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Possible effects of multiplicative noise on instabilities

Speaker: Dr. François Pétrélis (ENS-Paris, CNRS)
Date : Apr. 23 (Mon), 16:00-17:30
Place : Room 155B, Main Bldg.

Close to the onset of an instability, it is expected that fluctuations can play a role. In general, fluctuations act additively (broadly speaking, their effect do not depend on the amplitude of the unstable field) and are responsible for a variety of effects such as the well known anomalous critical exponents of equilibrium phase transition.

In out of equilibrium systems, fluctuations can be multiplicative: their effect vanish when the amplitude of the unstable mode is zero. Several new effects appear. During this seminar I will discuss in particular two topics: what happen to the onset (is it still defined, how to calculate
it)? and what happen above onset (focussing on the so-called on-off intermittent regime).

The presented results will be illustrated with examples in the context of instabilities in fluid dynamics and magneto hydrodynamics.

Macroscopic properties of ferromagnetic nematics

Speaker: Prof. Helmut R. Brand (University of Bayreuth)
Date : Mar. 19 (Mon), 16:30-18:00
Place : Room 155B, Main Bldg.

Already more than 4 decades ago the possibility of ferromagnetic nematic liquid crystals has been postulated [1] by combining ferromagnetic nanoparticles with a nematic solvent. First experiments along these ideas were carried out immediately [2], giving rise to ferronematics with no spontaneous magnetization. Only a few years ago [3], with the development of suitably well-characterized magnetic nanoparticles, truly ferromagnetic nematics could be synthesized and analyzed thus establishing the first room temperature multiferroic liquid system. The static properties including magneto-optic and converse magnetoelectric effects were demonstrated [4]. Quite recently the study of the dynamics of truly ferromagnetic nematic liquid crystals properties has started [5]. It was demonstrated in [5] that a dissipative cross-coupling between the two order parameters [6], magnetization and director, is essential to account for the dynamic experimental results quantitatively. Recent developments [7,8] in the dynamic domain include investigations of the light scattering behavior as well as the coupling to flow including shear flows and the analog of the Miesowicz viscosities familiar from usual nematic liquid crystals.

[1] F. Brochard and P.G. de Gennes, J. Phys. 31, 691 (1970).
[2] J. Rault, P.E. Cladis, and J.-P. Burger, Phys. Lett. A 32, 199 (1970).
[3] A. Mertelj, D. Lisjak, M. Drofenik, and M. Copic, Nature 504, 237 (2013).
[4] A. Mertelj, N. Osterman, D. Lisjak, and M. Copic, Soft Matter 10, 9065 (2014).
[5] T. Potisk et al., Phys. Rev. Lett. 119, 097802 (2017).
[6] E. Jarkova, H. Pleiner, H.-W. Mueller and H.R. Brand, J. Chem. Phys. 118, 2422 (2003).
[7] T. Potisk et al., Phys. Rev. E 97, 012701 (2018).
[8] T. Potisk et al., to be published.


Speaker: 藤芳 暁 氏 (東京工業大学)
Date : 3月5日(月) 13:30-15:00
Place : 本館156号室


[1] S. Fujiyoshi et al.; Phys. Rev. Lett. 100, 168101 (2008).
[2] S. Fujiyoshi et al.; Phys. Rev. Lett. 106, 078101 (2011).
[3] T. Furubayashi et al.; J. Am. Chem. Soc. 139, 8990 (2017).

Electrically Controlled Dynamics of Knotted Director Fields and Defects in Liquid Crystals

Speaker: Prof. Ivan I. Smalyukh (Univ. Colorado, Boulder)
Date : Nov. 30 (Thu), 11:00-12:30
Place : Room 239, Main Bldg.

Active colloids and liquid crystals are capable of locally converting the macroscopically-supplied energy into directional motion and promise a host of new applications, ranging from drug delivery to cargo transport at the mesoscale. In this presentation, I will discuss how knotted fields and defects in liquid crystals can locally transform electric energy to translational motion and allow for the transport of cargo along directions dependent on frequency of the applied electric field. By combining polarized optical video microscopy and numerical modeling that reproduces both the equilibrium structures of solitons and their temporal evolution in applied fields, we uncover the physical underpinnings behind this reconfigurable motion and study how it depends on the structure and topology of defects. In my lecture I will show that, unexpectedly, the directional motion of the studied defects with and without the cargo arises mainly from the asymmetry in rotational dynamics of molecular ordering in liquid crystal, rather than from the asymmetry of fluid flows, as in conventional active soft matter systems.

