Recently, research on particle migration in non-Newtonian viscoelastic fluids has gained considerable attention. In a viscoelastic fluid, three dimensional (3D) particle focusing can be easily realized in simple channels without the need for any external force fields or complex microchannel structures compared with that in a Newtonian fluid. Due to its promising properties for particle precise focusing and manipulation, this field has been developed rapidly, and research on the field has been shifted from fundamentals to applications. This review will elaborate the recent progress of particle migration in viscoelastic fluids, especially on the aspect of applications. The hydrodynamic forces on the micro/nano particles in viscoelastic fluids are discussed. Next, we elaborate the basic particle migration in viscoelasticity-dominant fluids and elasto-inertial fluids in straight channels. After that, a comprehensive review on the applications of viscoelasticity-induced particle migration (particle separation, cell deformability measurement and alignment, particle solution exchange, rheometry-on-a-chip and others) is presented; finally, we thrash out some perspectives on the future directions of particle migration in viscoelastic fluids.

/pdf/recent-progress-of-particle-migration-in-viscoelastic-fluids-4gu76jikon.pdf

Recent progress of particle migration in viscoelastic fluids

Cross-streamline noninertial migration of a vesicle in a bounded Poiseuille flow is investigated experimentally and numerically. The combined effects of the walls and of the curvature of the velocity profile induce a movement toward the center of the channel. A migration law (as a function of relevant structural and flow parameters) is proposed that is consistent with experimental and numerical results. This similarity law markedly differs from its analog in unbounded geometry. The dependency on the reduced volume ν and viscosity ratio λ is also discussed. In particular, the migration velocity becomes nonmonotonous as a function of ν beyond a certain λ.

Lateral migration of vesicles in bounded Poiseuille flow

Ferrofluids, also known as ferromagnetic particle suspensions, are materials with an excellent magnetic response, which have attracted increasing interest in both industrial production and scientific research areas. Because of their outstanding features, such as rapid magnetic reaction, flexible flowability, as well as tunable optical and thermal properties, ferrofluids have found applications in various fields, including material science, physics, chemistry, biology, medicine, and engineering. Here, a comprehensive, in-depth insight into the diverse applications of ferrofluids from material fabrication, droplet manipulation, and biomedicine to energy and machinery is provided. Design of ferrofluid-related devices, recent developments, as well as present challenges and future prospects are also outlined.

Flexible Ferrofluids: Design and Applications

Fabrication of artificial biomimetic materials has attracted abundant attention. As one of the subcategories of biomimetic materials, artificial cells are highly significant for multiple disciplines and their synthesis has been intensively pursued. In order to manufacture robust "alive" artificial cells with high throughput, easy operation, and precise control, flexible microfluidic techniques are widely utilized. Herein, recent advances in microfluidic-based methods for the synthesis of droplets, vesicles, and artificial cells are summarized. First, the advances of droplet fabrication and manipulation on the T-junction, flow-focusing, and coflowing microfluidic devices are discussed. Then, the formation of unicompartmental and multicompartmental vesicles based on microfluidics are summarized. Furthermore, the engineering of droplet-based and vesicle-based artificial cells by microfluidics is also reviewed. Moreover, the artificial cells applied for imitating cell behavior and acting as bioreactors for synthetic biology are highlighted. Finally, the current challenges and future trends in microfluidic-based artificial cells are discussed. This review should be helpful for researchers in the fields of microfluidics, biomaterial fabrication, and synthetic biology.

Microfluidics for Biosynthesizing: from Droplets and Vesicles to Artificial Cells

Controlling the fate of particles and cells in microfluidic devices is critical in many biomedical applications, such as particle and cell alignment and separation. Recently, viscoelastic polymer solutions have been successfully used to promote transversal migration of particles and cells toward fixed positions in straight microchannels. When inertia is negligible, numerical simulations have shown that strongly shear-thinning polymer solutions (fluids with a shear viscosity that decreases with increasing flow rates) promote transversal migration of particles and cells toward the corners or toward the centerline in a straight microchannel with a square cross section, as a function of particle size, cell deformability, and channel height. However, no experimental evidence of such shifting in the positions for particles or cells suspended in strongly shear-thinning liquids has been presented so far. In this work, we demonstrate that particle positions over the channel cross section can be shifted “from the e...

/pdf/from-the-edge-to-the-center-viscoelastic-migration-of-2exa9az9wq.pdf

From the Edge to the Center: Viscoelastic Migration of Particles and Cells in a Strongly Shear-Thinning Liquid Flowing in a Microchannel

We elucidate the phenomena of dynamic wetting, shape evolution and splitting of ferrofluid (FF) droplets on a hydrophobic surface under the influence of a magnetic field. In the case of a FF droplet interacting with a magnetic field, both surface energy and magnetic energy contribute to the total Gibb's free energy and hence the wetting phenomena. The nanoparticles in the FF droplet migrate and get accumulated at the apex of the droplet which enhances the magnetic interaction causing large deformation of the droplet. The FF droplet deformation and subsequent splitting are governed by the interplay between the magnetic Fm and surface tension Fs forces. The ratio of the forces km = (Fm/Fs) was found to be a function of the magnetic Bond number Bom and non-dimensional gap g* as km ∼ (Bom)0.3(g*)-0.86. Splitting of the FF droplets was observed for km > 1 and for km < 1, an equilibrium droplet shape was observed. The wetting behavior of the FF droplets was found to be strongly dependent on the FF concentration c - concentrated (c = 1.2%) FF droplets exhibit contact line (CL) pinning and decrease in contact angle (CA) θ with time throughout, while diluted (c = 0.6%) FF droplets show a mixed mode (CL pinning followed by constant CA). In splitting of FF droplets, the ratio of the volume of the daughter droplet to that of the parent droplet i.e. (Vd/Vp), was found to decrease with an increase in the parent droplet size Vp.

