Jets, ie collimated streams of matter, occur from the microscale up to the large-scale structure of the universe Our focus will be mostly on surface tension effects, which result from the cohesive properties of liquids Paradoxically, cohesive forces promote the breakup of jets, widely encountered in nature, technology and basic science, for example in nuclear fission, DNA sampling, medical diagnostics, sprays, agricultural irrigation and jet engine technology Liquid jets thus serve as a paradigm for free-surface motion, hydrodynamic instability and singularity formation leading to drop breakup In addition to their practical usefulness, jets are an ideal probe for liquid properties, such as surface tension, viscosity or non-Newtonian rheology They also arise from the last but one topology change of liquid masses bursting into sprays Jet dynamics are sensitive to the turbulent or thermal excitation of the fluid, as well as to the surrounding gas or fluid medium The aim of this review is to provide a unified description of the fundamental and the technological aspects of these subjects

Physics of liquid jets

Microbubble contrast agents and the associated imaging systems have developed over the past 25 years, originating with manually-agitated fluids introduced for intra-coronary injection. Over this period, stabilizing shells and low diffusivity gas materials have been incorporated in microbubbles, extending stability in vitro and in vivo. Simultaneously, the interaction of these small gas bubbles with ultrasonic waves has been extensively studied, resulting in models for oscillation and increasingly sophisticated imaging strategies. Early studies recognized that echoes from microbubbles contained frequencies that are multiples of the microbubble resonance frequency. Although individual microbubble contrast agents cannot be resolved-given that their diameter is on the order of microns-nonlinear echoes from these agents are used to map regions of perfused tissue and to estimate the local microvascular flow rate. Such strategies overcome a fundamental limitation of previous ultrasound blood flow strategies; the previous Doppler-based strategies are insensitive to capillary flow. Further, the insonation of resonant bubbles results in interesting physical phenomena that have been widely studied for use in drug and gene delivery. Ultrasound pressure can enhance gas diffusion, rapidly fragment the agent into a set of smaller bubbles or displace the microbubble to a blood vessel wall. Insonation of a microbubble can also produce liquid jets and local shear stress that alter biological membranes and facilitate transport. In this review, we focus on the physical aspects of these agents, exploring microbubble imaging modes, models for microbubble oscillation and the interaction of the microbubble with the endothelium.

http://iopscience.iop.org/article/10.1088/0031-9155/54/6/R01/pdf

Ultrasound contrast microbubbles in imaging and therapy: physical principles and engineering

This review presents recent technological advances in charge-coupled-device ultrahigh-speed video cameras and their applications in experimental fluid mechanics. Following a brief review of the various high-speed camera types, we point out the advantages of the new technology. Then we show examples of how these cameras are leading to new discoveries in the study of free-surface flows, emphasizing the dynamics of drops and bubbles. We specifically review work on the basic singularities occurring when liquid masses come into contact and coalesce, or break apart during the pinch-off of drops or bubbles from a vertical nozzle. We briefly discuss the imaging of cavitation bubbles and finish by outlining future prospects for these sensors.

High-Speed Imaging of Drops and Bubbles

▪ Abstract The more violent impacts of water waves on walls create velocities and pressures having magnitudes much larger than those associated with the propagation of ordinary waves under gravity. Insight into these effects has been gained by irrotational-flow computations and by investigating the role of entrained and trapped air in wave impacts. This review focuses on the results of theoretical work, making particular note of the value of considering pressure impulse, and highlights the aspects that are poorly understood.

Water-wave impact on walls

When bubbles expand and collapse close to boundaries, a shear flow is generated which is able to remove particles from the surface, thus locally cleaning it. Here the authors demonstrate experimentally with microparticle tracking velocimetry that the strongest forcing of particles occurs during a very brief time interval of the bubble oscillation period. During this interval a jet flow impacts and spreads radially along the surface, thus transporting the particles with it.

