Bubble breakup with permanent obstruction in an asymmetric microfluidic T‐junction

Bubble breakup with permanent obstruction in an asymmetric microfluidic T‐junction is investigated experimentally. The breakup process of bubbles can be divided into three stages: squeezing, transition, and pinch‐off stages. In the squeezing stage, the thinning of the bubble neck is mainly controlled by the velocity of the fluid flowing into the T‐junction, and the increase of the liquid viscosity can promote this process. In the transition stage, the minimum width of bubble neck decreases linearly with time. In the pinch‐off stage, the effect of the velocity of the fluid flowing into the T‐junction on the thinning of the bubble neck becomes weaker, and the increase of the liquid viscosity would delay this process. The evolution of the minimum width of the bubble neck with the remaining time before the breakup can be scaled by a power–law relationship. The bubble length has little influence on the whole breakup process of bubbles. © 2014 American Institute of Chemical Engineers AIChE J, 61: 1081–1091, 2015     

Breakup dynamics of slender bubbles in non‐newtonian fluids in microfluidic flow‐focusing devices

This study aims to investigate the breakup of slender bubbles in non‐Newtonian fluids in microfluidic flow‐focusing devices using a high‐speed camera and a microparticle image velocimetry (micro‐PIV) system. Experiments were conducted in 400‐ and 600‐μm square microchannels. The variation of the minimum width of gaseous thread with the remaining time before pinch‐off could be scaled as a power‐law relationship with an exponent less than 1/3, obtained for the pinch‐off of bubbles in Newtonian fluids. The velocity field and spatial viscosity distribution in the liquid phase around the gaseous thread were determined by micro‐PIV to understand the bubble breakup mechanism. A scaling law was proposed to describe the size of bubbles generated in these non‐Newtonian fluids at microscale. The results revealed that the rheological properties of the continuous phase affect significantly the bubble breakup in such microdevices. © 2012 American Institute of Chemical Engineers AIChE J,, 2012     

Droplet formation of H2SO4/alkane system in a T‐junction microchannel: Gravity effect

A special system of concentrated sulfuric acid (H₂SO₄) and n‐hexane was used to study the droplet formation in a glass T‐junction microchannel with H₂SO₄ as the continuous phase. The effects of capillary number, flow ratio, and viscosity ratio on the droplet formation were investigated. The effect of gravity was explored by changing the flow direction in the microchannel. Results showed that the formation of transition flow pattern from squeezing to dripping is much easier for this special system compared with common aqueous/organic systems. This phenomenon is due to the considerably higher viscosity of H₂SO₄ than that of common aqueous phase and the higher density difference of the system compared with those of common systems. In addition to capillary number and flow ratio, gravity evidently affects the formation of droplets and flow patterns. The droplet size is smaller than that during the horizontal flow when the flow direction is consistent with gravity. By contrast, flow direction contrary to that of gravity results in larger droplet size than that at horizontal flow. This phenomenon provides guidance on the operation of these special systems in microchannels. Finally, mathematical models of droplet size at different flow patterns have been established, and these models can predict droplet size very well. This study could be helpful to extend the application of microreactors to new working systems. © 2016 American Institute of Chemical Engineers AIChE J, 62: 4564–4573, 2016     

Phase separation of gas–liquid two‐phase stratified and plug flows in multitube T‐junction separators

Using air and water as the working fluids, phase separation phenomena for stratified and plug flows at inlet were investigated experimentally, at a simple T‐junction and specifically designed multitube T‐junction separators with two or three layers. The results show that for these two flow patterns the separation efficiency of the two phases for any multitube T‐junction separator is much higher than that of the simple T‐junction. Increasing the number of connecting tubes in the multitube T‐junction separator can increase the separation efficiency. Generally, for stratified flow, complete separation of the two phases can be achieved by the two‐layer multitube T‐junction separator with five or more connecting tubes and by the three‐layer separator; increasing the gas flow rate, the liquid flow rate, or the mixture velocity under plug flow is detrimental to phase separation with a drop in peak separation efficiency. © 2016 American Institute of Chemical Engineers AIChE J, 63: 2285–2292, 2017     

