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-diffusionadvection 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.

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.

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.

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.

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 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.

Creating a cavitation bubble in a thin liquid gap allows us to study a rich variety of interesting phenomena, such as cavitation nucleation, bubble-bubble interaction and viscous fingering. A thin liquid layer is sandwiched between two glass slides. A cavitation bubble is nucleated by a focused pulsed laser in the thin liquid layer and observed using high-speed imaging. Besides the induced cavitation bubble, further bubbles appear, which are arranged typically in radial rings around the main bubble. These additional bubbles are referred to as secondary cavitation bubbles. To understand the secondary bubble nucleation and hence, the pressure modulation in the thin liquid layer, numerical finite-volume simulations are performed. The simulations reveal that the induced cavitation bubble launches a transverse wave that exhibits a strong tension followed by a high-pressure region. The tension on the liquid, created by the rarefaction wave, strongly depends on the thickness of the liquid layer. Thus, the rarefaction wave nucleates further cavitation bubbles along its path only if the liquid gap is thin enough.　Secondary cavitation bubbles can be used to study cavitation nucleation on liquid impurities, such as oil, which have been brought into the liquid layer. Adjusting the　tension wave in the gap carefully, bubble nucleation　can be　observed mainly at the impurities, as they　decrease the cavitation threshold locally.　Finally, we found an instability of the expanding　cavitation bubble. The bubble　interface shows short, stubby fingers. As the denser medium is replaced by a lighter　one, i. e. water is replaced by a vapor phase, this instability is likely a Saffmann-Taylor　instability occurring in an elastic-walled Hele-Shaw cell. Again, the pressure field in the　liquid gap is analyzed with the help of numerical simulations. A linear stability analysis　has been performed to proof the origin of the observed instability of the bubble　interface.

The study of the dynamics of gas bubbles in liquids is justiied by the numerous applications and natural phenomena where this two-phase low is encountered. Gas bubbles moveas forces are applied to them ; their dynamics are full of nuances that need to be addressed carefully. Since the mass of gas bubbles is practically negligible, in comparison withthat of the surrounding liquid, their reaction to forces can be drastic. Furthermore, since their surface can be deformed by the same forces acting on them, their shape may changeleading to changes in their resistance to move, the drag force, and therefore affecting their speed. The liquid rheology, as well as its surfactant content can also affect the bubbleshape and motion as well. Understanding these issues, in addition to the effect of interactions with other bubbles, walls and non-uniform lows, provides suficient elements tomodel and predict bubble behavior through the solution of dynamics equations. In this talk, I will discuss some of these issues which have kept me busy for the past 20 years, toend with suggestions for research directions for the subject in the future.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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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Passcode: 813405

　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.

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　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.

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Passcode: 964857

　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.

　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).

　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.

　Fluids containing a small amount of polymer induce complex flow behavior, that is, hierarchical flow characteristics affected by a length scale. Elastic instability in a microfluidic device is one of an interesting phenomenon of complex fluids. In this seminar, I will introduce the observation of sodium hyaluronate (Hyaluronic Acid Sodium salt, Na-HA) solution in planar abrupt contraction-expansion microchannels to discuss the effects of polymer flexibility and entanglement on elastic instability. As the rigidity of Na-HA depends on the ionic strength of a solvent, Na-HA was dissolved in water and phosphate buffered saline. The flow regimes of the Na-HA solutions in several planar abrupt contraction-expansion channels were characterized by rheological properties of the solution. It was found that the entanglement of Na-HA in the solution is a more dominant factor affecting the flow regimes than the solution relaxation time and polymer rigidity.

　Particle flows injected as a beam and scattered by an intruder are numerically studied [1].
We find a crossover of the drag force from Epstein’s law to Newton’s law depending on the ratio of the speed to the thermal speed. These asymptotic laws can be reproduced by a simple analysis of a collision model between the intruder and particle flows. The crossover from Epstein’s law to Stokes' law is also found for the low speed regime as a time evolution of the drag force caused by beam particles.
We also show the existence of turbulent-like behavior of the particle flows behind the intruder with the aid of the second invariant of the velocity gradient tensor and the relative mean square displacement for the high speed regime and a large intruder.
[1] S. Takada and H. Hayakawa, arXiv:1904.12265.

