Academic Year 2022

• Nonuniform flows of dense suspensions【Onsite & Zoom】
  • Ryohei Seto (Wenzhou Institute, University of Chinese Academy of Sciences)
  • Date: 2023/03/08 16:00 --
  • Place: Conference Room K206, Main Building, Yukawa Institute, Kyoto U. & 【Zoom】
    *It will be held in hybrid.
  • Abstract:
    Suspension rheology is a classical subject. An idealized suspension looks rather simple, i.e., a mixture of Newtonian-type liquid and many rigid spheres dispersed there. The surface of undeformable, freely suspended particles merely give moving boundary conditions to the liquid. Such an idealized system contains no freedom to deform and store elastic energy like viscoelastic fluids. Thus, it is natural to consider a suspension as a viscous fluid. Historically, this idea has dominated suspension rheology studies.
    However, it has caused much confusion in explaining complex rheological behaviors at higher volume fractions, such as shear thickening and normal stress differences, in a conventional framework of viscous fluids. Recent research has progressed in a way that breaks with this conventional standard of suspension rheology and led to a situation where a basic consensus can be reached. The key has been to consider the mechanics of contacting particles rather than confine research to the hydrodynamic problems described abaove. Suspension rheology can now be reproduced by simulations with the Discrete Element Method [1].
    Rheology is the study of characterizing a material from the relationship between controlled, uniform flow and stress. However, even if this is known, it does not　immediately predict the flow for a given boundary condition. The constitutive law model for introducing the fluid equation of suspension is still undergoing trial and error [2, 3].
    We, therefore, extended the rheology simulations described above to understand how suspensions flow under nonuniform flow conditions. The simulation can reproduce the pressure-driven flow of a suspension through a channel. We have studied how the non-viscous fluid-like aspects of the suspension appear in such a nonuniform flow situation. Particle dispersion is no longer uniform as it flows through the channel. Particles move from the outside to the inside, and a plug is formed in the middle of the channel. This plug has a higher volume fraction than the jamming point, which is expected to be the maximum value due to viscosity divergence. We discuss the relationship between the pressure field and the mechanical contribution of normal stresses. In addition, we also observed Kelvin–Helmholtz-like instability, which invalidates predictions by the one-dimensional suspension balance model.
  • References
    [1] R. Mari, R. Seto, J. F. Morris, and M. M. Denn. Shear thickening, frictionless and frictional rheologies in non-Brownian suspensions. J. Rheol., 58(6):1693–1724, 2014.
    [2] C. Ness, R. Seto, and R. Mari. The physics of dense suspensions. Annu. Rev. Condens. Matter Phys., 13(1):97–117, March 2022.
    [3] M. M. Denn and J. F. Morris. Rheology of non-brownian suspensions. Annu. Rev. Chem. Biomol. Eng., 5(1):203–228, 2014.
• Liouvillian Skin Effect in an Interacting System 【Onsite & Zoom】
  • Samuel Begg (Asia Pacific Center for Theoretical Physics (APCTP))
  • Date: 2023/02/17 14:00 --
  • Place: Conference Room K206, Main Building, Yukawa Institute, Kyoto U. & Online (ZOOM*)
    *People interested in attending the seminar should register with google form.
  • Abstract:
    Recent years have seen a large amount of focus on non-Hermitian systems on a 1D lattice with asymmetric hopping amplitude (i.e., hopping amplitude to the right site is different from that to the left). Although such a non-reciprocal transport is prohibited in a closed equilibrium system, it was recently shown and demonstrated experimentally that such a situation can be implemented by carefully tailoring dissipation of the system [1,2]. Interestingly, some non-reciprocal systems were shown to exhibit an algebraic relaxation to the steady state even though the spectrum has a (dissipative) gap [3], which is a phenomenon called the Liouvillian skin effect [4]. However, prior works has focused on non-interacting systems, and whether such a ‘critical’ phenomenon can survive under the presence of many-body interactions has remained elusive. In this work, by numerically analyzing the dynamics of a quantum XY model engineered to exhibit non-reciprocal interactions, we show that the Liouvillian skin effect can indeed survive in the presence of dissipative interactions [5]. We demonstrate that the presence of the interactions alter the exponent of the algebraic　relaxation from the free fermion case, implying the emergence of a new universality class. Our results suggest that quantum non-reciprocal many body systems may host as　a novel playground to find non-equilibrium critical phenomena that has no equilibrium counterparts.
