Martin B. Nemer

Sandia National Laboratories, Albuquerque, New Mexico, United States

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Publications (14)25.14 Total impact

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    ABSTRACT: A model system has been developed for enabling a multi-scale understanding of centrifugal-contactor liquid-liquid extraction. The system consisted of Nd(III) + xylenol orange in the aqueous phase buffered to pH = 5.5 by KHP, and dodecane + thenoyltrifluroroacetone (HTTA) + tributyphosphate (TBP) in the organic phase. Diffusion constants were measured for neodymium in both the organic and aqueous phases, and the Nd(III) partition coefficients were measured at various HTTA and TBP concentrations. A microfluidic channel was used as a high-shear model environment to observe mass transfer on a droplet scale with xylenol orange as the aqueous-phase metal indicator; mass transfer rates were measured quantitatively in both diffusion and reaction limited regimes on the droplet scale. The microfluidic results were comparable to observations made for the same system in a laboratory scale liquid-liquid centrifugal contactor, indicating that single drop microfluidic experiments can provide information on mass-transfer in complicated flows and geometries. © 2014 American Institute of Chemical Engineers AIChE J, 2014
    AIChE Journal 05/2014; · 2.58 Impact Factor
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    ABSTRACT: Liquid droplets flowing through a rectangular microfluidic channel develop a vortical flow field due to the presence of shear forces from the surrounding fluid. In this paper, we present an experimental and computational study of droplet velocities and internal flow patterns in a rectangular pressure-driven flow for droplet diameters ranging from 0.1 to 2 times the channel height. Our study shows excellent agreement with asymptotic predictions of droplet and interfacial velocities for infinitesimally small droplets. As the droplet diameter nears the size of the channel height, the droplet velocity slows significantly, and the changing external flow field causes a qualitative change in the location of internal vortices. This behavior is relevant for future studies of mass transfer in microfluidic devices.
    Physics of Fluids 03/2014; 26:032105. · 1.94 Impact Factor
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    ABSTRACT: A study on the axisymmetric near-contact motion of drops with tangentially mobile interfaces under the action of a body force in a quiescent fluid is described. A long-time asymptotic analysis is presented for small-deformation conditions. Under these conditions the drops are nearly spherical, except in the near-contact region, where a flattened thin film forms. According to our analysis, a hydrostatic dome does not form in the near-contact region at long times, in contrast to the assumption underlying all previous analyses of this problem. Instead, the shape of the film in the near-contact region results from the absence of tangential stresses acting on it. As a result, the long-time behaviour of the system is qualitatively different than previously predicted. According to the theory presented herein, the minimum film thickness (rim region) decays with time as \${h}_{m} \sim {t}^{- 4/ 5} \$, and the thickness at the centre of the film decays as \${h}_{0} \sim {t}^{- 3/ 5} \$, which is a faster decay than predicted by prior analyses based on a hydrostatic dome. Numerical thin-film simulations quantitatively confirm the predictions of our small-deformation theory. Boundary-integral simulations of the full two-drop problem suggest that the theory also describes qualitatively the long-time evolution under finite-deformation conditions.
    Journal of Fluid Mechanics 08/2013; 728. · 2.29 Impact Factor
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    ABSTRACT: Understanding interfacial mass transport on a droplet scale is essential for modeling liquid-liquid extraction processes. A model system is investigated whereby neodymium is extracted from buffered aqueous droplets to a continuous phase of dodecane, tributyl phosphate and thenoyltrifluoroacetone. A thin flow-focusing channel is used to generate monodisperse aqueous phase droplets of varying size, with drop diameter to channel height ratios ranging from 0.5 to 2. Liquid streamlines within the droplets are inferred by particle tracking, revealing a fountain flow recirculation pattern. Larger droplets that are more confined in the channel move with a slower velocity relative to the continuous fluid and have a stronger recirculation pattern in the plane of the microfluidic chip. Knowledge of the recirculation pattern is then applied to understand species mass transfer from the droplet fluid. Quantitative mass transfer measurements are obtained using high speed imagery and spectrophotometry for various flow rates and droplet sizes. This research is supported by the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.
    12 AIChE Annual Meeting; 10/2012
  • Jeremy B. Lechman, Martin B. Nemer, David R. Noble
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    ABSTRACT: Particle suspensions play an important role in many engineering applications, yet their behavior in a number of respects remains poorly understood. In conjunction with careful experiments, modeling and simulation of these systems can provide key insight into their complex behavior. However, these two‐phase systems pose the challenge of simultaneously, accurately, and efficiently capturing the complex geometric structure, kinematics, and dynamics of the particulate discrete phase and the discontinuities it introduces into the variables (e.g., velocity, pressure, density) of the continuous phase. To this end, a new conformal decomposition finite element method (CDFEM) is introduced for solid particles in a viscous fluid. The method is verified in several simple test problems that are representative of aspects of particle suspension behavior. In all cases, we find the CDFEM to perform accurately and efficiently leading to the conclusion that it forms a prime candidate for application to the full direct numerical simulation of particle suspensions. Copyright © 2012 John Wiley & Sons, Ltd.
    International Journal for Numerical Methods in Fluids 04/2012; 68(11):1409-1421. · 1.35 Impact Factor
