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Zhu, Lailai
Publications (10 of 14) Show all publications
Shukla, I., Kofman, N., Balestra, G., Zhu, L. & Gallaire, F. (2019). Film thickness distribution in gravity-driven pancake-shaped droplets rising in a Hele-Shaw cell. Journal of Fluid Mechanics, 874, 1021-1040, Article ID PII S0022112019004531.
Open this publication in new window or tab >>Film thickness distribution in gravity-driven pancake-shaped droplets rising in a Hele-Shaw cell
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2019 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 874, p. 1021-1040, article id PII S0022112019004531Article in journal (Refereed) Published
Abstract [en]

We study here experimentally, numerically and using a lubrication approach, the shape, velocity and lubrication film thickness distribution of a droplet rising in a vertical Hele-Shaw cell. The droplet is surrounded by a stationary immiscible fluid and moves purely due to buoyancy. A low density difference between the two media helps to operate in a regime with capillary number $Ca$ lying between $0.03$ and $0.35$ , where $Ca=\unicode[STIX]{x1D707}_{o}U_{d}/\unicode[STIX]{x1D6FE}$ is built with the surrounding oil viscosity $\unicode[STIX]{x1D707}_{o}$ , the droplet velocity $U_{d}$ and surface tension $\unicode[STIX]{x1D6FE}$ . The experimental data show that in this regime the droplet velocity is not influenced by the thickness of the thin lubricating film and the dynamic meniscus. For iso-viscous cases, experimental and three-dimensional numerical results of the film thickness distribution agree well with each other. The mean film thickness is well captured by the Aussillous & Quere (Phys. Fluids, vol. 12 (10), 2000, pp. 2367-2371) model with fitting parameters. The droplet also exhibits the 'catamaran' shape that has been identified experimentally for a pressure-driven counterpart (Huerre et al., Phys. Rev. Lett., vol. 115 (6), 2015, 064501). This pattern has been rationalized using a two-dimensional lubrication equation. In particular, we show that this peculiar film thickness distribution is intrinsically related to the anisotropy of the fluxes induced by the droplet's motion.

Place, publisher, year, edition, pages
Cambridge University Press, 2019
Keywords
drops, Hele-Shaw flows, lubrication theory
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-255546 (URN)10.1017/jfm.2019.453 (DOI)000475480700001 ()2-s2.0-85073152731 (Scopus ID)
Note

QC 20190805

Available from: 2019-08-05 Created: 2019-08-05 Last updated: 2020-02-04Bibliographically approved
Pietrzyk, K., Nganguia, H., Datt, C., Zhu, L., Elfring, G. J. & Pak, O. S. (2019). Flow around a squirmer in a shear-thinning fluid. Journal of Non-Newtonian Fluid Mechanics, 268, 101-110
Open this publication in new window or tab >>Flow around a squirmer in a shear-thinning fluid
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2019 (English)In: Journal of Non-Newtonian Fluid Mechanics, ISSN 0377-0257, E-ISSN 1873-2631, Vol. 268, p. 101-110Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
Elsevier, 2019
Keywords
Locomotion, Shear-thinning, Carreau model
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-255331 (URN)10.1016/j.jnnfm.2019.04.005 (DOI)000472699500009 ()2-s2.0-85066325898 (Scopus ID)
Note

QC 20190730

Available from: 2019-07-30 Created: 2019-07-30 Last updated: 2019-07-30Bibliographically approved
Ungarish, M., Zhu, L. & Stone, H. A. (2019). Inertial gravity current produced by the drainage of a cylindrical reservoir from an outer or inner edge. Journal of Fluid Mechanics, 874, 185-209
Open this publication in new window or tab >>Inertial gravity current produced by the drainage of a cylindrical reservoir from an outer or inner edge
2019 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 874, p. 185-209Article in journal (Refereed) Published
Abstract [en]

We consider the time-dependent flow of a fluid of density rho(1) in a vertical cylindrical container embedded in a fluid of density rho(2) (<rho(1)) whose side boundary is suddenly removed and the fluid drains freely from the edge. We show that in the inertial-buoyancy regime (large initial Reynolds number) the flow is modelled by the shallow-water equations and bears similarities to a gravity current released from a lock (the dam-break problem) driven by the reduced gravity g' = (1 - rho(2)/rho(1))g. This formulation is amenable to an efficient finite-difference solution. Moreover, we demonstrate that similarity solutions exist, and show that the flow created by the dam break approaches the predicted self-similar behaviour when the volume ratio nu(t)/nu(0) approximate to 1/2 where t is time elapsed from the dam break. We considered two cases of drainage: (i) outward from the outer boundary in a full-radius reservoir; and (ii) inward from the inner radius in an annular-shaped reservoir. For the first case the similarity solution is expressed analytically, while the second case is more complicated and requires a numerical solution. In both cases nu(t)/nu(0) decays like t(-2), but the details are different. The similarity solutions admit an adjustable virtual-origin constant, which we determine by matching with the finite-difference solution. The analysis is valid for both Boussinesq and non-Boussinesq systems, and a wide range of geometric parameters (inner and outer radii, and height). The importance of the neglected viscous terms increases with time, and eventually the inertial-buoyancy model becomes invalid. An estimate for this occurrence is also provided. The predictions of the model are compared to results of direct numerical simulations; there is good agreement for the position of the interface and for the averaged radial velocity, and excellent agreement for nu(t)/nu(0). A box model is used for estimating the effect of a partial (over a sector) dam break. This study is an extension of the work for a rectangular reservoir of Momen et al. (J. Fluid Mech., vol. 827, 2017, pp. 640-663). We demonstrate that there are some similarities, but also significant differences, between the rectangular and the cylindrical reservoirs concerning the velocity, shape of the interface and rate of drainage, which are of interest in applications. The overall conclusion is that this simple model captures very well the flow field under consideration.

