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Two-phase flows in the presence of surfactant and elastic effects are encountered in a variety of emerging technologies such as biomedical applications in microfluidic devices, creation of aerosols for pulmonary therapies, and crystal growth in microgravity environment. Our research aims to understand multiphase flows of Newtonian and viscoelastic fluids using analytical, numerical, and experimental tools. The presence of surface tension gradients at the free surface can significantly modify the flow or in some cases, induce flow in the system. Surface tension gradients (Marangoni effects) are created by either temperature gradients along the interface or the presence of surface-active agents (surfactants). We study how surfactants and temperature gradients affect the dynamics of a fluid-fluid interface in various geometries. Some of the ongoing projects in my research group are:

Some of the ongoing projects in my research group are:

   Surfactant and Elastic Effects in Microfluidic Devices
     Sponsor: NSF-CTS

Tremendous progress has been made in the development of microfluidic devices in the last decade with applications such as a "lab-on-a-chip", ink-jet printer heads, cell sorters, DNA sequencers, and blood-testing systems. Microdroplets are being used in microfluidic devices as actuators, chemical microreactors, drug delivery devices, and bioterror detection tools. Typical applications of two-phase systems in microdevices involve viscoelastic fluids such as DNA solutions, blood, serum, and inks and suspensions with surfactants added as stabilizers. The stretching of polymer chains present in viscoelastic fluids causes large normal stresses to develop at deforming interfaces affecting the dynamics of drops. Surfactant molecules adsorbed at the interface can redistribute along the interface leading to a local reduction in surface tension. The major research objectives of this study are to identify parameters governing the shape and velocity of drops translating in rectangular microchannels and the effect of elastic stresses and surfactant mass transfer kinetics on the dynamics of drops in the microchannel. Microfluidic devices are fabricated using soft lithography techniques in our research laboratory (Figure 1). The deformation of drops and bubbles translating in a straight channel in the presence of surfactants and/or elasticity are studied in microchannels of varying sizes and cross-sections (Figure 2).

Figure 1: A (a) T-junction (80 microns to 40 microns), (b) diverging section (50 microns to 80 microns), and (c) SEM micrograph of the sidewall (50 microns high) of microdevices fabricated in our laboratory.

Figure 2: An (a) air bubble and (b) a train of water drops translating in a microdevice filled with silicone oil.

   Thermocapillary motion of drops in a Hele-Shaw cell
     Collaborator: A. Borhan

The flow of two immiscible fluids in a Hele-Shaw cell (two parallel plates with a very narrow gap) is of fundamental importance since, under appropriate conditions, it can serve as a model problem for the study of two-phase flow through porous media. A liquid drop in the absence of any driving force takes on a circular shape. The stability of a buoyancy-driven drop initially perturbed from a circular shape shows two regions of marginal stability. In the main branch, the drops with higher interfacial tension are more stable for the same initial deformation. In addition, a secondary branch of marginal stability is obtained where highly deformed drops with lower surface tension are found to stabilize. In the presence of a temperature gradient within a Hele-Shaw cell, the variations of interfacial tension along the interface induce Marangoni stresses that determine the shape and migration velocity of the drop. The effect of Marangoni stresses on drop deformation and mobility is taken into account using a physically based expression for the depth-averaged tangential stress exerted on the interface. It is found that Marangoni stresses induced by thermocapillarity has a destabilizing effect on the main branch of stability, that is, drops with the same initial deformation and lower interfacial tension break in the presence of temperature gradients. The stability of the drops can be increased by increasing the thermal conductivity or the viscosity of the drop phase as compared to the bulk phase. The effect of thermocapillarity on the stability of drops in the secondary branch is currently being pursued.

    Motion of drops and bubbles in confined domains at finite Reynolds numbers
     Sponsor: NSF-CTS

