| 1. |
- Balestra, G., et al.
(författare)
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Viscous Taylor droplets in axisymmetric and planar tubes : from Bretherton’s theory to empirical models
- 2018
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Ingår i: Microfluidics and Nanofluidics. - : Springer Verlag. - 1613-4982 .- 1613-4990. ; 22:6
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Tidskriftsartikel (refereegranskat)abstract
- The aim of this study is to derive accurate models for quantities characterizing the dynamics of droplets of non-vanishing viscosity in capillaries. In particular, we propose models for the uniform-film thickness separating the droplet from the tube walls, for the droplet front and rear curvatures and pressure jumps, and for the droplet velocity in a range of capillary numbers, Ca, from 10 - 4 to 1 and inner-to-outer viscosity ratios, λ, from 0, i.e. a bubble, to high-viscosity droplets. Theoretical asymptotic results obtained in the limit of small capillary number are combined with accurate numerical simulations at larger Ca. With these models at hand, we can compute the pressure drop induced by the droplet. The film thickness at low capillary numbers (Ca< 10 - 3) agrees well with Bretherton’s scaling for bubbles as long as λ< 1. For larger viscosity ratios, the film thickness increases monotonically, before saturating for λ> 10 3 to a value 2 2 / 3 times larger than the film thickness of a bubble. At larger capillary numbers, the film thickness follows the rational function proposed by Aussillous and Quéré (Phys Fluids 12(10):2367–2371, 2000) for bubbles, with a fitting coefficient which is viscosity-ratio dependent. This coefficient modifies the value to which the film thickness saturates at large capillary numbers. The velocity of the droplet is found to be strongly dependent on the capillary number and viscosity ratio. We also show that the normal viscous stresses at the front and rear caps of the droplets cannot be neglected when calculating the pressure drop for Ca> 10 - 3.
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| 2. |
- Gallino, Giacomo, et al.
(författare)
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The Hydrodynamics of a Micro-Rocket Propelled by a Deformable Bubble
- 2019
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Ingår i: Fluids. - : MDPI. - 2311-5521. ; 4:1
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Tidskriftsartikel (refereegranskat)abstract
- We perform simulations to study the hydrodynamics of a conical-shaped swimming micro-robot that ejects catalytically produced bubbles from its inside. We underline the nontrivial dependency of the swimming velocity on the bubble deformability and on the geometry of the swimmer. We identify three distinct phases during the bubble evolution: immediately after nucleation the bubble is spherical and its inflation barely affects the swimming speed; then the bubble starts to deform due to the confinement gradient generating a force that propels the swimmer; while in the last phase, the bubble exits the cone, resulting in an increase in the swimmer velocity. Our results shed light on the fundamental hydrodynamics of the propulsion of catalytic conical swimmers and may help to improve the efficiency of these micro-machines.
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| 3. |
- Hadikhani, P., et al.
(författare)
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Inertial manipulation of bubbles in rectangular microfluidic channels
- 2018
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Ingår i: Lab on a Chip. - : Royal Society of Chemistry. - 1473-0197 .- 1473-0189. ; 18:7, s. 1035-1046
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Tidskriftsartikel (refereegranskat)abstract
- Inertial microfluidics is an active field of research that deals with crossflow positioning of the suspended entities in microflows. Until now, the majority of the studies have focused on the behavior of rigid particles in order to provide guidelines for microfluidic applications such as sorting and filtering. Deformable entities such as bubbles and droplets are considered in fewer studies despite their importance in multiphase microflows. In this paper, we show that the trajectory of bubbles flowing in rectangular and square microchannels can be controlled by tuning the balance of forces acting on them. A T-junction geometry is employed to introduce bubbles into a microchannel and analyze their lateral equilibrium position in a range of Reynolds (1 < Re < 40) and capillary numbers (0.1 < Ca < 1). We find that the Reynolds number (Re), the capillary number (Ca), the diameter of the bubble (D), and the aspect ratio of the channel are the influential parameters in this phenomenon. For instance, at high Re, the flow pushes the bubble towards the wall while large Ca or D moves the bubble towards the center. Moreover, in the shallow channels, having aspect ratios higher than one, the bubble moves towards the narrower sidewalls. One important outcome of this study is that the equilibrium position of bubbles in rectangular channels is different from that of solid particles. The experimental observations are in good agreement with the performed numerical simulations and provide insights into the dynamics of bubbles in laminar flows which can be utilized in the design of flow based multiphase flow reactors.
