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Condensed Matter Theory

  • FUKUDA Jun-ichi, Professor
  • MATSUI Jun, Lecturer
  • YABUNAKA Shunsuke, Assistant Professor
Research topics in our group cover various phenomena in non-equilibrium systems and complex systems. Our focus is on theoretical and computational physics of soft condensed matter, and current research subjects include

  1. Self-organized structures and dynamics of liquid crystals
  2. Optical properties of ordered structures in soft matter
  3. Field theory of polymeric systems
  4. Poly-amorphism and crystallization
  5. Slowing dynamics near the glass transition
  6. Surface structure and dynamics in soft matters
  7. Theoretical models of active matter

1. Self-organized structures and dynamics of liquid crystals

We study various self-organized structures and dynamics of liquid crystals, mainly by numerical calculations based on continuum theories. Research topics of interest include, but are not limited to, exotic phases known as cholesteric blue phases [1], liquid crystal colloids (in part collaboration with Prof. Yasuyuki Kimura in our Department) [2], and liquid crystals in contact with sinusoidal grooves [3]. Structure of topological defects in liquid crystals is a major subject of interest.

Figure 1. Illustration of exotic structures exhibited by liquid crystals

References:
[1] Fukuda and Žumer, Phys. Rev. Lett. 104, 017801 (2010); Phys. Rev. Lett. 106, 097801 (2011); Nature Commun. 2, 246 (2011)
[2] Fukuda, J. Phys. Soc. Jpn. 78, 041003 (2009); Fukuda and Yokoyama, Phys. Rev. Lett. 94, 148301 (2005); Fukudaet al., Phys. Rev. E 69, 041706 (2004)
[3] Ohzono, Yamamoto and Fukuda, Nature Commun. 5,3735 (2014); Ohzono and Fukuda, Nature Commun. 3, 701 (2012)

2. Optical properties of ordered structures in soft matter -

Soft materials often exhibit self-organized structures whose periodicity is of the order of the wavelength of visible light. We investigate the properties of such structures as photonic crystals, and also how they can be observed by optical means such as confocal microscopy.

Figure 2. Example of the calculation of a confocal microscope image of a blue phase liquid crystal

References:
Fukuda and Žumer, Opt. Exp. 26, 1174 (2018); Nych, Fukudaet al., Nature Phys. 13, 1215 (2017); Fukudaet al., Proc. SPIE 10555, 105550A (2018); Proc. SPIE 9769, 976906 (2016)

3. Field theory of polymeric systems

From a microscopic model of semiflexible chains with bending elasticity, we made use of a field theory to derive the free energy functional and the equations of motion for compositional and orientational order parameters of the polymer component. We studied the coupling between phase separation and orientational ordering in the time evolution of these two order parameters.

Figure 3. Example of the calculation of a confocal microscope image of a blue phase liquid crystal

References:
Fukuda and Yokoyama, J. Phys. Soc. Jpn. 71, 1463 (2002); J. Chem. Phys. 115, 4930 (2001); Fukuda, Phys. Rev. E 59, 3275 (1999); Phys. Rev. E 58, 6939 (1998); Eur. Phys. J. B 7, 573 (1999)

4. Poly-amorphism and crystallization

We are interested in a model monatomic system, which is crystallized under cooling at high pressure and vitrified under cooling at low pressure. Using molecular dynamics simulation, we are calculating the T-P diagram and exploring the border between glass and crystalline in middle range of pressure.

Figure 4. The pentagonal bipyramid molecular geometry spreads over the whole system at temperature around Tg.

5. Slowing dynamics near the glass transition

We aim to understand the glass transition at the microscopic point of view; molecules are mostly trapped in cages formed by their surrounding molecules, which cause the diffusion coefficient decreasing and anomalous in highly supercooled liquids. Occasionally molecules hop to the neighbor's site and the neighbor do in the same way. These motions are collective and intermittent.

References:
T. Muranaka, J. Matsui and Y. Hiwatari, Mol. Sim. 41, 10-12, 822 (2015).

6. Surface structure and dynamics in soft matters

We theoretically study surface structure and dynamics in the presence of surface adsorption and charge effects. Recent topics are as follows: (1) Drag coefficient of a colloidal particle in a near critical binary mixture in the presence of preferential adsorption [1]. (2) Surface phase separation induced by preferential solvation effect of an antagonistic salt [2]

References:
[1] Shunsuke Yabunaka and Youhei Fujitani, Journal of Fluid Mechanics, 886 A2 (2020)
[2] Shunsuke Yabunaka and Akira Onuki, Phys. Rev. Lett. 119, 118001 (2017)

Figure 5. The deviation of the drag coefficient from the value without surface adsorption as a function of the correlation length far from the colloidal particle.
Figure 6. Schematic representation of lateral coexistence of two electric double layers.

7. Theoretical models of active matter

In recent years, self-propelled motion of active particles has attracted much interest in relationship with biology. As a very simple example, we constructed a theoretical model of self-propelled motion of a droplet due to the Marangoni effect under chemical reaction, and showed that there is a bifurcation from the motionless state to the self propelled state[3].

[3] S. Yabunaka, T. Ohta, and N. Yoshinaga, J. Chem. Phys. (2012)

Figure 7. Schematic figure of our model. Yellow dots represent the third component that is generated inside A-rich droplet formed by phase separation in an A-B binary mixture (shown by a blue circle and undergoing self-propelled motion). The third component influences the interface tension of the droplet.