第2回北大MMCセミナー

開催日時
2012年   7月 25日 16時 30分 ~ 2012年   7月 25日 18時 00分
場所
電子研 中央キャンパス総合研究棟2号館5階講義室(北12条西7丁目)
講演者
太田 隆夫(京都大学大学院理学研究科 ・教授)
 
Title   :Collective Motion of Self-Propelled Soft Particles

Abstract:
We investigate dynamics of deformable self-propelled particles based on the model equations for a single soft particle which has a coupling between migration and shape deformation [1]. The dynamics is governed by the time-evolution equations of the center of mass and the deformation tensor. Two models are introduced to study the collective motions of interacting particles. The first one (model I) has a repulsive interacting potential whose magnitude depends on the relative direction of elongation of a pair of particles [2, 3]. The force from other particles as well as a noise term are added to the equation for the center of mass. The other model (model II) has also a repulsive pair-wise potential but without an explicit alignment mechanism. This force enters both in the equation of the center of mass and in the equation for the deformation tensor so that existence of other particles causes directly shape deformation [4]. Numerical simulations are carried out in two dimensions by changing the noise intensity, the interaction strength and the particle density to obtain the phase diagram of the ordered and the disordered states. We take a Gaussian form of the interaction potential as a function of the distance between a pair of particles. In model I, the ordered state is a collective motion of the elongated particles which form a translational hexagonal lattice. By increasing the particle density and/or noise intensity, the collective motion is broken via a discontinuous transition. We show by a mean field analysis that deformability is a favorable origin for the transition [2, 3]. In model II, we show a variety of dynamics, not only the collapse of ordered state at high density, but also ”cluster lattices” and ”laning” depending on the density and the interaction strength [4]. It is noted that the instability of the ordered state at high density exhibited in both models might be related to the so-called reentrant fluids predicted in colloids with a Gaussian core potential [5, 6].

[1] T. Ohta and T. Ohkuma, Phys. Rev. Lett. 102, 154101 (2009).
[2] Y. Itino, T. Ohkuma and T. Ohta: J. Phys. Soc. Jpn. 80 033001 (2011).
[3] Y. Itino, and T. Ohta: J. Phys. Soc. Jpn. submitted.
[4] A. Menzel and T. Ohta: unpublished.
[5] F. H. Stillinger, J. Chem. Phys. 65, 3968 (1976).
[6] A. Lang, C. N. Likos, M. Watzlawek, and H. Loewen, J. Phys.: Condens. Matter 12, 5087 (2000).

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