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| 4. |
- Horvath, Daniel G., et al.
(författare)
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Sorting by interfacial tension (SIFT) : Label-free enzyme sorting using droplet microfluidics
- 2019
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Ingår i: Analytica Chimica Acta. - : ELSEVIER. - 0003-2670 .- 1873-4324. ; 1089, s. 108-114
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Tidskriftsartikel (refereegranskat)abstract
- Droplet microfluidics has the ability to greatly increase the throughput of screening and sorting of enzymes by carrying reagents in picoliter droplets flowing in inert oils. It was found with the use of a specific surfactant, the interfacial tension of droplets can be very sensitive to droplet pH. This enables the sorting of droplets of different pH when confined droplets encounter a microfabricated trench. The device can be extended to sort enzymes, as a large number of enzymatic reactions lead to the production of an acidic or basic product and a concurrent change in solution pH. The progress of an enzymatic reaction is tracked from the position of a flowing train of droplets. We demonstrate the sorting of esterase isoenzymes based on their enzymatic activity. This label-free technology, that we dub droplet sorting by interfacial tension (SIFT), requires no active components and would have applications for enzyme sorting in high-throughput applications that include enzyme screening and directed evolution of enzymes.
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| 5. |
- Kurzthaler, Christina, et al.
(författare)
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Particle motion nearby rough surfaces
- 2020
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Ingår i: Physical Review Fluids. - : AMER PHYSICAL SOC. - 2469-990X. ; 5:8
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Tidskriftsartikel (refereegranskat)abstract
- We study the hydrodynamic coupling between particles and solid, rough boundaries characterized by random surface textures. Using the Lorentz reciprocal theorem, we derive analytical expressions for the grand mobility tensor of a spherical particle and find that roughness-induced velocities vary nonmonotonically with the characteristic wavelength of the surface. In contrast to sedimentation near a planar wall, our theory predicts continuous particle translation transverse and perpendicular to the applied force. Most prominently, this motion manifests itself in a variance of particle displacements that grows quadratically in time along the direction of the force. This increase is rationalized by surface roughness generating particle sedimentation closer to or farther from the surface, which entails a significant variability of settling velocities.
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| 6. |
- Liu, Ying, et al.
(författare)
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Pattern formation in oil-in-water emulsions exposed to a salt gradient
- 2019
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Ingår i: Physical Review Fluids. - : AMER PHYSICAL SOC. - 2469-990X. ; 4:8
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Tidskriftsartikel (refereegranskat)abstract
- Flow instabilities can occur in a fluid system with two components that have significantly different diffusivities and that have opposite effects on the fluid density, as is the scenario in traditional double-diffusive convection. Here, we experimentally show that an oil-in-water emulsion exposed to salt concentration gradients generates a flowerlike pattern driven by vertical and azimuthal instabilities. We also report numerical and analytical studies to elaborate on the mechanism, the instability criteria, and the most unstable modes that determine the details of the observed patterns. We find that the instability is driven by buoyancy and stems from the differential transport between the dissolved salt and the suspended oil droplets, which have opposing effects on the density of the medium. Consequently, we identify a criterion for the development of the instability that involves the relative densities and concentrations of the salt and oil droplets. We also argue that the typical wave number of the pattern formed scales with the Peclet number of the salt, which here is equivalent to the Rayleigh number since the flow is driven by buoyancy. We find good agreement of these predictions with both experiments and numerical simulations.
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| 7. |
- Nadal, Francois, et al.
