Research

## Solvation effects and phase transitions

In soft matter physics, much attention has been paid to the consequences of the Coulombic interaction among charged objects, such as small ions, charged colloids, charged gels, and polyelectrolytes. However, not enough effort has been made on solvation effects among solutes (including hydrophobic particles) and polar solvent molecules. Solvation is also called hydration for water and for aqueous mixtures. In mixtures of a water-like fluid and a less polar fluid (including polymer solutions), the solvation is preferential or selective, depending on whether the solute is hydrophilic or hydrophobic. See Figure for its illustration. The typical solvation free energy much exceeds the thermal energy kT per solute particle. Hence selective solvation should strongly influence phase behavior or even induce a new phase transition. In experiments on aqueous mixtures, it is well known that a small amount of salt drastically alters phase behavior. In biology, preferential interactions between water and cosolvents with proteins are of crutial importance. Thus selective solvation is relevant in diverse fields, but its understanding from physics is still in its infancy. We menion some intereasting problems so far treated or under way.
1)Antagonistic salt
Surface tension of electrolytes: Hydrophilic and hydrophobic ions near an interface, J. Chem. Phys. 128, 224704 (2008). [PDF]
Dynamics of binary mixtures with ions: dynamic structure factor and mesophase formation, J. Phys.: Condens. Matt. 21, 424116 (2009). [PDF]
Sovation Effects in phase transitions in soft matter, J. Phys.: Condens. Matt. 23, 284113 (2011). [PDF]

2)Precipitation due to selective solvation
Precipitation in aqueous mixtures with addition of a strongly hydrophilic or hydrophobic solute, Phys. Rev. E 82, 051501 (2010). [PDF]
Selective solvation effects in phase separation in aqueous mixtures, Curr. Opin. Colloid In. 16, 525 (2011).[PDF]

3)Colloid interactions
Charged colloids in an aqueous mixture with a salt, Phys. Rev. E 84, 051401 (2011). [PDF]

4)Ionic surfactant
Nonionic and ionic surfactants at an interface, Europhys. Lett. 82, 58002 (2008). [PDF]

5)Polyelectrolytes
Ion distribution around a charged rod in one and two component solvents: Preferential solvation and first order ionization phase transition, J. Chem. Phys. 131, 094905 (2009). [PDF]
Solvation and Dissociation in Weakly Ionized Polyelectrolytes, J. Phys. Chem. B 113, 3988-3996 (2009). [PDF]

6)Ionic wetting

7)Selective hydrogen bonding

8)Predrying transition on hydrophobic surfaces
Predrying transition on a hydrophobic surface: Statics and dynamics, Phys. Rev. E 84, 041602 (2011). [PDF]

9)Nanobubbles

Lecture:"Solvation (hydration for water) in phase separation in soft matters"(PDF)
Review: Phase Transitions in Soft Matter Induced by Selective Solvation, Bull. Chem. Soc. Jpn. 84, 569 (2011). [PDF]

## Hydrodynamics with evaporation and condensation: Dynamic van der Waals theory

Lecture: "Dynamic van der Waals theory: Evaporation and condensation"(PDF)
Droplet evaporation in one-component fluids: Dynamic van der Waals theory, Europhys. Lett. 84, 36003 (2008). [PDF]
Spreading with evaporation and condensation in one-component fluids, Phys. Rev. E 82, 021603 (2010). [PDF]

## Dynamics of crystal, polycrystal, and glass

Heterogeneous dynamics in polycrystal and glass in a binary mixture with changing size dispersity and composition, Phys. Rev. E 75, 041503 (2007). [PDF]
Plastic deformations in crystal, polycrystal, and glass in binary mixtures under shear: Collective yielding, Phys. Rev. E 81, 051501 (2010). [PDF]
Construction of a disorder variable from Steinhardt order parameters in binary mixtures at high densities in three dimensions, J. Chem. Phys. 135, 174109 (2011). [PDF]

