On The Boundary Between Theory and Experiments: From chemically-driven viscous fingering to reaction fronts in microgravity.

Science is always challenging, particularly when one must choose between following a purely theoretical or experimental path. But sometimes, being right in between can help us find better solutions to scientific problems.

This presentation unfolds in two parts, each showcasing a unique problem at the interface of theory and experimentation. In the first part, we will examine a system where pattern formation is induced by complex chemical reactions [1, 2]. Here, we will study its behavior experimentally and use numerical simulations to understand the mechanism behind its dynamics. In the second part, we highlight the importance of prior experimental experience in shaping realistic mathematical models. Specifically, we investigate how the premixing of the chemical species influences the behavior of A + B → C reaction-diffusion- advection fronts under microgravity conditions. We illuminate how this experimental artifact impacts the characteristic properties of these fronts [3]. This talk extends beyond physical-chemical phenomena; it serves as guidance for those navigating their scientific paths, illustrating the richness of exploration at the confluence of theory and experimentation.

[1] D. M. Escala, A. De Wit, J. Carballido Landeira, A. P. Munuzuri,  Viscous fingering induced by apH-sensitive clock reaction, Langmuir 35, 4182 (2019).

[2] D. M. Escala, & A. P. Muñuzuri. Abottom- up approach to construct or deconstruct a fluid instability. Scientific reports, 11 (1), 1-16 (2021).

[3] Y. Stergiou, D. M. Escala, P. Papp, D. Horváth, M. J. B. Hauser, F. Brau, A. De Wit, Á. Tóth, K. Eckert, and K. Schwarzenberger, Unraveling dispersionand buoyancy dynamics around radial A+B → Creaction fronts: microgravity experiments and numerical simulations, NPJ microgravity (2024)  

分野横断研究のススメ 

分野横断研究を始めるきっかけとなった研究者初期の頃を振り返り，熱流体工学を専門とした学部・ 大学院時代や，海外の大学でPh.Dを取得した後帰国し分野横断をしてマイクロロボットの研究室へポ スドクとしてリスタートしたことなどを通して，研究者にとっ て重要なことは何か？日本でのキャリア形成，異分野の研究者 同士のネットワークの大切さについてお話します．後半は電界 誘起気泡を用いたインジェクターの研究のお話と分野横断研究 への展開のお話をします．個人研究(さきがけ)からチーム型研 究(CREST，ムーンショット)へシフトしていく中での様々な価 値観の考え方，纏め方などのお話ができればと思います．最後 に機械と生物と情報などの分野を繋げていく試みのプロジェク トの紹介をいたします． 

理系学生向け，企業における経営戦略の必要性と実務での活用

●本セミナーの目的
・実務における経営戦略の重要性を理解する ・日本マクドナルドホールディングスの開示情報を通じて経営戦略の具体例を 知る ・グループディスカッションを通じて店舗戦略を作成する過程を体感する ・理系学生の皆様がビジネスの世界で活躍するための基礎知識を習得する

●経営戦略の意義
・経営戦略とは 

・企業は利益を最大化する目標を持つ。経営者はどのような状況でも利益最大化のため効率的、効果的に組織を動かす必要あり 

・変化の時代(VUCAの時代)では、「正解」が変わるため、環境変化を捉えて柔軟に応じる経営戦略が必要になる

●戦略の全体像、優秀なビジネスパーソンになるために全体像を理解しよう

 ・戦略の全体像(上層部の意図の理解と実行が重要)

 ・経営戦略とその下位戦略の関連性 ・経営戦略、事業戦略、マーケティング戦略、店舗戦略、等

●経営戦略の立案プロセス ・ミッション、ビジョン、バリューの定義、長期的な方向性を定義 

・SWOT分析、クロスSWOT分析(積極戦略、改善戦略、差別化戦略、撤退 戦略) 

・外部環境(PEST、3C、ファイブフォース、プロダクトライフサイクル) 

・内部環境(コアコンピタンス、経営資源、7S、4P)

●経営戦略に基づく事業計画の作成
・経営戦略を事業計画に落とし込む 

・事業計画の作成方法の説明(売上高、コスト(変動費、固定費)、投資、等)

●経営戦略の実践

・定期的に事業計画の数値と実際の数値を比較 ・予算実績差異分析⇒PDCAサイクルを回す⇒目標達成を目指す

・KGI(Key Goal Indicator)、KPI(Key Performance Indicator)、KDI (Key Doing Indicator)の利用法

●ケーススタディ:日本マクドナルドホールディングス 

・有価証券報告書、決算報告資料等の活用

 ・どのような公開情報があるか、公開情報をどのように読むか 

・経営戦略と決算数値の関連性

●フランチャイズ店舗の戦略

 ・具体的な店舗の店舗戦略を検討。同業他社、ライバル店舗と比較

 ・日本マクドナルドホールディングス全体の経営戦略の中で店舗戦略を構築

●グループディスカッションの実施 

・マクドナルドの店舗戦略を考えてみよう! 

