On the flow topology inside droplets moving in rectangular microchannels

Ma, Shaohua; Sherwood, Joseph M.; Huck, Wilhelm T. S.; Balabani, Stavroula

doi:10.1039/c4lc00671b

Cited by 100 publications

(74 citation statements)

References 40 publications

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“…Another situation analogous to the ganglia in porous media is the movement of microfluidic droplets in micro-channels 37–41 . Experimental and numerical works on liquid-liquid slug flows in capillary channels have shown internal recirculations within both phases, which were affected by capillary forces 42 , the viscosity ratio between the fluids 43,44 , the channel flow velocity and the slug size 45,46 . Low capillary numbers resulted in slower channel flows and large slugs caused the attenuation of recirculation zones inside the mobilised slugs.…”

Section: Introductionmentioning

confidence: 99%

Velocity distributions in trapped and mobilized non-wetting phase ganglia in porous media

Zarikos

Terzis

Hassanizadeh

2018

Sci Rep

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Understanding the mobilisation of trapped globules of non-wetting phase during two-phase flow has been the aim of numerous studies. However, the driving forces for the mobilisation of the trapped phases are still not well understood. Also, there is little information about what happens within a globule before, at the onset and during mobilization. In this work, we used micro-particle tracking velocimetry in a micro-fluidic model in order to visualise the velocity distributions inside the trapped phase globules prior and during mobilisation. Therefore, time-averaged and instantaneous velocity vectors have been determined using fluorescent microscopy. As a porous medium, we used a polydimethylsiloxane (PDMS) micro-model with a well-defined pore structure, where drainage and imbibition experiments were conducted. Three different geometries of trapped non-wetting globules, namely droplets, blobs and ganglia were investigated. We observed internal circulations inside the trapped phase globules, leading to the formation of vortices. The direction of circulating flow within a globule is dictated by the drag force exerted on it by the flowing wetting phase. This is illustrated by calculating and analyzing the drag force (per unit area) along fluid-fluid interfaces. In the case of droplets and blobs, only one vortex is formed. The flow field within a ganglion is much more complex and more vortices can be formed. The circulation velocities are largest at the fluid-fluid interfaces, along which the wetting phase flows and decreases towards the middle of the globule. The circulation velocities increased proportionally with the increase of wetting phase average velocity (or capillary number). The vortices remain stable as long as the globules are trapped, start to change at the onset of mobilization and disappear during the movement of globules. They reappear when the globules get stranded. Droplets are less prone to mobilization; blobs get mobilised in whole; while ganglia may get ruptured and get mobilised only partially.

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Section: Introductionmentioning

confidence: 99%

Velocity distributions in trapped and mobilized non-wetting phase ganglia in porous media

Zarikos

Terzis

Hassanizadeh

2018

Sci Rep

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“…In recent years, it has been widely used to study the droplet flow topology of liquid-liquid systems in microfluidics. [23][24][25][26][27][28][29] However, to the best of our knowledge no µPIV study has reported on the flow conditions in double emulsions, especially those in inner cores of w/o/w double emulsions. All chemicals were purchased from Sigma Aldrich Co. UK, unless noted otherwise and used without further purification.…”

Section: Introductionmentioning

confidence: 99%

“…Microfluidic PDMS devices were fabricated by combined photo-and softlithography, 30 and have been described in detail previously. 29 particles per ml before flow delivery. The volume fraction of these seeding particles in flow was only 4 × 10 -6 , and hence no effect on the measured flow topology was expected.…”

Section: Introductionmentioning

confidence: 99%

“…A µPIV system developed by Sherwood et al 32 was used for measurements of the flow field inside the droplets. 29 The microfluidic device was mounted onto the stage of an inverted microscope (Leica DM ILM, Germany). The (inner) aqueous and oil phases were pumped into the microchannel at various flow rates with a syringe pump (Fusion 400, Chemyx Inc., USA), and the continuous aqueous phase in the double emulsions was perfused from a pressure controlled aqueous reservoir.…”

Section: Introductionmentioning

confidence: 99%

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The microenvironment of double emulsions in rectangular microchannels