Interface‐shrinkage‐driven breakup of droplets in microdevices with different dispersed fluid channel shape

A new droplet breakup mechanism is previously proposed—interface‐shrinkage‐driven breakup. In coaxial microdevices, when the contact angle between the continuous phase and dispersed fluid channel (DFC) is sufficiently low, the new mechanism instead of the classic shear‐driven mechanism dominates the breakup. The present study further investigated the new mechanism in microdevices with DFCs of different shape. Critical contact angles in different devices were determined by theoretical analysis and verified by experiments. It was found that the critical contact angle for the new mechanism depends on the shape of the DFC. The droplet size was measured for different devices when the new mechanism dominated the breakup. In contrast to the case for the shear‐driven mechanism, the droplet size is little affected by the capillary number. Mathematical models were established to predict the droplet size in different devices and results were found to agree well with experimental results. © 2017 American Institute of Chemical Engineers AIChE J, 63: 367–375, 2018     

A unified theoretical model for breakup of bubbles and droplets in turbulent flows

Pressure has a significant effect on bubble breakup, and bubbles and droplets have very different breakup behaviors. This work aimed to propose a unified breakup model for both bubbles and droplets including the effect of pressure. A mechanism analysis was made on the internal flow through the bubble/droplet neck in the breakup process, and a mathematical model was obtained based on the Young–Laplace and Bernoulli equations. The internal flow behavior strongly depended on the pressure or gas density, and based on this mechanism, a unified breakup model was proposed for both bubbles and droplets. For the first time, this unified breakup model gave good predictions of both the effect of pressure or gas density on the bubble breakup rate and the different daughter size distributions of bubbles and droplets. The effect of the mother bubble/droplet diameter, turbulent energy dissipation rate and surface tension on the breakup rate, and daughter bubble/droplet size distribution was discussed. This bubble breakup model can be further used in a population balance model (PBM) to study the effect of pressure on the bubble size distribution and in a computational fluid dynamics‐population balance model (CFD‐PBM) coupled model to study the hydrodynamic behaviors of a bubble column at elevated pressures. © 2014 American Institute of Chemical Engineers AIChE J, 61: 1391–1403, 2015     

Gas/liquid/liquid three‐phase flow patterns and bubble/droplet size laws in a double T‐junction microchannel

The double T‐junction microchannel is a classical microstructured chemical device used to generate gas/liquid/liquid three‐phase microflows. An experimental study that focused on the three‐phase flow phenomena and bubble/droplet generation rules in a double T‐junction microchannel was introduced. Based on the published knowledge of gas/liquid and liquid/liquid two‐phase microflows, new flow patterns were carefully defined: bubble cutting flow, spontaneous break‐up and bubble cutting coupling flow, and bubble/droplet alternate break‐up flow. According to the classical correlations of bubble and droplet volumes and their generation frequency ratio, the operating criteria for creating different three‐phase flow patterns were established and a model for the dimensionless average bubble and droplet volumes in the three‐phase microflows was developed. These various three‐phase microflows have great application potential in material science and flow chemistry synthesis. © 2015 American Institute of Chemical Engineers AIChE J, 61: 1722–1734, 2015     

Bubble splitting under gas–liquid–liquid three‐phase flow in a double T‐junction microchannel

Gas–aqueous liquid–oil three‐phase flow was generated in a microchannel with a double T‐junction. Under the squeezing of the dispersed aqueous phase at the second T‐junction (T2), the splitting of bubbles generated from the first T‐junction (T1) was investigated. During the bubble splitting process, the upstream gas–oil two‐phase flow and the aqueous phase flow at T2 fluctuate in opposite phases, resulting in either independent or synchronous relationship between the instantaneous downstream and upstream bubble velocities depending on the operating conditions. Compared with two‐phase flow, the modified capillary number and the ratio of the upstream velocity to the aqueous phase velocity were introduced to predict the bubble breakup time. The critical bubble breakup length and size laws of daughter bubbles/slugs were thereby proposed. These results provide an important guideline for designing microchannel structures for a precise manipulation of gas–liquid–liquid three‐phase flow which finds potential applications among others in chemical synthesis. © 2017 American Institute of Chemical Engineers AIChE J, 63: 376–388, 2018     

