In this paper we study the transient surface cavity which is created by the controlled impact of a disk of radius h 0 on a water surface at Froude numbers below 200. The dynamics of the transient free surface is recorded by high-speed imaging and compared to boundary integral simulations giving excellent agreement. The flow surrounding the cavity is measured with high-speed particle image velocimetry and is found to also agree perfectly with the flow field obtained from the simulations.We present a simple model for the radial dynamics of the cavity based on the collapse of an infinite cylinder. This model accounts for the observed asymmetry of the radial dynamics between the expansion and the contraction phases of the cavity. It reproduces the scaling of the closure depth and total depth of the cavity which are both found to scale roughly as ∝ Fr 1/2 with a weakly Froude-number-dependent prefactor. In addition, the model accurately captures the dynamics of the minimal radius of the cavity and the scaling of the volume V bubble of air entrained by the process, namely, V bubble /h 3 0 ∝ (1 + 0.26Fr 1/2 )Fr 1/2 . † Present address:

Inkjet printing deposits droplets with a well-controlled narrow size distribution. This paper aims at improving experimental and numerical methods for the optimization of drop formation. We introduce a method to extract the one-dimensional velocity profile inside a single droplet during drop formation. We use a novel experimental approach to capture two detailed images of the very same droplet with a small time delay. The one-dimensional velocity within the droplet is resolved by accurately determining the volume distribution of the droplet. We compare the obtained velocity profiles to a numerical simulation based on the slender jet approximation of the Navier-Stokes equation and we find very good agreement.

A long, smooth cylinder is dragged through a water surface to create a cavity with an initially cylindrical shape. This surface void then collapses due to the hydrostatic pressure, leading to a rapid and axisymmetric pinch-off in a single point. Surprisingly, the depth at which this pinch-off takes place does not follow the expected Froude 1=3 power law. Instead, it displays two distinct scaling regimes separated by discrete jumps, both in experiment and in numerical simulations (employing a boundary integral code). We quantitatively explain the above behavior as a capillary wave effect. These waves are created when the top of the cylinder passes the water surface. Our work thus gives further evidence for the nonuniversality of the void collapse. DOI: 10.1103/PhysRevLett.100.084502 PACS numbers: 47.55.Dÿ, 47.11.Hj, 47.35.Pq Many phenomena in fluid dynamics are known to be self-similar [1] and universal, allowing physicists to describe their final outcome without precise knowledge of the initial conditions. Prime examples for such universality are the breakup of an elongated fluid filament inside another viscous fluid [2] and the pinch-off of a liquid droplet in air [3][4][5]. For the inverse problem [6 -10], i.e., when an air bubble pinches off inside a liquid, the dynamics retains a memory of its creation until the very end, indicating nonuniversality. As an example for such a breakup, we examine the air-filled cavity created when a solid object is rapidly submerged through a water surface. The walls of the cavity subsequently collapse due to hydrostatic pressure from the liquid bulk. When the colliding walls meet, a violent jet shoots up into the air. Regardless of the nonuniversality of the pinch-off [7], the location at which it takes place has been reported (experimentally and theoretically) to scale in a continuous fashion with the object velocity for such different systems as spheres on prefluidized sand [11], solid disks [12], spheres and cylinders [13] on water, and even water columns on water [14]. Our experimental and numerical evidence shows the lower limit where this universal scaling is broken through the interference of a second phenomenon unrelated to hydrostatic pressure. Surface waves created as the object passes the water surface significantly alter the pinch-off location in a noncontinuous manner. Similar effects for the breakdown of a universal behavior due to wave interaction have been observed in, e.g., magnetohydrodynamics [15] and turbulence [16].In our experiment we drag a cylinder with radius R 0 20 mm and length l 147 mm through the surface of a large water tank using a linear motor connected to the cylinder bottom by a rod. We prescribe a constant cylinder velocity V between 0.5 and 2:5 m=s. With the kinematic viscosity the global Reynolds number Re R 0 V= is of the order of 10 4 , while the local Reynolds number Re R _ R= defined with the cavity radius R for the point of minimum radius lies between 10 2 and 10 5 , demonstrating that inertia dominates viscous effects. Further, wi...

The challenge in visualizing fast microscale fluid motion phenomena is to record high-quality images free of motion-blur. Here, we present an illumination technique based on laser-induced fluorescence which delivers high-intensity light pulses of 7 ns. The light source consists of a Q-switched Nd:YAG laser and a laser dye solution incorporated into a total internal reflection lens, resulting in a uni-directional light beam with a millimetersized circular aperture and 3°divergence. The laser coherence, considered undesirable for imaging purposes, is reduced while maintaining a nanoseconds pulse duration. The properties of the illumination by laser-induced fluorescence (iLIF) are quantified, and a comparison is made with other high-intensity pulsed and continuous light sources.

The volume of a bubble in a piezoinkjet printhead is measured acoustically. The method is based on a numerical model of the investigated system. The piezo not only drives the system but it is also used as a sensor by measuring the current it generates. The numerical model is used to predict this current for a given bubble volume. The inverse problem is to infer the bubble volume from an experimentally obtained piezocurrent. By solving this inverse problem, the size and position of the bubble can thus be measured acoustically. The method is experimentally validated with an inkjet printhead that is augmented with a glass connection channel, through which the bubble was observed optically, while at the same time the piezocurrent was measured. The results from the acoustical measurement method correspond closely to the results from the optical measurement.