Ivan U Vakarelski

King Abdullah University of Science and Technology, Djidda, Makkah, Saudi Arabia

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Publications (77)264.58 Total impact

  • E. Q. Li · I. U. Vakarelski · S. T. Thoroddsen ·
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    ABSTRACT: When a drop impacts onto a solid surface, the lubrication pressure in the air deforms its bottom into a dimple. This makes the initial contact with the substrate occur not at a point but along a ring, thereby entrapping a central disc of air. We use ultra-high-speed imaging, with 200 ns time resolution, to observe the structure of this first contact between the liquid and a smooth solid surface. For a water drop impacting onto regular glass we observe a ring of microbubbles, due to multiple initial contacts just before the formation of the fully wetted outer section. These contacts are spaced by a few microns and quickly grow in size until they meet, thereby leaving behind a ring of microbubbles marking the original air-disc diameter. On the other hand, no microbubbles are left behind when the drop impacts onto molecularly smooth mica sheets. We thereby conclude that the localized contacts are due to nanometric roughness of the glass surface, and the presence of the microbubbles can therefore distinguish between glass with 10 nm roughness and perfectly smooth glass. We contrast this entrapment topology with the initial contact of a drop impacting onto a film of extremely viscous immiscible liquid, where the initial contact appears to be continuous along the ring. Here, an azimuthal instability occurs during the rapid contraction at the triple line, also leaving behind microbubbles. For low impact velocities the nature of the initial contact changes to one initiated by ruptures of a thin lubricating air film.
    Journal of Fluid Mechanics 12/2015; 785. DOI:10.1017/jfm.2015.643 · 2.38 Impact Factor
  • Ivan U Vakarelski · Derek Y C Chan · Sigurdur T Thoroddsen ·
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    ABSTRACT: We report measurements of the effects of a melting ice surface on the hydrodynamic drag of ice-shell-metal-core spheres free falling in water at a Reynolds of number Re∼2×10^{4}-3×10^{5} and demonstrate that the melting surface induces the early onset of the drag crisis, thus reducing the hydrodynamic drag by more than 50%. Direct visualization of the flow pattern demonstrates the key role of surface melting. Our observations support the hypothesis that the drag reduction is due to the disturbance of the viscous boundary layer by the mass transfer from the melting ice surface.
    Physical Review Letters 07/2015; 115(4):044501. DOI:10.1103/PhysRevLett.115.044501 · 7.51 Impact Factor
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    Saifullah Lone · Ivan U. Vakarelski · Basil Chew · Zhihong Wang · Sigurdur T. Thoroddsen ·
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    ABSTRACT: We describe a hybrid methodology that combines a two dimensional (2D) monolayer of latex particles (with a pitch size down to 1 μm) prepared by horizontal dry deposition, lift-up of a 2D template onto flat surfaces and evaporation lithography to fabricate metal micro- and nano wire-networks.
    RSC Advances 11/2014; 4(103). DOI:10.1039/C4RA11278D · 3.84 Impact Factor
  • Ivan U. Vakarelski · Er Qiang Li · Sigurdur T. Thoroddsen ·
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    ABSTRACT: Here we introduce an extension of the atomic force microscopy (AFM) colloidal probe technique, as a simple and reliable experimental approach to measure the interaction forces between small water droplets (∼80–160 μm) dispersed in oil. Small water droplets are formed by capillary breakup of a microscale water jet in air, which is forced out of a fine capillary nozzle, and deposited on a superhydrophobic substrate immersed in tetradecane oil medium. In these conditions the water droplets are very loosely attached to the superhydrophobic substrate and are easily picked up with a hydrophobic AFM cantilever to form a soft colloidal probe. Sample force measurements are conducted to demonstrate the capability of the technique.