Effect of zeta potentials of oil droplets and bubbles on flotation of oil-in-water mixtures

The effects of surface charges of oil droplets and bubbles in the oil-in-water flotation process were studied by a zeta potential apparatus which was specially designed to measure the zeta potential of bubbles. To measure the electrophoretic velocity of bubbles accurately, small bubbles whose rising velocities were less than 1 mm/s were introduced into an electrophoresis cell by a method of pressure reduction of a solution containing dissolved air, and the movement of the bubble in the cell was followed by moving the cell vertically at the same velocity as that of the rising bubble. It was found that the separation efficiency, n, for the flotation was strongly dependent on the zeta potentials of both the oil droplets and the bubbles. n increased as the zeta potentials of both the oil droplets and the bubbles decreased. Also n became the largest when the oil droplets were negatively charged and the bubbles were positively charged. Furthermore, a critical adhesion parameter, m=4π?ζ_pζ_b/kA, between oil droplets and bubbles was calculated by the heterocoagulation theory, using the value of the experimentally determined Hamaker constant, A=3.38x 10^?20J, and the dependence of n on m was examined.     

Surfactant‐free microdispersion process of gas in organic solvents in microfluidic devices

The scaling of bubble/slug formation in organic solvents at microscale without surfactant was initially investigated by using T‐junction and symmetrically cross‐shaped microfluidic devices. Four unique organic solvents and three dispersion methods were used, forming different flow patterns and dispersion size. The flow pattern of uniform slug flow was investigated. Both the gas–liquid flow and dispersion size, which ranged from 400 to 1400 μm in length and 270 to 430 μm in diameter, depended on several factors including dispersion method, two‐phase flow rates, physical properties of the liquid phase, and structure of microchannels. A general equation L/w = k(Q_G/Q_L)^αCa^β was used to characterize the dispersion size with modification of Q_G/Q_L for different dispersion methods, considering the influences of breakup rate and transformation of the interface shape on the dispersion process. Three models were developed to predict the dispersion size for different dispersion methods, and calculated data were in good agreement with the experimental results. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Time‐resolved ultrasonic spectroscopy for bubbles

We show that ultrasound can provide time‐resolved measurements of the size distribution and the concentration of bubbles in a liquid. The potential of the technique is demonstrated by following disappearance of bubbles having an average radius of 20 μm with a 10 ms time resolution. We show that our technique can detect small concentrations of bubbles, with a large spectrum of accessible bubble radii (from 80 nm to 40 μm for a gas volume fraction of ), and with a sub‐millisecond time resolution. This new technique could be a valuable tool for investigating rapid processes such as nucleation or dissolution of bubbles. © 2017 American Institute of Chemical Engineers AIChE J, 63: 4666–4672, 2017     

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

Bubble 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 μm 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. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1920–1929, 2014     

Evaporation of pinned droplets containing polymer – an examination of the important groups controlling final shape

Controlling the final shape resulting from evaporation of pinned droplets containing polymer, is important in the fabrication of P‐OLED displays by inkjet printing. Typically, a coffee ‐ ring shape arises, due to the pinning and associated outward capillary flow. For operational reasons, this is undesirable – a flat topography is required. The aim of this work is to understand the important groups governing the shape, to provide a practical guide to ink selection. The theory presented is based on a thin‐film lubrication model. The governing equations are solved numerically and continuously track the lateral progression of a liquid/gel front. A large capillary number or large ratio of initial to maximal polymer volume fraction can suppress the coffee‐ring. White light interferometry is used to confirm these findings experimentally. © 2015 The Authors AIChE Journal published by Wiley Periodicals, Inc. on behalf of American Institute of Chemical Engineers AIChE J, 61: 1759–1767, 2015     

Self‐similar breakup of viscoelastic thread for droplet formation in flow‐focusing devices