  • References
    [1] A. Clerk, SciPost Phys. Lect. Notes 44 (2022).
    [2] Q. Liang, et al., Phys. Rev. Lett. 129, 070401 (2022).
    [3] F. Song, et al., Phys. Rev. Lett. 123, 170401 (2019).
    [4] T. Haga, et al., Phys. Rev. Lett. 127, 070402 (2021).
    [5] S. Begg and R. Hanai, in preparation.
• YITP Colloquium: Active matter, or the macroscopic consequences of breaking the arrow of time at the microscopic scale 【Onsite & Zoom】
  • Frédéric van Wijland (Universite Paris Cite)
  • Date: 2023/02/16 14:00 --
  • Place: Panasonic Auditorium, Yukawa Hall, Yukawa Institute, Kyoto U. & Online (ZOOM*)
    *People interested in attending the seminar should register with google form.
  • Abstract:
    After a general introduction to what active matter is made of, I'll explain in what sense active systems depart from their passive equilibrium counterparts. I will illustrate the similarities and the differences between the active and the passive versions of a single particle, and then of many interacting particles. After reviewing why purely repulsive active particles like to stick together I will point the finger to the rather complex microscopic mechanism at work, in spite of simple equilibrium-like mesoscopic features.
• Speed-up sampling with nonreciprocal interactions : a theoretical, quantitative evaluation 【Onsite & Zoom】
  • Federico Ghimenti (Universite Paris Cite)
  • Date: 2023/02/08 16:00 -
  • Place: Conference Room K206, Main Building, Yukawa Institute, Kyoto U. & Online (ZOOM*)
    *People interested in attending the seminar should register with google form.
  • Abstract:
    We consider a class of nonequilibrium dynamics that nevertheless sample the Boltzmann distribution in their steady state. For these dynamics, it is known that the relaxation time is shorter than the one of their equilibrium counterpart, though a quantitative understanding of the acceleration in systems with many interacting degrees of freedom is lacking. We contribute to this framework with theoretical computation of the speed-up in several model systems, with emphasis on temperature dependence, optimality and saturation of the acceleration.
• 静的かつ非熱的な剪断下での摩擦のあるアモルファス固体に対する固有値解析 【Onsite & Zoom】
  • 井嶋大輔(YITP)
  • Date: 2023/01/11 14:00 -
  • Place: Conference Room K206, Main Building, Yukawa Institute, Kyoto U. & Online (ZOOM*)
    *People interested in attending the seminar should register with google form.
  • Abstract:
    粉体，コロイド，気泡，エマルションなどの粒子が分散したアモルファス系は，自然界に遍在している。特に、アモルファス系のダイナミクスは、薬物の混合、瓦礫の流れ、雪崩など、日常生活で観察されるため、その特性を理解することは非常に重要である。 これまでの研究では粒子間に摩擦が働かない分散した粒子からなるアモルファス固体の固有関数解析が行われている[1]。特に非熱的かつ準静的剪断下では剛性率を固有関数解析によって求められる上、応力降下の際に最小固有値が0に近づくことが報告されている。 しかし、一般に粉体といった粒子系では粒子間摩擦を無視することは出来ない。そこで本研究では、摩擦の粉体粒子からなる2次元アモルファス固体の非熱的かつ準静的せん断下での固有値解析を行った。具体的には(i)線形応答領域における粒子間接触力をヘルツ力とした場合[2]と(ii)有限せん断ひずみ下での粒子間接触力を調和ポテンシャルに相当する線形ばねにした場合[3]に対し、固有値解析を行った。 解析の結果、(i)については、接線方向と法線方向のバネ定数の比が十分に小さい場合の状態密度において、2つの分離したモードが観測された。ここで粒子の回転から生じるモードは低い周波数で見られ、並進から生じるモードは高い周波数で見られた。さらに、動的行列を用いた固有値解析により、線形応答下での剛性を求めることに成功した。(ii)では、(i)の固有値解析を有限ひずみの場合にも拡張した。安定な粒子配置の下での固有値解析によって、応力-歪み曲線は、塑性変形を引き起こす応力降下の有無にかかわらず、数値計算によって得られたものと一致することが分かった。さらに、調和ポテンシャルを用いた系の最小固有値は、応力降下の前兆を示さないことが明らかとなった。
  • References
    [1] C. Maloney, and A. Lemaître, Phys. Rev. E 74, 016118 (2006)
    [2] Daisuke Ishima, Kuniyasu Saitoh, Michio Otsuki, and Hisao Hayakawa, arXiv:2207.06632.
