Papers by Michael Loewenberg
Journal of Fluid Mechanics, Jul 25, 2002
The effect of an insoluble surfactant on the rheological behaviour of a dilute emulsion is theore... more The effect of an insoluble surfactant on the rheological behaviour of a dilute emulsion is theoretically studied under low-capillary-number conditions. The dynamics depends on three dimensionless time-scale parameters that characterize the strength of the mechanisms that control the magnitude of the distortion of the surfactant distribution on the drop interface. These mechanisms include Marangoni relaxation, drop rotation by the imposed flow, and oscillations of the imposed flow. The interaction of the time scales gives rise to a complex rheological behaviour. The evolution of the system is described by a nonlinear matrix equation derived by expanding the fluid velocity and surfactant distribution in spherical harmonics. Analytical expansions are developed for conditions where the surfactant distribution is only slightly perturbed, which occurs when one of the time-scale parameters is small.
A Simplified Description of Char Combustion
Chemical Engineering Communications, Aug 1, 1987
ABSTRACT
Journal of Fluid Mechanics, Sep 25, 1999
Journal of Fluid Mechanics, Aug 25, 1996
A three-dimensional computer simulation of a concentrated emulsion in shear flow has been develop... more A three-dimensional computer simulation of a concentrated emulsion in shear flow has been developed for low-Reynolds-number finite-capillary-number conditions. Numerical results have been obtained using an efficient boundary integral formulation with periodic boundary conditions and up to twelve drops in each periodically replicated unit cell. Calculations have been performed over a range of capillary numbers where drop deformation is significant up to the value where drop breakup is imminent. Results have been obtained for dispersed-phase volume fractions up to 30% and dispersed-to continuous-phase viscosity ratios in the range of 0 to 5. The results reveal a complex rheology with pronounced shear thinning and large normal stresses that is associated with an anisotropic microstructure that results from the alignment of deformed drops in the flow direction. The viscosity of an emulsion is only a moderately increasing function of the dispersed-phase volume fraction, in contrast to suspensions of rigid particles or undeformed drops. Unlike rigid particles, deformable drops do not form large clusters.
Soil biotechnology (SBT)is a system for water renovation which makes use of a formulated media wi... more Soil biotechnology (SBT)is a system for water renovation which makes use of a formulated media with culture of soil micro and macro-organisms to process water and wastewater. The process gives advantage in terms of applicability for very small to large scale; natural aeration, no moving parts except pumps, no sludge, no odor and all green environment. Computational Fluid Dynamics (CFD) is used to study the hydrodynamics as well as rate limiting features of the system. Simulations are performed for different configurations of the bioreactor and the results are compared with laboratory and field experimental data. It is shown that this CFD model can be used to predict behaviour of the process.
Conference on Lasers and Electro-Optics, 2016
Cilia are organelles that generate microfluidic flows in the lungs, central nervous system, and F... more Cilia are organelles that generate microfluidic flows in the lungs, central nervous system, and Fallopian tubes. Quantitative flow diagnostic remain immature. I will present a comprehensive optical imaging-based approach for quantifying cilia flow physiology. OCIS codes: (110.4500) Optical coherence tomography; (170.1650) Coherence imaging; (170.3880) Medical and biological imaging; (170.5380) Physiology General; Cilia are organelles that generate microscale, surface-driven fluid flows in the lungs, central nervous system, and Fallopian tubes. Quantitative flow diagnostic remain immature. Moreover, there is a growing appreciation that advances in optical imaging are opening up new areas of investigation in clinically-relevant ciliary biology. In this invited talk, I will present our comprehensive optical imaging-based approach for quantifying cilia-driven fluid flow physiology. Central to our approach is the incorporation of principles of fluid mechanics into the development of novel imaging-based flow performance metrics. I also will discuss our work in novel coherent imaging methods that is complementary to the development of novel performance metrics in ciliary biology.
Structure and oxidation of carbonaceous cenospheres
Carbonaceous cenosphere particles from an oil furnace are characterized by scanning electron micr... more Carbonaceous cenosphere particles from an oil furnace are characterized by scanning electron microscopy, mercury intrusion and nitrogen adsorption porosimetry. The porosimetry data are used to construct a random pore model describing the evolution of surface area and pore volume distribution. The pore model is combined with gas phase transport to formulate the equations for transient particle combustion and some numerical solutions are presented.
Physical Review E, Apr 30, 2004
The growth of spherical drops by coalescence in simple shear and axisymmetric straining flows has... more The growth of spherical drops by coalescence in simple shear and axisymmetric straining flows has been numerically investigated, and the long-time scaling behavior of the system was explored. It is shown that hydrodynamic interactions qualitatively modify the the collision kernel in the population balance equation and thus alter the evolution of the drop size distribution at long times. In the presence of hydrodynamic interactions, the number of drops in the system decays as t Ϫ1 , and the average drop size grows as e ͱt ; in the absence of hydrodynamic interactions, these quantities evolve exponentially at long times. Hydrodynamic interactions lead to broader drop size distributions, and cause the influence of initial conditions to decay with time. Drops undergoing thermocapillary migration are shown to exhibit similar features. Our results are shown to be consistent with the established theory for the scaling behavior of aggregating systems. It is shown that the theory applies even in certain cases where the binary collision kernel does not have the assumed form. In the presence of hydrodynamic interactions, the scaling regime is attained slowly ͑logarithmically͒.
