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Statistical Physics

  • Hiizu Nakanish, Professor
  • Kiyohide Nomura, Associate Professor
  • Takahiro Sakaue, Assistant Professor
Our group studies equilibrium and non-equilibrium statistical physics theoretically on various systems. Recent research topics include granular media, polymer physics, stochastic processes, bio-physics, low-dimensional quantum systems, application of quantum field theory on statistical physics.
  1. Shear thickening in granular-fluid mixture
  2. Non-equilibrium dynamics of single polymers
  3. Statistical mechanics of ring polymer solutions
  4. Electro-rheological effect in binary fluids
  5. Low dimensional quantum systems

1. Shear thickening in granular-fluid mixture:

Figure 1. Dilatant fluid in theTaylor-Couette cell: The thickened regions are colored as red (compressive) and blue (tensile).

Dense granule-fluid mixture is called dilatant fluid and often shows severe shear thickening; its viscosity discontinuously increases by orders of magnitude under shear stress. We have developed a phenomenological model for its macroscopic behavior and found that the medium may show shear thickening oscillation, namely, the oscillation between thickened and relaxed states under a constant external state in a certain range[1]. This oscillation had never been reported, thus we constructed a experimental setup for this purpose and demonstrated the medium actually shows the oscillation as we predicted[2]. We also demonstrated that the thickening appear in the region of tensile stress[3].

[1] H. Nakanishi, S. Nagahiro, and N. Mitarai, Phys. Rev. E 85, 011401 (2012).
[2] S. Nagahiro, H. Nakanishi and N. Mitarai, EPL 104 (2013) 28002.
[3] S. Nagahiro and H. Nakanishi, submitted. arXiv:1511.05250

2. Non-equilibrium dynamics of single polymers:

Figure 2. Illustration of the non-equilibrium process of polymer translocation.

We study the dynamic of a long flexible polymer driven by various external forces. A key example is the translocation of biopolymers, i.e., the threading of DNA, RNA and proteins through a narrow pore. We have pointed out that in typical experiments, the translocation process takes place in the condition far-from-equilibrium accompanying of large conformational distortion (Fig. 1). We elucidated the underlying physics in term of the tension propagation along the chain backbone, and constructed a basic framework to describe such a highly non-equilibrium process [1].

The concept of tension propagation is important in various problems in polymer dynamics, including a stretching process by tensile force [2] and a rotation around a bar driven by torque [3]. A conceptually similar dynamics can be seen in the compression dynamics of nano-channel confined DNA, where the stress propagation manifests itself as the development of concentration shock wave [4].

[1] T. Sakaue, Phys. Rev. E. vol. 76 (2007) 021803; ibid, vol. 81 (2010) 041808.
[2] T. Sakaue. T. Saito and H. Wada, Phys. Rev. E. vol. 86 (2012) 011804.
[3] M. Laleman et al., Macromolecules. vol. 49 (2016) 405.
[4] A. Khorshid et al. Phys. Rev. Lett. vol. 113 (2014) 268104.