I am interested in structural phase transitions. X-ray and neutron scattering, especially a small-angle scattering and neutron spin echo spectroscopy are my main experimental methods.

I. Mesoscopic structure induced by coupling of the solvation effect and critical concentration fluctuation
Small-angle neutron scattering experiments were performed on mixtures of 3-methylpyridine, D2O, and antagonistic salt NaBPh4 to investigate concentration fluctuations and structures in the vicinity of the critical point. The SANS profiles of ternary mixtures with large critical temperature shifts can be analyzed by the sum of the concentration fluctuation and a scattering from spherical clusters. The size of the clusters increases as the critical point is approached. These results can be interpreted in terms of a strong coupling of the critical fluctuation and the solvation effect of salt ions. The critical behavior of the ternary mixture with the salt seems to belong to 2D-Ising like universality class. In water-rich mixtures, we discovered a phase transition from disordered phase to an ordered lamellar phase by decreasing temperature. Onion morphologies are observed by optical microscope below the phase transition temperature.

II. Spontaneous deformation of an oil droplet through the generation and the destruction of a gel-like structure
Spontaneous deformation of a tetradecane droplet with palmitic acid on an aqueous phase with a surfactant has been investigated by means of optical microscope and small-angle X-ray scattering. Palmitic acid is transported from the oil droplet to the aqueous phase by the concentration difference between the organic and the aqueous phases. The transport of palmitic acid causes the oil droplet interface to undergo various spontaneous deformations. When the oil droplet is placed on an aqueous surface, its diameter shrinks. Several tens of seconds later, the oil droplet suddenly expands and then shrinks in a second. After such a dramatic deformation, the oil droplet undergoes blebbing on its oil-water interface for over 1 h. We discuss the cause of these deformations in terms of the spatiotemporal variation of the interfacial tension and elucidate that the blebbing deformation is due to the surfactant aggregate generated by cationic and anionic surfactants.

    Y. Sumino, H. Kitahata, H. Seto, and K. Yoshikawa, "Blebbing dynamics in a oil-water-surfactant system through the generation and the destruction of a gel-like structure", Physical Review E 76, 055202(R) (2007).

    Y. Sumino, H. Kitahata, H. Seto, S. Nakata, and K. Yoshikawa, "Spontaneous Deformation of an Oil Droplet Induced by the Cooperative Transport of Cationic and Anionic Surfactants through the Interface", Journal of Physical Chemistry B, 113 (48), 15709-15714 (2009).

    Y. Sumino, H. Kitahata, H. Seto, and K. Yoshikawa, "Dynamical blebbing at a droplet interface driven by instability in elastic stress: a novel self-motile system", Soft Matter, 7, 3204-3212 (2011).

III. The structures and the vesicle formation of lipids
Origin of the unbinding transition of lipid bilayers was investigated by small-angle scattering and neutron spin echo. Cooperation of repulsive forces between bilayers, for example, Helfrich steric force and electrostatic force, induces the unbinding of lipids. A large uni-lamellar vesicle formation from lipid dry film on solid substrate is investigated in terms of time-resolved small-angle X-ray scattering.

IV. The relative form factor method and the relative intermediate form factor method
New methods to interprete SANS and NSE data with contrast variation method are introduced.
  • M. Nagao, H. Seto, and N. L. Yamada, "Interaction between droplets in a ternary microemulsion evaluated by the relative form factor method", Physical Review E, 75, 061401 (2007).
  • M. Nagao and H. Seto, "Concentration dependence of shape and structure fluctuations of droplet microemulsions investigated by neutron spin echo spectroscopy", Physical Review E, 78, 011507 (2008).
V. Dynamical behaviors of micelles and microemulsions
Membrane dynamics of micelles and microemulsions have been investigated by NSE, and the experimental results were explained by a concept of membrane fluctuations. VI. Pressure induced phase transition in a ternary microemulsion
A pressure-induced phase transition from a dense droplet to a lamellar structure was investigated by means of SANS, SAXS and NSE. The difference from the similar phase transition induced by temperature was clarified.

VII. Critical phenomena in a ternary microemulsion
Critical phenomena in a ternary microemulsion system composed of AOT, decane and D2O were investigated by means of small-angle neutron scattering. A crossover from the mean-field behavior to the 3D-Ising behavior was observed.

VIII. Precursor phenomena in martensitic transformation
A martensitic transformation in Fe-Pd alloy was investigated by means of x-ray scattering and neutron scattering. A new precursor phenomenon, namely "two-tetragonal-mixed phase" was observed for the first time, and explained by a concept of "crest-riding periodon".

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