Hydrodynamic feedback on bubble breakup at a T-junction within an asymmetric loop

International audienceBubble breakup at a microfluidic T-junction by taking into consideration the hydrodynamic feedback at the downstream channels is presented. Experiments are conducted in square microchannels with 400 mu m in width. The splitting ratio of the bubble size in the bifurcations varies nonmonotonically with the flow rate ratio of gas/liquid phases, and it is also affected by the liquid viscosity. A critical size of the mother bubble determines the variation trend of the splitting ratio of bubble size with flow rates of both phases and the liquid viscosity, which is related to the different breakup mechanisms for long and short bubbles at the junction and the different additional resistances induced by long and short bubbles in downstream channels. A theoretical model is proposed to predict the tailoring size of bubbles at the T-junction by taking into account of the additional resistance in the presence of bubbles in downstream channels

Ferrofluid droplet formation and breakup dynamics in a microfluidic flow-focusing device

International audienceThis work aims at studying the expanding and breakup dynamics of the thread of a controllable dispersed phase under different flow rates in a microfluidic flow-focusing device. The whole formation process of ferrofluid droplets under no magnetic field (NM), a radial magnetic field (RM) and an axial magnetic field (AM) were investigated and compared. It was found that the volume of the ferrofluid droplets can be actively controlled by the applied magnetic field. The radial magnetic field and axial magnetic field affect mainly the expanding and breakup processes of the thread, respectively. The influence of the flow rates, magnetic flux density and magnetic field direction on the formation and breakup processes were extensively studied. The variation of the minimum width of the ferrofluid thread with the remaining time could be scaled with a power law

Scaling of the bubble formation in a flow-focusing device: Role of the liquid viscosity

International audienceThe present work studies the bubble formation in viscous liquids with the viscosity ranging from 5 to 400 mPa s by using a high-speed digital camera. The experiment was carried out in a flow-focusing device with square cross-section of 600 x 600 mu m. Results show that the viscous shear stress strongly influences the dynamics of bubble formation, including the shape and size of bubbles. The bubble size follows power-law relations with the gas flow rate, the flow rate and viscosity of the liquid phase respectively, indicating that bubbles formed in viscous fluids are controlled by a combination of squeezing mechanism and shearing mechanism. Therefore, the bubble size can be predicted by a power law function depending on the flow rate ratio of gas and liquid phases phi representing the squeezing mechanism and capillary number Ca representing the shearing mechanism. In addition, the dynamics of bubble formation in viscous liquids in a flow focusing device is also analyzed

Numerical simulation of the interactions between three equal-interval parallel bubbles rising in non-Newtonian fluids

International audienceThe motion and interactions of three equal-interval parallel bubbles in non-Newtonian fluids were numerically simulated by volume of fluid method (VOF), in which the continuous surface tension model and the power-law model were adopted to represent surface tension and rheological properties of non-Newtonian fluids, respectively. The computational method was validated by the comparison of the processes of coalescence of two in-line bubbles and rising of two parallel bubbles between experiment and simulation. This method was then applied to study the effect of initial bubble diameter, initial horizontal bubble interval and rheological properties of non-Newtonian fluids on lateral coalescence and rising of three parallel bubbles. The dimensionless critical horizontal interval of bubble coalescence was obtained under different physical property conditions. The critical horizontal interval of bubble coalescence decreases with the increase of initial bubble diameter and flow index of non-Newtonian fluids. When the initial horizontal bubble interval is less than the critical horizontal interval of bubble coalescence, three bubbles will coalesce into a bigger bubble. The coalescing bubble could breakup into two identical daughter bubbles when the initial bubble diameter was increased or the flow index of non-Newtonian fluids was decreased. Three parallel bubbles rising in non-Newtonian fluids will experience repulsive interactions once the initial horizontal bubble interval is greater than the critical horizontal interval of bubble coalescence, the horizontal bubble interval increased gradually owing to the repulsive effect, while the vertical distance between bubbles varied dramatically for spherical bubble and ellipsoidal bubble due to the differences of their flow field structures

Formation mechanism and size prediction of bubble in opposite-flowing T-shaped microchannel

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Pinch-off mechanism for Taylor bubble formation in a microfluidic flow-focusing device

3rd European Conference on Microfluidics (Mu Flu), Heidelberg, GERMANY, DEC, 2012International audienceThe present work aims at studying the nonlinear breakup mechanism for Taylor bubble formation in a microfluidic flow-focusing device by using a high-speed digital camera. Experiments were carried out in a square microchannel with cross section of 600 x 600 mu m. During the nonlinear collapse process, the variation of the minimum radius of bubble neck (r(0)) with the remaining time until pinch-off (tau) can be scaled by a power-law relationship: r(0) proportional to tau(alpha). Due to the interface rearrangement around the neck, the nonlinear collapse process can be divided into two distinct stages: liquid squeezing collapse stage and free pinch-off stage. In the liquid squeezing collapse stage, the neck collapses under the constriction of the liquid flow and the exponent alpha approaches to 0.33 with the increase in the liquid flow rate Q(1). In the free pinch-off stage, the value of alpha is close to the theoretical value of 0.50 derived from the Rayleigh-Plesset equation and is independent of Q(1)

Three-dimensional numerical simulation of coalescence and interactions of multiple horizontal bubbles rising in shear-thinning fluids

International audienceThe dynamics of multiple horizontal bubbles rising from different orifice arrangements in shear-thinning fluids was simulated numerically by three-dimensional Volume of Fluid method. The effects of bubble size, rheological properties of shear-thinning fluids, and orifice structure arrangements on multiple bubbles interaction and coalescence were analyzed, and the mechanisms of bubble coalescence and breakup were fully discussed and elucidated. The variation of bubble rising velocity during coalescence process and freely rising processes for different orifice arrangements was also deeply investigated. The critical initial horizontal intervals for coalescence of multiple horizontal bubbles with various orifice arrangements were attained by simulation, which could serve as the critical criterion of bubble coalescence or noncoalescence. Furthermore, the critical bubble interval was predicted based on the film drainage model, the prediction accords well with the simulation result and is quite conducive for the design and optimization of perforated gas-liquid contact equipment