Surface cleaning from laser-induced cavitation bubbles

Finite-time singularities—local divergences in the amplitude or gradient of a physical observable at a particular time—occur in a diverse range of physical systems. Examples include singularities capable of damaging optical fibres and lasers in nonlinear optical systems1, and gravitational singularities2 associated with black holes. In fluid systems, the formation of finite-time singularities cause spray and air-bubble entrainment3, processes which influence air–sea interaction on a global scale4,5. Singularities driven by surface tension have been studied in the break-up of pendant drops6,7,8,9 and liquid sheets10,11,12. Here we report a theoretical and experimental study of the generation of a singularity by inertial focusing, in which no break-up of the fluid surface occurs. Inertial forces cause a collapse of the surface that leads to jet formation; our analysis, which includes surface tension effects, predicts that the surface profiles should be describable by a single universal exponent. These theoretical predictions correlate closely with our experimental measurements of a collapsing surface singularity. The solution can be generalized to apply to a broad class of singular phenomena.

Singularity dynamics in curvature collapse and jet eruption on a fluid surface

An approach using interdigitated capacitors for electrical characterization of CYCLOTENE, a spin-on low-k benzocyclobutene (BCB)-based polymer is introduced and the effect of moisture uptake is investigated. The dielectric constant of CYCLOTENE is extracted from capacitance measurements with a systematic error less than 0.1%, giving an average value of 2.49 with a standard deviation of 1.5%. The dielectric constant increases by 1.2% after a humidity stress of 85% RH at 85/spl deg/C. The I-V characteristics of CYCLOTENE show a dependency of breakdown strength and leakage current on the geometrical dimensions of the device under test. A breakdown strength of 225V//spl mu/m and 320 V//spl mu/m for 2-/spl mu/m and 3-/spl mu/m finger spacing, respectively, and a leakage current of a few to tens of pA are measured. The I-V characteristics degrade drastically after the humidity stress, showing a breakdown strength of 100 V//spl mu/m and 180 V//spl mu/m for 2-/spl mu/m and 3-/spl mu/m finger spacing, respectively, and a maximum increase in the leakage current as large as one order of magnitude. The maximum performance and long-term reliability of an electric micromachine are adversely affected by the degradation of the breakdown voltage and the leakage current after moisture absorption. It is expected, however, that the electrical efficiency is improved using BCB-based polymers with negligible dependency on moisture absorption.

Electrical characterization of benzocyclobutene polymers for electric micromachines

Electrical properties and thickness of insulating dielectric films directly affect electrical energy loss and electrical breakdown limit in electric micromachines. A thick, low-k film exhibits low parasitic capacitive effects that help with the reduction of electrical energy loss. The electrical performance can be deteriorated due to degradation of the electrical properties of the insulating dielectric material caused by process, device structure, and moisture. In this paper, we introduce the application of CYCLOTENE, a spin-on, low-k, BCB-based polymer in electric micromachines as insulating layer and interlevel dielectric. A novel approach using interdigitated capacitors for electrical characterization of CYCLOTENE and the effect of moisture absorption is introduced in this paper. The dielectric constant of CYCLOTENE is extracted from two steps of capacitance measurements, giving an average value of 2.49 with a standard deviation of 1.5 %. The dielectric constant increases by 1.2 % after a humidity stress of 85 %RH at 85 °C. The measured I-V characteristics of CYCLOTENE show a dependency of the breakdown strength and leakage current on the geometrical dimensions of the device under test. A breakdown strength of 225 V/μm for 2 μm finger spacing and 320 V/μm for 3 μm finger spacing, and a leakage current of a few to tens of pA are measured. The I-V characteristics degrades drastically after a humidity stress of 85 %RH at 85 °C, showing a breakdown strength of 100 V/μm for 2 μm finger spacing and 180 V/μm for 3 μm finger spacing. Based on the results of this study, it is expected that the electrical efficiency of an electric micromachine is improved using BCB-based polymers with negligible dependency on moisture absorption. On the other hand, the maximum performance that depends on the maximum operating voltage is adversely affected by the degradation of the breakdown voltage after moisture absorption.

Benjamin Kleber

Papers

Singularity dynamics in curvature collapse and jet eruption on a fluid surface

Electrical characterization of benzocyclobutene polymers for electric micromachines

Electrical Characterization of BCB for Electrostatic Microelectromechanical Devices