・クロスSWOT分析、ChatGPTの活用 

・どうしたら利益(=収益-費用)が増えるか

●グループディスカッションの結果発表 

・学生たちによる事業計画のプレゼンテーション 

・店舗戦略の発表と評価、各グループへのコメントとアドバイス

● まとめ:理系学生の皆様への期待

・経営戦略を数字に落とし込むことの重要性

・事業計画は、必ず実績数値と比較してPDCAサイクルを回す

・PDCAサイクルを回すためにも、なぜこの計画か、数字で説明が必要

・数字に強い理系学生の皆様へ期待

Solid surface energy in elasto-capillary systems

It is expected that in elastocapillary systems the deformation of soft materials and/or slender structures abutting the fluid interface will be nontrivially affected by the surface energies at the interfaces between the solid and fluid/air. This makes understanding the role of solid surface energies practically important. While authoritative thermodynamic treatments from the time of Gibbs make the case of solid surface energy clear, debate still persists about how to experimentally measure these surface energies at the solid's various interfaces, how to incorporate them in the mechanics of elastocapillary systems, and whether such interfacial energies will affect the solid structure’s deformation and/or stress field. A recent experiment [Kumar et al. 2020, Nature Materials, vol. 19, pp. 690-693] is cited to claim that solid surface energies cannot affect thickness-averaged stress within membranes and, consequently, the membrane's deformation as a solid body is independent of its surface energies. I will discuss a framework in which solid surface energies may be included in elastocapillary systems in a systematic way, show how the internal stress field in the solid is modified by solid surface energies, and thus arrive at a consistent interpretation of experiments, such as those of Kumar et al.

How the growth of lake and sea ice depends on the fluid dynamics underneath

Landscapes resulting from ice–water interactions coupled with solidification/melting are ubiquitous in nature. We study the coupling dynamics of the growth of lake and sea ice with the fluid motion underneath. In the first work, we investigate the solidification of freshwater, considering phase transition, water density anomaly, and the real physical properties of ice and water phases, which we show to be essential for correctly predicting various qualitative and quantitative behaviors. Despite the complex interaction between the ice front and fluid motions, remarkably, the average ice thickness and growth rate can be well captured with a theoretical model. In the second study, we experimentally investigate the complete freezing process of water with dissolved salt. We identify possible modes of heat transport and emphasize the role of the presence of brine convection through the mushy ice in influencing icing dynamics.

Faraday Instability at Liquid-Fluid Interfaces- Physical Phenomena, Mechanical Modeling, Experimental Evaluation

Resonance driven instability or Faraday instability occurs when vertically stacked fluid bilayers are subject to periodic forcing in a direction that is normal to their common interface. The forcing can arise from several means, for example, by mechanical motion, by acoustic means, or even via electrostatic fields. The instability at the interface, which is manifested by ordered patterns, has its origins in the resonance between the imposed frequency and the system’s natural frequency. Our talk will focus on a comparison between theory and experiments showing remarkable agreement between the two.

We also show how and why electrostatic forced resonance is an excellent candidate to determine interfacial tension between fluids such as liquid semi-conductors and encapsulants.

Cavitation Bubble interaction with hard and soft matter: from erosion to nucleation

Non-spherical cavitation bubble dynamics in liquids are known to create enormous shear stresses which may be even enhanced when the bubbles are entrained in a low. Yet the mechanism by which cavitation creates erosion is through shock wave focusing and not related to the jet impact or a spherical energy focusing. It is actually the opposite; erosion results from the loss of axisymmety. Non-spherical cavitation in elastic solids generate shear waves that transport deformation energy to distances much larger than the local strain ield. We will show that two shear waves with different orientation are generated near a boundary and their individual amplitude is a function of the stand-off distance. These waves can be recorded with acoustic plane wave imaging at very high frame rates. I’ ll end the presentation with a mechanism through which cavitation is nucleated from shock-wave gas-bubble interaction in tissue.