Journal of Colloid and Interface Science, Mar 1, 1997
The stability and pairwise aggregation rates of small spherical potentials may also occur . parti... more The stability and pairwise aggregation rates of small spherical potentials may also occur . particles in a heterogeneous suspension under the collective effects Gravity-induced particle aggregation has been studied by of gravitational motion and electrophoretic migration are ana-Melik and Fogler (3) and Davis (4), who employed trajeclyzed. The particles are assumed to be non-Brownian, with thin, tory analyses to predict pairwise particle collision rates. Reunpolarized double layers and different zeta potentials. The gravcently, Nichols et al. (5) analyzed the stability criteria and ity vector and the electric field are assumed to be oriented in either pairwise aggregation rates for electrophoretic motion of colthe same direction or opposite directions. The particle aggregation loidal spheres with differing zeta potentials and thin, unporates are always enhanced by the presence of an electric field for larized electric double layers. Their results indicate that hyparallel alignment of the gravitational and electrophoretic veloci- ties. For antiparallel alignment with the magnitude of the gravita-drodynamic interactions generally have a weaker effect on tional relative velocity exceeding the magnitude of the electrophothe collision rate for electrophoretic motion than for gravity retic relative velocity between two widely separated particles, the motion. As a result, the collision efficiencies for electrophoparticle aggregation rates are reduced by the presence of the elecresis generally exceed those for gravitational motion by an tric field, and there is a ''collision-forbidden'' region in parameter order of magnitude. In these previous studies, the effects of space due to stronger hydrodynamic interactions of the particles gravity motion and electrophoretic migration were examined for gravitational motion than for electrophoretic motion. For antiindependently; however, gravity and electrophoresis may act parallel alignment with the magnitude of the electrophoretic relasimultaneously in some applications. One example is the tive velocity exceeding the magnitude of the gravitational relative separation of slowly sedimenting suspensions, for which an velocity between two widely separated particles, the particle aggreelectric field may be employed to accelerate the initial formagation rates are enhanced by the presence of the electric field. tion of aggregates; once the aggregates become large
Journal of Fluid Mechanics, Nov 1, 1993
The axisymmetric, thermocapillary-driven motion of a pair of non-conducting, spherical drops in n... more The axisymmetric, thermocapillary-driven motion of a pair of non-conducting, spherical drops in near contact is analysed for conditions of small Reynolds and Marangoni numbers. The pairwise motion and an associated contact force are computed by considering touching drops in point contact. Relative motion for nearly touching drops results from the contact force balanced by a lubrication resistance. A new, analytical solution is obtained for the axisymmetric temperature field around an unequal pair of non-conducting, tangent spheres embedded in an ambient temperature gradient. Numerical results for the painvise migration velocity, contact force, and the relative and individual drop velocities are presented for all size ratios and a wide range of viscosity ratios, and asymptotic formulae are derived for small size ratios. For nearly equisized drops, the ratio of the relative velocity for two drops in near contact to that for widely separated drops is similar for thermocapillary-driven and gravity-driven motion. For small and moderate size ratios, however, this ratio is much larger for thermocapillary-driven relative motion than for gravity-driven relative motion, indicating that the former represents a more efficient coalescence mechanism. An explanation for this finding is provided in terms of the thermocapillary motion of the interface of the larger drop aiding the withdrawal of continuous phase from between the two drops.
Physics of fluids, Nov 1, 1993
The unsteady (oscillatory) Stokes resistance is numerically computed for finite-length, circular ... more The unsteady (oscillatory) Stokes resistance is numerically computed for finite-length, circular cylinders using a first-kind, boundary integral formulation for low and moderate dimensionless frequencies. Translational oscillations parallel and perpendicular to the cylinder axis (axial and transverse oscillations) are considered. For arbitrarily oriented cylinders with length-to-width ratios in the range: l/lO<a/b<lO, a simple, previously proposed, expression accurately ( f 5%) describes the oscillatory Stokes resistance at all frequencies in terms of steady Stokes and inviscid flow resistance parameters. Numerical calculations for spheroids agree with previous results; the results for cylinders and transverse oscillations of oblate spheroids are new.
Soft Matter, 2012
The evolution of linear arrays of rigid spheres and deformable drops in a Poiseuille flow between... more The evolution of linear arrays of rigid spheres and deformable drops in a Poiseuille flow between parallel walls is investigated to determine the effect of particle deformation on the collective dynamics in confined particulate flows. We find that linear arrays of rigid spheres aligned in the flow direction exhibit a particlepairing instability and are unstable to lateral perturbations. Linear arrays of deformable drops also undergo the pairing instability but also exhibit additional dynamical features, including formation of transient triplets, cascades of pair-switching events, and the eventual formation of pairs with equal interparticle spacing. Moreover, particle deformation stabilizes drop arrays to lateral perturbations. These pairing and alignment phenomena are qualitatively explained in terms of hydrodynamic far-field dipole interactions that are insensitive to particle deformation and quadrupole interactions that are deformation induced. We suggest that quadrupole interactions may underlie the spontaneous formation of droplet strings in confined emulsions under shear [Phys. Rev. Lett., 2001, 86, 1023.].