    Colloids and Surfaces A Physicochemical and Engineering Aspects 11/2014; 462:259–263. DOI:10.1016/j.colsurfa.2014.09.028 · 2.75 Impact Factor
  • Ivan U Vakarelski · Derek Y C Chan · Sigurdur T Thoroddsen ·
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    ABSTRACT: We investigate the dynamic effects of a Leidenfrost vapour layer sustained on the surface of heated steel spheres during free fall in water. We find that a stable vapour layer sustained on the textured superhydrophobic surface of spheres falling through 95 °C water can reduce the hydrodynamic drag by up to 75% and stabilize the sphere trajectory for the Reynolds number between 10(4) and 10(6), spanning the drag crisis in the absence of the vapour layer. For hydrophilic spheres under the same conditions, the transition to drag reduction and trajectory stability occurs abruptly at a temperature different from the static Leidenfrost point. The observed drag reduction effects are attributed to the disruption of the viscous boundary layer by the vapour layer whose thickness depends on the water temperature. Both the drag reduction and the trajectory stabilization effects are expected to have significant implications for development of sustainable vapour layer based technologies.
    Soft Matter 05/2014; 10(31). DOI:10.1039/c4sm00368c · 4.03 Impact Factor
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    Er Qiang Li · Ivan U Vakarelski · Derek Y C Chan · Sigurdur T Thoroddsen ·
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    ABSTRACT: Using high-speed video recording of bubble rise experiments we study the stability of thin liquid films trapped between a rising bubble and a surfactant-free liquid-liquid meniscus interface. Using different combinations of non-polar oils and water that are all immiscible, we investigate the extent to which film stability can be predicted by attractive and repulsive van der Waals (vdW) interactions that are indicated by the relative magnitude of the refractive indices of the liquid combinations e.g. water (refractive index, n = 1.33), perfluorohexane (n = 1.23) and tetradecane (n = 1.43). We show that when the film-forming phase was oil (perfluorohexane or tetradecane) the stability of the film could always be predicted from the sign of the vdW interaction with a repulsive vdW force resulting in a stable film and an attractive vdW force resulting in film rupture. However, if aqueous electrolyte is the film-forming bulk phase between the rising air bubble and the upper oil phase, the film always ruptured even when a repulsive vdW interaction was predicted. We interpret these results as supporting the hypothesis that a short-ranged hydrophobic attraction determines the stability of the thin water film formed between an air phase and a non-polar oil phase.
    Langmuir 04/2014; 30(18). DOI:10.1021/la500868y · 4.46 Impact Factor
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    E. Q. Li · S. A. Al-Otaibi · I. U. Vakarelski · S. T. Thoroddsen ·
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    ABSTRACT: When a bubble rises to an interface between two immiscible liquids, it can pass through the interface, if this is energetically favourable, i.e. the bubble preferring the side of the interface with the lower air-liquid surface tension. Once the intermediate film between the bubble and the interface has drained sufficiently, the bubble makes contact with the interface, forming a triple-line and producing strong capillary waves which travel around the bubble and can pinch off a satellite on the opposite side, akin to the dynamics in the coalescence cascade. We identify the critical Ohnesorge numbers where such satellites are produced and characterize their sizes. The total transition time scales with the bubble size and differential surface tension, while the satellite pinch-off time scales with the capillary-inertial time of the pool liquid, which originally surrounds the bubble. We also use high-speed video imaging to study the motion of the neck of the contact. For low viscosity we show that it grows in time with a power-law exponent between 0.44 and 0.50, with a prefactor modified by the net sum of the three interfacial tensions. Increasing the viscosity of the receiving liquid drop drastically slows down the motion of the triple-line, when the Ohnesorge number exceeds similar to 0.08. This differs qualitatively from the coalescence of two miscible drops of different viscosities, where the lower viscosity sets the coalescence speed. We thereby propose a strong resistance from the triple-line.