Self-similarity and intermediate asymptonics on the dynamical impact on PDMS viscoelastic board

Self-similarity is a fundamental concept in physics. In this seminar, I introduce the concept of self-similarity and intermediate asymptotics that were formalized by Barenblatt and show how effective these concepts are to describe and understand the phenomena in soft matter physics in which their behviors and scaling laws vary depending on the scale of physical parameters. In this seminar, I will show the experiments of dynamical impact of solid spheres onto viscoelastic boards. Its scaling laws varied at different impact-velocity. I have succeeded in describing a self-similar solution which governs the scaling behaviors completely. The self-smilar solution has the dimensionless parameter derived from a solution in elastic regime and an inverse Deborah number. The solution shows that the behaviors are fully determined by the competition between elasticity, viscosity and inertia. It is a typical framework of crossover of scaling law.

Self-similarity and intermediate asymptonics on the dynamical impact on PDMS viscoelastic board

Self-similarity is a fundamental concept in physics. In this seminar, I introduce the concept of self-similarity and intermediate asymptotics that were formalized by Barenblatt and show how effective these concepts are to describe and understand the phenomena in soft matter physics in which their behviors and scaling laws vary depending on the scale of physical parameters. In this seminar, I will show the experiments of dynamical impact of solid spheres onto viscoelastic boards. Its scaling laws varied at different impact-velocity. I have succeeded in describing a self-similar solution which governs the scaling behaviors completely. The self-smilar solution has the dimensionless parameter derived from a solution in elastic regime and an inverse Deborah number. The solution shows that the behaviors are fully determined by the competition between elasticity, viscosity and inertia. It is a typical framework of crossover of scaling law.

Self-similarity and intermediate asymptonics on the dynamical impact on PDMS viscoelastic board

Self-similarity is a fundamental concept in physics. In this seminar, I introduce the concept of self-similarity and intermediate asymptotics that were formalized by Barenblatt and show how effective these concepts are to describe and understand the phenomena in soft matter physics in which their behviors and scaling laws vary depending on the scale of physical parameters. In this seminar, I will show the experiments of dynamical impact of solid spheres onto viscoelastic boards. Its scaling laws varied at different impact-velocity. I have succeeded in describing a self-similar solution which governs the scaling behaviors completely. The self-smilar solution has the dimensionless parameter derived from a solution in elastic regime and an inverse Deborah number. The solution shows that the behaviors are fully determined by the competition between elasticity, viscosity and inertia. It is a typical framework of crossover of scaling law.

Hydroelasticity of High-Speed Planing Craft Subjected to Slamming Events by Vertical Water Entry Experiments

Small high-speed craft endure repeated slams into waves in the ocean. To better design these craft, it is important to understand this fluid-structure interaction when materials such as composites are used in their construction. In this talk, the slamming of composite high-speed craft is experimentally investigated by early free-falling water entry experiments. These are conducted on a wedge with flexible bottom panels. Kinematics, hydrodynamics, structural deformation, and spray-root propagation on the bottom of the wedge are measured and analyzed. It was found that for a composite panel, small outward (toward the water) deflections are observed at very early times in the impact event and decaying vibrations are observed after chinewetting. Soon, we will begin planning of non-traditional towing-tank tests that will be conducted at the VT Advanced Towing Tank Facility. These future VT experiments will use controlled motion experiments to replicate single slamming events of a planning hull boat on calm water. The motions will be controlled in surge, heave, and pitch and fixed in all other degrees of freedom.

混沌にもがけ！

・自己紹介
・大学院時代の勉強と研究
・液体衝撃波管の開発とキャビテーション気泡力学
・気泡力学から気液界面の物理へ
・私の研究方法は大学院・学振研究員・助手の時代に形成された
・科学研究には虚空に飛び込む勇気が必要！ (W. ハイゼンベルク)
・北大での私の研究:気液界面現象の階層構造
・私設 流体物理学研究所での研究と仲間たち
・皆さんへ：研究に苦しみ，研究を楽しめ!

