Browsing by Author "Sun, Chao"
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Item Drop Fragmentation at Impact onto a Bath of an Immiscible Liquid(Physical Review Letters, 6/28/2013) Lhuissier, Henri; Sun, Chao; Prosperetti, Andrea; Lohse, DetlefThe impact of a drop onto a deep bath of an immiscible liquid is studied with emphasis on the drop fragmentation into a collection of noncoalescing daughter drops. At impact the drop flattens and spreads at the surface of the crater it transiently opens in the bath and reaches a maximum deformation, which gets larger with increasing impact velocity, before surface tension drives its recession. This recession can promote the fragmentation by two different mechanisms: At moderate impact velocity, the drop recession converges to the axis of symmetry to form a jet which then fragments by a Plateau-Rayleigh mechanism. At higher velocity the edge of the receding drop destabilizes and shapes into radial ligaments which subsequently fragment. For this latter mechanism the number N?We3 and the size distribution of the daughter drops p(d)?d?4 as a function of the impact Weber number We are explained on the basis of the observed spreading of the drop. The universality of this model for the fragmentation of receding liquid sheets might be relevant for other configurations.Item Drop Impact on Superheated Surfaces(Physical Review Letters, 1/20/2012) Tran, Tuan; Staat, Hendrik J.J.; Prosperetti, Andrea; Sun, Chao; Lohse, DetlefAt the impact of a liquid droplet on a smooth surface heated above the liquid’s boiling point, the droplet either immediately boils when it contacts the surface (“contact boiling”), or without any surface contact forms a Leidenfrost vapor layer towards the hot surface and bounces back (“gentle film boiling”), or both forms the Leidenfrost layer and ejects tiny droplets upward (“spraying film boiling”). We experimentally determine conditions under which impact behaviors in each regime can be realized. We show that the dimensionless maximum spreading ? of impacting droplets on the heated surfaces in both gentle and spraying film boiling regimes shows a universal scaling with the Weber number We (??We 2/5), which is much steeper than for the impact on nonheated (hydrophilic or hydrophobic) surfaces (??We 1/4). We also interferometrically measure the vapor thickness under the droplet.Item Dynamic Leidenfrost effect: relevant time and length scales(Physical Review Letters, 2/10/2016) Shirota, Minori; van Limbeek, Michiel A.J.; Sun, Chao; Prosperetti, Andrea; Lohse, DetlefWhen 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. By this method we are able to reveal the processes responsible for the transitional regime between the fully wetting and the fully levitated droplet as the solid temperature increases, thus shedding light on the characteristic time and length scales setting the dynamic Leidenfrost temperature for droplet impact on an isothermal substrate.Item Growing bubbles in a slightly supersaturated liquid solution(Review of Scientific Instruments, 6/26/2013) Enríquez, Oscar R.; Hummelink, Christian; Bruggert, Gert-Wim; Lohse, Detlef; Prosperetti, Andrea; van der Meer, Devaraj; Sun, ChaoWe have designed and constructed an experimental system to study gas bubble growth in slightly supersaturated liquids. This is achieved by working with carbon dioxide dissolved in water, pressurized at a maximum of 1MPa and applying a small pressure drop from saturation conditions. Bubbles grow from hydrophobic cavities etched on silicon wafers, which allows us to control their number and position. Hence, the experiment can be used to investigate the interaction among bubbles growing in close proximity when the main mass transfer mechanism is diffusion and there is a limited availability of the dissolved species.Item Highly focused supersonic microjets(Physical Review X, 7/9/2012) Tagawa, Yoshikuyi; Oudalov, Nikolai; Visser, Claas Willem; Peters, Ivo R.; van der Meer, Devaraj; Sun, Chao; Prosperetti, Andrea; Lohse, DetlefThis paper describes the production of thin, focused microjets with velocities of up to 850 m/s by the rapid vaporization of a small mass of liquid in an open liquid-filled capillary. The vaporization is caused by the absorption of a low-energy laser pulse. A likely explanation of the observed phenomenon is based on the impingement of the shock wave caused by the nearly instantaneous vaporization on the free surface of the liquid. We conduct an experimental study of the dependence of the jet velocity on several parameters and develop a semiempirical relation for its prediction. The coherence of the jets and their high velocity, good reproducibility, and controllability are unique features of the system. A possible application is to development of needle-free drug-injection systems that would be of great importance for health care worldwide.Item Highly focused supersonic microjets: numerical simulations(Journal of Fluid Mechanics, 9/28/2012) Peters, Ivo R.; Tagawa, Yoshikuyi; Oudalov, Nikolai; Sun, Chao; Prosperetti, Andrea; Lohse, Detlef; van der Meer, DevarajBy focusing a laser pulse inside a capillary partially filled with liquid, a vapour bubble is created which emits a pressure wave. This pressure wave travels through the liquid and creates a fast, focused axisymmetric microjet when it is reflected at the meniscus. We numerically investigate the formation of this microjet using axisymmetric boundaryintegral simulations, where we model the pressure wave as a pressure pulse applied on the bubble. We find a good agreement between the simulations and experimental results in terms of the time evolution of the jet and on all parameters that can be compared directly. We present a simple analytical model that accurately predicts the velocity of