Research Interests

The focus of my research is to understand multiphase flows with surface tension gradients and elastic effects using analytical, numerical, and experimental tools.  The prediction of two-phase flow of Newtonian fluids is difficult because the location of the interface between the two immiscible phases is not known a priori and needs to be determined as part of the solution. The fundamental issues of how surface tension gradients affect free surface flows, how surfactants can be added to control flows, and how elasticity in the bulk phase changes the deformation and break up of a dispersed fluid phase are explored in confined domains such as a rectangular or cylindrical capillary. More recently I have started focusing on applying my expertise of two-phase flows for particle-laden flows.  In particular, I am interested in developing techniques to synthesize nanoparticles and microparticles of controlled size and shape for biochemical and biomedical applications.  Some of the ongoing projects in my research group are:

Continuous Flow Synthesis of Nanoparticles

Collaborator: Dale Barkey

Nanoparticles made of precious metals or metal oxides are useful for a variety of applications such as inks, energy generation, and biomedical therapies. Reactions performed in batch reactors generate nanoparticles with a large particle size distribution due to concentration and temperature variations within the reactor. Microfluidic reactors are ideal for continuous synthesis of uniform nanoparticles due to their small volume and high surface-area to volume ratio. We are developing techniques for continuous synthesis of nanoparticles using microfluidic or millifluidic channels which provide precise control of particle size, shape, and particle size distribution. Furthermore, we measure and characterize the rheology of these suspensions for processing of particle-laden flows such as screen printing and drop breakup.

  • Zukas, B. G. and Gupta, N. R., Interphase synthesis of zinc oxide nanoparticles in a droplet flow reactor,'' Ind. Eng. Chem.  Res., 56, 7184−7191 (2017).  DOI: 10.1021/acs.iecr.7b00407

ZnO nanoparticles

  • Hemmati, S,. Barkey, D. P., Eggleston, L., Zukas, B., Gupta, N., and Harris, M. Silver nanowire synthesis in a continuous millifluidic reactor,'' ECS J. Solid State Sci. Technol., 6(4), P144-P149 (2017). DOI: 10.1149/2.0171704jss
  • Hemmati, S., Barkey, D. P, and Gupta, N., Rheological behavior of silver nanowire conductive inks during screen printing,'' J. Nanopart. Res., 18, 249 (2016). DOI: 10.1007/s11051-016-3561-4
  • Hemmati, S., Barkey, D. P, Gupta, N., and Banfield, R., Synthesis and characterization of silver nanowire suspensions for printable conductive media,'' ECS J. Solid State Sci. Technol., 4(4), P3075-P3079 (2015). DOI: 10.1149/2.0121504jss

Capsule Dynamics in Channels

Manipulation of single cells in microfluidic devices has gained momentum in the last decade with the development of novel microdevices for in vitro fertilization, cell-culture studies, forensic analysis, and diagnosis of diseases such as cancer, diabetes, and malaria. The overall research objectives of this project are to determine: (a) the effect of inertia on the three-dimensional non-axisymmetric motion of capsules in channels, (b) the effect of a non-uniform confining wall on capsule dynamics, and (c) the effect of the presence of another capsule in the vicinity.

  • Carroll, R. M. and Gupta, N. R., Inertial effects on the flow of capsules in cylindrical channels,'' Int. J. Multiphase Flow, 87, 114-123 (2016).  DOI: 10.1016/j.ijmultiphaseflow.2016.07.016

Stress in a Capsule

Motion of drops and bubbles in confined domains at finite Reynolds numbers

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.

  • Li, W. and Gupta, N. R., "Buoyancy-driven motion of bubbles in the presence of soluble surfactants in a Newtonian fluid," Ind. Eng. Chem. Res., Article ASAP (2019).  DOI: 10.1021/acs.iecr.8b04788
  • Cui, Y. and Gupta, N. R., Numerical study of surfactant effects on the buoyancy-driven motion of a drop in a tube,'' Chem. Eng. Sci., 144, 48-57 (2016).  DOI: 10.1016/j.ces.2016.01.020
  • Carroll, R. M. and Gupta, N. R. Inertial and surfactant effects on the steady droplet flow in cylindrical channels,'' Phys. Fluids, 26, 122102 (2014).   DOI: 10.1063/1.4902902

Re effect

  • Cui, Y. and Gupta, N. R. Surfactant effects on drop formation in co-flowing fluid streams,'' Colloids and Surfaces A: Physicochem. Eng. Aspects, 393, 111-121 (2012).  DOI: 10.1016/j.colsurfa.2011.11.008
  • Cui, Y. and Gupta, N. R. Drop formation in co-flowing fluid streams,'' Int. J. Trans. Phenomena, 12(3-4), 217-226 (2011).
  • Li, J., Bulusu, V., and Gupta, N. R., Buoyancy-driven motion of bubbles in square channels,'' Chem. Eng. Sci., 63(14), 3766-3774 (2008).  DOI: 10.1016/j.ces.2008.04.041

Thermocapillary Flow in Double-layer Fluid Structures  

Collaborator: A. Borhan, H. Haj-Hariri

Thermocapillary flows are of considerable technological importance in materials processing applications such as crystal growth particularly under microgravity conditions where the influence of buoyancy-driven convection is minimized. We numerically analyze the thermally-driven convection within a differentially-heated rectangular or cylindrical cavity containing two immiscible liquid layers in the absence of gravity. We have developed a single-layer model which accurately predicts the flow in the double-layer system even for large aspect ratios.

  • Gupta, N. R., Haj-Hariri, H., and Borhan, A. , Effect of free surface heat transfer on thermocapillary flow in double-layer fluid structures,'' Heat Mass Transfer , 50, 333-339 (2014).  DOI: 10.1007/s00231-013-1268-y
  • 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).  DOI: 10.1016/j.jcis.2007.06.017