(författare)
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Rotational propulsion enabled by inertia
- 2014
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Ingår i: The European Physical Journal E Soft matter. - : Springer Science and Business Media LLC. - 1292-8941 .- 1292-895X. ; 37:7, s. 1-14
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Tidskriftsartikel (refereegranskat)abstract
- The fluid mechanics of small-scale locomotion has recently attracted considerable attention, due to its importance in cell motility and the design of artificial micro-swimmers for biomedical applications. Most studies on the topic consider the ideal limit of zero Reynolds number. In this paper, we investigate a simple propulsion mechanism --an up-down asymmetric dumbbell rotating about its axis of symmetry-- unable to propel in the absence of inertia in a Newtonian fluid. Inertial forces lead to continuous propulsion for all finite values of the Reynolds number. We study computationally its propulsive characteristics as well as analytically in the small-Reynolds-number limit. We also derive the optimal dumbbell geometry. The direction of propulsion enabled by inertia is opposite to that induced by viscoelasticity.
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| 8. |
- Nason, F., et al.
(författare)
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Numerical simulation of a deformable cell in microchannels
- 2013
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Ingår i: Computational Methods for Coupled Problems in Science and Engineering V. - 9788494140761 ; , s. 685-695
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Konferensbidrag (refereegranskat)abstract
- The main goal of this work is to numerically investigate the behavior of a cell flowing in a microfluidic system. In particular, we want to model flow-induced deformations of an isolated cell to quantitatively evaluate the cell response when subjected to a representative range of flow rates in a realistic geometry, with specific interest in the case of cell trapping. This research will help optimize operating conditions as well as the design of cell manipulation/culture micro-devices, so as to guarantee cell viability and ultimately improve high-throughput performance.
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| 9. |
- Pak, Shun, et al.
(författare)
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Micropropulsion and microrheology in complex fluids via symmetry breaking
- 2012
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Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 24:10, s. 103102-
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Tidskriftsartikel (refereegranskat)abstract
- Many biological fluids have polymeric microstructures and display non-Newtonian rheology. We take advantage of such nonlinear fluid behavior and combine it with geometrical symmetry-breaking to design a novel small-scale propeller able to move only in complex fluids. Its propulsion characteristics are explored numerically in an Oldroyd-B fluid for finite Deborah numbers while the small Deborah number limit is investigated analytically using a second-order fluid model. We then derive expressions relating the propulsion speed to the rheological properties of the complex fluid, allowing thus to infer the normal stress coefficients in the fluid from the locomotion of the propeller. Our simple mechanism can therefore be used either as a non-Newtonian micro-propeller or as a micro-rheometer.
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| 10. |
- Pietrzyk, Kyle, et al.
(författare)
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Flow around a squirmer in a shear-thinning fluid
- 2019
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Ingår i: Journal of Non-Newtonian Fluid Mechanics. - : Elsevier. - 0377-0257 .- 1873-2631. ; 268, s. 101-110
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Tidskriftsartikel (refereegranskat)abstract
- Many biological fluids display shear-thinning rheology, where the viscosity decreases with an increasing shear rate. To better understand how this non-Newtonian rheology affects the motion of biological and artificial micro swimmers, recent efforts have begun to seek answers to fundamental questions about active bodies in shear-thinning fluids. Previous analyses based on a squirmer model have revealed non-trivial variations of propulsion characteristics in a shear-thinning fluid via the reciprocal theorem. However, the reciprocal theorem approach does not provide knowledge about the flow surrounding the squirmer. In this work, we fill in this missing information by calculating the non-Newtonian correction to the flow analytically in the asymptotic limit of small Carreau number. In particular, we investigate the local effect due to viscosity reduction and the non-local effect due to induced changes in the flow; we then quantify their relative importance to locomotion in a shear-thinning fluid. Our results demonstrate cases where the non-local effect can be more significant than the local effect. These findings suggest that caution should be exercised when developing physical intuition from the local viscosity distribution alone around a swimmer in a shear-thinning fluid.
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