Physica D: Nonlinear Phenomena, Feb 1, 2000
The rheology of a diluted emulsion of surfactant-covered spherical drops has been investigated. A... more The rheology of a diluted emulsion of surfactant-covered spherical drops has been investigated. A diluted ÿlm of insoluble surfactant is assumed. A matrix formulation of the problem is derived and analyzed by perturbation expansions for low-and high-shear rates, and for high-viscosity drops; the high-viscosity expansion converges rapidly for a wide range of parameters. Our theory provides a quantitative description of shear thinning and normal stress di erences that occur as a result of surfactant redistribution.
The dynamic mobility of nonspherical particles
Journal of Colloid and Interface Science, Apr 1, 1992
Abstract This paper is concerned with the electrophoretic mobility of particles in alternating el... more Abstract This paper is concerned with the electrophoretic mobility of particles in alternating electric fields. We refer to this mobility as the “dynamic mobility,” to distinguish it from the standard electrophoretic mobility, which is defined for static fields. This dynamic mobility can now be measured at frequencies up to 1 MHz with the aid of electroacoustic devices. In this paper we present formulae for calculating the dynamic mobility of spheroidal particles with thin double layers. With the aid of these formulae it should be possible to obtain information on particle charge, size, and shape from electroacoustic measurements on dilute suspensions.
Electrohydrodynamic Thermal Oscillators for Waste Heat Harvesting Applications
The Effect of Flow on Drop Coalescence
Drop coalescence is a complex process due to the nonlinear dynamics of a system with deformable i... more Drop coalescence is a complex process due to the nonlinear dynamics of a system with deformable interfaces. In earlier studies the effect of an external flow on near-contact motion of drops was assumed to be equivalent to an external body force. Accordingly, the direct coupling between thin-film flow (in the near-contact region) and flow inside the drops was neglected. These
Dynamics of a surfactant-covered drop and the non-Newtonian rheology of emulsions
APS, Mar 1, 2004
Complex fluids exhibit rich dynamic macroscopic behavior that stems from relaxation of the materi... more Complex fluids exhibit rich dynamic macroscopic behavior that stems from relaxation of the material microstructure. Emulsions are examples of complex fluids with drop-like morphology. Predicting the rheology of emulsions relies on a detailed understanding of the drop-level microhydrodynamics. The focus of this talk is on the dynamics of a drop in linear viscous flows. The drop is covered with a
Bulletin of the American Physical Society, Nov 19, 2012
The evolution of linear arrays of rigid spheres and deformable drops in a Poiseuille flow between... more The evolution of linear arrays of rigid spheres and deformable drops in a Poiseuille flow between parallel walls is investigated to determine the effect of particle deformation on the collective dynamics in confined particulate flows. We find that linear arrays of rigid spheres aligned in the flow direction undergo a particle-pairing instability and are unstable to lateral perturbations. Linear arrays of deformable drops, in addition to the pairing instability, exhibit other dynamical features, including formation of transient triplets, cascades of pair-switching events, and formation of pairs with equal interparticle spacing. Particle deformation also stabilizes drop arrays to lateral perturbations. These pairing and alignment phenomena are qualitatively explained in terms of hydrodynamic far-field dipole interactions (insensitive to particle deformation) and quadrupole interactions (deformation induced). We suggest that quadrupole interactions underlie the spontaneous formation of droplet strings in confined emulsions under shear [Phys. Rev. Lett., 2001, 86, 1023].
Near-Contact, Thermocapillary Migration of a Nonconducting, Viscous Drop Normal to a Planar Interface
Journal of Colloid and Interface Science, Oct 1, 1993
Abstract The near-contact, thermocapillary motion of a nonconducting, spherical drop normal to a ... more Abstract The near-contact, thermocapillary motion of a nonconducting, spherical drop normal to a planar, isothermal interface has been analyzed using a lubrication solution for conditions of small Reynolds and Marangoni numbers. The interface separates two viscous fluids; a third is contained within the drop. A closed-form expression is obtained for the near-contact drop velocity. The drop motion results from a "contact force" that is opposed by a lubrication resistance; the contact force is determined by considering a force balance on a drop tangent to the interface. For drops of moderate viscosity, the contact force is comparable to the thermocapillary force that acts on an isolated, stationary drop; for highly viscous drops, the contact force dominates as the logarithm of the drop viscosity ratio. If both the drop surface and the planar interface are fully mobile or free, the near-contact drop velocity, normalized by the value for a drop in an unbounded fluid, decreases with drop viscosity; however, if the interface is immobile and/or the drop is highly viscous, the normalized velocity increases with drop viscosity. The cases of a drop moving toward rigid and free surfaces and of a drop moving toward a semi-infinite fluid with the same viscosity are examined explicitly. A comparison with buoyancy-driven motion reveals that thermocapillary migration is the more efficient near-contact migration mechanism.
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Papers by Michael Loewenberg