    Journal of Fluid Mechanics 03/2014; 744. DOI:10.1017/jfm.2014.67 · 2.38 Impact Factor
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    ABSTRACT: We report results from an experimental study of cavity formation during the impact of superhydrophobic spheres onto water. Using a simple splash-guard mechanism, we block the spray emerging during initial contact from closing thus eliminating the phenomenon known as 'surface seal', which typically occurs at Froude numbers Fr= V02/(gR0) = O(100). As such, we are able to observe the evolution of a smooth cavity in a more extended parameter space than has been achieved in previous studies. Furthermore, by systematically varying the tank size and sphere diameter, we examine the influence of increasing wall effects on these guarded impact cavities and note the formation of surface undulations with wavelength λ =O(10)cm and acoustic waves λa=O(D0) along the cavity interface, which produce multiple pinch-off points. Acoustic waves are initiated by pressure perturbations, which themselves are generated by the primary cavity pinch-off. Using high-speed particle image velocimetry (PIV) techniques we study the bulk fluid flow for the most constrained geometry and show the larger undulations ( λ =O (10cm)) have a fixed nature with respect to the lab frame. We show that previously deduced scalings for the normalized (primary) pinch-off location (ratio of pinch-off depth to sphere depth at pinch-off time), Hp/H = 1/2, and pinch-off time, τ α (R0/g) 1/2, do not hold for these extended cavities in the presence of strong wall effects (sphere-to-tank diameter ratio), ε = D 0/Dtank 1/16. Instead, we find multiple distinct regimes for values of Hp/H as the observed undulations are induced above the first pinch-off point as the impact speed increases. We also report observations of 'kinked' pinch-off points and the suppression of downward facing jets in the presence of wall effects. Surprisingly, upward facing jets emanating from first cavity pinch-off points evolve into a 'flat' structure at high impact speeds, both in the presence and absence of wall effects.
    Journal of Fluid Mechanics 01/2014; 743:295-326. DOI:10.1017/jfm.2014.35 · 2.38 Impact Factor
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    ABSTRACT: We investigate the onset of cavitation during the collision of a sphere with a solid surface covered with a layer of Newtonian liquid. The conventional theory dictates cavitation to initiate during depressurization, i.e. when the sphere rebounds from the solid surface. Using synchronized dual-view high-speed imaging, we provide conclusive experimental evidence that confirms this scenario—namely—that cavitation occurs only after the sphere makes initial contact with the solid surface. Similar to previous experimental observations for spheres released above the liquid surface, bubbles are formed on the sphere surface during entry into the liquid layer. These were found to squeeze radially outwards with the liquid flow as the sphere approached the solid surface, producing an annular bubble structure unrelated to cavitation. In contrast, spheres released below the liquid surface did not exhibit these patterns.
    Experiments in Fluids 01/2014; 55(1). DOI:10.1007/s00348-013-1648-6 · 1.67 Impact Factor
  • J. O. Marston · Y. Zhu · I. U. Vakarelski · S. T. Thoroddsen ·

    Physics of Fluids 09/2013; 25(9):1107-. DOI:10.1063/1.4820020 · 2.03 Impact Factor
  • Ivan U Vakarelski · Derek Y C Chan · Jeremy O Marston · Sigurdur T Thoroddsen ·
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    ABSTRACT: We provide an experimental demonstration that a novel macroscopic, dynamic continuous air layer or plastron can be sustained indefinitely on textured superhydrophobic surfaces in air-supersaturated water by a natural gas influx mechanism. This type of plastron is an intermediate state between Leidenfrost vapor layers on superheated surfaces and the equilibrium Cassie-Baxter wetting state on textured superhydrophobic surfaces. We show that such plastron can be sustained on the surface of a centimeter size superhydrophobic sphere immersed in heated water and variations of its dynamic behavior with air saturation of the water can be regulated by rapid changes of the water temperature. The simple experimental set-up allows for quantification of the air flux into the plastron and identification of the air transport model of the plastron growth. Both the observed growth dynamics of such plastrons and of millimeter-sized air bubbles seeded on hydrophilic surface under identical air-supersaturated solution condition are consistent with the predictions of a well-mixed gas transport model.
    Langmuir 08/2013; 29(35). DOI:10.1021/la402306c · 4.46 Impact Factor
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    J.O. Marston · J.E. Sprittles · Y. Zhu · E.Q. Li · I.U. Vakarelski · S.T. Thoroddsen ·
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    ABSTRACT: We present results from an experimental study of the impact of liquid drops onto powder beds which are pre-wetted with the impacting liquid. Using high-speed video imaging, we study both the dynamics of the initial spreading regime and drainage times once the drop has reached its maximum spread on the surface. During the initial spreading stage, we compare our experimental data to a previously developed model which incorporates imbibition into the spreading dynamics and observe reasonable agreement. We find that the maximum spread is a strong function of the moisture content in the powder bed and that the total time from impact to complete drainage is always shorter than that for dry powder. Our results indicate that there is an optimum moisture content (or saturation) which leads to the fastest penetration. We use simple scaling arguments which also identify an optimum moisture content for fastest penetration, which agrees very well with the experimental result.