Interactions of cavitation bubbles with surface-attached bubbles, particles, and droplets

Cavitation near boundaries can cause severe damage to surfaces due to the existence of cavitation nuclei and contaminations like particles. It can also be harnessed in surface cleaning, emulsification, and medical treatment. This talk focuses on cavitation bubbles near rigid boundaries, investigating cavitation inception from surface-attached micro- and nanobubbles, and interactions between cavitation bubbles and surface-attached rigid particles or oil droplets. (i) First, we design experiments on cavitation inception from surface micro- and nanobubbles due to strong shear flows and strong ultrasonic fields, respectively. In response to strong shear flows, surface micro- and nanobubbles deform, pinch off, and release free gaseous nuclei. At a driving frequency of about 100 kHz, surface micro- and nanobubbles are observed to merge with ultrasonic cavitation bubbles and then detach from the substrate, thus becoming free gaseous nuclei. In summary, we prove that surface micro- and nanobubbles can evolve into free gaseous nuclei, which provides new ways to remove the attached gaseous bubbles from surfaces. (ii) Second, we find by experiments that laser- induced cavitation bubbles can accelerate spherical metal particles from surfaces. By theoretical analysis, we reveal that the lift forces on the particles are originated from the decelerating expansion of the cavitation bubbles. Our findings provide an invasive way to manipulate particles immersed in water. (iii) Finally, we find by experiments that laser-induced cavitation bubbles can interact with surface-attached hemispherical oil droplets in four typical ways: oil droplet rupture, water droplet entrapment in oil droplet, oil droplet large deformation, and oil droplet mild deformation. We successfully predict in theory the direction of the migration of the collapsing cavitation bubbles, and establish the phase diagram for oil droplet responses.

大学院生活を楽しみ、その後のキャリアを考えるために

私の大学院〜海外ポスドク生活は、素晴らしい環境でのびのびと研究生活を過ごさせていただいた一方で、進路や留学については、決断できずに悩むことばかりだったように思います。皆さんの普段の研究生活はいかがでしょうか。また、皆さんはそのような時どのように乗り越えているでしょうか。私は、予想外の出来事に対して右往左往する ことばかりで、攻略本が欲しいとさえ、常々思っています。そこで今回は、私自身がこれまでにそれぞれのタイミングで遭遇してきたイベントをご紹介します。(奏功したかはともかく)ひとつのアプローチ例をご提供しますので、皆さんにとって万一の備えとなればと思います。また、「もし今、そのような場面に遭遇したらどうするか」という観点からも考えます。これらを通して、失敗例も多いと思いますが、皆さんの参考になる情報がご提供できればと思いますので、楽な気持ちで聞いていただけたら幸いです。くわえて、大学院時代の学びを振り返り、いまの生活において役に立っていることや、 反対に、大学院時代にやっておけばよかった!と感じた(感じる)ことなど、私の率直な考えをお伝えできればと思います。

Stick-slip Dynamics for Unsteady Wetting on Soft Solids

In this talk, I will discuss studies for unsteady spreading of liquids on soft solids, a situation in which liquid spreading and solid deformation occur spontaneously at the three-phase contact line (CL). I show that stick-slip results from a competition between liquid spreading rate and solid viscoelastic reacting rate. I demonstrate that this CL behavior can be described and ontrolled by considering growing deformation of soft solids as dynamic surface heterogeneities. This may provide an understanding of unsteady wetting phenomena that have broad applicability in additive manufacturing, fabrication of flexible electronics, and biomedicine.

未来のキャリアを拓くための道しるべ: 田川研から自動車業界へ

みなさんは将来どんな仕事に就きたいでしょうか.アカデミック，一般企業，公務員などに決めている方もいれば，まだ迷っている方もいらっしゃいますよね.やっと進路を決めて入社をしたのもつかの間，今後のキャリアを問われるタイミングがみなさんにもやってきます.1つの組織で定年まで同じ仕事をする人は少数です.自動車業界は100年に1 度の変革期と言われています.ビジネスの急激な変化に伴い，求められる業務も変化していきます.これは他業界でもいつでも起こり得ます.少し不安になってしまったかもしれませんが，大丈夫です!本講演では，田川研で培った経験が会社でどのように活用できているかをご紹介し，「自分もやっていけそう!」とポジティブに思っていただけるような内容をお伝えします.まず，田川研での4年間の研究生活を経て学んできたことを整理します.つぎに，自動車業界で私が辿ってきたキャリアのターニングポイントを振り返り，自分が大切にしてきた思いをご紹介します.ひとつの田川研OBのケースとして，自分の将来を想像しながら気楽にお聞きいただければと思います.

