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Publications (5 of 5) Show all publications
Rosti, M. E., Ge, Z., Jain, S. S., Dodd, M. S. & Brandt, L. (2019). Droplets in homogeneous shear turbulence. Journal of Fluid Mechanics, 876, 962-984
Open this publication in new window or tab >>Droplets in homogeneous shear turbulence
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2019 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 876, p. 962-984Article in journal (Refereed) Published
Abstract [en]

We simulate the flow of two immiscible and incompressible fluids separated by an interface in a homogeneous turbulent shear flow at a shear Reynolds number equal to 15 200. The viscosity and density of the two fluids are equal, and various surface tensions and initial droplet diameters are considered in the present study. We show that the two-phase flow reaches a statistically stationary turbulent state sustained by a non-zero mean turbulent production rate due to the presence of the mean shear. Compared to single-phase flow, we find that the resulting steady-state conditions exhibit reduced Taylor-microscale Reynolds numbers owing to the presence of the dispersed phase, which acts as a sink of turbulent kinetic energy for the carrier fluid. At steady state, the mean power of surface tension is zero and the turbulent production rate is in balance with the turbulent dissipation rate, with their values being larger than in the reference single-phase case. The interface modifies the energy spectrum by introducing energy at small scales, with the difference from the single-phase case reducing as the Weber number increases. This is caused by both the number of droplets in the domain and the total surface area increasing monotonically with the Weber number. This reflects also in the droplet size distribution, which changes with the Weber number, with the peak of the distribution moving to smaller sizes as the Weber number increases. We show that the Hinze estimate for the maximum droplet size, obtained considering break-up in homogeneous isotropic turbulence, provides an excellent estimate notwithstanding the action of significant coalescence and the presence of a mean shear.

Place, publisher, year, edition, pages
Cambridge University Press, 2019
Keywords
drops, multiphase flow, turbulence simulation
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-257426 (URN)10.1017/jfm.2019.581 (DOI)000480242100001 ()2-s2.0-85070481433 (Scopus ID)
Note

QC 20190902

Available from: 2019-09-02 Created: 2019-09-02 Last updated: 2019-09-02Bibliographically approved
Ge, Z., Tammisola, O. & Brandt, L. (2019). Flow-assisted droplet assembly in a 3D microfluidic channel. Soft Matter, 15(16), 3451-3460
Open this publication in new window or tab >>Flow-assisted droplet assembly in a 3D microfluidic channel
2019 (English)In: Soft Matter, ISSN 1744-683X, E-ISSN 1744-6848, Vol. 15, no 16, p. 3451-3460Article in journal (Refereed) Published
Abstract [en]

Self-assembly of soft matter, such as droplets or colloids, has become a promising scheme to engineer novel materials, model living matter, and explore non-equilibrium statistical mechanics. In this article, we present detailed numerical simulations of few non-Brownian droplets in various flow conditions, specifically, focusing on their self-assembly within a short distance in a three-dimensional (3D) microfluidic channel, cf. [Shen et al., Adv. Sci., 2016, 3(6), 1600012]. Contrary to quasi two-dimensional (q2D) systems, where dipolar interaction is the key mechanism for droplet rearrangement, droplets in 3D confinement produce much less disturbance to the underlying flow, thus experiencing weaker dipolar interactions. Using confined simple shear and Poiseuille flows as reference flows, we show that the droplet dynamics is mostly affected by the shear-induced cross-stream migration, which favors chain structures if the droplets are under an attractive depletion force. For more compact clusters, such as three droplets in a triangular shape, our results suggest that an inhomogeneous cross-sectional inflow profile is further required. Overall, the accelerated self-assembly of a small-size droplet cluster results from the combined effects of strong depletion forces, confinement-mediated shear alignments, and fine-tuned inflow conditions. The deterministic nature of the flow-assisted self-assembly implies the possibility of large throughputs, though calibration of all different effects to directly produce large droplet crystals is generally difficult.

Place, publisher, year, edition, pages
ROYAL SOC CHEMISTRY, 2019
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-252637 (URN)10.1039/c8sm02479k (DOI)000468007600014 ()30958490 (PubMedID)2-s2.0-85064601819 (Scopus ID)
Note

QC 20190610

Available from: 2019-06-10 Created: 2019-06-10 Last updated: 2019-06-10Bibliographically approved
Ge, Z., Loiseau, J. C., Tammisola, O. & Brandt, L. (2018). An efficient mass-preserving interface-correction level set/ghost fluid method for droplet suspensions under depletion forces. Journal of Computational Physics, 353, 435-459
Open this publication in new window or tab >>An efficient mass-preserving interface-correction level set/ghost fluid method for droplet suspensions under depletion forces
2018 (English)In: Journal of Computational Physics, ISSN 0021-9991, E-ISSN 1090-2716, Vol. 353, p. 435-459Article in journal (Refereed) Published
Abstract [en]

Aiming for the simulation of colloidal droplets in microfluidic devices, we present here a numerical method for two-fluid systems subject to surface tension and depletion forces among the suspended droplets. The algorithm is based on an efficient solver for the incompressible two-phase Navier–Stokes equations, and uses a mass-conserving level set method to capture the fluid interface. The four novel ingredients proposed here are, firstly, an interface-correction level set (ICLS) method; global mass conservation is achieved by performing an additional advection near the interface, with a correction velocity obtained by locally solving an algebraic equation, which is easy to implement in both 2D and 3D. Secondly, we report a second-order accurate geometric estimation of the curvature at the interface and, thirdly, the combination of the ghost fluid method with the fast pressure-correction approach enabling an accurate and fast computation even for large density contrasts. Finally, we derive a hydrodynamic model for the interaction forces induced by depletion of surfactant micelles and combine it with a multiple level set approach to study short-range interactions among droplets in the presence of attracting forces.