　Viscous fingering (VF) is one of hydrodynamic instabilities, which is observable while displacing a less-mobile fluid by another more-mobile fluid through porous media, and it is ubiquitous to the transport phenomena in several porous media flows application. Moreover, instabilities at the interface of two distinct fluids remain a major challenge for enhanced oil recovery processes such as polymer flooding, On the other hand, this instability is likely to be detrimental to the separation efficiency in chromatographic separation process and can improve mixing in non-turbulent systems and micro-fluidic devices. The fact that depending on the application either a stable or an unstable interface is desirable makes the ability to control interfacial fingering instabilities. essential in design and technology. Such control mechanism of instabilities in fluid–fluid systems can be achieved by manipulating the various physio-chemical properties of the underlying fluids as well as the porous medium. This talk will be based upon the following three aspect, the possibilities of the liquid adsorption on the porous medium, the competition between advection and diffusion, as well as the chemical reaction, to control the VF.

　A water drop deposited on a hydrophobic micrometric roughness is highly mobile. This property known as superhydrophobicity arises from the air trapped under the drop. However, when condensation forms within the textures, repellency is most often destroyed.
　In this work, we explore the possibility to induce antifogging abilities using nanotextured materials, following the example of cicada’s wings, shown to expel micrometric drops as they merge. Using model surfaces, we discuss the resistance of nanostructured materials to breath figures and show the full efficiency of nanocones: almost all the merging drops jump off the surface. We will then explore the mechanism responsible for this jumping behaviour with millimetric drops, and how viscosity limits antifogging abilities for microdroplets.
　We also describe the adhesion of hot water drops on model nanotextures with various sizes. Our study shows that the denser the textures are, the more resistant the surface is to temperature effects. Surprisingly, we observe the opposite in dynamic conditions: higher structures let more to hot drops bounce. The time needed for condensation to fill the air gap under the drop can be greater than the bouncing time: condensation has no effect on adhesion.

　Drops and their evaporation are not only ubiquitous in nature but also relevant to many industrial applications e.g. material processing, patterning, DNA chip manufacturing and thermal management. Although it looks like a simple system at a glance, drop evaporation results from the complex interplay between heat and mass transfer and fluid dynamics. In my talk, I shall introduce our thermographic visualisation of evaporating drops (organic solvents or water) in various situations (environment, heating), revealing a few of them. The following topics will be covered: spontaneous thermocapillary hydrothermal waves on ethanol drops, the effect of secondary component present in the atmosphere i.e. ethanol drops evaporating in humid air, a way to actively control mixing in pure water drops.

　Phase separation induced by solvent addition is ubiquitous in many technologic and industrial processes, from preparation of pharmaceutical products, to formulation of cosmetics and insecticides, to liquid–liquid microextraction. The new microscopic phase formation induced by solvent addition takes place under the conditions far out-of-equilibrium. The growth dynamics of individual domains is determined not only by the concentration of compositions (thermodynamic aspects), but also by the temporal and spatial characteristics of the mixing process of the solvents (dynamic aspects). We have experimentally and theoretically investigated the effects from the mixing dynamics on the droplet formation under controlled flow conditions. A universal femtoliter droplet-based platform is developed for determination of partition coefficient in water and oil phases and for fast and sensitive nanoextraction of trace of hydrophobic compounds in aqueous solutions. We further revealed the droplet formation from liquid-liquid phase separation in a quasi-2D chamber. Remarkably, the droplets exhibit significantly enhanced the mass transfer in confined spaces. This finding may of relevance to the interfacial process during oil extraction from underground by a displacing fluid.

　A few tricks (hydrophobic texture, heat, etc.) allow us to keep water (or even oil) with a spherical shape, making drops looking like pearls. This induces spectacular dynamical properties. We shall describe a few of them, ranging from anti-rain and anti-fog properties to self-propulsion.