Turbulent air-jet impact on granular surfaces

Crater formation due to an impact is an extremely complex phenomenon. Jet-impact crater formation further involves the interaction between continuously impinging pressurized fluid and granular surface, which results in erosion of the surface, outward transportation of grains, and formation of bowl-shaped depression. The cratering process is dynamic, and the crater shape varies with the properties of eroding material and the impinging fluid. Landing rockets on planetary surfaces and erosion near hydraulic structures are some important examples relating to impact-induced cratering. Here, we consider the former application to focus on the cratering phenomenon that may cause problems during the landing and re-launching of rockets, like tilting of rockets, hardware damage to rocket bodies, etc. We investigate crater morphology and thrust forces in a rocket landing scenario, where a turbulent air-jet impinges on the granular surfaces. To reveal the fundamental aspect of this phenomenon, systematic experiments are performed with various air velocities, nozzle positions, grain sizes, shapes, and densities. We characterize the crater morphology using an aspect ratio, Rc = Dc/Hc, where Dc and Hc are crater diameter and depth, respectively. As a result, we find Rc is governed by the product of Mn = vn/C and Gn = dn/hn, where vn, C, dn, hn are air velocity at the nozzle, speed of sound in the air, nozzle diameter, and nozzle distance from the surface, respectively. We find that the non-dimensional thrust force is solely governed by Mn. These results suggest that the turbulent nature of air-jet mainly controls both the crater shape and thrust force. Moreover, we report a novel drop-shaped sub-surface cratering phenomenon.

田川研究室での学びが生み出すキャリア: 博士号取得後の変化

田川研究室で過ごした博士前期課程の2年間と博士後期課程の1年間 (社会人ドクター)の経験が、私のキャリアにどのような影響を与えたのかについてお話します。また、社会人の立場から感じた「アカデミア と民間企業とのギャップ」や、「博士号がキャリア選択に及ぼした効 果」についても詳しく触れていきます。田川研究室での経験を糧に、私 自身が社会に出てどのように変化し、成長していったのかを振り返り、 皆さんにも参考になるようなお話をお届けできればと思います。

細胞検体を利用した肺癌診療の最前線

近年の腫瘍免疫学の進歩に伴い、原発性肺癌の治療に際しては、診断時の遺伝子変異の検索が必須となり、遺伝子変異陽性肺癌に対しては分子標的薬の使用が標準治療に位置付けられ、患者予後は劇的に改善した。診断時の遺伝子一括検査はNGSおよびPCR法でのパネル検査が主流であるが、原則的に十分量の組織検体が必要であることが課題であった。2020年から取り組んだ細胞診検体活用の研究において、我々は細胞診検体でも十分の核酸収量を得られることだけでなく、組織検体との比較で核酸クオリティも高いことを示し、肺がんコンパクトパネルは、第3の肺癌CDxとして2022年10月に薬事承認され普及している。気管支鏡下に採取できる擦過検体や針洗浄液、胸水等の液性検体のみならず、気管支洗浄液や喀痰など、従来遺伝子パネル解析に使用すると想定されていない検体でも解析が成功しており、今後低侵襲かつ高精度な検査法がますます普及することが予想されている。

高粘度インクジェット吐出技術

~博士後期課程の研究と現職の繋がり~

インクジェット技術は，低粘度ニュートン性インクを紙への印刷から，高粘度・非ニュートン性インクをさまざまなメディア(金属，樹脂，生体など)への印刷など幅広い分野で用いられる.講演者は博士後期課程で高粘度液体の吐出技術の開発およびメカニズムの解明を行っており，現在は株式会社リコーで産業用インクジェットの設計・開発に従事している.本セミナーでは，博士後期課程での研究内容である高粘度液体の吐出技術の紹介と現職の繋がりについて説明する.

Harnessing acousitically driven two-phase fluids

Bubbles oscillate volumetrically under the effect of pressure fluctuations, such as those produced by underwater sound waves. When driven into a violent collapse, they can yield strong sound emissions, high-speed jets, and extreme heating – a behaviour known as cavitation. Here, we will present ongoing research efforts to reach a fundamental understanding of the intriguing dynamics of acoustically driven bubbles across a wide range of scales from a single bubble level to that of bubble clouds. For this, we generally combine theory with ultra-high-speed experiments ranging from videomicroscopy and synchrotron X-ray imaging to laser-based measurement techniques. More specifically, we will report on our progress in temporally resolving acoustically driven microbubble oscillations and microdroplet vaporisation, elucidating the role of cavitation in ultrasound-mediated tissue adhesion and kidney stone destruction, and describing shock-driven bubble collapses. The broad aim of this research lies in the quest for harnessing the power of acoustically driven two-phase fluid media for a variety of engineering applications, including medical ultrasound diagnostics, drug delivery, lithotripsy, sonochemistry, surface cleaning and micro-fluidics.