Recent results on dense bacterial suspensions

Speaker: Dr. Hugues Chaté (CEA-Saclay & Computational Science Research Center)
Date : Nov. 28 (Tue), 16:30-18:00
Place : Room 239, Main Bldg.

I will present recent experimental results on dense bacterial suspensions obtained in the groups of Masaki Sano (University of Tokyo), Yilin Wu (Chinese University of Hong Kong), and Hepeng Zhang (Shanghai Jiaotong University). I will put them in context, situating them within our current knowledge of active matter, stressing differences and similarities. Particular attention will be paid to the modeling efforts already deployed or to be developed in order to understand the fascinating large-scale phenomena observed by these 3 groups.

Two stories on kinetic roughening: non-equilibrium cluster diffusion and interface collisions

Speaker: Dr. Olivier Pierre-Louis (ILM, Univ. Lyon 1, CNRS)
Date : Oct. 23 (Mon), 13:30-15:00
Place : Room 156, Main Bldg.

Kinetic roughening is the process by which interfaces develop a self-affine roughness in non-equilibrium systems. The interfaces can represent domain walls, the surface of a growing crystal, the edge of a bacterial colony, etc. We present two consequences of non-equilibrium kinetic roughening in two dimensions.

Our first story reports on the non-equilibrium diffusion of two-dimensional cluster. These clusters can represent e.g. an Ising droplet driven by a field, a monolayer island growing on a facet during crystal growth or dissolution, or an expanding bacterial colony. We find that the mean square displacement of the center of mass of clusters exhibit a transition from superdiffusive to subdiffusive diffusion during growth, with exponents controlled by the kinetic roughening of the cluster edge.

The second story focuses of the collision between growth fronts. We here aim to model e.g. the process by which grain boundaries form in graphene, or by which different expanding bacterial films collide. We claim that this process can be seen as non-trivial generalization of first passage processes. We show that the spatio-temporal roughness of the collision is controlled by the roughness accumulated before the collision. The distribution of times of collision, and the roughness of interface after collision are shown to obey dynamic scaling, and combine linearly the distributions of the two fronts before collision.

1. Non-equilibrium cluster diffusion during growth and evaporation in two dimensions (editor's suggestion), Y. Saito, M. Dufay, O. Pierre-Louis, Phys Rev. Lett. 108, 245504 (2012)
2. Non-equilibrium interface collisions, F.A. Reis, O. Pierre-Louis, preprint (2016)

Statistical mechanics of skin homeostasis

Speaker: Dr. Kyogo Kawaguchi (Harvard Medical School, Univ. of Tokyo)
Date : Aug. 2 (Wed.) 16:00-17:30
Place : Room 155B, Main Bldg.

Adult tissues undergo rapid turnover as mature cells are continuously lost, and new cells arise through cell division. The balance between gain and loss of cells must be finely orchestrated to maintain tissues, but how this balance is achieved remains largely unknown. For the skin, it had been assumed that the fate choices of stem cells (division or differentiation) are made strictly cell-autonomously. Here we recorded every stem cell fate choice within mouse skin epidermal regions over one week and found that, far from being cell-autonomous, stem cell loss by differentiation was compensated by direct neighboring division[1]. Furthermore, division events were triggered by neighbor differentiation and not vice versa, showing differentiation-dependent division as the core feature of homeostatic control.

In this presentation, we will formalize the problem of tissue homeostasis using a macroscopic nonequilibrium model setup[2]. Starting from an interacting particle system with Brownian motion, we show how the coarse-graining of our model will lead to the effective dynamics of the Voter model (DP2). We will then explain the pitfall in two-dimensions of using scaling relations of the type used before for the clonal fate trace of cells, and illustrate the workaround used in the new data analysis to definitively show the existence of cell-to-cell fate correlation.

[1] Mesa, Kawaguchi et al., Biorxiv (2017) doi:
[2] Yamaguchi, Kawaguchi, and Sagawa, Phys. Rev. E 96, 012401 (2017)

Numerical simulations with CUDA of critical dynamics on Ising-like systems

Speaker: Prof. Francisco Sastre (Univ. of Guanajuato, Mexico)
Date : Jul. 14 (Fri.), 16:30-18:00
Place : Room 155B, Main Bldg.