　In the present work we focus on a single bouncing event of a drop on a solid surface. The dynamics of drop depends on three important non-dimensional numbers namely Reynolds number (𝑅𝑒 = 𝜌𝑈𝑅0/𝜇𝑙), Weber number (𝑊𝑒 = 𝜌𝑙𝑈2𝑅0/𝜎) and capillary number (𝐶𝑎𝑔 = 𝜇𝑔𝑈/𝜎 based on gas viscosity). To facilitate bouncing, we set 𝑊𝑒~𝑂(1), 𝑅𝑒~𝑂(100) and contact angle 𝜃 = 170°. We find that in these range of parameters, drop undergoes a complete rebound.
　Using axisymmetric simulations and a few experiments, several different bouncing regimes are found which can be depicted in the form of a phase diagram. For all Weber numbers and moderate values of Re, bouncing of the droplet occurs without any assistance of the surface characteristics such as super-hydrophobicity. This is referred to as wettability independent bouncing. For relatively higher Reynolds number, the droplet makes physical contact with the surface making the process wettability dependent. For wettability independent bouncing process, the drop is supported a thin cushion of gas underneath it. Using high resolution simulations, a detailed characterization of the gas film has been carried out revealing that the entire bouncing process can be divided into five regimes. The talk will shed light on scaling laws for gas film thickness, the shape of the gas film in each of the five regimes and the energetics of the bouncing process. The work rationalises many of the earlier experiments carried out in this area.
　If time permits, other problems of interest currently being pursued in my research group will also be briefly highlighted.

　A premixed flame is a sub-millimeter interface through which a mixture of reacting gas is transforming into burned gas with an increase in temperature and molar volume by a factor 8. This is due to the global exothermicity of hundreds of chemical reactions at play. The gas expansion at the reactive interface induces hydrodynamical instabilities that are wrinkling the interface in a complex dynamics. Some cells are forming and merging in a fashion similar to bubbles in Rayleigh-Taylor instability. However, contrary to that instability, the propagation of the flame induces a limitation of the wrinkling amplitude and it is then possible to describe the evolution of the interface with a PDE equation: the Michelson-Sivashinsky equation. Moreover some analytical solutions consisting of pole trajectories in the complex plane are capable of describing all the dynamics, even in the fully nonlinear regime. This is what we demonstrate experimentally in a quasi-2D burner.

　The dynamics of acoustic cavitation bubbles play an essential role in "efficient" ultrasonic cleaning, but may cause damage to cleaning surface in case bubble collapse is violent enough to accompany strong shock emission and re-entrant liquid jet collision against the surface. Here, the use of dissolved oxygen (DO) supersaturated water is proposed toward "damageless" low-intensity ultrasonic cleaning where mild bubble dynamics are expected to clean surfaces softly. In this talk, visualization of ultrasound-induced acoustic and hydrodynamic phenomena in water whose DO supersaturation is an experimental parameter will be presented to show the importance of dissolved gas supersaturation for ultrasonic cleaning to be both efficient and damageless.

　Here we provide a self-consistent analytical solution describing the unsteady flow in the slender thin film which is expelled radially outwards when a drop hits a dry solid wall. Thanks to the fact that the fluxes of mass and momentum entering into the toroidal rim bordering the expanding liquid sheet are calculated analytically, we show here that our theoretical results closely follow the measured time-varying position of the rim with independence of the wetting properties of the substrate. The particularization of the equations describing the rim dynamics at the instant the drop reaches its maximal extension which, in analogy with the case of Savart sheets, is characterized by a value of the local Weber number equal to one, provides an algebraic equation for the maximum spreading radius also in excellent agreement with experiments. The self-consistent theory presented here, which does not make use of energetic arguments to predict the maximum spreading diameter of impacting drops, provides us with the time evolution of the thickness and of the velocity of the rim bordering the expanding sheet. This information is crucial in the calculation of the diameters and of the velocities of the droplets ejected radially outwards for drop impact velocities above the splashing threshold. We will use the results obtained to the particular case the substrate is superhydrophobic and will show that the theory is able to predict the splash threshold velocity and also the velocities and the diameters of the droplets ejected for impact velocities above this threshold.

　When a drop of a low viscosity liquid impacts against a smooth solid substrate at a velocity V, a liquid sheet of thickness very small compare to the drop radius is expelled tangentially to the substrate at high velocity compare to V. If the impact velocity is such that V > V* with V* the critical velocity for splashing, the edge of the expanding liquid sheet lifts off from the wall as a consequence of the gas lubrication force at the wedge region created between the advancing liquid front and the substrate. In the present talk, we show that the magnitude of the gas lubrication force is limited by the values of the slip lengths at the gas-liquid interface and at the solid. We demonstrate that the splashing regime changes depending on the value of the ratio of the slip lengths, a fact explaining the spreading-splashing-spreading-splashing transition for a reduced value of the surrounding gas pressure as the drop impact velocity increases. We also provide an expression for V* as a function of the inclination angle of the substrate, the drop radius, the material properties of the liquid and the gas and the mean free path of gas molecules.