High humidity enhances the evaporation of non-aqueous volatile sprays

We experimentally investigate the evaporation of very volatile liquid droplets (Novec 7000 Engineered Fluid, chemical name hydrofluoroethers HFE-7000) in a turbulent spray. Droplets with diameters of the order of a few micrometres are produced by a spray nozzle and then injected into a purpose-built enclosed dodecahedral chamber filled with air containing various amounts of water vapour. The ambient temperature and relative humidity in the chamber are carefully controlled. We observe water condensation on the rapidly evaporating droplet, both for the spray and for a single acoustically levitated millimetric Novec 7000 droplet. We further examine the effect of humidity, and reveal that a more humid environment leads to faster evaporation of the volatile liquid, as well as more water condensation. This is explained by the much larger latent heat of water as compared with that of Novec 7000. We extend an analytical model based on Fick’s law to quantitatively account for the data.

Laser-induced cavitation for needle-free jet injection

Over the last 15 years, laser-actuated jet injectors have been investigated as an alternative to injection needles. These injectors use a laser to generate a bubble inside a microfluidic channel. This expanding bubble pushes out the remaining liquid, forming a fast microfluidic jet that is able to penetrate the outer layers of skin under certain conditions. To create this bubble, two different lasers can be used: a nanosecond-pulsed, or a low-power continuous- wave laser. Although both of these lasers have been thoroughly used, a direct comparison between these two has never been made. In this talk, I will present our recent work on a comparison of the bubble formation between a low-power continuous-wave and a pulsed laser. This comparison shows that both lasers create similar bubble growth and collapse dynamics, although the bubbles created by the pulsed laser require slightly less optical energy. Furthermore, I will present some of our recent work on selectively coated microchannels and their influence on the jet formation and stability.

Rheoloy of dense granular suspensions: from Newtonian to Bagnoldian rheology

Dense granular suspensions that consist of concentrated mixtures of non Brownian particles suspended in a liquid are ubiquitous in many natural phenomenon (e.g. landslides, debris flows, and sediment transport) and industrial processes (e.g. concrete and pastes). Their rheology is not fully understood and establishing a unified theoretical framework across the different flowing regimes is still challenging. The present work describes precise rheological measurements of granular suspensions in the dense regime utilizing a unique custom-built rheometer able to perform pressure- and volume-imposed rheometry. It addresses the transition from a Newtonian rheology in the Stokes limit to a Bagnoldian rheology when inertia is increased and examines whether the inertial and viscous regimes can be unified as a function of a single dimensionless number based on stress additivity. This work has been done in collaboration with M. Ichihara, O. Pouliquen and F. Tapia.

管理会計セミナー

>会計とは?
・会計の意味は，報告，説明すること
・財務会計と管理会計がある，外か内か
・会計では
・管理会計の切り口
>財務諸表分析
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高速度赤外線カメラで観察した沸騰熱伝熱メカニズム

沸騰熱伝熱は単相の対流熱伝熱やふく射伝熱などの他の伝熱形態と比較して高い熱伝熱率を有するため，電動車両用パワー半導体や高輝度光源など，さまざまな高発熱密度体への冷却への応用が考えられている．高い熱伝熱率をもつことは広く知られている事実であるが，その理由を明確に答えることは今でも難しい．我々のグループでは，MEMSセンサや高速度赤外線カメラ，感温塗料を用いた現象の直せいつ観察で熱伝達メカニズムを調べることを方針として研究を行ってきた．本講演では，高速度赤外線カメラを用いた沸騰伝熱面表面の温度・熱輸送分布の高時空間分解能計測を通じて調べた壁面熱伝達メカニズムについて説明する．

Large droplet formation and deep-pool impact

Water droplet break-up has been studied extensively but typically focuses on droplets with radii near the capillary length (< 3 mm). Droplets larger than this typically break up when their velocity approaches terminal velocity, with larger droplets breaking up well before they reach terminal speeds. In general, studies involving large droplet break-ups are scarce and part of the reason is the difficulty of performing such experiments. We propose a method to suspend droplets of radii up to 60 mm using a series of release mechanisms and release speeds to illustrate the complexity and parameter space for practical solutions to these problems. The research details the effects of release acceleration, surface geometry, and surface features on the initial droplet perturbations. The shape of a droplet impact onto a liquid pool may greatly influence the dynamics of the cavity formation and splashing. Although droplet impact onto a liquid pool
has been investigated for relatively small droplets, behaviors greatly change when the droplet is large enough that significant oscillation deformation occurs. The shape and dynamics of the cavity formed by the large droplets are significantly affected by the deformation of the droplet at impact. In general, three different shapes of the impacting droplets occur prolate, oblate, and circular. We show that, for a fixed liquid volume and a fixed Weber number of an impacting droplet, prolate-shaped droplets produce the maximum cavity depth whereas an oblate-shaped droplet results in a minimum cavity depth.