## Orientation-stain glass

Nonspherical molecules such as KCN can form a crystal without long-range orientational order for mild molecular anisotropy, while liquid crystal phases can appear for large molecular anisotropy. Such crystals are called plastic crystals in a rotator phase. They undergo an orientational phase transition as the temperature $T$ is further lowered, where the crystal structure is cubic at high T and non-cubic at low T. With inclusion of impurities in such solids, the so-called orientational glass has been realized. Around the transitions, a peak in the specific heat and softening of the shear modulus have been observed. In real systems, the molecules often have dipolar moments, yielding dielectric anomaly. %where the impurities serve to pin the heterogeneous %orientation fluctuations. As a similar example, metallic ferroelectric glass, called relaxor, has been studied extensively. We propose a microscopic model of molecular dynamics simulation to study orientational glass in three dimensions. We present simulation results for mixtures of mildly anisotropic particles and spherical impurities. We also include the dipolar interaction to study ferroelectric glass with anomalously large dielectric constant.
K. Takae and A. Onuki
Molecular dynamics simulation of orientation glass formation in anisotropic particle systems in three dimensions
Europhys. Lett. 100, 16006 (2012).[PDF]

Preprint: "Structural phase transition and orientation-strain glass formation in anisotropic particle systems with impurities in two dimensions" (Kyohei Takae and Akira Onuki) [PDF]

## Stress-diffusion coupling and shear-induced phase separation

We investigate viscoelastic phase separation in polymer solutions under shear numerically using a time-dependent Ginzburg-Landau model. The gross variables in our model are the polymer volume fraction and a conformation tensor. The latter represents chain deformations and relaxes slowly on the rheological time giving rise to a large viscoelastic stress. The polymer and the solvent obey two-fluid dynamics in which the viscoelastic stress acts asymmetrically on the polymer and, as a result, the stress and the diffusion are dynamically coupled. Above the coexistence curve and for shear rates larger than a crossover value, phase separation is induced incompletely and the solution behaves chaotically. Below the coexistence curve, sharp interfaces appear with increasing the quench depth and the solvent regions act as a lubricant. In these cases the composition heterogeneity causes more enhanced viscoelastic heterogeneity and the macroscopic stress is decreased at fixed applied shear rate. We find steady two-phase states composed of the polymer-rich and solvent-rich regions, where the characteristic domain size is inversely proportional to the average shear stress for various shear rates. The space averages of the deviatoric stress components exhibit large temporal fluctuations. The average first normal stress difference frequently takes negative values at weak shear.
Viscoelastic Phase Separation in Shear Flow, Phys. Rev. E 70, 051503 (2004). [PDF]
Spatio-temporal structures in sheared polymer systems(A. Furukawa and A. Onuki) Physica D 205 195-206 (2005).

Viscoelastic phase separation with polymer-rich network without shear flow
Viscoelastic phase separation with polymer-rich network with shear rate=0.005
Shear-induced phase separation with shear rate=0.05 above coexistence curve
Viscoelastic phase separation with shear rate=0.05 inside coexistence curve, similar to gel breakage
Viscoelastic phase separation with shear rate=0.05 deep inside coexistence curve with increased polymer fraction

## Heat flow effect near the superfluid transition

We investigate the nonlinear dynamics of He4 heated from above under gravity by integrating model F equations in three dimensions. When a superfluid slightly below the transition is heated from above, vortex tangles and a sheetlike phase slip are generated near the bottom plate. Then a self-organized superfluid containing high-density vortices and phase slips grows upward into an ordinary superfluid. The thermal resistance due to these defects produces a constant temperature gradient equal to the gradient of the pressure-dependent transition temperature $T_{\lambda}(p)$. The temperature deviation T-T_lambda(p) consists of a negative constant independent of the height and time-dependent fluctuations in the self-organized region. Its time-average is calculated in good agreement with the experiment (W.A. Moeur et al., Phys. Rev. Lett. 78, 2421 (1997)).

Lecture: "Singular hydrodynamics: Piston effect near gas-liquid transition and self-organized states near superfluid transition", [PDF]
The HeI-HeII Interface in He4 and He3 -He4 near the Superfluid Transition, Jap. J. Applied Phys. 26 (1987) 365-366. [PDF]
Self-organization in 4He near the superfluid transition in heat flow and gravity, Phys. Rev. B 82, 024501 (2010). [PDF]
Superfluid helium heated from above under gravity. Superfluid with vortices and phase slips appears from bottom
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