Place, publisher, year, edition, pages
Academic Press, 2018
Keywords
Colloidal droplet, Depletion force, Ghost fluid method, Level set method, Multiphase flow
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-218922 (URN)10.1016/j.jcp.2017.10.046 (DOI)000418229800020 ()2-s2.0-85032258558 (Scopus ID)
Funder
Swedish e‐Science Research CenterSwedish Research Council, 2013-5789EU, Horizon 2020
Note

QC 20171201

Available from: 2017-12-01 Created: 2017-12-01 Last updated: 2019-09-18Bibliographically approved
Ge, Z., Holmgren, H., Kronbichler, M., Brandt, L. & Kreiss, G. (2018). Effective slip over partially filled microcavities and its possible failure. Physical Review Fluids, 3(5), Article ID 054201.
Open this publication in new window or tab >>Effective slip over partially filled microcavities and its possible failure
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2018 (English)In: Physical Review Fluids, E-ISSN 2469-990X, Vol. 3, no 5, article id 054201Article in journal (Refereed) Published
Abstract [en]

Motivated by the emerging applications of liquid-infused surfaces (LIS), we study the drag reduction and robustness of transverse flows over two-dimensional microcavities partially filled with an oily lubricant. Using separate simulations at different scales, characteristic contact line velocities at the fluid-solid intersection are first extracted from nanoscale phase field simulations and then applied to micronscale two-phase flows, thus introducing a multiscale numerical framework to model the interface displacement and deformation within the cavities. As we explore the various effects of the lubncant-toouter-fluid viscosity ratio A2/A0 th(mu)over tilde( )c(mu)over tilde(1), thary number Ca, the static contact angle A> and t theta(s), filling fraction of the cavity <5, we f delta d that the effective slip is most sensitive to the parameter S. The effects of A2/A1 an(mu)over tilde( )A(mu)over tilde(a )re ge theta(s)erally intertwined but weakened if <5 < 1. delta M 1er, for an initial filling fraction S = 0.94 delta our results show that the effective slip is nearly independent of the capillary number when it is small. Further increasing Ca to about O.OIA1/A20.01(mu)over tilde(1)/(mu)over tilde(2)ntify a possible failure mode, associated with lubricants draining from the LIS, for A2/A1 A (mu)over tilde(2)1(mu)over tilde(1)V less than or similar to y viscous lubricants (e.g., A2/A1 > (mu)over tilde()),(mu)over tilde(h)owever, are immune to such failure due to their generally larger contact line velocity.

Place, publisher, year, edition, pages
American Physical Society, 2018
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-230443 (URN)10.1103/PhysRevFluids.3.054201 (DOI)000433036100003 ()2-s2.0-85049006815 (Scopus ID)
Funder
Swedish Research Council, 621-2012-2360EU, FP7, Seventh Framework Programme, 664823Swedish e‐Science Research Center
Note

QC 20180613

Available from: 2018-06-13 Created: 2018-06-13 Last updated: 2019-04-04Bibliographically approved
Fouxon, I., Ge, Z., Brandt, L. & Leshansky, A. (2017). Integral representation of channel flow with interacting particles. Physical review. E, 96(6), Article ID 063110.
Open this publication in new window or tab >>Integral representation of channel flow with interacting particles
2017 (English)In: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 96, no 6, article id 063110Article in journal (Refereed) Published
Abstract [en]

We construct a boundary integral representation for the low-Reynolds-number flow in a channel in the presence of freely suspended particles (or droplets) of arbitrary size and shape. We demonstrate that lubrication theory holds away from the particles at horizontal distances exceeding the channel height and derive a multipole expansion of the flow which is dipolar to the leading approximation. We show that the dipole moment of an arbitrary particle is a weighted integral of the stress and the flow at the particle surface, which can be determined numerically. We introduce the equation of motion that describes hydrodynamic interactions between arbitrary, possibly different, distant particles, with interactions determined by the product of the mobility matrix and the dipole moment. Further, the problem of three identical interacting spheres initially aligned in the streamwise direction is considered and the experimentally observed "pair exchange" phenomenon is derived analytically and confirmed numerically. For nonaligned particles, we demonstrate the formation of a configuration with one particle separating from a stable pair. Our results suggest that in a dilute initially homogenous particulate suspension flowing in a channel the particles will eventually separate into singlets and pairs.

National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-220595 (URN)10.1103/PhysRevE.96.063110 (DOI)000417834600010 ()2-s2.0-85038211455 (Scopus ID)
Note

QC 20180117

Available from: 2018-01-17 Created: 2018-01-17 Last updated: 2018-01-17Bibliographically approved
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0002-4222-012x

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