Microuidics control the ballistic energy of thermocavitation liquid jets for needle-free injections

Illuminating a water solution with a focused continuous wave laser produces a strong local heating of the liquid that leads to the nucleation of bubbles, also known as thermocavitation. During the growth of the bubble, the surrounding liquid is expelled from the constraining microfluidic channel through a nozzle, creating a jet. The characteristics of the resulting liquid jet was imaged using ultra-fast imaging techniques. Here, we provide a phenomenological description of the jet shapes and velocities, and compare them with a Boundary Integral numerical model. We define the parameter regime, varying jet speed, taper geometry and liquid volume, for optimal printing, injection and spray applications. These results are important for the design of energy-efficient needle-free jet injectors based on microfluidic thermocavitation

Advanced Cryo-Tanks Research in CHATT

An EU-funded study called CHATT (Cryogenic Hypersonic Advanced Tank Technologies) has been initiated early 2012. The project CHATT is part of the European Commission’s Seventh Framework Programme and run on behalf of the Commission by DLR-SART in a multinational collaboration. One of the core objectives is to investigate Carbon Fiber Reinforced Plastic (CFRP) cryogenic pressure tanks. Four different subscale CFRP-tanks are planned to be designed, manufactured, and tested. The paper outlines the study logic of CHATT, gives a detailed presentation of the technology de-velopment tasks, and summarizes all major research results already available

Coalescence of diffusively growing gas bubbles

Under slightly supersaturated conditions, bubbles need many minutes to grow due to the low gas diffusivity in liquids. When coalescence occurs, the fact that the bubbles have diffusively grown on top of a surface allows for control with precision of the location and the timing at which the coalescence takes place. Numerous coalescences of two CO2 microbubbles in water are recorded at a frame rate of ∼65 000fps. The evolution of the coalescing process is analysed in detail, differentiating among three phases: neck formation, wave propagation along the bubble surface and bubble detachment. First of all, the formation of the collapsing neck between both bubbles is compared to a capillary-inertial theoretical model. Afterwards, the propagating deformation along the surface is characterised measuring its evolution, velocity and dominant wavelength. Once bubbles coalesce, the perturbing waves and the final shape of the new bubble breaks the equilibrium between buoyancy and capillary forces. Consequently, the coalesced bubble detaches and rises due to buoyancy, oscillating with its natural Minnaert frequency. In addition to the experiments, a boundary integral code has been used to obtain numerical results of the coalescence under similar conditions, showing excellent agreement with the experimental data

Shortwave infrared imaging setup to study entrained air bubble dynamics in a MEMS-based piezo-acoustic inkjet printhead

Abstract: Piezo-acoustic inkjet printing is the method of choice for high-frequency and high-precision drop-on-demand inkjet printing. However, the method has its limitations due to bubble entrainment into the nozzle, leading to jetting instabilities. In this work, entrained air bubbles were visualized in a micrometer scale ink channel inside a silicon chip of a MEMS-based piezo-acoustic inkjet printhead. As silicon is semi-transparent for optical imaging with shortwave infrared (SWIR) light, a highly sensitive SWIR imaging setup was developed which exploited the optical window of silicon at 1550 nm. Infrared recordings of entrained bubbles are presented, showing rich phenomena of acoustically driven bubble dynamics inside the printhead. Graphic abstract: [Figure not available: see fulltext.]

Shortwave infrared imaging setup to study entrained air bubble dynamics in a MEMS-based piezo-acoustic inkjet printhead

\u3cp\u3eAbstract: Piezo-acoustic inkjet printing is the method of choice for high-frequency and high-precision drop-on-demand inkjet printing. However, the method has its limitations due to bubble entrainment into the nozzle, leading to jetting instabilities. In this work, entrained air bubbles were visualized in a micrometer scale ink channel inside a silicon chip of a MEMS-based piezo-acoustic inkjet printhead. As silicon is semi-transparent for optical imaging with shortwave infrared (SWIR) light, a highly sensitive SWIR imaging setup was developed which exploited the optical window of silicon at 1550 nm. Infrared recordings of entrained bubbles are presented, showing rich phenomena of acoustically driven bubble dynamics inside the printhead. Graphic abstract: [Figure not available: see fulltext.].\u3c/p\u3

Secondary Tail Formation and Breakup in Piezoacoustic Inkjet Printing: Femtoliter Droplets Captured in Flight