The self‐similarity of the breakup of viscoelastic dispersed thread for droplet formation in flow‐focusing devices is investigated experimentally. A high‐speed camera is used to capture the evolution and angles of the cone‐shaped liquid‐liquid interface. The self‐similar profiles for the liquid‐liquid interface are obtained by normalizing the interface with the minimum width of the dispersed thread. The breakup of the dispersed thread transfers from a self‐similar power law scaling stage with an exponent of 0.36 to a self‐similar exponential scaling stage. The asymptotic cone angles prior to final breakup are consistent with the value of 125.5° and 151°, respectively. The viscoelasticity inhibits the development of finite‐time singularity for the breakup of the liquid‐liquid interface at microscale, similar to the capillary breakup at macroscale. The results demonstrate that the breakup of the viscoelastic dispersed thread for droplet formation exhibits self‐similarity at microscale. © 2017 American Institute of Chemical Engineers AIChE J, 2017     

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

The 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. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3528–3546, 2015     

Measuring enthalpy of fast exothermal reaction with micro‐reactor‐based capillary calorimeter

This work presents a new micro‐reactor‐based capillary calorimeter for the enthalpy measurement of fast exothermal reactions. The new calorimeter was operated in the continuous way and the reaction enthalpy can be easily measured with the online temperatures from detached sensor chips. A standard reaction system and an industrial reaction system were selected to test this new calorimeter with homogeneous and heterogeneous reaction processes. The measurement was taken place at nearly adiabatic situations and the reaction enthalpy was calculated from the rising of temperature. High accuracy and good repeatability were obtained from this new calorimeter with relative experimental errors less than 3.5% and 2.4%, respectively. The temperature response was quick in this new calorimeter too, which was benefit to the low cost of reactive component. The fast and accurate measurement was contributed to the nice mixing performance and strict plug flowing in the calorimeter. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

Einsatzgrenzen von T‐Stücken mit Abzweigen gleichen Durchmessers

Limitations of Tees with Branches of Equal Diameter Welded and necked‐out tees are frequently used in pressure vessel and piping design. Standard calculations based on the Kellog method are merely applicable for pressure loading. Supplementary forces and moments are excluded in this case. The application of numerical methods of structural analysis is extremely helpful in this situation. However, detailed modelling of the geometry is quite complicated and standard models of the cylinder‐cylinder intersection type cannot be used. Special models of the welded and necked‐out design variants have to be generated and parametrized in order to be applicable for different nominal sizes. Tees as structural components have to be considered under various aspects with respect to their mechanical behaviour. The following investigation proposes methods for design under static and cyclic loading conditions based on detailed FE analyses. The results reveal a considerable potential of tees which has to be proved by analysis.     

Instability and breakup of cavitation bubbles within diesel drops

A modified mathematical model is used to study the effects of various forces on the stability of cavitation bubbles within a diesel droplet. The principal finding of the work is that viscous forces of ...     

Dynamic formation and scaling law of hollow droplet with gas/oil/water system in dual‐coaxial microfluidic devices

Based on the one‐step microfluidic method of producing hollow droplet with thin film, this article studies the effect of water and oil flow rate, gas pressure, and viscosity of aqueous phase on the dynamic formation and size of hollow droplet by analyzing large amounts of data acquired automatically. The results show that the filling stage of hollow droplet is similar to that of microbubble formation, while the necking stage is similar to that of droplet formation process. Furthermore, based on the data and mathematical model describing droplet formation mechanism, a filling stage model including Capillary number of continuous phase is developed. Considering the dynamic interface breakup and displacement of droplet in necking stage, a necking stage model is developed. The results show that the model results considering filling and necking stage fit well with the experimental data, and the relative error is less than 5%. Finally, the same model with parameters is used to predict the size of hollow droplet with other systems and devices, and the model is proved to be relative precise in our experimental conditions. The results presented in this work provide a more in‐depth understanding of the dynamic formation and scaling law of hollow droplet with G/L/L systems in microfluidic devices. © 2017 American Institute of Chemical Engineers AIChE J, 64: 730–739, 2018