Place, publisher, year, edition, pages
CAMBRIDGE UNIV PRESS, 2019
Keywords
geophysical and geological flows, gravity currents, shallow water flows
National Category
Geophysical Engineering
Identifiers
urn:nbn:se:kth:diva-255396 (URN)10.1017/jfm.2019.452 (DOI)000473909800001 ()2-s2.0-85068527773 (Scopus ID)
Note

QC 20190814

Available from: 2019-08-14 Created: 2019-08-14 Last updated: 2019-08-14Bibliographically approved
Liu, Y., Rallabandi, B., Zhu, L., Gupta, A. & Stone, H. A. (2019). Pattern formation in oil-in-water emulsions exposed to a salt gradient. Physical Review Fluids, 4(8), Article ID 084307.
Open this publication in new window or tab >>Pattern formation in oil-in-water emulsions exposed to a salt gradient
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2019 (English)In: Physical Review Fluids, E-ISSN 2469-990X, Vol. 4, no 8, article id 084307Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2019
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-259437 (URN)10.1103/PhysRevFluids.4.084307 (DOI)000483355400002 ()2-s2.0-85072025648 (Scopus ID)
Note

QC 20190923

Available from: 2019-09-23 Created: 2019-09-23 Last updated: 2019-09-23Bibliographically approved
Zhu, L. & Stone, H. A. (2019). Propulsion driven by self-oscillation via an electrohydrodynamic instability. Physical Review Fluids, 4(6), Article ID 061701.
Open this publication in new window or tab >>Propulsion driven by self-oscillation via an electrohydrodynamic instability
2019 (English)In: Physical Review Fluids, ISSN 2469-990X, Vol. 4, no 6, article id 061701Article in journal (Refereed) Published
Abstract [en]

Oscillations of flagella and cilia play an important role in biology, which motivates the idea of functional mimicry as part of bioinspired applications. Nevertheless, it still remains challenging to drive their artificial counterparts to oscillate via a steady, homogeneous stimulus. Combining theory and simulations, we demonstrate a strategy to achieve this goal by using an elastoelectrohydrodynamic instability (based on the Quincke rotation instability). In particular, we show that applying a uniform dc electric field can produce self-oscillatory motion of a microrobot composed of a dielectric particle and an elastic filament. Upon tuning the electric field and filament elasticity, the microrobot exhibits three distinct behaviors: a stationary state, undulatory swimming, and steady spinning, where the swimming behavior stems from an instability emerging through a Hopf bifurcation. Our results imply the feasibility of engineering self-oscillations by leveraging the elastoviscous response to control the type of bifurcation and the form of instability. We anticipate that our strategy will be useful in a broad range of applications imitating self-oscillatory natural phenomena and biological processes.

Place, publisher, year, edition, pages
American Physical Society, 2019
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-255312 (URN)10.1103/PhysRevFluids.4.061701 (DOI)000473044300001 ()2-s2.0-85069747426 (Scopus ID)
Available from: 2019-08-07 Created: 2019-08-07 Last updated: 2019-08-07Bibliographically approved
Horvath, D. G., Braza, S., Moore, T., Pan, C. W., Zhu, L., Pak, O. S. & Abbyad, P. (2019). Sorting by interfacial tension (SIFT): Label-free enzyme sorting using droplet microfluidics. Analytica Chimica Acta, 1089, 108-114
Open this publication in new window or tab >>Sorting by interfacial tension (SIFT): Label-free enzyme sorting using droplet microfluidics
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2019 (English)In: Analytica Chimica Acta, ISSN 0003-2670, E-ISSN 1873-4324, Vol. 1089, p. 108-114Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
ELSEVIER, 2019
Keywords
Microfluidics, Droplet, Lab-on-a-chip, Sorting, Label-free, Enzyme activity
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-263328 (URN)10.1016/j.aca.2019.08.025 (DOI)000490138000012 ()31627807 (PubMedID)2-s2.0-85071109685 (Scopus ID)
Note

QC 20191206

Available from: 2019-12-06 Created: 2019-12-06 Last updated: 2019-12-06Bibliographically approved
Gallino, G., Zhu, L. & Gallaire, F. (2019). The Hydrodynamics of a Micro-Rocket Propelled by a Deformable Bubble. FLUIDS, 4(1), Article ID 48.
Open this publication in new window or tab >>The Hydrodynamics of a Micro-Rocket Propelled by a Deformable Bubble
2019 (English)In: FLUIDS, ISSN 2311-5521, Vol. 4, no 1, article id 48Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
MDPI, 2019
Keywords
catalytic microswimmers, bubble-propelled microswimmers, microrockets, numerical simulations, self-propulsion
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-251353 (URN)10.3390/fluids4010048 (DOI)000464463800001 ()2-s2.0-85063372821 (Scopus ID)
Note

