Droplet breakup in the square microchannel with a short square constriction to generate slug flow

Droplet breakup in micro-constrictions is an important phenomenon in industrial applications. This work aimed to investigate the droplet breakup in the square microchannel with a short square constriction to generate the slug flow, which drew little attention before. Mechanism analysis indicated that this breakup process included the shear-force-dominated, squeezing-force-dominated, and pinch-off stages. Non-uniform daughter droplets were generated in the constriction with their interface restricted in the horizontal and perpendicular directions by the microchannel walls. The average relative deviation of the daughter droplet size was < 30%, much lower than that for the breakup with the daughter droplet restricted only in one direction. An empirical equation with a deviation of < 20% was provided to show the dependence of the daughter droplet size on the operation conditions. The comparison results suggested that the different restriction effects of microchannel wall on daughter droplets led to the different breakup mechanisms in different constrictions.

Numerical study of droplet breakup in an asymmetric T-junction microchannel with different cross-section ratios

In this study, numerical simulations are conducted to investigate droplet breakup in an asymmetric [Formula: see text]-junction microchannel with different cross-section ratios. To this approach, a two-phase model based on the volume of fluid (VOF) method is adopted to study the three-dimensional feature of droplet motion inside [Formula: see text]-junctions. The comparison reveals that the present results are in good agreement with previous studies. The effects of the capillary number (Ca), the non-dimensional droplet length ([Formula: see text]), and the non-dimensional width ratio ([Formula: see text]) on the breakup time and splitting ratio of daughter droplets are studied. Five distinct regimes are observed involving the non-breakup, breakup with tunnel, breakup without tunnel, asymmetric breakup, and sorting. Achieved results indicate that the time of breakup ([Formula: see text]) increases about 15% when the Ca is increased from 0.0134 to 0.0268 (about 100%). It is also found that the mass center of the mother droplet in the primary channel is shifted to a larger wide branch, which facilitates the asymmetric breakup of the droplet in a [Formula: see text]-junction microchannel.

Water Droplets Translocation and Fission in a 3D Bi-Planar Multifurcated T-Junction Microchannels

Droplet fission has gained notable interest in drug delivery applications due to its ability to perform parallel operations in single device. Hitherto, droplet flow behavior in a 3D constriction was scarcely investigated. This study aims to investigate droplets fission inside a 3D bi-planar multifurcated microfluidic device. The flow behavior and droplet size distribution were studied in trifurcated microchannels using distilled water as dispersed phase (1 mPa·s) and olive oil (68 mPa·s) as continuous phase. Various sizes of subordinate daughter droplets were manipulated passively through the modulation of flowrate ratio (Q) (0.15 < Q < 3.33). Overall, we found droplet size coefficient of variations (CV%) ranging from 0.72% to 69%. Highly monodispersed droplets were formed at the upstream T-junction (CV% < 2%) while the droplet fission process was unstable at higher flowrate ratio (Q > 0.4) as they travel downstream (1.5% < CV% < 69%) to splitting junctions. Complex responses to the non-monotonic behavior of mean droplet size was found at the downstream boundaries, which arose from the deformations under nonuniform flow condition. CFD was used as a tool to study the preliminary maximum velocity (Umax) profile for the symmetrical (0.01334 m/s < Umax < 0.0153 m/s) and asymmetrical branched channels (0.0223 m/s< Umax < 0.00438 m/s), thus complementing the experimental model studies.

Effects of tip streaming on the prediction of droplet size distribution in the presence of dispersants during subsea blowouts

(2017-193) With the presence of surfactants in the fluid mixture, tip streaming phenomenon often occurs where daughter droplets of micron or sub-micron size are ejected from thin threads of the droplet poles. Recent experimental and modeling studies of tip streaming phenomenon have been focusing on the formation of individual droplets. However, effects of tip streaming on the prediction of droplet formation during subsurface oil blowouts have not been thoroughly investigated. Due to the high intensity flow in the blowout, the amount of micron or sub-micron size droplets resulting from tip streaming could be substantial and cannot be ignored. In this study, a new empirical-numerical scheme is developed in the thoroughly-validated droplet formation model, VDROP-J, to account for the tip streaming phenomenon when dispersants are presence. Calibration of the new scheme and model validations are performed in association with the underwater oil jet experiments. The new model development improves the capability of VDROP-J model in application to the cases when dispersants are used, which would provide valuable information of droplet formation during subsea blowouts for decision makers and research groups.

PREDICTION OF OIL DROPLET MOVEMENT AND SIZE DISTRIBUTION: LAGRANGIAN METHOD AND VDROP-J MODEL

ABSTRACT (2017-306): During subsurface oil releases, oil disperses into droplets whose trajectories depend on the droplet size. We report the measurements of the droplet size distribution (DSD) obtained from the release of diesel at 135 GPM from a horizontal pipe in the Ohmsett tank. The DSD was predicted using the model VDROP-J and matched the observation. Subsequently, the movement of the droplets was tracked using a Lagrangian Particle Tracking (LPT) approach. Various forces affecting the migration of the droplets were considered, these include drag, buoyancy, lift, and added mass force. It was found that the lift force is negligible. The added mass force was negligible for droplets smaller than 500 μm. Visual observation and modeling indicated that large droplets (larger than 300 μm) tend to separate from the plume and migrate upward independently, which affects, not only the DSD of large droplets but also the resulting daughter droplets. This is an issue that has not been addressed in the literature. Our findings indicate that the DSD is needed to better predict the trajectory of oil blowouts.

Controllable geometry-mediated droplet fission using “off-the-shelf” capillary microfluidics device

We describe a cheap, easily assembled, controllable droplet fission device to obtain a variety of uniform daughter droplets.

Microfluidic Combinatorial Screening Platform

We propose a microfluidic droplet-based platform that accepts an unlimited number of sample plugs from a multi-well plate, performs splitting of these sample droplets into smaller daughter droplets and subsequent synchronization-free, reliable fusion of sample daughter droplets with multiple reagents simultaneously. This system consists of two components: 1) a custom autosampler which generates a linear array of sub-microliter plugs in a microcapillary from a multi-well plate and 2) A microfluidic chip with channels for sample plug introduction, reagent merging and droplet incubation. This novel system generates large arrays of heterogeneous droplets from hundreds to thousands of samples while concurrently screening these arrays against a large array of reagents. This high throughput system minimizes sample and reagent consumption and can be applied to a gamut of biological assays, ranging from SNP detection to forensic screening.

Generation of secondary droplets in coalescence of a drop at a liquid–liquid interface

When a droplet of liquid 1 falls through liquid 2 to eventually hit the liquid 2–liquid 1 interface, its initial impact on the interface can produce daughter droplets of liquid 1. In some cases, a partial coalescence cascade governed by self-similar capillary-inertial dynamics is observed, where the fall of the secondary droplets in turn continues to produce further daughter droplets. Results show that inertia and interfacial surface tension forces largely govern the process of partial coalescence. The partial coalescence is suppressed by the viscous force when Ohnesorge number is below a critical value and also by gravity force when Bond number exceeds a critical value. Generation of secondary drop is observed for systems of lower Ohnesorge number for liquid 1, lower and intermediate Ohnesorge number for liquid 2 and for low and intermediate values of Bond number. Whenever the horizontal momentum in the liquid column is more than the vertical momentum, secondary drop is formed. A transition regime from partial to complete coalescence is obtained when the neck radius oscillates twice. In this regime, the main body of the column can be fitted to power-law scaling model within a specific time range. We investigated the conditions and the outcome of these coalescence events based on numerical simulations using a coupled level set and volume of fluid method (CLSVOF).