The deformation and breakup of drops in a confined fluid medium is a problem of great practical importance in a variety of natural and physical processes such as motion of blood cells in capillaries as well as a host of industrial applications such as polymer processing, two-phase flows in microfluidic devices. We are conducting a combined experimental and numerical study of the three-dimensional motion of a drop/bubble in capillaries and channels of varying length scales. Numerically, the coupling between the momentum balance and the surfactant balance in two-phase flows will be handled using a hybrid volume-of-fluid (VOF) algorithm developed in our group. Unlike the VOF method, the interface is tracked explicitly as a Lagrangian grid and this scheme ensures that the shape and the normals and curvatures at the interface are represented accurately. Hence, the numerical technique can model strongly deforming interfaces with large density and viscosity ratios. Many fluids of interest such as glass-forming liquids, polymer melts and solutions, and micellar solutions exhibit viscoelastic behavior. Unlike Newtonian fluids, viscoelastic materials have the ability to store energy to some extent and thereby display partial recovery upon the removal of applied stress. Many interesting two-phase phenomena such as two-dimensional cusps at the trailing end of bubbles and velocity discontinuity have been attributed to the elasticity of the fluid phases. Experiments and numerical studies of the effects of elasticity on the motion of fluid particles in confined domains is currently underway. Being able to predict the mobility and deformation of viscoelastic drops in confined domains helps us to choose appropriate operating conditions to stabilize (as in emulsions) or eliminate (as in polymer systems) fluid droplets in a bulk fluid medium.

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Graduate Students

Yuanyuan Cui (PhD: Expected 2011)
Weihua Li (MS: Expected 2011)

Undergraduate Students

Xiameng Liu (BS: Expected 2011)
Brian Zukas (BS: Expected 2012)

Former Students

Michael O'Connor (MS 2009) - Wyeth
Vinod Bulusu (MS 2006) - Amgen Inc.
Glareh Azadi (BS 2008) - PhD at Brown
Scott Luczko (BS 2008) - Woodward and Curran
Jingyan Li (BS 2007) - PhD at UCLA
Eric C. Beauregard (BS 2006) - Aspen Aerogels

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    Gupta, N. R., Haj-Hariri, H., and Borhan, ``Effect of heat transfer on the thermocapillary flow in double-layered fluid structures,'' J. Colloid Interface Sci., submitted (2010).

    Cui, Y. and Gupta, N. R., ``Drop formation in co-flowing fluid systems,'' Annals N. Y. Acad. Sci., submitted (2010).

    Li, J., Bulusu, V., and Gupta, N. R., ``Buoyancy-driven motion of bubbles in square channels,'' Chem. Eng. Sci., 63(14), 3766-3774 (2008).

    Gupta, N. R., Haj-Hariri, H., and Borhan A., ``Thermocapillary convection in double-layer fluid structures within a two-dimensional open cavity,'' J. Colloid Interface Sci., 315(1), 237-247 (2007).

    Gupta, N. R., Haj-Hariri, H., and Borhan A., ``Thermocapillary Flow in Double-Layer Fluid Structures,'' Ann. N. Y. Acad. Sci., 1077, 395-414 (2006).

    Jin, F., Gupta, N. R., and Stebe, K. J., ``The detachment of a viscous drop in a viscous solution in the presence of a soluble surfactant,'' Phys. Fluids, 18(2), 022103 (2006).

    Gupta, N. R., Haj-Hariri, H., and Borhan A., ``Thermocapillary Flow in Double-Layer Fluid Structures: An Effective Single-Layer Model,'' J. Colloid Interface Sci., 293(1), 158-171 (2006).

    Gupta, N. R., Balasubramaniam, R., and Stebe, K. J., ``Rapidly expanding viscous drop from a submerged orifice at finite Reynolds numbers,'' Ann. N. Y. Acad. Sci., 974 , 398-409 (2002).

    Gupta, N. R., Nadim, A., Haj-Hariri, H., and Borhan, A., ``A Numerical study of the effect of insoluble surfactants on the stability of a viscous drop translating in a Hele-Shaw cell,'' J. Colloid Interface Sci., 252(1), 236-248 (2002).

    Gupta, N. R., Nadim, A., Haj-Hariri, H., and Borhan, A., ``Stability of the shape of viscous drops in a Hele-Shaw cell,'' J. Colloid Interface Sci., 222 (1), 107-116 (2000).

    Gupta, N. R., Nadim, A., Haj-Hariri, H., and Borhan, A., ``On the linear stability of a circular drop translating in a Hele-Shaw cell,'' J. Colloid Interface Sci., 218, 338 (1999).


    Borhan, A. and Gupta, N. R., ``Capsule motion and deformation in tube and channel flow,'' in Modeling and Simulation of Capsules and Biological Cells, Ed. C. Pozrikidis, Chapman & Hall / CRC (2003).

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Last Updated: April 20, 2010