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Colin J Patterson, Samira Shiri, James C Bird
Liquid drops can bounce when they impact non-wetting surfaces. Recently, studies have demonstrated that the time that the bouncing drop contacts a superhydrophobic surface can be reduced by incorporating ridged macrotextures on the surface. Yet the existing models aimed at explaining this phenomenon offer incompatible predictions of the contact time when a drop impacts multiple intersecting macrotextures, or spokes. Furthermore, it is unclear whether the effects of the macrotexture on the drop hydrodynamics extend to non-wetting surfaces in which direct contact is avoided by a thin vapor layer...
February 15, 2017: Journal of Physics. Condensed Matter: An Institute of Physics Journal
Sander Wildeman, Chao Sun
Leidenfrost droplets, i.e. droplets whose mobility is ensured by a thin vapor film between the droplet and a hot plate, are exposed to an external electric field. We find that in a strong vertical electric field the droplet can start to bounce progressively higher, defying gravitational attraction. From the droplet's trajectory we infer the temporal evolution of the amount of charge on the droplet. This reveals that the charge starts high and then decreases in steps as the droplet slowly evaporates. After each discharge event the charge is in a fixed proportion to the droplet's surface area...
December 6, 2016: Soft Matter
Tatiana M Matlasz, Jamie L Brylski, Corey M Leidenfrost, Matt Scalco, Samuel J Sinclair, Ronald M Schoelerman, Valerie Tsang, Daniel Antonius
Cognitive impairment among seriously mentally ill offenders has implications for legal matters (e.g., competency to stand trial), as well as clinical treatment and care. Thus, being able to identify potential cognitive concerns early in the adjudication process can be important when deciding on further interventions. In this study, we examined the validity scales of the Personality Assessment Inventory (PAI), scores on the Wechsler Adult Intelligence Scale-IV (WAIS-IV), and competency findings in male inmates (n=61) diagnosed with a serious mental illness...
October 29, 2016: International Journal of Law and Psychiatry
Dhananjai Saranadhi, Dayong Chen, Justin A Kleingartner, Siddarth Srinivasan, Robert E Cohen, Gareth H McKinley
Skin friction drag contributes a major portion of the total drag for small and large water vehicles at high Reynolds number (Re). One emerging approach to reducing drag is to use superhydrophobic surfaces to promote slip boundary conditions. However, the air layer or "plastron" trapped on submerged superhydrophobic surfaces often diminishes quickly under hydrostatic pressure and/or turbulent pressure fluctuations. We use active heating on a superhydrophobic surface to establish a stable vapor layer or "Leidenfrost" state at a relatively low superheat temperature...
October 2016: Science Advances
Evgeniy Khain, Leonard M Sander
The granular Leidenfrost effect [B. Meerson, et al., Phys. Rev. Lett. 91, 024301 (2003)PRLTAO0031-900710.1103/PhysRevLett.91.024301; P. Eshuis et al., Phys. Rev. Lett. 95, 258001 (2005)PRLTAO0031-900710.1103/PhysRevLett.95.258001] is the levitation of a mass of granular matter when a wall below the grains is vibrated, giving rise to a hot granular gas below the cluster. We find by simulation that for a range of parameters the system is bistable: the levitated cluster can occasionally break and give rise to two clusters and a hot granular gas above and below...
September 2016: Physical Review. E
Jeong Tae Ok, Junseo Choi, Emily Brown, Sunggook Park
Leidenfrost droplets, liquid droplets placed on a hot flat surface above the Leidenfrost temperature of the liquid, are an interesting model system to understand and achieve frictionless motion of droplets on a surface. Controlled unidirectional motion of otherwise random Leidenfrost droplets can be achieved by replacing the flat surface by a surface with topological ratchets. In this study, we show how an increase in the vapor layer thickness below the Leidenfrost droplet influences the droplet motion for underlying ratchets with various periods ranging from 1...
June 1, 2016: Microelectronic Engineering
Beom Seok Kim, Geehong Choi, Sangwoo Shin, Thomas Gemming, Hyung Hee Cho
The enhancement of boiling heat transfer, the most powerful energy-transferring technology, will lead to milestones in the development of high-efficiency, next-generation energy systems. Perceiving nano-inspired interface functionalities from their rough morphologies, we demonstrate interface-induced liquid refreshing is essential to improve heat transfer by intrinsically avoiding Leidenfrost phenomenon. High liquid accessibility of hemi-wicking and catalytic nucleation, triggered by the morphological and hydrodynamic peculiarities of nano-inspired interfaces, contribute to the critical heat flux (CHF) and the heat transfer coefficient (HTC)...