    [3] Daisuke Ishima, Kuniyasu Saitoh, Michio Otsuki, and Hisao Hayakawa, arXiv:2212.04628.
• Non-reciprocal many-body physics: phase transitions and frustration physics【Onsite & Zoom】
  • Ryo Hanai (YITP, Kyoto University/APCTP)
  • Date: 2022/11/01 13:30 -
  • Place: Conference Room K206, Main Building, Yukawa Institute, Kyoto U. & Online (ZOOM*)
    *People interested in attending the seminar should register with google form.
  • Abstract:
    In equilibrium, the system typically dissipates toward its minimum free-energy state. This implies that the interaction is always reciprocal (like in Newton’s third law). However, once driven out of equilibrium, this action-reaction symmetry is often broken. In fact, they appear ubiquitously in Nature in a wide discipline of science, ranging from open quantum systems, active matter, ecology, social science, neuroscience, to robotics. In this talk, I will introduce the following two classes of collective phenomena that arise due to non-reciprocity in driven many-body systems.
    In the first part of the talk, I will introduce a novel class of nonequilibrium phase transitions [1-2] and critical phenomena [3] that can only occur in the presence of macroscopic non-reciprocity, by generalizing the Ginzburg-Landau theory to driven systems. Remarkably, the discovered phase transition is controlled by spectral singularity called exceptional points that can only occur by breaking the detailed balance and therefore has no equilibrium counterparts. The emergent collective phenomena range from active time (quasi)crystals, hysteresis, to anomalous critical phenomena that exhibit anomalously large phase fluctuations that diverge at d≤4 [3]. These phenomena occur in a broad class of many-body interacting nonequilibrium systems both in quantum and classical matter, where examples include exciton-polariton condensates [2], pattern formation [1], flocking [1], and synchronization [1].
    In the second part of the talk, I will view non-reciprocal interacting many-body systems from the perspective of frustration physics. I argue that there is a direct analogy between geometrical frustration and non-reciprocity-induced frustration physics [4], by pointing out the marginal orbits that emerge in anti-symmetric coupled non-reciprocal matter that can be regarded as the dynamical counterpart of accidental degeneracy. I will show that the dynamical counterpart of order-by-disorder phenomena and spin glass occurs due to non-reciprocal frustration.
    These frameworks lay the foundation of the general theory of critical phenomena and frustration physics in systems whose dynamics are not governed by an optimization principle.
  • Reference
    [1] M. Fruchart*, R. Hanai*, P. B. Littlewood, and V. Vitelli, Nature 592, 363 (2021).
    [2] R. Hanai, et al, Phys. Rev. Lett. 122, 185301 (2019).
    [3] R. Hanai and P. B. Littlewood, Phys. Rev. Res. 2, 033018 (2020).
    [4] R. Hanai, arXiv:2208.08577.
• Dynamics of dense non-Brownian suspensions under impact【Onsite & Zoom】
  • Pradipto (YITP, Kyoto University)
  • Date: 2022/07/04 16:00 -
  • Place: Conference Room K202, Main Building, Yukawa Institute, Kyoto U. & Online (ZOOM*)
    *People interested in attending the seminar should register with google form.
