The initial impact of drops cushioned by an air or vapour layer with applications to the dynamic Leidenfrost regime

This work is devoted to the study of the conditions under which a drop directed normally towards a superheated or isothermal smooth substrate prevents the initial contact with the solid by skating over a micrometre-sized vapour or air layer. The results have been obtained analysing the gas flow at the spatio-temporal region where the maximum liquid pressure is attained, which is also where and when the minimum values of the film thickness are reached. For the common case in which WeSt−1/6≳1, where We=ρlU2R/γ and St=ρlUR/ηa denote, respectively, the Weber and Stokes numbers, we find that capillary effects are negligible and the ratio between the minimum film thickness and the local drop radius of curvature is hm/R∝St−7/6, with ρl, γ, ηa, U and R indicating the liquid density, interfacial tension coefficient, gas viscosity, impact velocity and drop radius, respectively. In contrast, when WeSt−1/6≲1, capillary effects can no longer be neglected and hm/R∝We−1/3St−10/9. The predicted values of the minimum film thickness are compared with published experimental data, finding good agreement between predictions and measurements for the cases of both isothermal and superheated substrates. In addition, using mass conservation, we have also deduced an equation providing the minimum value of the substrate temperature for which a cylindrical central vapour bubble of constant height hd/R∝St−2/3, with hd≫hm, grows radially at the wetting velocity deduced in Riboux & Gordillo (Phys. Rev. Lett., vol. 113, 2014, 024507). The predicted values are in good agreement with the dynamic Leidenfrost temperatures reported by Shirota et al. (Phys. Rev. Lett., vol. 116, 2016, 064501).Ministerio de Ciencia e Innovación PID2020-115655

Direction of the microjet produced by the collapse of a cavitation bubble located in a corner of a wall and a free surface

In this paper, we present a simplified theoretical model based on the method of images that predicts the direction of the microjet produced after the implosion of a cavitation bubble created in a corner of a free interface and rigid wall. Our theoretical predictions have been verified by means of a thorough experimental study in which the distances of the pulsed-laser cavitation bubble to the wall and the free surface are varied in a systematic manner. In addition, we extend the predictions to arbitrary values of the corner angle, pi/(2n) with n a natural number. The present analytical solution might be a hint to a practical design for preventing cavitation-induced damage.Ministerio de Educación, Cultura, Deportes, Ciencia y Tecnología de Japón 17H01246Ministerio de Educación, Cultura, Deportes, Ciencia y Tecnología de Japón 20H0022

Scaling the drop size in coflow experiments

We perform extensive experiments with coflowing liquids in microfluidic devices and provide a closed expression for the drop size as a function of measurable parameters in the jetting regime that accounts for the experimental observations; this expression works irrespective of how the jets are produced, providing a powerful design tool for this type of experiment

Large impact velocities suppress the splashing of micron-sized droplets

Article number 023605Here we investigate the transition from spreading to splashing of drops with radii R varying from millimeters to tens of microns impacting onto a smooth and dry partially wetting substrate at normal atmospheric conditions. Experiments show that the smaller R is, the larger the impact velocity V for the drop to splash needs to be but also that splash is inhibited if Weλ = ρV 2λ/σ 0.5, with σ, ρ, and λ indicating the interfacial tension coefficient, the liquid density, and the mean free path of gas molecules. This result has been validated for two different values of the Ohnesorge number Ohλ = μ/√ρλσ, with μ indicating the liquid viscosity, defined using only the material properties of the liquid and of the surrounding gaseous atmosphere. The underlying reason for this a priori unexpected finding results from the fact that the thin liquid film ejected after the drop touches the substrate is, under many practical conditions, Ht σ/(ρV 2 ) Riboux and Gordillo [Phys. Rev. Lett. 113, 024507 (2014)]. Then, for sufficiently large values of V , the thickness of the lamella becomes similar to the mean free path of gas molecules, i.e., Ht ≈ λ, and, under these conditions, the splash of the drop is inhibited because the lift force causing the liquid to dewet the partially wetting solid is negligible. The spreading to splashing and the splashing to spreading transitions observed experimentally as the impact velocity is increased and the radii of the droplets is above a certain threshold value are very well predicted by the theory in G. Riboux and J. M. Gordillo [Phys. Rev. Lett. 113, 024507 (2014)] and J. M. Gordillo and G. Riboux [J. Fluid Mech. 871, R3 (2019)] once the aerodynamic lift force is set to zero for Ht /λ 2, i.e., when Weλ 0.5Ministerio de Economía y Competitividad (MINECO) DPI2017-88201-C3-1-RMinisterio de Economía y Competitividad (MINECO) RED2018-102829-TJapan Society for the Promotion of Science 20H00222Japan Society for the Promotion of Science 20H0022

Dispositivo y procedimiento para romper gotas y burbujas de tamaño milimetrico y micrometrico.

