10:30-11:30, December 27th Thursday, 2018
Building 6 -Room 501:
Prof. Dr. Xuehua Zhang
Department of Chemical & Materials Engineering
University of Alberta
Formation of nanodroplets in ternary liquid mixtures
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.
13:30-14:30, May 25th Friday, 2018
Building 6 -Room 501:
Prof. Dr. Yanrong Li
Department of Mechanical Engineering, College of Engineering
Frequency characteristics and fluid flow property of flow-induced vibration
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.
January 22nd Monday, 2018: Prof. Tadd T. Truscott
Department of Mechanical and Aerospace Engineering
Utah State University
Water skipping of elastomeric spheres
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).
January 22nd Monday, 2018: Dr. M. M. Mansoor
Department of Mechanical and Aerospace Engineering
Utah State University
Stable-streamlined and helical cavities following the impact of Leidenfrost spheres
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).
January 22nd Monday, 2018: Dr. Z. Pan
Department of Mechanical and Aerospace Engineering
Utah State University
Water collection and transport on Syntrichia caninervis: a fluid mechanics study on multi-scale multi-functional morphology of a desert moss
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.
January 18th Thursday, 2018:
Dr. Andres FernandoFranco-Gomez
School of Physics and Astronomy
The University of Manchester
Semi-infinite and finite bubble propagation in the presence of a channel-depth perturbation
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.
June 1st Thursday: Dr. Ivo Peters
Aerodynamics and Flight Mechanics Research Group
University of Southampton
Dynamic shear jamming in dense suspensions
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.
May 24th Wednesday: Prof. Dr. Claus-Dieter Ohl
School of Physical and Mathematical Sciences,
Division of Physics and Applied Physics, Nanyang Technological University
The look and touch of nanobubbles
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.
April 6th Thursday: Prof. Dr. Xuehua Zhang
School of Engineering, RMIT University, Melbourne, Australia
Formation of surface bubbles under flow conditions
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.
March 22nd Wednesday: Prof. Dr. Marc K. Smith
George W. Woodruff School of Mechanical Engineering
Georgia Institute of Technology
Multiphase flows from bloodstain pattern analysis to acoustically forced condensation
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.
February 27th Monday: Prof. Dr. Tuan Tran
School of Mechanical & Aerospace Engineering, College of Engineering
Nanyang Technological University
Universality of oscillating in Leidenfrost transition
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.
February 21st Tuesday: Prof. Dr. Chihiro Inoue
Department of Aeronautics and Astronautics, University of Tokyo
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 ”
February 20th Monday:
Prof. Dr. Anne-Laure Biance
Institut Lumière Matière, Université Lyon 1 et CNRS
Morphology of drop impact
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