Toshiyuki Shikata

Viscoelastic behavior of concentrated spherical suspensions

Experimental results on the linear viscoelastic behavior of concentrated suspensions are presented. The materials studies were prepared by dispersing submicron silica spheres at volume fractions φ ranging from 0.3 to 0.6, in a highly viscous liquid. The response to oscillatory shearing was determined over a wide range of frequency, ω. The zero frequency viscosity η0 and the limiting high‐frequency viscosity η’∞ for all particle radii studied were essentially identical to results previously obtained for suspensions having hard sphere interactions. In this range of concentrations, the frequency dependence of the dynamic moduli, G’ and G‘−ωη’∞, appears to be described by universal functions of ωτw where τw is the mean longest relaxation time proportional to a characteristic (Peclet) time defined as τp=a2/6Ds(φ). Herein a denotes the particle radius and Ds(φ) is a φ dependent short‐time self‐diffusion constant. We also found that at sufficiently high frequencies, G’ has a plateau, G∞, that is given by kT/a3 t...

Systematic Dielectric and NMR Study of the Ionic Liquid 1‐Alkyl‐3‐Methyl Imidazolium

The dynamic behaviors of ionic liquid samples consisting of a series of 1-alkyl-3-methylimidazolium cations and various counteranionic species are investigated systematically over a wide frequency range from 1 MHz to 20 GHz at room temperature using dielectric relaxation (DR) and nuclear magnetic resonance (NMR) spectroscopies. DR spectra for the ionic liquids are reasonably deconvoluted into two or three relaxation modes. The slowest relaxation times are strongly dependent upon sample viscosity and cation size, whereas the relaxation times of other modes are almost independent of these factors. We attribute the two slower relaxation modes to the rotational relaxation modes of the dipolar cations because the correlation times of the cations evaluated using longitudinal relaxation time (T(1) (13)C NMR) measurements corresponded to the dielectric relaxation times. On the other hand, the fastest relaxation mode is presumably related to the inter-ion motions of ion-pairs formed between cationic and anionic species. In the case of the ionic liquid bis(trifluoromethanesulfonyl)imide, the system shows marked dielectric relaxation behavior due to rotational motion of dipolar anionic species in addition to the relaxation modes attributed to the dipolar cations.

Enormously elongated cationic surfactant micelle formed in CTAB-aromatic additive systems

Abstract A cationic surfactant, cetyltrimethylammonium bromide (CTAB), when added to an aromatic substance such as benzoic, salicylic, or phthalic acid or their alkaline metal salt, induces a remarkable viscoelasticity in solution even in very diluted states. In these systems the authors, using an electron microscope, have been able to detect images of enormously elongated micelles which might cause this spectacular behavior in solution.

Nonlinear viscoelastic behavior of aqueous detergent solutions

Abstract Nonlinear viscoelastic behavior of aqueous solutions consisting of a cationic detergent, cetyltrimethylammonium bromide, and sodium salicylate as an added salt was examined with varying salt concentration. The nonlinear properties varied markedly with varying C*S, the concentration of free salicylate ion not caught in the detergent-salt complex. The systems with low C*S exhibited a kind of strain-hardening: the shear relaxation modulus, G(t, γ), increased with increasing magnitude of strain, γ; the shear stress, σI(t, γ . ) increased enormously and exhibited a marked overshoot at the start of shear flow of high rate of shear, γ . . The nonlinear behavior of systems with high C*S is somewhat similar to those of polymeric liquids: G(t, γ) decreased with increasing γ; the ratio σI(t, γ . / γ . was always a decreasing function of γ . . On the other hand, σI(t, γ . ) at high γ . exhibited a damped oscillatory behavior, which has not been reported for most of the polymeric systems. Evidently the viscoelastic properties cannot be described by the integrated constitutive equation with strain-dependent memory function which are suitable for polymeric liquids.

Microstructural changes in a colloidal liquid in the shear thinning and shear thickening regimes

The structure of a colloidal suspension under shear flow was studied by in situ small angle neutron scattering (SANS). This suspension exhibited shear thinning at low shear rates and shear thickening at high shear rates. Under quiescent conditions, the SANS profiles were azimuthally symmetric and contained a well-defined scattering maximum. This is due to local, liquidlike correlations between neighboring particles. Increasing shear rate lead to changes in the interparticle correlations. These changes are quantified by obtaining the anisotropic structure factor of the suspension under shear flow. We found an increased probability for the formation of inter-particle clusters in the gradient-vorticity plane. This results in an increase in the low angle scattering intensity in the flow direction, and the scattering peak, observed under quiescent conditions, is reduced to a shoulder. We found no evidence for a shear-induced phase transition in our experimental window. At low shear rates (γ), the microstructu...

Viscoelastic behavior of surfactant threadlike micellar solutions: effects of additives, 4.