　In this talk, I will discuss about some specific research problems in the area of cavitation bubble dynamics that we are working on in my laboratory at IIT Hyderabad, India. I will discuss two specific problems a) The entrapment of an air bubble on the free surface of water due to the expansion and collapse of a non-equilibrium bubble and b) The effect of viscosity on the dynamics of a non-equilibrium bubble. We have used a low-voltage spark circuit wherein a capacitor is first charged and then discharged through two contacting electrodes within a liquid to create a bubble. As the bubble expands and collapses, we observe dynamics of the bubble under different surroundings in both of the above mentioned problems. The entire dynamics were recorded using a high-speed camera which helped to understand the mechanism behind the observed behaviour.

　In this talk, I will demonstrate how to alter macroscopic flows via interfacial modifications at micro scales, highlighting associated energy, environmental, and technological applications. Several flow scenarios are scrutinized and presented, namely microfluidic flows, drop impact, and macroscopic fluid-fluid displacement. In microfluidic laminar flow, our pore-scale measurements reveal that the geometry of the liquid-gas interface strongly influences the hydrodynamic slippage, i.e., drag reduction, using hydrophobic micro-structures. In drop impact on a solid substrate, the impact outcomes can be tuned by varying the microstructures of the surface. In a large-scale flow configuration, a viscous-fingering in a Hele-Shaw cell—a convenient framework for modeling a homogeneous porous medium—can be controlled through a capillary effect. From these results, we learn that the interfacial conditions play an important role, thereby offering strategic controls of flow motion.

　This talk will deal with cellulose fibrils, a class of materials that can be obtained from various sources, including plants, tunicates and bacteria. This talk will present the extraction and characterization of cellulose fibrils obtained from a marine resource and how membranes with highly negative surfaces charges at pH < 4 can be obtained. Potential application of these membranes will be discussed. Also, we will show how cellulose fibrils obtained from bacteria and plants can be used as a reinforcement material for the design of composite materials with improved mechanical properties. Experimental data on the quantification of the interfacial interaction between nanocellulose and various polymer matrices by Raman spectroscopy will be shown.

　Heterogeneous nucleation and growth of droplets on surfaces under different flow conditions has recently advanced to a quantitative level. This talk will give an overview of our latest contributions to the formation and properties of surface droplets by solvent exchange and to point out the potential applications of surface nanodroplets in highly sensitive chemical analysis. We will present a universal femtoliter surface droplet-based platform for direct quantification of trace of hydrophobic compounds in aqueous solutions. In-situ quantification of the extracted analytes is achieved by surface-enhanced Raman scattering (SERS) spectroscopy by nanoparticles on the functionalized droplets.

　The present study focused on the frequency properties of flow-induced vibration, i.e. clarifying lock-in condition and flow fluctuation propagation of flow-induced vibration with resonator and frequency characteristics and flow oscillation pattern of flow- induced vibration without resonator. On this basis, flow-induced acoustic resonance inside closed coaxial side branches, which is a typical case of flow-induced vibration with resonator, and periodical feedback flow structure inside a feedback type fluidic oscillator, which is a classical type of flow-induced vibration without resonator, was investigated. In order to find out the jet oscillation properties, the high time resolved PIV technique was utilized to extract the two-dimensional flow fields.

　Elastomeric silicone rubber spheres ricochet from a water surface when rigid spheres and disks (or skipping stones) cannot, but why? High speed cameras allow us to see that these unique spheres deform significantly as they impact the water surface, flattening into oblate shapes at impact and then returning to spherical shapes. We present a regime diagram which enables the prediction of ricochet from sphere impact conditions. Our experiments and mathematical models show that these deformable spheres skip more readily because deformation momentarily increases cross-sectional area and produces an attack angle with the water which is favorable to skipping. Predictions from our mathematical model agree strongly with observations from experiments. Even when a sphere was allowed to skip multiple times in the laboratory, the mathematical model showed good agreement with measured impact conditions through subsequent skipping events. While studying multiple impact events in an outdoor setting, we discovered a previously unidentified means of skipping, which is unique to deformable spheres. This new skipping occurs when a relatively soft sphere impacts the water at a high speed and low impact angle wherein the sphere begins to rotate very quickly. This large angular velocity causes the sphere to stretch into and maintain a disk-like shape with an oval cross section. The sphere is observed to move nearly parallel with the water surface with the tips of the major axis of the spheroid dipping into the water as it rotates with the sides passing just over the surface. This sequence of rapid impact events give the impression that the sphere is walking across the water surface (collaborated with R. Hurd & J. Belden).

