

During drying of a chemically crosslinked poly(vinyl alcohol) gel, we observed, by atomic force microscopy (AFM) and scanning electronic microscopy, micron-order wrinkling which remains until the end of drying. We show that while at high humidity drying is homogeneous and no surface instability is observed, fast drying at low humidity induces the formation of a glassy skin layer on the soft bulk that wrinkles because the skin is compressed as the bulk contracts. The role of the glass transition is not only to create a mismatch of the elasticity with the bulk but also to remove the in-plane tensile tension which stretches the skin before drying. We proved experimentally the presence of this thin glassy skin on the wet bulk by approach–retract curve measurements in AFM. We also comment on the effect of mechanical restraint and the observation of hierarchical wrinkling over different length scales.

Polymer–surfactant interactions: their cooperativity and stoichiometry

Abstract Surfactant bindings with solvated and crosslinked polyelectrolytes having charges on the side chain as well as chain backbone have been studied using various kinds of oppositely charged surfactants. We have found that there are three categories of the surfactant binding: they are cooperative and stoichiometric, non-cooperative and stoichiometric and cooperative and non-stoichiometric. The modes of these categories are predominantly determined by the steric chemical structure of the surfactant, hydrophobicity and the charge densities of the polyelectrolytes. Process and mechanism of the surfactant binding have been discussed.

Chemo-osmotically driven inhomogeneity growth during the enzymatic gelation of gelatin

We present an extensive study of the enzyme-mediated, isothermal formation of covalently cross-linked gelatin gels. We find that the enzymatic activity in the forming network is drastically reduced compared with that in solution, and show that this can be attributed to the growing level of cross-link induced geometric constraints which impede translational and rotational motions. Thanks to the slowness of these kinetics, we monitor the concomitant build-up of the shear modulus G′ and of the optical turbidity , which indicates that gelation is associated with the development of a high level of inhomogeneity. We find that, as the gelatin concentration cG is varied, the levels of G′ and are strongly anti-correlated. Moreover, the lower cG, the more precocious the emergence of . We are able to analyze inhomogeneity development in terms of the amplification of structural fluctuations via the coupling between the kinetics of the cross-linking reaction and the osmotic flow driven by swelling pressure fluctuations. We expect this positive feedback mechanism to be efficient in any slow, irreversible gelation process.