    Powder Technology 05/2013; 239:128–136. DOI:10.1016/j.powtec.2013.01.062 · 2.35 Impact Factor
  • Ivan U Vakarelski · Jeremy O Marston · Sigurdur T Thoroddsen ·
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    ABSTRACT: Evaporative lithography using latex particle templates is a novel approach for the self-assembly of suspension-dispersed nanoparticles into ordered microwire networks. The phenomenon that drives the self-assembly process is the propagation of a network of interconnected liquid bridges between the template particles and the underlying substrate. With the aid of video-microscopy, we demonstrate that these liquid bridges are in fact the border zone between the underlying substrate and foam films vertical to the substrate, which are formed during the evaporation of the liquid from the suspension. The stability of the foam films and thus the liquid bridge network stability are due to the presence of a small amount of surfactant in the evaporating solution. We show that the same type of foam film stabilised liquid bridge network can also propagate in 3D clusters of spherical particles, which has important implications for the understanding of wet granular matter.
    Langmuir 03/2013; 29(16). DOI:10.1021/la400662n · 4.46 Impact Factor
  • G. Bahar Basim · Ayse Karagoz · Long Chen · Ivan Vakarelski ·
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    ABSTRACT: In this study, slurry formulations in the presence of self-assembled surfactant structures were investigated for Ge/SiO2 CMP applications in the absence and presence of oxidizers. Both anionic (sodium dodecyl sulfate-SDS) and cationic (cetyl trimethyl ammonium bromide-C12TAB) micelles were used in the slurry formulations as a function of pH and oxidizer concentration. CMP performances of Ge and SiO2 wafers were evaluated in terms of material removal rates, selectivity and surface quality. The material removal rate responses were also assessed through AFM wear rate tests to obtain a faster response for preliminary analyses. The surfactant adsorption characteristics were studied through surface wettability responses of the Ge and SiO2 wafers through contact angle measurements. It was observed that the self-assembled surfactant structures can help obtain selectivity on the silica/germanium system at low concentrations of the oxidizer in the slurry.
    MRS Online Proceeding Library 01/2013; 1560. DOI:10.1557/opl.2013.971
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    ABSTRACT: The nano-scale wear and friction of silica and silicon nitride surfaces in aqueous electrolyte solutions were investigated by using sharp atomic force microscope (AFM) cantilever tips coated with silicon nitride. Measurements were carried out in aqueous solutions of varying pH and in monovalent and divalent cations chloride and nitrate solutions. The silica surface was shown to wear strongly in solutions of high pH (≈ 11.0), as expected, but the presence of simple cations, such as Cs(+) and Ca(2+), were shown to dramatically effect the wear depth and friction force for the silica surface. In the case of monovalent cations, their hydration enthalpies correlated well with the wear and friction. The weakest hydrated cation of Cs(+) showed the most significant enhancement of wear and friction. In the case of divalent cations, a complex dependence on the type of cation was found, where the type of anion was also seen to play an important role. The CaCl(2) solution showed the anomalous enhancement of wear depth and friction force, although the solution of Ca(NO(3))(2) did not. The present results obtained with an AFM tip were also compared with previous nanotribology studies of silica surfaces in electrolyte solutions, and possible molecular mechanisms as to why cations enhance the wear and friction were also discussed.
    Langmuir 10/2012; 28(46). DOI:10.1021/la303223q · 4.46 Impact Factor
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    Jeremy Marston · Ying Zhu · Ivan Vakarelski · Siggi Thoroddsen ·
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    ABSTRACT: This document accompanies fluid dyanmics video entry V83911 for APS DFD 2012 meeting. In this video, we present experiments on how drop oscillations can be "frozen" using hydrophobic powders.