the jet after the pressure pulse and its maximum velocity.Item Leidenfrost temperature increase for impacting droplets on carbon nanofiber surfaces(Soft Matter, 9/3/2013) Nair, Hrudya; Staat, Hendrik J.J.; Tran, Tuan; van Houselt, Arie; Prosperetti, Andrea; Lohse, Detlef; Sun, ChaoDroplets impacting on a superheated surface can either exhibit a contact boiling regime, in which they make direct contact with the surface and boil violently, or a film boiling regime, in which they remain separated from the surface by their own vapor. The transition from the contact to the film boiling regime depends not only on the temperature of the surface and kinetic energy of the droplet, but also on the size of the structures fabricated on the surface. Here we experimentally show that surfaces covered with carbon-nanofibers delay the transition to film boiling to much higher temperature compared to smooth surfaces. We present physical arguments showing that, because of the small scale of the carbon fibers, they are cooled by the vapor flow just before the liquid impact, thus permitting contact boiling up to much higher temperatures than on smooth surfaces. We also show that, as long as the impact is in the film boiling regime, the spreading factor of impacting droplets follows the same We 3/10 scaling (with We the Weber number) found for smooth surfaces, which is caused by the vapor flow underneath the droplet.Item Optimal Taylor–Couette flow: radius ratio dependence(Journal of Fluid Mechanics, 2013-10) Ostilla-Mónico, Rodolfo; Huisman, Sander G.; Jannink, Tim J. G.; Van Gils, Dennis P. M.; Verzicco, Roberto; Grossmann, Siegfried; Sun, Chao; Lohse, DetlefTaylor–Couette flow with independently rotating inner ( i ) and outer ( o ) cylinders is explored numerically and experimentally to determine the effects of the radius ratio on the system response. Numerical simulations reach Reynolds numbers of up to Rei=9.5 x 10^3 and Re0= 5 x 10^3 , corresponding to Taylor numbers of up to Ta=10^8 for four different radius ratios n=ri/ro between 0.5 and 0.909. The experiments, performed in the Twente Turbulent Taylor–Couette ( T^3C ) set-up, reach Reynolds numbers of up to Rei= 2 x 10^6 and Reo= 1.5 x 10^6 , corresponding to Ta= 5 x 10^12 for n=.714--0.909 . Effective scaling laws for the torque J^w(Ta) are found, which for sufficiently large driving Ta are independent of the radius ratio n . As previously reported for n=0.714 , optimum transport at a non-zero Rossby number Ro=ri|wi-wo|/ ]2(ro-ri)wo] is found in both experiments and numerics. Here Ro(opt) is found to depend on the radius ratio and the driving of the system. At a driving in the range between Ta~3 x 10^8 and Ta~10^10 , Ro(opt) saturates to an asymptotic n -dependent value. Theoretical predictions for the asymptotic value of Ro(opt) are compared to the experimental results, and found to differ notably. Furthermore, the local angular velocity profiles from experiments and numerics are compared, and a link between a flat bulk profile and optimum transport for all radius ratios is reported.Item Salinity transfer in bounded double diffusive convection(Journal of Fluid Mechanics, 2015-03) Yang, Yantao; Van Der Poel, Erwin P.; Ostilla-Mónico, Rodolfo; Sun, Chao; Verzicco, Roberto; Grossmann, Siegfried; Lohse, DetlefThe double diffusive convection between two parallel plates is numerically studied for a series of parameters. The flow is driven by the salinity difference and stabilised by the thermal field. Our simulations are directly compared with experiments by Hage & Tilgner (Phys. Fluids, vol. 22, 2010, 076603) for several sets of parameters and reasonable agreement is found. This, in particular, holds for the salinity flux and its dependence on the salinity Rayleigh number. Salt fingers are present in all simulations and extend through the entire height. The thermal Rayleigh number seems to have a minor influence on the salinity flux but affects the Reynolds number and the morphology of the flow. In addition to the numerical calculation, we apply the Grossmann–Lohse theory for Rayleigh–Bénard flow to the present problem without introducing any new coefficients. The theory successfully predicts the salinity flux both with respect to the scaling and even with respect to the absolute value for the numerical and experimental results.Item Spatial dependence of fluctuations and flux in turbulent Rayleigh-B ?enard convection(Physical Review E, 11/26/2012) Lakkaraju, Rajaram; Stevens, Richard J.A.M.; Verzicco, Roberto; Grossmann, Siegfried; Prosperetti, Andrea; Sun, Chao; Lohse, DetlefItem Tribonucleation of bubbles(Proceedings of the National Academy of Sciences, 7/15/2014) Wildeman, Sander; Lhuissier, Henri; Sun, Chao; Lohse, Detlef; Prosperetti, AndreaWe report on the nucleation of bubbles on solids that are gently rubbed against each other in a liquid. The phenomenon is found to depend strongly on the material and roughness of the solid surfaces. For a given surface, temperature, and gas content, a trail of growing bubbles is observed if the rubbing force and velocity exceed a certain threshold. Direct observation through a transparent solid shows that each bubble in the trail results from the early coalescence of several microscopic bubbles, themselves detaching from microscopic gas pockets forming between the solids. From a detailed study of the wear tracks, with atomic force and scanning electron microscopy imaging, we conclude that these microscopic gas pockets originate from a local fracturing of the surface asperities, possibly enhanced by chemical reactions at the freshly created surfaces. Our findings will be useful either for preventing undesired bubble formation or, on the contrary, for “writing with bubbles,” i.e., creating controlled patterns of microscopic bubbles.