複合化した輸送現象における熱力学解析

二つ以上の非可逆過程が複合化した輸送現象は、時に創発的な時空間構造を示す。化学反応と拡散過程が干渉するBelousov–Zhabotinsky反応や熱と運動量輸送が干渉するRayleigh–Bénard対流や拡散と運動量輸送が干渉するMarangoni対流は、平衡から離れると空間的な対称性が崩れ、安定した創発的な時空間構造を形成する。この創発的な時空 間構造の特徴は、巨視的な駆動力に直交する方向に熱力学流束が創発し、その創発流束が存在する直交面内でエントロピーを生成しながら安定な構造を形成する点にある。時空間構造の発展や安定性は、非線形力学により詳細に議論できるが、異なる現象間の構造に質的な共通点がない限り、時空間構造を統一的に理解することはできない。本セミナーでは、様々な複合化した輸送現象における時空間構造を、熱力学流束を用いて平衡系の相のように規定し、エントロピー生成を用いて時空間構造の発展および安定性を解析する方法について、概説する。

生体内流動の階層性と物性を繋ぐ

　力学の解明にむけた膜-流体連成解析

生命動態の基本は対流と拡散であり、環境刺激の下でこれらのダイナミクスが異なる階層で進行し、結果としてより高次の生体機能を創出する。流れ場中の構成要素の各階層における詳細なダイナミクスは、生体機能を成立させる力学場と生体システムの関係を理解する上で重要であるだけでなく、疾病機序の理解にも繋がると期待している。本セミナーでは、血液中の細胞流動を例に、膜-流体連成解析手法を用いた赤血球濃厚懸濁液のレオロジー解析について述べる。また、異なる階層の流れ場を描像する数値的アプローチについても紹介したい。

Shear rate imaging usin a polarization camera and birefringent aqueous cellulose nanocrystal suspensions

A fluid that is composed of anisotropic particles may show birefringence when under the effect of shear. This phenomenon is known as shear induced birefringence and can be used to study and visualize fluid flows. The most common way to measure birefringence is to send polarized light through the birefringent material and to measure the change in polarization. In this seminar I would like to talk about four aspects of flow birefringence measurements. First, we are going to discuss factors influencing the measurement quality of a polarization camera. Second, a two-dimensional birefringence measurement technique that is based on a rotatable linear polarizer and a polarization camera is presented. Third, birefringence measurements of aqueous cellulose nanocrystal suspensions are discussed. Finally, I argue that the study of shear rates in a two-dimensional shearing flow by means of flow birefringence is feasible and therefore encourage the use of aqueous CNC suspensions for birefringent flow studies.

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Lab-in-a-drop approach: 音響場における浮遊液滴の非接触マニピュレーション

　近年、超音波を用いた「非接触流体マニピュレーション」が分析化学、生物、医学・創薬分野等に積極導入され、社会実装を目指す研究が世界中で活性化している。その実現に向けて、音響場で浮遊する液滴が内在する界面ダイナミクスならびに連成する液滴内外部の流動現象および熱物質輸送現象の解明が求められている。現象解明には計測手法の開発および解析技術の確立が必要不可欠であるものの、その現象の複雑さ(非定常性、非線形性、非平衡性)から、浮遊液滴のダイナミクスに対する包括的理解は未だ途上にある。本セミナーでは、その実現の道標となる統合的な現象把握と物理解明への挑戦、音響場による非接触流体マニピュレーションを用いたLab-in-a-dropの概念実証過程および今後の展望を紹介する。

物理現象の対称性を反映させ，任意のメッシュを扱える機械学習モデルによる流動現象の学習について

　流動現象の機械学習による予測は、理論・実用の双方の観点から重要な課題である。 また、数値シミュレーションでよく扱われるメッシュは Graph neural network (GNN) と呼ばれる機械学習モデルの枠組みで取り扱えることが知られている。しかし ながら、通常の GNN では解析領域の形状や物理状態の複雑さのため、汎用性が問題 となることが多い。そこで本研究では、合同変換群同変性と呼ばれる、物理現象に見 られる回転・平行移動・鏡映に対する対称性を反映させた性質を持つ GNN について 議論する。また、提案手法が流動現象の予測を安定的に行えることを示す。