Ageing phenomena may arise in systems quenched, from some initial state, either (i) into a coexistence phase with more than one stable equilibrium state or else (ii) onto a critical point of the stationary state. Studies of dynamical critical properties, in the same way that the steady state properties, are neccesarily limited to samples of finite size. For this reason it is important to explore computational alternatives to obtain reliable results. In this work we explain how the critical dynamic can be implemented correctly in the Majority voter model, an non-equilibrium Ising-like model, using CUDA. By means of Monte Carlo simulations of the critical Ising and Majority voter models with Glauber dynamics on two dimensional honeycomb lattices we found that the dynamic critical exponents for the Majority voter model are in good agreement with the reported values of the Ising model.

The most effective model for describing the universal behavior of unstable surface growth

Speaker: Dr. Yuki Minami (AIST)
Date : Jun. 12 (Mon.), 16:00-17:30
Place : Room 155B, Main Bldg.

Anomalous transportation is characterized by a scale dependent transportation coefficient. It has been conjectured that such behavior is effectively described by a stochastic model. However, the parameter values of the model are determined only by measurements, and it is not established to connect the parameter value of such effective models with microscopic descriptions. It may be obvious that this can be studied by the renormalization group (RG) method. However, standard RG analysis, in which a fixed point and scaling exponents are studied, cannot determine the effective model for scale-dependent parameters.

In this talk, we propose a new theoretical framework to determine parameter values of an effective stochastic model for anomalous transportation [1]. We discuss that the model is determined by identifying ''a specific trajectory'' of solutions of the RG equation. The trajectory represents the minimum flow from an effective model to an infrared universal behavior. Specifically, we determine the Kardar-Parisi-Zhang equation that effectively describes the universal behavior of the Kuramoto-Sivashinsky equation. Furthermore, we discuss an application of our theory to other systems that have anomalous transportation.

[1] Y. Minami, S. Sasa, arXiv:1703.08946 [cond-mat.stat-mech].

Long-range nematic order and anomalous fluctuations in collective motion of swimming filamentous bacteria

Speaker: Dr. Daiki Nishiguchi (The University of Tokyo)
Date : Apr. 28 (Fri) 17:00-18:30
Place : Room 155B, Main Bldg.

Collective motion of self-propelled elements, as seen in bird flocks, fish schools, bacterial swarms, etc., is so ubiquitous that it has driven physicists to search for its possibly universal properties. Evidence for such universality has been provided by many theoretical and numerical studies using simple flocking models such as Vicsek-style models and hydrodynamic theories [1-2]. However, no experiments so far have been fully convincing in demonstrating the existence of this universality.

In this seminar, after introducing standard models of collective motion and giving state-of-the-art interpretations on previous experimental studies, I show our experiments on elongated bacteria swimming in a quasi-two-dimensional fluid layer [3]. Strong confinement and the high aspect ratio of bacteria induce weak nematic alignment upon collision, which gives rise to spontaneous breaking of rotational symmetry and global nematic order at sufficiently high density of bacteria. This homogeneous but fluctuating ordered phase has turned out to exhibit true long-range orientational order, non-trivial giant number fluctuations, and algebraic correlations associated with Nambu-Goldstone modes, which verifies the existence of an active phase predicted to emerge in standard flocking models. Such properties contrast our system with usual bacterial experiments that end up with turbulent states without any global orientational order.

Through our experiments, I will also discuss (i) what might be crucial for the emergence of such universality in reality and (ii) possible discrepancy between our experiments and theoretical predictions with approximations.

[1] F. Ginelli, “The Physics of the Vicsek model”, Eur. Phys. J.: Special Topics, 225, 2099 (2016).
[2] J. Toner, Y. Tu, and S. Ramaswamy. “Hydrodynamics and phases of flocks”, Ann. Phys. (N.Y.), 318, 170 (2005).
[3] D. Nishiguchi, K. H. Nagai, H. Chaté, and M. Sano, “Long-range nematic order and anomalous fluctuations in suspensions of swimming filamentous bacteria”, Phys. Rev. E, 95, 020601(R) (2017).

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