October 6, 2016: Scientific Reports
Ivan U Vakarelski, Joseph D Berry, Derek Y C Chan, Sigurdur T Thoroddsen
The drag coefficient C_{D} of a solid smooth sphere moving in a fluid is known to be only a function of the Reynolds number Re and diminishes rapidly at the drag crisis around Re∼3×10^{5}. A Leidenfrost vapor layer on a hot sphere surface can trigger the onset of the drag crisis at a lower Re. By using a range of high viscosity perfluorocarbon liquids, we show that the drag reduction effect can occur over a wide range of Re, from as low as ∼600 to 10^{5}. The Navier slip model with a viscosity dependent slip length can fit the observed drag reduction and wake shape...
September 9, 2016: Physical Review Letters
Simeng Chen, Volfango Bertola
When a droplet of water impacts a heated surface, the drop may be observed to bounce. Recently is has been found that small quantities (∼100 ppm) of polymer additives such as polyethylene oxide can significantly increase the maximum bouncing height of drops. This effect has been explained in terms of the reduction of energy dissipation caused by polymer additives during the drop retraction and rebound, resulting in higher mechanical energy available for bouncing. Here we demonstrate, by comparing three types of fluids (Newtonian, shear-thinning, and viscoelastic), that the total kinetic energy carried by low-viscosity Newtonian drops during retraction is partly transformed into rotational kinetic energy rather than dissipated when compared with high-viscosity or non-Newtonian drops...
August 2016: Physical Review. E
Rui Liu, Mingcheng Yang, Ke Chen, Meiying Hou, Kiwing To
Using an event-driven molecular dynamics simulation, we show that simple monodisperse granular beads confined in coupled columns may oscillate as a different type of granular clock. To trigger this oscillation, the system needs to be driven against gravity into a density-inverted state, with a high-density clustering phase supported from below by a gaslike low-density phase (Leidenfrost effect) in each column. Our analysis reveals that the density-inverted structure and the relaxation dynamics between the phases can amplify any small asymmetry between the columns, and lead to a giant oscillation...
August 2016: Physical Review. E
Simeng Chen, Volfango Bertola
The impact morphology of viscoplastic drops on a heated surface in the Leidenfrost regime is investigated experimentally by high-speed imaging. In particular several important parameters which characterize the impact morphology (such as maximum spreading diameter, minimum retracting diameter and maximum bouncing height etc.) are measured by analysing the impact process, recorded using a high-speed camera. It is shown that as the yield stress grows, surface forces are no longer able to minimize the free surface of the drop, and the inertial deformation upon impact becomes permanent...
September 28, 2016: Soft Matter
Philip E Mason, Tillmann Buttersack, Sigurd Bauerecker, Pavel Jungwirth
Alkali metals in water are always at the brink of explosion. Herein, we show that this vigorous reaction can be kept in a non-exploding regime, revealing a fascinating richness of hitherto unexplored chemical processes. A combination of high-speed camera imaging and visible/near-infrared/infrared spectroscopy allowed us to catch and characterize the system at each stage of the reaction. After gently placing a drop of a sodium/potassium alloy on water under an inert atmosphere, the production of solvated electrons became so strong that their characteristic blue color could be observed with the naked eye...
October 10, 2016: Angewandte Chemie
Ryan M Bain, Christopher J Pulliam, Fabien Thery, R Graham Cooks
Leidenfrost levitated droplets can be used to accelerate chemical reactions in processes that appear similar to reaction acceleration in charged microdroplets produced by electrospray ionization. Reaction acceleration in Leidenfrost droplets is demonstrated for a base-catalyzed Claisen-Schmidt condensation, hydrazone formation from precharged and neutral ketones, and for the Katritzky pyrylium into pyridinium conversion under various reaction conditions. Comparisons with bulk reactions gave intermediate acceleration factors (2-50)...
August 22, 2016: Angewandte Chemie
N Kouraytem, E Q Li, S T Thoroddsen
We use high-speed video imaging to investigate vapor explosions during the impact of a molten Field's metal drop onto a pool of water. These explosions occur for temperatures above the Leidenfrost temperature and are observed to occur in up to three stages as the metal temperature is increased, with each explosion being more powerful that the preceding one. The Field's metal drop breaks up into numerous microbeads with an exponential size distribution, in contrast to tin droplets where the vapor explosion deforms the metal to form porous solid structures...