Sherwood

Huck

et al. 2015

Lab Chip

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The flow environment in inner cores of water-in-oil-in-water (w/o/w) microfluidic double emulsions has a significant impact on industrial applications of such systems. For example, in the case of shear sensitive cells compartmentalised in the cores, high shear conditions may be deleterious. This study reports on the flow characteristics of w/o/w inner cores in comparison to those in single water -in-oil (w/o) microdroplets of equal size moving in the same microchannel, resolved by means of micro -particle image velocimetry (μPIV). The multiphase flow system employed in the study had a viscosity ratio, λ, between aqueous and oil phase of the order of unity (λ = 0.78) and both single droplets and inner cores of double emulsions filled the channel. This configuration resulted in a weak recirculating flow inside the w/o single droplet: the measured flow field exhibited a uniform low velocity flow field in the central region surrounded by small regions of reversed flow near the channel walls. This flow topology was maintained in the inner cores of w/o/w double emulsions for intermediate capillary numbers ( Ca) ranging from 10 -3 to 10 -2 , and core morphologies varying from large plugs to pancake cores. The core morphology affected the magnitude and distribution of the velocity in the droplets. The similarity in the flow topology resulted from the fact that inner cores were located at the back of the outer droplet in such a way that inner and outer interfaces were in contact for over half of core surface area and separated only by a thin lubricating film. Both single droplets and inner cores exhibited a narrow shear rate distribution characterised by small regions of maximum shear confined near the channel walls. Shear rate magnitude values were found to be an order of magnitude lower than those in the channel and hence capable of reducing stresses in flow cytometry to far below reported values for cell damage. Hence, it can be concluded that double emulsions are suitable candidates to substitute single droplets in flow cytometry to protect the screened items and are compatible with the commercial flow cytometry systems.

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“…This can be explained qualitatively from the simulations, which clearly show that strong and uniform focusing occurs in systems with good acoustic matching between the droplet and the continuous phase (olive oil), while focusing is only maintained near the center of droplets having a poor acoustic matching with the continuous phase (fluorinated oil). The lack of focusing forces near the droplet ends is crucial in explaining the poor overall acoustophoretic behavior: as droplets move in microfluidic channels flow rolls are induced inside them (Kinoshita et al 2007;Ma et al 2014), and these flow rolls counteract the acoustic focusing force in the front of the droplet . Consequently, in acoustically mismatched droplets, the acoustic focusing is too weak to balance the defocusing Stokes' drag force from the flow rolls, and such droplets are unlikely to exhibit good acoustic particle focusing.…”

Section: Concluding Discussionmentioning

confidence: 99%

Intra-droplet acoustic particle focusing: simulations and experimental observations

Fornell

Garofalo

Nilsson

et al. 2018

Microfluid Nanofluid

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The aim of this paper is to study resonance conditions for acoustic particle focusing inside droplets in two-phase microfluidic systems. A bulk acoustic wave microfluidic chip was designed and fabricated for focusing microparticles inside aqueous droplets (plugs) surrounded by a continuous oil phase in a 380-μm-wide channel. The quality of the acoustic particle focusing was investigated by considering the influence of the acoustic properties of the continuous phase in relation to the dispersed phase. To simulate the system and study the acoustic radiation force on the particles inside droplets, a simplified 3D model was used. The resonance conditions and focusing quality were studied for two different cases: (1) the dispersed and continuous phases were acoustically mismatched (water droplets in fluorinated oil) and (2) the dispersed and continuous phases were acoustically matched (water droplets in olive oil). Experimentally, we observed poor acoustic particle focusing inside droplets surrounded by fluorinated oil while good focusing was observed in droplets surrounded by olive oil. The experimental results are supported qualitatively by our simulations. These show that the acoustic properties (density and compressibility) of the dispersed and continuous phases must be matched to generate a strong and homogeneous acoustic field inside the droplet that is suitable for high-quality intra-droplet acoustic particle focusing.

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On the flow topology inside droplets moving in rectangular microchannels

Cited by 100 publications

References 40 publications

Velocity distributions in trapped and mobilized non-wetting phase ganglia in porous media

Velocity distributions in trapped and mobilized non-wetting phase ganglia in porous media

The microenvironment of double emulsions in rectangular microchannels

Intra-droplet acoustic particle focusing: simulations and experimental observations

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