The formation of bubbles by flow focusing of a gas and a liquid in a rectangular channel is shown to depend strongly on the channel aspect ratio. Bubble breakup consists in a slow linear 2D collapse of the gas thread, ending in a fast 3D pinch-off. The 2D collapse is predicted to be stable against perturbations of the gas-liquid interface, whereas the 3D pinch-off is unstable, causing bubble polydispersity. During 3D pinch-off, a scaling w_(m) approximately tau(1/3) between the neck width w_(m) and the time tau before breakup indicates that breakup is driven by the inertia of both gas and liquid, not by capillarity.

A new regime of operation of PDMS-based flow-focusing microfluidic devices is presented. We show that monodisperse microbubbles with diameters below one-tenth of the channel width (here w = 50 μm) can be produced in low viscosity liquids thanks to a strong pressure gradient in the entrance region of the channel. In this new regime bubbles are generated at the tip of a long and stable gas ligament whose diameter, which can be varied by tuning appropriately the gas and liquid flow rates, is substantially smaller than the channel width. Through this procedure the volume of the bubbles formed at the tip of the gas ligament can be varied by more than two orders of magnitude. The experimental results for the bubble diameter d(b) as function of the control parameters are accounted for by a scaling theory, which predicts d(b)/w ∝ (μ(g)/μ(l))(1/12)(Q(g)/Q(l))(5/12), where μ(g) and μ(l) indicate, respectively, the gas and liquid viscosities and Q(g) and Q(l) are the gas and liquid flow rates. As a particularly important application of our results we produce monodisperse bubbles with the appropriate diameter for therapeutic applications (d(b) ≃ 5 μm) and a production rate exceeding 10(5) Hz.

Discharging a liquid from a nozzle at sufficient large velocity leads to a continuous jet that due to capillary forces breaks up into droplets. Here we investigate the formation of microdroplets from the breakup of micron-sized jets with ultra high-speed imaging. The diminutive size of the jet implies a fast breakup time scale $\tau_\mathrm{c} = \sqrt{\rho r^3 / \gamma}$ of the order of 100\,ns{}, and requires imaging at 14 million frames per second. We directly compare these experiments with a numerical lubrication approximation model that incorporates inertia, surface tension, and viscosity [Eggers and Dupont, J. Fluid Mech. 262, 205 (1994); Shi, Brenner, and Nagel, Science 265, 219 (1994)]. The lubrication model allows to efficiently explore the parameter space to investigate the effect of jet velocity and liquid viscosity on the formation of satellite droplets. In the phase diagram we identify regions where the formation of satellite droplets is suppressed. We compare the shape of the droplet at pinch-off between the lubrication approximation model and a boundary integral (BI) calculation, showing deviations at the final moment of the pinch-off. Inspite of this discrepancy, the results on pinch-off times and droplet and satellite droplet velocity obtained from the lubrication approximation agree with the high-speed imaging results

B-lines are ultrasound-imaging artifacts, which correlate with several lung-pathologies. However, their understanding and characterization is still largely incomplete. To further study B-lines, lung-phantoms were developed by trapping a layer of microbubbles in tissue-mimicking gel. To simulate the alveolar size reduction typical of various pathologies, 170 and 80 µm bubbles were used for phantom-type 1 and 2, respectively. A normal alveolar diameter is approximately 280 µm. A LA332 linear-array connected to the ULA-OP platform was used for imaging. Standard ultrasound (US) imaging at 4.5 MHz was performed. Subsequently, a multi-frequency approach was used where images were sequentially generated using orthogonal sub-bands centered at different frequencies (3, 4, 5, and 6 MHz). Results show that B-lines appear predominantly with phantom-type 2. Moreover, the multi-frequency approach revealed that the B-lines originate from a specific portion of the US spectrum. These results can give rise to significant clinical applications since, if further confirmed by extensive in-vivo studies, the native frequency of B-lines could provide a quantitative-measure of the state of the lung.

International audienceWe investigate the gas jet breakup and the resulting microbubble formation in a microfluidic flow-focusing device using ultra high-speed imaging at 1 × 106 frames/s. In recent experiments [Dollet et al., Phys. Rev. Lett. 100, 034504 (2008)], it was found that in the final stage of the collapse the radius of the neck scales with time with a 1/3 power-law exponent, which suggested that gas inertia and the Bernoulli suction effect become important. Here, ultra high-speed imaging was used to capture the complete bubble contour and quantify the gas flow through the neck. The high temporal resolution images enable us to approach the final moment of pinch-off to within 1 μs. It revealed that during the collapse, the flow of gas reverses and accelerates towards its maximum velocity at the moment of pinch-off. However, the resulting decrease in pressure, due to Bernoulli suction, is too low to account for the accelerated collapse. We observe two stages of the collapse process. At first, the neck collapses with a scaling exponent of 1/3 which is explained by a ''filling effect.'' In the final stage, the collapse is characterized by a scaling exponent of 2/5, which can be derived, based on the observation that during the collapse the neck becomes less slender, due to the driving through liquid inertia. However, surface tension forces are still important until the final microsecond before pinch-off

