Taotao Fu

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

This 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.

The viscosity distribution around a rising bubble in shear-thinning non-newtonian fluids

The viscosity distribution of the liquid around a rising bubble in carboxymethyl cellulose (CMC) aqueous solutions was measured experimentally by particle image velocimetry (PIV). The effect of the concentration of CMC solutions on the viscosity distribution around a bubble and the coupling relations between the viscosity field, flow field and shear stress field were also studied. Results indicated that the specific viscosity (non-dimensionalized by η0) decreases with the increase in CMC solution concentration due to a shear thinning effect. Within the experimental range, similar viscosity distributions of liquids around a rising bubble were found: a hollow cylindrical low viscosity region around the bubble wake and a high viscosity region in the central bubble wake.

Magnetofluidic control of the breakup of ferrofluid droplets in a microfluidic Y-junction

This paper is mainly focused on the investigation of the magnetofluidic control of the breakup of ferrofluid droplets in a symmetric Y-junction. The asymmetric breakup of the ferrofluid droplet or non-breakup with filtering the mother droplet into a desired branch to separate it from the satellite droplet was implemented by an external magnetic field. The breakup processes of ferrofluid droplets with and without the magnetic field were studied systematically. The influences of both the flow rate ratio between the continuous phase and dispersed phase and the magnetic flux density on the sizes of daughter droplets were determined. It was found that the attractive magnetic force shifted the mass center of mother droplet in the upstream main channel, which accordingly facilitated the asymmetric breakup of the droplet at the downstream Y-junction. A power function correlation for precisely predicting the sizes of daughter droplets was proposed by introducing the magnetic Bond number (Bom). Moreover, we also found that the controllable magnetic force could promote the pattern transition between the breakup and non-breakup of ferrofluid droplets.

Experimental investigation of the liquid volumetric mass transfer coefficient for upward gas-liquid two-phase flow in rectangular microchannels

The gas-liquid two-phase mass transfer process in microchannels is complicated due to the special dynamical characteristics. In this work, a novel method was explored to measure the liquid side volumetric mass transfer coefficient kLa. Pressure transducers were utilized to measure the pressure variation of upward gas-liquid two-phase flow in three vertical rectangular microchannels and the liquid side volumetric mass transfer coefficient kLa was calculated through the Pressure-Volume-Temperature correlation of the gas phase. Carbon dioxide-water, carbon dioxide-ethanol and carbon dioxide-n-propanol were used as working fluids, respectively. The dimensions of the microchannels were 40 µm×240 µm (depth×width), 100 µm×800 µm and 100 µm×2000 µm, respectively. Results showed that the channel diameter and the capillary number influence kLa remarkably and that the maximum value of kLa occurs in the annular flow regime. A new correlation of kLa was proposed based on the Sherwood number, Schmidt number and the capillary number. The predicted values of kLa agreed well with the experimental data.

Dynamics of bubble breakup at a T junction.

The gas-liquid interfacial dynamics of bubble breakup in a T junction was investigated. Four regimes were observed for a bubble passing through the T junction. It was identified by the stop flow that a critical width of the bubble neck existed: if the minimum width of the bubble neck was less than the critical value, the breakup was irreversible and fast; while if the minimum width of the bubble neck was larger than the critical value, the breakup was reversible and slow. The fast breakup was driven by the surface tension and liquid inertia and is independent of the operating conditions. The minimum width of the bubble neck could be scaled with the remaining time as a power law with an exponent of 0.22 in the beginning and of 0.5 approaching the final fast pinch-off. The slow breakup was driven by the continuous phase and the gas-liquid interface was in the equilibrium stage. Before the appearance of the tunnel, the width of the depression region could be scaled with the time as a power law with an exponent of 0.75; while after that, the width of the depression was a logarithmic function with the time.

The Drag Coefficient and the Shape for a Single Bubble Rising in Non-Newtonian Fluids

The dynamical characteristic of a single bubble rising in non-Newtonian fluid was investigated experimentally. The bubble as-pect ratio and rising velocity were measured by high speed cam-era. The shape regimes for bubbles in non-Newtonian fluids wasplotted by means of Reynolds number Re, Eo¨tvo¨s number Eo andMorton number Mo. The effects of bubble shape and liquid rheo-logical property on the total bubble drag coefficient were studied.A new empirical drag coefficient correlation covering sphericalbubble and deformed bubble was proposed, the predicted resultsshows good conformity to experimental values over a wide rangeof 0.05<Re<300. [DOI: 10.1115/1.4007073]Keywords: bubble, drag coefficient, non-Newtonian fluids, bubbleshape

Velocity Evolution for the Coalescence of Two In-Line Bubbles Rising in Non-Newtonian Fluids

This study experimentally investigates velocity evolution for the coalescence of two in-line bubbles rising in non-Newtonian fluids by using a high speed camera. Due to the wake of the leading bubble and the shear-thinning effect of non-Newtonian fluids, the following bubble is accelerated to approach the leading bubble, leading to the coalescence of the two bubbles. Based on the Newton’s second law and Schlichting’s wake theory, a theoretical model was developed to estimate the instantaneous rising velocity of the following bubble in the coalescence process of the two bubbles. The predicted values by the present model showed a good agreement with the experimental data.

Dynamics of bubble breakup in a microfluidic Y-bifurcation

A highspeed camera was used to investigate experimentally the bubble breakup and dynamics in a symmetric microfluidic Y-junction divergence. N2 and deionized water with 0.3% surfactant sodium dodecyl sulfate (SDS)-glycerol were adopted as dispersed and continuous phases, respectively. The symmetric breaking with permanent obstruction, symmetric breaking with gaps and non-breaking regimes were observed in experiments. Experimental results showed that: whether bubbles breakup or not depends primarily on both bubble initial lengths and capillary number. And the higher the viscosity of the fluid is, the higher is the capillary number needed to break bubbles. Dynamics of bubbles breakup were investigated under different regimes. The breakup with permanent obstruction included two stages: squeezing stage and rapid breakup stage. In squeezing stage, the breakup process was only controlled by the augmented pressure. The rapid breakup stage was affected by surface tension. The breakup with gaps consisted of three stages: squeezing stage, squeezing-stretching stage and rapid breakup stage. The squeezing stage and the rapid breakup stage were similar to the stage of the breakup with permanent obstruction. The squeezing-stretching was commonly controlled by the viscous force, inertia force and the augment pressure.