Dispositivo y procedimiento para romper gotas y burbujas de tamaño milimétrico y micrométrico.La presente invención describe un dispositivo y un procedimiento para producir la rotura de pequeñas gotas o burbujas en fragmentos más pequeños. El rango de ta

Theory of the jets ejected after the inertial collapse of cavities with applications to bubble bursting jets

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license.The dynamics of the axisymmetric jets originated from the bursting of bubbles in a liquid of density ρ, viscosity μ, and interfacial tension coefficient σ can be rationalized as a two-stage process in which, initially, the pressure jump ∼σ/Rb accelerates the liquid towards the axis of symmetry, inducing a far-field flow rate per unit length Q∞VcRb, with Rb and Vc=σ/(ρRb) indicating the radius of the bubble and the capillary velocity, respectively. The second stage, during which a fast jet of radius Rjet(T) Rb and velocity Vjet(T)≫Vc is ejected, is driven by the far-field radial velocity field established initially, which forces the collapse of the cavity walls while keeping Q∞ practically constant in time because liquid inertia and mass conservation prevent appreciable changes of this quantity during the very short timescale characterizing the ejection of the jet. Our theoretical predictions for Rjet(T) and Vjet(T) reproduce fairly well the time evolution of the jet width and of the jet velocity for over three decades in time, obtaining good agreement with numerical simulations from the instant of jet inception until Rjet∼Rb. The analytical expressions for the jet width and for the jet velocity provided here constitute the initial conditions for the explicit solution of the ballistic equations deduced in Gekle and Gordillo [J. Fluid Mech. 663, 293 (2010)0022-112010.1017/S0022112010003526], which, hence, can be straightforwardly used in order to quantify the size and velocity of the first drop ejected and the fluxes of mass, momentum, and energy transferred from the ocean into the atmosphere. In addition, motivated by the results obtained for the particular case of bubble bursting jets, we also present here a unified theoretical framework aimed at quantifying the dynamics of the type of generic jets produced by the collapse of axisymmetric gas cavities of arbitrary shape when their implosion is forced by the radial velocity induced by a far-field boundary condition expressing that the dimensionless liquid flow rate per unit length directed towards the axis of symmetry, q∞, remains constant in time. Making use of theory and of full numerical simulations, we first analyze the case of the collapse of a conical bubble with a half-opening angle β finding that, when the value of q∞ is fixed to a constant, this type of axisymmetric jets converge towards a purely inertial β-dependent self-similar solution of the inviscid Navier-Stokes equations, described here for the first time, which is characterized by the fact that the jet width and velocity are respectively given, in the limit β 1, by rjet≈2.25tanβq∞τ and vjet≈3q∞/(2tanβq∞τ), with τ indicating the dimensionless time after the jet is ejected. For the case of parabolic cavities with a dimensionless radius of curvature at the plane of symmetry rc, our theory predicts that rjet(2rc)-1/2(q∞τ)3/4 and vjetq∞(2rc)1/2(q∞τ)-3/4, a result which is also in good agreement with full numerical simulations. The present results might also find applications in the description of the very fast jets, with velocities reaching up to 1000 m s-1, produced after a bubble cavitates very close to a wall and in the quantification of the so-called bazooka effect

Impulsive generation of jets by flow focusing

This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited.Here we characterize the origin and subsequent disintegration into droplets of the type of high-speed jets formed after the sudden implosion of a locally spherical cavity. The full spatio-temporal evolution of these types of impulsively generated jets is described here making use of just the initial values of the interfacial normal velocity at the axis of symmetry and of its corresponding second derivative along the azimuthal direction, obtained straightforwardly from the solution of the Laplace equation subjected to standard boundary conditions. The predicted time evolutions of the jet tip radius and velocity, and of the radii of the ejected droplets, are shown to agree well with experimental observations

On the jets produced by drops impacting a deep liquid pool and by bursting bubbles

Article number A37Here we provide a unified theoretical description of two different physical situations in which liquid jets are expelled out of the bulk of a liquid as a consequence of the capillary collapse of a void. We demonstrate that the velocity field giving rise to the emergence of these jets can be calculated as the flow generated by a line of sinks with a length and an intensity that can be expressed in terms of the initial cavity radius and the wavelength and velocity of the capillary waves propagating along the cavity walls. The predicted jet speeds, which are expressed through algebraic equations, are in good quantitative agreement with those obtained from experiments and from the simulations of bubbles bursting on a free surface or after the implosion of the crater formed when a drop impacts a liquid pool.Ministerio de Economía y Competitividad DPI2017-88201-C3-1-R,IJCI 2016-3012

Sistema agitador y difusor de un gas en líquidos

Sistema agitador y difusor de un gas en líquidos que comprende un eje rotatorio, accionado por un sistema motriz de velocidad variable a voluntad y una o más palas unidas al eje rotatorio. La sección transversal de las palas es un perfil aerodinámico y s