The so-called "Kelvin water dropper" is a simple experiment demonstrating the spontaneous appearance of induced free charge in droplets emitted through a tube. As Lord Kelvin explained, water droplets spontaneously acquire a net charge during detachment from a faucet due to the presence of electrical fields in their surrounding created by any metallic object. In his experiment, two streams of droplets are allowed to drip from separated nozzles into separated buckets, which are at the same time interconnected through the dripping needles. In this paper we build a microfluidic water dropper and demonstrate that the droplets get charged and break-up due to electrohydrodynamic instabilities. A comparison with recent simulations shows the dependence of the acquired charge in the droplets on different parameters of the system. The phenomenon opens a door to cheap and accessible transformation of pneumatic pressure into electrical energy and to an enhanced control in microfluidic and biophysical manipulation of capsules, cells and droplets via self-induced charging of the elements.In 1867, Sir William Thomson (later known as Lord Kelvin) devised an apparatus to "illustrate the voltaic theory" as he literally stated 1 . The apparatus has become very popular as a simple demonstration of electrostatic processes, since it can safely generate high voltages and electrical discharges by just letting water falling from a couple of faucets. The physical mechanism is however complex and still intriguing: two faucets are dripping water into two separate metallic buckets (figure 1) which have to be connected in a particular way. When a drop detaches from a metallic faucet at high enough frequency it will acquire a tiny residual amount of charge. The amount of acquired charge depends on the material of the faucet, on the water electrical conductivity and on the local electrical field at the detachment point. A metallic ring figure 1), so when a droplet passes with a small charge, it will induce an opposite charge in the metallic ring. The droplet continues and ends up in the metallic bucket (C1 in figure 1), with its tiny charge dispersed in it. The idea of Lord Kelvin was to make a self-feeding system with the help of the second faucet: the metallic ring is connected to a second metallic bucket (C2 in figure 1). Therefore, the ring will be slightly charged with the charge induced in the second faucet. Assuming it is negatively charged, the ring will then induce positive charges in the droplets of the first faucet. They will fall down with their positive charges into bucket I1. The bucket is connected to the second metallic ring (I2 in figure 1) in the adjacent system, in such a way that negative charge induction is enhanced in the second faucet's droplets (see figure 1). A voltage difference of order of several kilovolts can be created between the two buckets in a matter of seconds. In popular demonstrations like the classical videos from Melcher, Zahn and Silva 2 or the more recent by Walter Lewin in MIT 3 , electrical sparks ca...

In this paper, the size of bubbles formed through the breakup of a gaseous jet in a co-axial microfluidic device is derived. The gaseous jet surrounded by a co-flowing liquid stream breaks up into monodisperse microbubbles and the size of the bubbles is determined by the radius of the inner gas jet and the bubble formation frequency.We obtain the radius of the gas jet by solving the Navier-Stokes equations for low Reynolds number flows and by minimization of the dissipation energy. The prediction of the bubble size is based on the system's control parameters only, i.e. the inner gas flow rate Q i , the outer liquid flow rate Q o , and the tube radius R. For a very low gas-to-liquid flow rate ratio (Q i /Q o → 0) the bubble radius scales as r b /R ∝ Q i /Q o , independently of the inner to outer viscosity ratio η i /η o and of the type of the velocity profile in the gas, which can be either flat or parabolic, depending on whether high-molecular-weight surfactants cover the gas-liquid interface or not. However, in the case in which the gas velocity profiles are parabolic and the viscosity ratio is sufficiently low, i.e. η i /η o 1, the bubble diameter scales as r b ∝ (Q i /Q o ) β , with β smaller than 1/2. PACS numbers: 47.55.db,47.61.Jd,47.15.Rq 1 arXiv:1103.0096v1 [physics.flu-dyn]

In this study, the neoplastic drug frequently used in the treatment of lung cancer, carboplatin is loaded to microbubbles via a microfluidic platform. In order to increase the drug loading capacity of microbubbles, carboplatin is encapsulated into alginate polymer layer. The phospholipid microbubbles (MBs) are synthesized by MicroSphere Creator, which is connected with T‐junction and micromixer for the treatment with CaCl2 solution to provide gelation of the alginate coated phospholipid microbubbles (AMBs). The carboplatin loaded alginate coated phospholipid microbubbles (CAMBs) result in 12.2 ± 0.21 µm mean size, obtained by mixing with 0.05% CaCl2 using T‐junction. The cytotoxic activities of the synthesized MBs, AMBs, and CAMBs are also investigated with the 3‐(4,5‐Dimethyl‐2‐thiazolyl)‐2,5‐diphenyl‐2H‐tetrazolium bromide) (MTT) and live/dead fluorescent dying assays in the A549 and BEAS‐2B cell lines. The one‐step microfluidic coating of lipid microbubbles with natural alginate polymer appears to be a promising strategy for enhanced drug reservoir properties.