Fluid dynamics of needle-free intradermal injection

　Delivery of medicines and vaccines has been largely unchanged for the past 150 years, with hypodermic needles being the vector of choice. In contrast, there has been considerable innovation in drug development, with explosive growth in the field of injectables (i.e. liquid-based drugs delivered across the skin). In this talk, I will present my groups’recent work in this area, with the main focus on needle-free intradermal jet injection, where we use a combination of experimental (high-speed video) observations, theory and numerical simulations to elucidate fundamental fluid dynamics of the process. I will also briefly discuss my groups other efforts related to (i) laser-induced and spark-induced micro-jets, (ii) drop impact for ophthalmic drug delivery, and (iii) using tattoo devices for intradermal injection.

Experiments on Cavitation Nuclei and Nanobubbles suspensions

　Pulsed lasers are extremely useful to generate and expand small bubbles in liquids and surfaces. In this seminar I will present two simple experimental systems to explore the nature of nanobubbles. The first utilizes a thin liquid gap sandwiched by two stiff transparent materials, i.e. glass. The laser pulse launches a surface wave in the gap that is able to induce tension of up to the homogenous nucleation threshold. We explore this system and provide a novel way to nucleate bubbles by supersaturating the liquid in the gap locally. In the second experimental system we create by some yet unknown mechanism with a pulsed laser submicron sized bubbles in bulk water. Utilizing a rarefaction wave some time after the pulsed laser demonstrates the successful generation of nanobubbles. Both systems are easy to setup in a lab that has a pulsed laser source. I invite you to join this quest in understanding nanobubbles and their nucleation.

Beyond Six Feet: Respiratory flows and airborne disease transmission

　We first describe respiratory flows associated with breathing, coughing and sneezing, giving particular attention to both the liquid and gas phases. We demonstrate that the widely implemented 6-foot-rule safety guideline for COVID-19 was based on a physical picture in which the gas-phase was neglected. Consideration of the gas-phase flows makes clear that the range of droplet-borne pathogen may greatly exceed 6 feet, and introduces the possibility of long-range airborne transmission. Evidence is presented that airborne transmission was the dominant mode of transmission of COVID-19. We develop a guideline for mitigating airborne disease transmission that provides a limit for the time spent in indoor spaces with infected individuals. We further demonstrate that carbon- dioxide may serve as a proxy for concentration of airborne pathogen; thus, carbon-dioxide monitoring allows for a real-time assessment of the risk of COVID-19 in indoor settings.

Running on top of a liquid: The impact-induced hardening and the viscoelastic response of the impact process on dense suspensions

　A suspension is a mixture of macroscopic and undissolved particles in a liquid. Such suspensions surround our daily life. One can observe the impact-induced hardening in dense suspensions, where the suspensions behave liquid-like at rest and abruptly turn solid-like under impact. This phenomenon enables people to run on top of the suspensions. In this talk, we will discuss the numerical approach to study the impact-induced hardening in dense suspensions with the lattice Boltzmann method that incorporates the contact between suspended particles and the free surface of the suspension. Then, we will discuss the rebound motion of a free falling spherical impactor on top of the suspensions, how the frictional interactions between suspended particles affect the rebound, and how the impact induced the emergence of a dynamically jammed region beneath the impactor [1]. We will also discuss the connection between the rebound motion, the impact speed, and the maximum force acting on the impactor. Finally, we will discuss our proposed phenomenology that can recover the viscoelastic response of the impactor during the impact [2].
[1] Pradipto and Hayakawa, Phys. Rev. Fluids 6, 033301 (2021).
[2] Pradipto and Hayakawa, Phys. Fluids 33, 093110 (2021).

Flexibility breaks the symmetry of the microswimmer hydrodynamics

　A microswimmer is a self-propelled particle in a fluid at low Reynolds number, ranging from swimming microorganisms to artificial active colloids and microbots. Due to the linearity of the Stokes equations for the flow around microswimmers, the dynamics are accordingly time-reversal as highlighted by the scallop theorem, which states that a microswimmer cannot generate net locomotion via a reciprocal motion. This symmetry, however, can be broken by soft dynamics such as fluid elasticity and swimmer elasticity. In this talk, after a brief introduction to the low-Reynolds-number hydrodynamics with moving boundaries, we will discuss locomotion in viscoelastic fluids and structure-coupled dynamics, showcasing our recent studies on swimming bacteria and sperm cells.