Viscoelastic behavior of aqueous solutions of a cationic surfactant, cetyltrimethylammonium p-toluene sulfonate (CTApTS), and an organic salt, sodium p-toluene sulfonate (NapTS), was examined at room temperature by changing concentrations of CTApTS (CD) and NapTS (CS). CTApTS forms long threadlike micelles in aqueous solution even at low CD in spite of the absence of additives. Aqueous CTApTS solutions show pronounced viscoelastic behavior, which is perfectly described by Maxwell model type viscoelasticity, keeping a single set of relaxation parameters (GN and tau). The magnitude of relaxation strength, GN, is proportional to the square of CD irrespective of the value of CS. The value of a relaxation time, tau, is independent of CD, while the value remarkably reduces with increasing concentration (CS*) of excess pTS- ions in the bulk aqueous phase. The relationship between tau and CS* for the system is not identical with that for aqueous solutions of cetyltrimethylammonium bromide (CTAB) and NapTS at CS* < 30 mmolkg(-1). However, the relationships between tau and CS* for these systems agree well with each other at CS* >/= 30 mmolkg(-1). This implies that formed threadlike micelles in both systems have similar structure and the relaxation mechanism is controlled by CS*, but not influenced by the presence of Br- ions above CS* = 30 mmolkg(-1).

Structure and dynamics of a bisurea-based supramolecular polymer in n-dodecane.

The structure and dynamics of a supramolecular polymer formed by a bisurea-type compound, 2,4-bis(2-ethylhexylureido)toluene (EHUT), in an apolar solvent, n-dodecane (C12), were examined in detail. The EHUT/C12 organo-gel system forms long, dynamic chain-like supramolecular polymers, which lead to an entangled network showing remarkable viscoelastic behavior with two major relaxation modes. A slow relaxation mode with an approximately constant relaxation time, tauS, was observed in a flow region and the other, fast, relaxation mode with a time tauF1 (<tauS) was observed in a high-frequency range. Because no dielectric relaxation behavior was observed over a frequency region including the mechanical tauS and tauF1 relaxation modes, the formed supramolecular polymer does not possess any total dipole moment due to antiparallel intermolecular hydrogen bonding of the two ureido groups of each EHUT unit. A structural model for the supramolecular polymer formed in EHUT/C12 is proposed based on force-field simulations. This proposed model is consistent with all the experimental data concerning this system: flow birefringence measurements, dielectric spectroscopy, SANS, and FTIR.

Electron microscope study of viscoelastic cationic surfactant systems

Some cationic surfactants such as cetyltrimethylammonium bromide (CTAB) show a very spectacular viscoelasticity in solution, in even very diluted states, with some aromatic substances added, such as salicylic acid. Formerly, the authors established that the remarkable solution behaviour was the result of the entanglement of such enormously elongated giant micelles, based on recognition of the existence of large aggregates, through direct observation of them as substantial images under the electron microscope.In this report, we further confirm the relation between micelle length and solution viscoelasticity after arbitrarily obtaining different size aggregates by altering the media pH, or by admixing shorter chain length surfactants than CTAB, which alone did not induce viscoelasticity.

Reconsideration of the anomalous dielectric behavior of dimethyl sulfoxide in the pure liquid state

Although many vibrational spectroscopic studies using infrared absorption and Raman scattering techniques reveal that dimethyl sulfoxide (DMSO) forms intermolecular associations, such as dimers, in the pure liquid state, the results of many dielectric relaxation studies deny the presence of such associations and claim very little orientational correlation between the dipoles of DMSO molecules because of a Kirkwood correlation factor close to unity in the pure liquid state and in solution. Recently, we found reasons for the inconsistency and elucidated the presence of dimeric DMSO associations via dielectric relaxation measurements from 50 MHz to 50 GHz. The dissociation of DMSO dimers is the major dielectric relaxation process with a relaxation time of 19 ps, while the relaxation of monomeric DMSO is a minor mode with a relaxation time of 4.5 ps at 25 °C and slightly increasing strength with increasing temperature.

Rheo-SANS Studies on Shear-Thickening/Thinning in Aqueous Rodlike Micellar Solutions

Shear-induced thickening/thinning phenomena of aqueous rodlike micellar solutions of cetyltrimethylammonium bromide (CTAB) and sodium p-toluene sulfonate (NapTS) were investigated by means of simultaneous measurements of rheology and small-angle neutron scattering (SANS), the so-called Rheo-SANS. The aqueous CTAB/NapTS solutions were classified into five different categories dependent on their flow behavior and micellar structure. By increasing salt concentration and/or shear rates, the micelles underwent morphological transition from (i) spherical or short rodlike micelles to (ii) long rodlike micelles without entanglements, followed by (iii) those with entanglements. These transitions were recognized as changes in flow behavior from Newtonian to shear-thickening and shear-thinning flow, respectively. In the latter two cases, anisotropic SANS patterns appeared around these critical shear rates. The physical meaning of the anisotropic SANS patterns accompanied by shear-thickening flow behavior is discussed in conjunction with other shear-thickening systems.