  • Abstract:
    Non-Brownian suspensions with high volume fractions are ubiquitous in nature and exhibit interesting behaviors. One of them is the fact that people can run on top of cornstarch suspensions, but sink if they walk. Such impact-induced hardening is less studied since most studies of the dense suspensions are conducted under shear. Thus, we extend the coupled lattice Boltzmann method and discrete element method (LBM-DEM) for dense suspensions to include the free surface of the suspensions and investigate the rebound motion of a free-falling spherical impactor. We find that the rebound depends on the impact speed, the volume fraction of the suspensions, and frictional contact between suspended particles [1]. We also visualize local quantities inside the suspensions after the impact, which are not accessible yet by experiments. Then, we analyze the topological structure of the contact network between suspended particles using persistent homology. Second, we propose a simple phenomenology to capture the viscoelastic response of the impact process. As a result, we succeeded to explain the power-law relationships among the impact velocity, the maximum force exerted on the impactor, and the time to reach the maximum force at high impact speed [2]. Third, we delineate the dynamically jammed region induced by the impact and quantify its viscosity and elasticity [3]. Finally, we discuss the impact of a foot-spring-body system to mimic the hopping motion on dense suspensions.
  • Reference
    [1] Pradipto and H. Hayakawa, Phys. Rev. Fluids 6, 033301 (2021).
    [2] Pradipto and H. Hayakawa, Phys. Fluids 33, 093110 (2021).
    [3] Pradipto and H. Hayakawa, arXiv:2205.13822.
• Some recent results on the response and the relaxation in small stochastic systems【Onsite & Zoom】
  • Naoto Shiraishi (The University of Tokyo)
  • Date: 2022/06/13 16:45 -
  • Place: Conference Room K206, Main Building, Yukawa Institute, Kyoto U. & Online (ZOOM*)
    *People interested in attending the seminar should register with google form.
  • Abstract:
    熱ゆらぎが無視できない小さな系における熱力学の研究は、集中講義でも触れているように、ゆらぎの定理、情報熱力学、熱力学的不確定性関係など、さまざまな普遍的関係式を明らかにしてきた。この談話会では、その最近の結果として、まず時間反転対称なカレントの導入とその揺動応答関係の話[1]をしたい。通常のカレントは時間反転反対称だが、ここでは時間反転対称なカレントの対応物をうまく定義し、それがある種の非平衡定常状態周りで揺動応答関係を満たすことを証明する。この対称カレントは、理論的興味のみならず、生の遷移レートの実験的推定への応用も期待できる。 時間の余裕があれば、緩和過程特有の、第二法則よりも強い熱力学的制限を与える不等式[2]についても議論したい。
  • Reference
    [1] N. Shiraishi, arXiv:2111.09477.
    [2] N. Shiraishi and K. Saito, Phys. Rev. Lett. 123, 110603 (2019).
• Lecture series: YITP Intensive Lecture ２ "stochastic thermodynamics"【Onsite & Zoom】
  • Naoto Shiraishi (The University of Tokyo)
  • Date: 2022/06/10 - 2022/06/14
  • Place: Conference Room K206, Main Building, Yukawa Institute, Kyoto U. & Online (ZOOM*)
    *People interested in attending the seminar should register with google form.
  • Abstract:
    本講義では、ここ20～30年の非平衡統計力学の一つの重要な研究領域である、ゆらぐ系の熱力学について概観する。通常の熱力学はマクロな系を対象とするが、ゆらぐ系の熱力学は熱ゆらぎの無視できない小さな系を対象とする。このような系においてはゆらぎの定理に代表されるさまざまな普遍的な関係式が成り立つ。本講義では、ゆらぐ小さな系においてどのように熱力学を定式化するのかという問題から初めて、ゆらぎの定理、情報熱力学、熱力学的不確定性関係などの重要な結果を解説する。
  • 到達目標
    講義を通じて、ゆらぐ系の熱力学の様々な関係式がどのように導かれ、その背景にはどのような性質があるのかを学ぶ。
  • 授業計画と内容
    1.確率過程による熱力学の定式化
    2.ゆらぎの定理
    3.既知の非平衡関係式のゆらぎの定理からの導出
    4.ゆらぎの定理タイプの等式
    5.情報熱力学
    6.熱力学的不確定性関係
    7.スピードに対するトレードオフ不等式
  • 時間割
    6/10(金)　13:15-14:45　15:00-16:30 　16:45-18:15
    6/13(月)　10:30-12:00　13:15-14:45　 15:00-16:30 　16:45-18:15(基研セミナー)
    6/14(火)　13:15-14:45　15:00-16:30 　16:45-18:15