The role of meniscus motion and ink viscosity in the formation of a secondary tail and its breakup are studied experimentally during the picoliter-droplet formation process of a MEMS piezoacoustic inkjet print head using laser-induced 8-ns single-flash stroboscopic imaging with a temporal resolution of 100 ns. It is found that the formation of the secondary tail is driven by meniscus motion and that the secondary tail forms reproducibly between the primary tail and the meniscus in the final microseconds before pinchoff. We demonstrate that the stability of the secondary tail can be controlled through the motion of the meniscus after the primary tail has formed. A 4 times increase in stretching rate results in a 2.2 times increase in the secondary-tail length and a 3 times higher number of femtoliter satellites. Furthermore, as expected for Rayleigh breakup, a 43% increase in ink viscosity is found to increase the secondary-tail length by 50%. Finally, it is found that, during inkjet printing, the secondary tail cascades into tertiary and quaternary tails. We show that the formation of higher-order tails is irreproducible and therefore driven by noise. The formation of thicker secondary and thinner higher-order tails results in a bimodal satellite size distribution, where the secondary satellites with a volume greater than or equal to 4fL are located closer to the primary-tail droplet, while satellites with a volume less than 4fL are located closer to the nozzle. The main findings of the present work, that the stability of the secondary tail decreases with a decrease in stretching rate and ink viscosity, can be employed in the inkjet-printing community for waveform design to minimize internal contamination of inkjet printers

Inkjet Nozzle Failure by Heterogeneous Nucleation: Bubble Entrainment, Cavitation, and Diffusive Growth

Piezoacoustic drop-on-demand (DOD) inkjet printing is widely applied in high-end digital printing due to its unprecedented precision and reproducibility. Micron-sized droplets of a wide range of chemical compositions can be deposited; however, the stability of piezoacoustic DOD inkjet printing can sometimes be compromised through the stochastic entrainment of bubbles within the ink channel. Here, bubble nucleation, translation, and growth are studied in an experimental silicon-based printhead with a glass nozzle plate using high-speed imaging that is triggered by changes in the ink-channel acoustics. It is found that impurities in the ink can trigger bubble nucleation upon their interaction with the oscillating meniscus. Cavitation inception on a dirt particle during the rarefaction pressure wave is identified as a second mechanism for bubble formation. The acoustic driving pressure within the ink channel, and its change upon bubble nucleation, are obtained from a fit of a Rayleigh-Plesset-type bubble-dynamics equation to the measured time-resolved radial dynamics of the bubble. The measured decrease in channel resonance frequency after bubble entrainment results in a 24% increased ink-jet length. The nucleated bubbles translate toward the ink-channel walls due to acoustic radiation forces and ink streaming. The convective ink flow is characterized using high-speed particle-tracking velocimetry. The vortical flow near the oscillating meniscus is shown to trap the impurities, thereby increasing the particle-to-meniscus interaction probability and, correspondingly, the bubble-entrainment probability

Inkjet Nozzle Failure by Heterogeneous Nucleation:Bubble Entrainment, Cavitation, and Diffusive Growth

\u3cp\u3ePiezoacoustic drop-on-demand (DOD) inkjet printing is widely applied in high-end digital printing due to its unprecedented precision and reproducibility. Micron-sized droplets of a wide range of chemical compositions can be deposited; however, the stability of piezoacoustic DOD inkjet printing can sometimes be compromised through the stochastic entrainment of bubbles within the ink channel. Here, bubble nucleation, translation, and growth are studied in an experimental silicon-based printhead with a glass nozzle plate using high-speed imaging that is triggered by changes in the ink-channel acoustics. It is found that impurities in the ink can trigger bubble nucleation upon their interaction with the oscillating meniscus. Cavitation inception on a dirt particle during the rarefaction pressure wave is identified as a second mechanism for bubble formation. The acoustic driving pressure within the ink channel, and its change upon bubble nucleation, are obtained from a fit of a Rayleigh-Plesset-type bubble-dynamics equation to the measured time-resolved radial dynamics of the bubble. The measured decrease in channel resonance frequency after bubble entrainment results in a 24% increased ink-jet length. The nucleated bubbles translate toward the ink-channel walls due to acoustic radiation forces and ink streaming. The convective ink flow is characterized using high-speed particle-tracking velocimetry. The vortical flow near the oscillating meniscus is shown to trap the impurities, thereby increasing the particle-to-meniscus interaction probability and, correspondingly, the bubble-entrainment probability.\u3c/p\u3