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    ABSTRACT: A thin flow-focusing microfluidic channel is evaluated for generating monodisperse liquid droplets. The microfluidic device is used in its native state, which is hydrophilic, or treated with OTS to make it hydrophobic. Having both hydrophilic and hydrophobic surfaces allows for creation of both oil-in-water and water-in-oil emulsions, facilitating a large parameter study of viscosity ratios (droplet fluid/continuous fluid) ranging from 0.05 to 96 and flow rate ratios (droplet fluid/continuous fluid) ranging from 0.01 to 2 in one geometry. The hydrophilic chip provides a partially-wetting surface (contact angle less than 90°) for the inner fluid. This surface, combined with the unusually thin channel height, promotes a flow regime where the inner fluid wets the top and bottom of the channel in the orifice and a stable jet is formed. Through confocal microscopy, this fluid stabilization is shown to be highly influenced by the contact angle of the liquids in the channel. Non-wetting jets undergo breakup and produce drops when the jet is comparable to or smaller than the channel thickness. In contrast, partially-wetting jets undergo breakup only when they are much smaller than the channel thickness. Drop sizes are found to scale with a modified capillary number based on the total flow rate regardless of wetting behavior.
    Lab on a Chip 03/2012; 12(8):1540-7. · 5.70 Impact Factor
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    ABSTRACT: Understanding interfacial mass transport on a droplet scale is essential for modeling liquid-liquid extraction processes. A thin flow-focusing microfluidic channel is evaluated for generating monodisperse liquid droplets for microscale mass transport studies. Surface treatment of the microfluidic device allows creation of both oil in water and water in oil emulsions, facilitating a large parameter study of viscosity and flow rate ratios. The unusually thin channel height promotes a flow regime where no droplets form. Through confocal microscopy, this regime is shown to be highly influenced by the contact angle of the liquids with the channel. Drop sizes are found to scale with a modified capillary number. Liquid streamlines within the droplets are inferred by high speed imagery of microparticles dispersed in the droplet phase. Finally, species mass transfer to the droplet fluid is quantitatively measured using high speed imaging.
    11/2011;
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    ABSTRACT: The linear viscoelastic response of an ordered dense emulsion is explored by numerical simulation. At concentrations below maximum packing, the stress relaxation is dominated by a single time scale associated with lubrication, which diverges at maximum packing. For concentrations above maximum packing, the stress relaxation is dominated by fast time scales of the order of the drop relaxation time. A slow time scale appears but does not dominate.
    05/2007: pages 75-84;
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    ABSTRACT: From an analysis of tangent spherical drops in straining flow, Baldessari and Leal conclude that the drop-scale internal circulation, driven by the ambient flow, has a negligible influence on the drainage of the thin liquid film between drops under small-deformation conditions [F. Baldessari, L.G. Leal, J. Colloid Interface Sci. 289 (2005) 262]. However, their conclusion is incorrect as explained in this letter.
    Journal of Colloid and Interface Science 05/2007; 308(1):1-3. · 3.55 Impact Factor
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    ABSTRACT: We analyze axisymmetric near-contact motion of two drops under the action of an external force or imposed flow. It is shown that hydrodynamic stresses in the near-contact region that are associated with the outer (drop-scale) flow can qualitatively affect the drainage of the thin fluid film separating the drops. If this far-field stress acts radially inward, film drainage is arrested at long times; exponential film drainage occurs if this stress acts outward. An asymptotic analysis of the stationary long-time film profile is presented for small-deformation conditions, and the critical strength of van der Waals attraction for film rupture is calculated. The effect of an insoluble surfactant is also considered. Hindered and enhanced drop coalescence are not predicted by the current theories, because the influence of the outer flow on film drainage is ignored.
    Physical Review Letters 04/2004; 92(11):114501. · 7.73 Impact Factor
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    ABSTRACT: 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 assumptions have been used in calculations of collisional efficiencies and analyses of experimental results. Our investigations show that for drops with tangentially mobile interfaces the above assumptions do not hold. The velocity field produced inside the drops by the external flow couples to the film motion through tangential stress f (infinity) acting on the film interface. For sufficiently thin films (e.g., long times), this stress qualitatively alters the dynamics of the lubrication region by arresting or enhancing film drainage.
    01/2002;
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    ABSTRACT: At long times, a thin liquid film between two deformable drops forms a central dimpled region which is separated from an outer hydrostatic region by a narrow rim region where the film thickness is minimal. We present a long-time asymptotic analysis of this problem. Previous scaling analyses were based on the assumption that the dimple assumes a hydrostatic shape at long times. Matching of the hydrostatic dimple to the inner rim solution requires that the film thickness in the matching region varies linearly with the distance x from the minimal gap. However, our solution of the integro-differential equation describing the rim indicates that nonlocal contributions give rise to a film thickness that varies as x^1/2. The dimple shape is governed by another integro-differential equation that minimizes the stresses due to the flow inside the drops. Based on the new matching conditions, we find that the central gap decreases as t-3/5 and the minimum gap as t-4/5, in contrast to the results of earlier studies. Evidence from thin film numerical simulations supports our assertions.
    11/2001;
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    ABSTRACT: Imported oil exacerabates our trade deficit and funds anti-American regimes. Nuclear Energy (NE) is a demonstrated technology with high efficiency. NE's two biggest political detriments are possible accidents and nuclear waste disposal. For NE policy, proliferation is the biggest obstacle. Nuclear waste can be reduced through reprocessing, where fuel rods are separated into various streams, some of which can be reused in reactors. Current process developed in the 1950s is dirty and expensive, U/Pu separation is the most critical. Fuel rods are sheared and dissolved in acid to extract fissile material in a centrifugal contactor. Plants have many contacts in series with other separations. We have taken a science and simulation-based approach to develop a modern reprocessing plant. Models of reprocessing plants are needed to support nuclear materials accountancy, nonproliferation, plant design, and plant scale-up.