　In this talk I will report results from an experimental study on the formation of stable–streamlined and helical cavity wakes following the free-surface impact of Leidenfrost spheres. The Leidenfrost effect encapsulates the sphere by a vapor layer to prevent any physical contact with the surrounding liquid. This phenomenon is essential for the pacification of acoustic rippling along the cavity interface to result in a stable-streamlined cavity wake. This streamlined configuration is found to experience drag coefficients an order of a magnitude lower than those acting on room-temperature spheres. A striking observation is the formation of helical cavities which occur for impact Reynolds numbers Re0 ≥ 1.4×105 and are characterized by multiple interfacial ridges, stemming from and rotating synchronously about an evident contact line around the sphere equator. Using sphere trajectory measurements, this helical cavity wake configuration is shown to have 40%–55% smaller force coefficients than those obtained in the formation of stable cavity wakes. (published in the J. Fluid Mech. 823, 2017). The teardrop-shaped gas cavities are also shown to be reproducible in water with room-temperature spheres where the gas-layer around the sphere can be sustained by making the sphere surface superhydrophobic (published in Science Advances, 3(9) 2017).

　Syntrichia caninervis is one of the most abundant desert mosses in the world and thrives in an extreme environment with multiple but limited water resources (such as dew, fog, snow and rain), yet the mechanisms for water collection and transport have never been completely revealed. S. caninervis has a unique adaptation: it uses a tiny hair (awn) on the end of each leaf to collect water, in addition to that collected by the leaves themselves. Here we show that the unique multiscale structures of the hair are equipped to collect and transport water in four modes: nucleation of water droplets and films on the leaf hair from humid atmospheres; collection of fog droplets on leaf hairs; collection of splash water from raindrops; and transportation of the acquired water to the leaf itself. Fluid nucleation is accomplished in nanostructures, whereas fog droplets are gathered in areas where a high density of small barbs are present and then quickly transported to the leaf at the base of the hair. Our observations reveal nature’s optimization of water collection by coupling relevant multiscale physical plant structures with multiscale sources of water. This research could inspire the design of fog harvest devices to help the people living in arid areas and high-efficiency dehumidifiers.

　The two-phase flow displacement of a viscous fluid by a less viscous one in a confined environment leads to a viscous fingering instability commonly encountered in natural systems, for example, in flows through porous media or pulmonary airways, with important applications in oil recovery or respiratory treatments. The classical study of viscous fingering has been conducted in rectangular channels of high aspect ratio (large width/height), known as Hele-Shaw channels where a unique, steady symmetric, semi-infinite bubble (finger) emerges. Here, we consider the propagation of semi-infinite (open) and finite (closed) air bubbles in Hele-Shaw channels where thin, axially-uniform occlusions are introduced. This configuration is known to generate symmetric, asymmetric and oscillatory modes with complex interactions and rich behaviour. Experimental results of finger propagation in these constricted channels is found to be in quantitative agreement with numerical computations using a depth- averaged model once the aspect ratio reaches a value of α ≥ 40 and capillary numbers (ratio between viscous and surface tension forces) below Ca ≤ 0.012. The same evolution of the bifurcation structure between multiple modes is found, however, it occurs for decreasing values of occlusion height as the value of aspect ratio is increased, that the system exhibits sensitivity to small but finite depth-variations. Moreover, deviations from the single symmetric mode are predicted when depth-variations of order of the roughness of the channel walls (∼ 1 μm) are introduced for larger aspect ratios of α ≥ 155.
The second part of this talk considers strongly confined finite bubbles in a channel with constant aspect ratio of α = 30 and where the height of the occlusion, termed rail, is 1/40 of the channel height. Recent investigations (Abbyad et al. 2011) have suggested that local variations in drop confinement can be used as an effective means of passively guiding or trapping droplets using microfabricated grooves. Here, we investigate bubble propagation in two regimes of bubble sizes, one on the same order as the rail-width and the other of the order of the channel-width. We find that when surface tension forces (small capillary number Ca) are dominant the bubbles tend to localise on the channel-sides. However, a subtle balance between viscous and surface tension forces means that for intermediate capillary numbers the bubbles can be stabilised on top of the rail, before a further symmetry breaking bifurcation pushes them into one of the channel-sides at higher Ca. Moreover, for bubbles comparable with the rail-width, we find that they can travel stably either centred or in a channel-side beyond a critical capillary number. These bifurcation phenomena are investigated using a combination of experiments and numerical computations with a view to passively control and sort bubbles by size.