June 2016: Physical Review. E
Meng Shi, Xing Ji, Shangsheng Feng, Qingzhen Yang, Tian Jian Lu, Feng Xu
The Leidenfrost phenomenon of liquid droplets levitating and dancing when placed upon a hot plate due to propulsion of evaporative vapor has been extended to many self-propelled circumstances. However, such self-propelled Leidenfrost devices commonly need a high temperature for evaporation and a structured solid substrate for directional movements. Here we observed a "cold Leidenfrost phenomenon" when placing a dry ice device on the surface of room temperature water, based on which we developed a controllable self-propelled dry ice hovercraft...
2016: Scientific Reports
Istafaul H Ansari, Meheboob Alam
Experiments are conducted in a two-dimensional monolayer vibrofluidized bed of glass beads, with a goal to understand the transition scenario and the underlying microstructure and dynamics in different patterned states. At small shaking accelerations (Γ=Aω^{2}/g<1, where A and ω=2πf are the amplitude and angular frequency of shaking and g is the gravitational acceleration), the particles remain attached to the base of the vibrating container; this is known as the solid bed (SB). With increasing Γ (at large enough shaking amplitude A/d) and/or with increasing A/d (at large enough Γ), the sequence of transitions/bifurcations unfolds as follows: SB ("solid bed") to BB ("bouncing bed") to LS ("Leidenfrost state") to "2-roll convection" to "1-roll convection" and finally to a gas-like state...
May 2016: Physical Review. E
Dong-Wook Lee, Min-Ho Jin, Young-Joo Lee, Ju-Hyoung Park, Chun-Boo Lee, Jong-Soo Park
The development of green synthesis methods for supported noble metal catalysts remains important challenges to improve their sustainability. Here we first synthesized carbon-supported Pd catalysts in a green Leidenfrost droplet reactor without reducing agents, high-temperature calcination and reduction procedures. When the aqueous solution containing Pd nitrate precursor, carbon support, and water is dripped on a hot plate, vapor layer is formed between a solution droplet and hot surface, which allow the solution droplet to be levitated on the hot surface (Leidenfrost phenomena)...
2016: Scientific Reports
M Adda-Bedia, S Kumar, F Lechenault, S Moulinet, M Schillaci, D Vella
We explore the interaction between a liquid drop (initially at room temperature) and a bath of liquid nitrogen. In this scenario, heat transfer occurs through film-boiling: a nitrogen vapor layer develops that may cause the drop to levitate at the bath surface. We report the phenomenology of this inverse Leidenfrost effect, investigating the effect of the drop size and density by using an aqueous solution of a tungsten salt to vary the drop density. We find that (depending on its size and density) a drop either levitates or instantaneously sinks into the bulk nitrogen...
May 3, 2016: Langmuir: the ACS Journal of Surfaces and Colloids
Minori Shirota, Michiel A J van Limbeek, Chao Sun, Andrea Prosperetti, Detlef Lohse
When a liquid droplet impacts a hot solid surface, enough vapor may be generated under it to prevent its contact with the solid. The minimum solid temperature for this so-called Leidenfrost effect to occur is termed the Leidenfrost temperature, or the dynamic Leidenfrost temperature when the droplet velocity is non-negligible. We observe the wetting or drying and the levitation dynamics of the droplet impacting on an (isothermal) smooth sapphire surface using high-speed total internal reflection imaging, which enables us to observe the droplet base up to about 100 nm above the substrate surface...
February 12, 2016: Physical Review Letters
Sumith Y D, Shalabh C Maroo
A novel surface-heating algorithm for water is developed for molecular dynamics simulations. The validated algorithm can simulate the transient behavior of the evaporation of water when heated from a surface, which has been lacking in the literature. In this work, the algorithm is used to study the evaporation of water droplets on a platinum surface at different temperatures. The resulting contact angles of the droplets are compared to existing theoretical, numerical, and experimental studies. The evaporation profile along the droplet's radius and height is deduced along with the temperature gradient within the drop, and the evaporation behavior conforms to the Kelvin-Clapeyron theory...
September 17, 2015: Journal of Physical Chemistry Letters
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