QC 20190521

Available from: 2019-05-21 Created: 2019-05-21 Last updated: 2019-05-23Bibliographically approved
Hadikhani, P., Hashemi, S. M., Balestra, G., Zhu, L., Modestino, M. A., Gallaire, F. & Psaltis, D. (2018). Inertial manipulation of bubbles in rectangular microfluidic channels. Lab on a Chip, 18(7), 1035-1046
Open this publication in new window or tab >>Inertial manipulation of bubbles in rectangular microfluidic channels
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2018 (English)In: Lab on a Chip, ISSN 1473-0197, E-ISSN 1473-0189, Vol. 18, no 7, p. 1035-1046Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
Royal Society of Chemistry, 2018
National Category
Biochemistry and Molecular Biology
Identifiers
urn:nbn:se:kth:diva-225928 (URN)10.1039/c7lc01283g (DOI)000428569400004 ()29512658 (PubMedID)2-s2.0-85044649947 (Scopus ID)
Funder
Swedish Research Council, 2015-06334Swedish e‐Science Research Center
Note

QC 20180411

Available from: 2018-04-11 Created: 2018-04-11 Last updated: 2018-04-19Bibliographically approved
Zhu, L. & Stone, H. A. (2018). Rotation of a low-Reynolds-number watermill: theory and simulations. Journal of Fluid Mechanics, 849, 57-75
Open this publication in new window or tab >>Rotation of a low-Reynolds-number watermill: theory and simulations
2018 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 849, p. 57-75Article in journal (Refereed) Published
Abstract [en]

Recent experiments have demonstrated that small-scale rotary devices installed in a microfluidic channel can be driven passively by the underlying flow alone without resorting to conventionally applied magnetic or electric fields. In this work, we conduct a theoretical and numerical study on such a flow-driven 'watermill' at low Reynolds number, focusing on its hydrodynamic features. We model the watermill by a collection of equally spaced rigid rods. Based on the classical resistive force (RF) theory and direct numerical simulations, we compute the watermill's instantaneous rotational velocity as a function of its rod number N, position and orientation. When N >= 4, the RF theory predicts that the watermill's rotational velocity is independent of N and its orientation, implying the full rotational symmetry (of infinite order), even though the geometrical configuration exhibits a lower-fold rotational symmetry; the numerical solutions including hydrodynamic interactions show a weak dependence on N and the orientation. In addition, we adopt a dynamical system approach to identify the equilibrium positions of the watermill and analyse their stability. We further compare the theoretically and numerically derived rotational velocities, which agree with each other in general, while considerable discrepancy arises in certain configurations owing to the hydrodynamic interactions neglected by the RP theory. We confirm this conclusion by employing the RP-based asymptotic framework incorporating hydrodynamic interactions for a simpler watermill consisting of two or three rods and we show that accounting for hydrodynamic interactions can significantly enhance the accuracy of the theoretical predictions.

Place, publisher, year, edition, pages
CAMBRIDGE UNIV PRESS, 2018
Keywords
low-Reynolds-number flows, MEMS/NEMS, microfluidics
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-238929 (URN)10.1017/jfm.2018.416 (DOI)000448745500003 ()2-s2.0-85048779241 (Scopus ID)
Note

QC 20181114

Available from: 2018-11-14 Created: 2018-11-14 Last updated: 2018-11-16Bibliographically approved
Yu, Y. E., Zhu, L., Shim, S., Eggers, J. & Stone, H. A. (2018). Time-dependent motion of a confined bubble in a tube: transition between two steady states. Journal of Fluid Mechanics, 857, Article ID R4.
Open this publication in new window or tab >>Time-dependent motion of a confined bubble in a tube: transition between two steady states
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2018 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 857, article id R4Article in journal (Refereed) Published
Abstract [en]

When a confined bubble translates steadily in a cylindrical capillary tube, without the consideration of gravity effects, a uniform thin film of liquid separates the bubble surface and the tube wall. In this work, we investigate how this steady state is established by considering the transitional motion of the bubble as it adjusts its film thickness profile between two steady states, characterized by two different bubble speeds. During the transition, two uniform film regions coexist, separated by a step-like transitional region. The transitional motion also requires modification of the film solution near the rear of the bubble, which depends on the ratio of the two capillary numbers. These theoretical results are verified by experiments and numerical simulations.

Place, publisher, year, edition, pages
Cambridge University Press, 2018
Keywords
bubble dynamics, lubrication theory, thin films
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-239089 (URN)10.1017/jfm.2018.835 (DOI)000448522900001 ()2-s2.0-85055848357 (Scopus ID)
Note

QC 20181121

Available from: 2018-11-21 Created: 2018-11-21 Last updated: 2018-11-21Bibliographically approved
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