　インクジェット技術はインクを微小なノズルから液滴化して吐出させ描画する技術であるが、その吐出させる原理としてピエゾ方式とサーマル方式の二種類が存在する。同じ目的でありながらその滴形成の原理は大きく異なり、設計論もジオメトリーも大きく異なることが物理的にはとても興味深い。今回は特にサーマル方式インクジェットの撃力による吐出メカニズムを中心にインクジェット技術の概要を紹介する。

　Shear a dense suspension of cornstarch and water hard enough, and the system seems to solidify as a result. Indeed, previous studies have shown that a jamming front propagates through these systems until, after interaction with boundaries, a jammed solid spans across the system. Because these fully jammed states are only observed if the deformation is fast enough, a natural question to ask is how this phenomenon is related to the discontinuous shear thickening (DST) behavior of these suspensions. We present a single experimental setup in which we on the one hand can measure the rheological flow curves, but on the other hand also determine if the suspension is in a jammed state. This we do by using a large-gap cylindrical Couette cell, where we control the applied shear stress using a rheometer. Because our setup only applies shear, the jammed states we observe are shear-jammed, and cannot be a result of an overall increase in packing fraction. We probe for jammed states by dropping small steel spheres on the surface of the suspension, and identify elastic responses. Our experiments reveal a clear distinction between the onset of DST and Shear-Jammed states, which have qualitatively different trends with packing fraction close to the isotropic jamming point.

　Nanobubbles challenged the scientific community twofold: they are difficult to distinguish from contamination and they - according to physics should not be existent. In this seminar I'll convince you that nanobubbles are a reality, as long as they are pinned to a substrate and they have a special touch. The key to a successful study of the mysterious gaseous objects was to combine the touch from atomic force microscopy and with the optical image. I'll present a preliminary theory on their stability, the measurement of the pinning force, and how you can make them yourself by mixing water with alcohol.

　Regulating formation and growth of microscopic bubbles at solid-liquid interfaces is essential in many physical, chemical and catalytic processes. The growth of bubbles in a group is influenced by the neighbouring bubbles as well as the overall gas concentration in the system. In this work, we have investigated the growth of multiple microbubbles on highly ordered hydrophobic microcavity arrays, seeded by pre-existing gas pockets trapped inside the cavities. A pulse of gas oversaturation at an extremely low level was supplied by solvent exchange. Our results show that the distance between the seeding air pockets has significant effects on the location, number density and size of bubbles on the array. With closely spaced microcavities, growing microbubbles self-organized into symmetric patterns. Their growth rate was enhanced at the corners and edges of the array, and interior bubbles dissolved due to the competitive growth. By contrast, no symmetric patterns were observed when the space between the microcavities was large. The findings reported in this work provide important insights into solvent exchange and collective interactions in the formation of surface bubbles.