Shape evolution and splitting of ferrofluid droplets on a hydrophobic surface in the presence of a magnetic field

Droplets can be used as carrier vehicles for the transportation of biological and chemical reagents. Manipulation of water- and oil-based ferromagnetic droplets in the presence of a magnetic field has been well-studied. Here, we elucidate the transport of a sessile aqueous (diamagnetic) droplet placed over spikes of oil-based ferrofluid (FF) in the presence of a nonuniform magnetic field. An oil-based FF droplet, dispensed over a rigid oleophilic surface, interacts with a magnetic field to get transformed into an array of spikes which then act as a carrier for the transportation of the aqueous droplet. Our study reveals that transportation phenomena is governed by the interplay of three different forces: magnetic force Fm, frictional force Ff, and interfacial tension force Fi, which is expressed in terms of the magnetic Laplace number (Lam) and magnetic Bond number (Bom) as Lam?1 = (Ff1/Fm,x) and BomLam?1 = (Ff2/Fi). Based on the values of the dimensionless numbers, three different regimes, steady droplet...

Transport of a Sessile Aqueous Droplet over Spikes of Oil Based Ferrofluid in the Presence of a Magnetic Field.

We report the dynamics of aqueous droplets of different size and viscosity at the interface of a coflowing stream of immiscible oils (i.e., primary and secondary continuous phases) in a microchannel, at low Re. The lateral migration of droplets introduced into the primary continuous phase toward the interface and subsequent selective migration of droplets across the interface into the secondary continuous phase is investigated. The interplay between the competing noninertial lift and interfacial tension forces, which govern the interfacial migration of the droplets, is presented and discussed. The velocity and strain rate profiles, and interface location, which are critical for calculating the lift force and migration behavior of droplets, are presented. The trajectories of droplets of different size and viscosity in the primary continuous phase are obtained for different interface locations. During interfacial migration, the deformation behavior of droplets of different viscosities is studied. Finally, s...

Dynamics of Aqueous Droplets at the Interface of Coflowing Immiscible Oils in a Microchannel.

In this work, we report experimental and numerical studies of alternating and merged droplets in a double T-junction microchannel. The microchannel device is fabricated using PDMS substrate and experiments are performed with mineral oil with surfactant as the continuous phase and aqueous glycerol as the discrete phase. Based on the flow rate fraction ϕ and Capillary number Ca, four different flow regimes are identified: merging, stable alternating droplets, alternating droplets with transition and laminar. A numerical model that employs volume-of-fluid formulations is used to predict the alternating droplet generation process. In the stable alternating droplet regime, the effect of the discretephase flow rate ratio α on the droplet diameter ratio β is experimentally studied and compared with that predicted from the simulations. It is observed that the droplet diameter ratio β increases linearly with increase in the flow rate ratio α and a good match between experiments and simulations is observed. The diameters of droplets at different Capillary numbers Ca generated using single and double T-junction microchannels are compared and it is observed that, at low Ca, the double T-junction generates larger droplets as compared to single T-junction. In merged droplet regime, the effect of the continuous phase flow rate Qc and discrete phase viscosity μd on diameter dm and interdistance between the droplets λ of the merged droplets are studied. It is observed that the merged droplet diameter dm is reduced and interdistance between the droplets λ increases with increase in the continuous phase flow rate Qc. As the viscosity of the discrete phase μd increases, the diameter dm and interdistance between the droplets λ of the merged droplets decreases.

Alternating and merged droplets in a double T-junction microchannel

Manipulation of aqueous droplets in microchannels has great significance in various emerging applications such as biological and chemical assays. Magnetic-field based droplet manipulation that offers unique advantages is consequently gaining attention. However, the physics of magnetic field-driven cross-stream migration and the coalescence of aqueous droplets with an aqueous stream are not well understood. Here, we unravel the mechanism of cross-stream migration and the coalescence of aqueous droplets flowing in an oil based ferrofluid with a coflowing aqueous stream in the presence of a magnetic field. Our study reveals that the migration phenomenon is governed by the advection (τa) and magnetophoretic (τm) time scales. Experimental data show that the dimensionless equilibrium cross-stream migration distance δ* and the length Lδ* required to attain equilibrium cross-stream migration depend on the Strouhal number, St = (τa/τm), as δ* = 1.1 St0.33 and Lδ*=5.3 St−0.50, respectively. We find that the droplet-stream coalescence phenomenon is underpinned by the ratio of the sum of magnetophoretic (τm) and film-drainage time scales (τfd) and the advection time scale (τa), expressed in terms of the Strouhal number (St) and the film-drainage Reynolds number (Refd) as ξ = (τm + τfd)/τa = (St−1 + Refd). Irrespective of the flow rates of the coflowing streams, droplet size, and magnetic field, our study shows that droplet-stream coalescence is achieved for ξ ≤ 50 and ferrofluid stream width ratio w* < 0.7. We utilize the phenomenon and demonstrated the extraction of microparticles and HeLa cells from aqueous droplets to an aqueous stream.

U. Banerjee

Papers

Shape evolution and splitting of ferrofluid droplets on a hydrophobic surface in the presence of a magnetic field

Transport of a Sessile Aqueous Droplet over Spikes of Oil Based Ferrofluid in the Presence of a Magnetic Field.

Dynamics of Aqueous Droplets at the Interface of Coflowing Immiscible Oils in a Microchannel.

Alternating and merged droplets in a double T-junction microchannel

Cross-stream migration and coalescence of droplets in a microchannel co-flow using magnetophoresis