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    ABSTRACT: In 1756, Leidenfrost observed that water drops skittered on a sufficiently hot skillet, owing to levitation by an evaporative vapour film. Such films are stable only when the hot surface is above a critical temperature, and are a central phenomenon in boiling. In this so-called Leidenfrost regime, the low thermal conductivity of the vapour layer inhibits heat transfer between the hot surface and the liquid. When the temperature of the cooling surface drops below the critical temperature, the vapour film collapses and the system enters a nucleate-boiling regime, which can result in vapour explosions that are particularly detrimental in certain contexts, such as in nuclear power plants. The presence of these vapour films can also reduce liquid-solid drag. Here we show how vapour film collapse can be completely suppressed at textured superhydrophobic surfaces. At a smooth hydrophobic surface, the vapour film still collapses on cooling, albeit at a reduced critical temperature, and the system switches explosively to nucleate boiling. In contrast, at textured, superhydrophobic surfaces, the vapour layer gradually relaxes until the surface is completely cooled, without exhibiting a nucleate-boiling phase. This result demonstrates that topological texture on superhydrophobic materials is critical in stabilizing the vapour layer and thus in controlling--by heat transfer--the liquid-gas phase transition at hot surfaces. This concept can potentially be applied to control other phase transitions, such as ice or frost formation, and to the design of low-drag surfaces at which the vapour phase is stabilized in the grooves of textures without heating.
    Nature 09/2012; 489(7415):274-7. DOI:10.1038/nature11418 · 41.46 Impact Factor
  • J. O. Marston · Y. Zhu · I. U. Vakarelski · S. T. Thoroddsen ·
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    ABSTRACT: In this work we show that when a liquid drop impacts onto a fine-grained hydrophobic powder, the final form of the drop can be very different from the spherical form with which it impacts. In all cases, the drop rebounds due to the hydrophobic nature of the powder. However, we find that above a critical impact speed, the drop undergoes a permanent deformation to a highly non-spherical shape with a near-complete coverage of powder, which then freezes the drop oscillations during rebound.
    Powder Technology 09/2012; 228:424–428. DOI:10.1016/j.powtec.2012.06.003 · 2.35 Impact Factor
  • J. O. Marston · I. U. Vakarelski · S. T. Thoroddsen ·
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    ABSTRACT: We present experimental results for the penetration of a solid sphere when released onto wet sand. We show, by measuring the final penetration depth, that the cohesion induced by the water can result in either a deeper or shallower penetration for a given release height compared to dry granular material. Thus the presence of water can either lubricate or stiffen the granular material. By assuming the shear rate is proportional to the impact velocity and using the depth-averaged stopping force in calculating the shear stress, we derive effective viscosities for the wet granular materials.
    Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics 08/2012; 86(2). DOI:10.1103/PhysRevE.86.020301 · 2.81 Impact Factor
  • J. O. Marston · I. U. Vakarelski · S. T. Thoroddsen ·
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    ABSTRACT: We report observations of cavity formation and subsequent collapse when a heated sphere impacts onto a liquid pool. When the sphere temperature is much greater than the boiling point of the liquid, we observe an inverted Leidenfrost effect where the sphere is encompassed by a vapour layer that prevents physical contact with the liquid. This creates the ultimate non-wetting scenario during sphere penetration through a free surface, producing very smooth cavity walls. In some cases during initial entry, however, the liquid contacts the sphere at the equator, leading to the formation of a dual cavity structure. For cold sphere impacts, where a contact line is observed, we reveal details of the contact line pinning, which initially forms a sawtooth pattern. We also observe surface waves on the cavity interface for cold spheres. We compare our experimental results to previous studies of cavity dynamics and, in particular, the influence of hydrophobicity on the entry of the sphere.
    Journal of Fluid Mechanics 05/2012; 699:465-488. DOI:10.1017/jfm.2012.124 · 2.38 Impact Factor

Publication Stats

1k Citations
264.58 Total Impact Points


  • 2011-2014
    • King Abdullah University of Science and Technology
      • Division of Physical Sciences and Engineering (PSE)
      Djidda, Makkah, Saudi Arabia
  • 2000-2012
    • Kyoto University
      • Department of Chemical Engineering
      Kioto, Kyōto, Japan
  • 2010
    • National Institute of Advanced Industrial Science and Technology
      • Advanced Manufacturing Research Institute
      Tsukuba, Ibaraki, Japan
  • 2009-2010
    • University of Melbourne
      • Department of Chemical and Biomolecular Engineering
      Melbourne, Victoria, Australia
  • 2001-2010
    • University of Florida
      • Department of Materials Science and Engineering
      Gainesville, Florida, United States