　Multiphase flows appear in a broad range of systems and applications. In this presentation, two very different applications will be described. The first is a bloodstain pattern. A blood-letting event such as a cut, blunt trauma, or gunshot wound will release many blood droplets from the body that produce a complex bloodstain pattern upon impact with a solid surface. Bloodstain pattern analysis is a set of techniques used in forensic crime-scene reconstruction that examines this pattern to help determine the point of origin of the blood droplets as well as the method of their creation. In one technique, the pattern analyst examines individual bloodstains to infer the size, impact angle, and impact velocity of the blood droplet that produced the stain. This technique was explored and extended in an experimental study of the impact and spreading of a single droplet of a blood simulant on planar surfaces of variable roughness and wettability oriented at various angles with respect to the velocity vector of the approaching droplet. The primary focus of the work was to use the shape of the fully spread droplet to determine the initial conditions of the droplet impact. Three different solid surfaces were used and the impact process was explored over a range of Reynolds numbers (1,000 – 5,500) and Weber numbers (200 – 2,000) typical of some forensic situations. Results of this work will be presented and compared to the common forensics practice used in the field.
　A second application of multiphase flow is the enhanced condensation of vapor bubbles in a quiescent subcooled liquid using ultrasonic actuation. In this set of experiments, the vapor bubbles are formed by direct injection from a pressurized steam reservoir through a nozzle with a specified diameter. The bubbles then rise in the liquid through a constant or pulsed acoustic field of programmable intensity. Low frequency acoustic actuation creates capillary waves on the vapor-liquid interface (known as Faraday waves), while ultrasonic actuation leads to the formation of a liquid jet that penetrates the bubble and significantly increases its surface area and therefore its condensation rate. The ultrasonic beam also self-focusses within the jet leading to the ejection of small-scale droplets from the jet that are propelled to the opposite surface of the bubble, which further enhances condensation. High-speed Schlieren video is used to investigate the effects of ultrasonic actuation on the thermal boundary layer at the vapor-liquid interface and its effect on the enhanced condensation rate. PIV measurements on the liquid side are used to assess liquid motion during condensation.

　The Leidenfrost transition leads a boiling system to the boiling crisis, a state in which the liquid loses contact with the heated surface due to excessive vapor generation. Here, using experiments of liquid droplets boiling on a heated surface, we report a new phenomenon, termed oscillating boiling, at the Leidenfrost transition. We show that oscillating boiling is resulted from the competition between two effects: separation of liquid from the heated surface due to localized boiling, and rewetting. We argue theoretically that the Leidenfrost transition can be predicted based on its link with the oscillating boiling phenomenon, and verify the prediction experimentally for various liquids.

　戦国時代後期に,火縄銃とともに黒色火薬が伝来した。江戸時代になると,黒色火薬を用いて花火が作られるようになった。線香花火は,わずか0.1gの黒色火薬と和紙からなる最も単純な玩具花火である。同時に,最も馴染み深い花火の一つであろう。紙縒りの上端を持ち,下端に点火すると,火薬が勢い良く燃えてオレンジ色の火球ができる。一呼吸置いた後,火球から四方に飛び出した火花が,遠方でパッと弾けて,松葉状の美しい模様を描き出す。やがて火花は弱々しく出ては消え,一生を終える。この独特の儚い美しさは,数世紀にわたって親しまれてきたものの,なぜ火花が出るのか?なぜ火花が破裂するのか?なぜ独特の色味が表れるのか?といった,誰もが思う素朴な疑問への答えは明らかにされてこなかった。2012年8月以降, 我々は,高速度可視化計測や熱分析,理論解析を行うことで,長年の謎の一端を解き明かすことに成功した。講演では,線香花火研究を始めるきっかけになった映像や,初めて撮影された火花連鎖分裂の高速度可視化結果などを紹介しながら, 線香花火の美の物理を解説する。また, 火花は溶融塩液滴の軌跡であり,線香花火現象の律速過程が液滴内部の熱拡散であることなど,理論的側面について述べる。あわせて,横浜国立大学と共同で実施した熱分析によって特定された,液滴連鎖分裂を引き起こす物質ついても紹介する。
　　” Senko-hanabi as Dancing Drops ”
　　(https://www.youtube.com/watch?v=kMrvKMciELs)

　Droplet impact on substrates is highly used in many practical situations, for cooling processes, solid sphere fabrication, police investigation or ink-jet printing among others… Depending on the liquid characteristics (surface tension, viscosity) and the droplet size and velocity, different regimes can be observed (viscous, capillary or inertia dominated ones.). In this talk, I will review some work we performed on this subject focusing on the specific nature of the substrate. We will consider the cases of:
-Drop impact on smooth substrate which can be heated (we are then in the so-called Leidenfrost regime) or not (the viscous dissipation is then crucial)
-Drop impact on multiscale super hydrophobic substrates
-Drop impact on a heated substrate that bears some topological defects.
In both cases, we will rely on high-speed camera characterizations, adsorption measurements (to probe the lamella thickness) and particle tracking velocimetry to understand the different behaviors observed.
Collaborators C. Ybert, C. Lee, C. Pirat
Students H. Lastakowski, Q. Ehlinger