Principal Investigator

Arezoo Motavalizadeh Ardekani
O’Hara, C.S.C., Assistant Professor
369 Fitzpatrick Hall
Notre Dame, IN 46556


Dr. Ardekani is currently an assistant professor at the University of Notre Dame. Prior to joining Notre Dame, she was a Shapiro Postdoctoral Fellow at the Massachusetts Institute of Technology. She graduated from University of California Irvine with her Ph.D. in 2009. She received the Society of Women Engineers and Amelia Earhart awards in 2007, Schlumberger Foundation faculty for the future grant in 2009, and NSF CAREER award in 2012. Her research interests are fluid dynamics, complex fluids, biofluids, micro/nanofluids, and multiphase flows that are relevant to biomedical devices, energy, and environmental applications.

Settling in stratified fluids

Funded by NSF Grants CBET-1066545 and CBET-1414581

The transient settling dynamics of a spherical particle sedimenting in a linearly stratified fluid is investigated by performing fully resolved direct numerical simulations. The settling behaviour is quantified for different values of Reynolds, Froude and Prandtl numbers. It is demonstrated that the transient settling dynamics is correlated to the induced Lagrangian drift of flow around the settling particle. The peak velocity of the particle can be followed by the oscillation of the settling velocity and the particle can even reverse its direction of motion before reaching to its neutrally buoyant level. The frequency of oscillation of settling velocity scales with the Brunt–Väisälä frequency and the motion of the particle can lead to the formation of secondary and tertiary vortices following the primary vortex.

  • A. Doostmohammadi, S. Dabiri, A.M. Ardekani, “A numerical study of the dynamics of a particle settling at moderate Reynolds numbers in a linearly stratified fluid,” accepted, Journal of Fluid Mechanics, 2014.

Gyrotactic Bioconvection at Pycnoclines

Funded by NSF Grant No. CBET-1066545

Bioconvection is a complex biological phenomenon causing spontaneous pattern formation and self-organization in the suspensions of motile bacteria and algae. It occurs as a result of the collective behavior of up-swimming microorganisms in response to the certain types of physical stimuli. We are interested in the special case of gyrotaxis where the swimming is directed by the balance of the viscous and gravitational torques. We investigate gyrotactic bioconvection in presence of stratification arising from thermal or solutal gradients in aquatic environments. Using large-scale numerical simulations, we explore different regimes of the flow by varying the boundary conditions and the strength of stratification. Also, we discuss the inhibition threshold of bioconvection in light of a linear stability analysis. The result can shed light on the characteristics of double-diffusive convection engendered by active swimmers in a stratified environment.

Bacterial aggregation and biofilm formation in a vortical flow

Funded by NSF Grant No. CBET-1150348- CAREER

Microbial habitats are rarely at rest (e.g. ocean, blood stream, flow in porous media and flow through membrane filtration processes). In order to study the hydrodynamics of bacterial response in a vortical flow, we utilize a microfluidic system to mimic some of the important microbial conditions at ecologically relevant spatiotemporal scales. We experimentally demonstrate the formation of “ring”-shaped bacterial collection patterns and subsequently the formation of biofilm streamers in a microfluidic system. Acoustic streaming of a microbubble is used to generate a vortical flow in a microchannel. Due to bacteria’s finite-size, the microorganisms are directed to closed streamlines and trapped in the vortical flow. The collection of bacteria in the vortices occurs in a matter of seconds and, unexpectedly, triggers the formation of biofilm streamers within minutes.

Particle Interaction in Stratified Fluids

Funded by NSF Grant No. CBET-1066545

The anisotropic structure of fluidized suspensions is governed by their microstructures which are in turn determined by the dynamics of particle pair interactions. In this study, we present numerical simulations of particle interaction in linearly stratified fluids. It is shown that unlike homogeneous fluids, stratification results in attraction of particles settling abreast. The interaction of the particles settling in tandem can be fundamentally altered due to the presence of the background density gradients and the drafting-kissing-tumbling behavior in a homogeneous fluid can be replaced by drafting-kissing-separation or drafting-separation phenomenon depending on the strength of the stratification. In case of weak stratification, drafting-kissing-tumbling occurs, however, a prolonged kissing time is observed and the rate of change of the orientation of particles is reduced. Attraction of particles settling side-by-side and prolonged kissing time for particles settling in tandem can lead to accumulation of particles and aggregation of organisms at local hot spots in aquatic environments characterized by density stratification.

Flow-induced aggregation of microorganisms in polymeric fluids

Funded by NSF Grant No. CBET-1150348- CAREER

Spatial distribution of microorganisms
in a vortical flow of a complex fluid.

We have shown that the rheological properties of exopolysaccharides secreted by microorganisms play an important role in their interaction with background flow field. The normal stresses generated due to the presence of polymer molecules lead to aggregation of microorganisms in a vortical flow field. The shape and formation rate of these aggregation patterns depend on motility, vorticity, and rheological properties of exopolysaccharides. Given the viscoelastic nature of extracellular polymeric substances, these results suggest new mechanisms for ubiquitous processes among microorganisms, such as bacterial aggregation and biofilm formation.

Unsteady propulsion

Funded by NSF grant No. CBET-1066545

Propulsion speed of small organisms is strongly
affected by unsteady hydrodynamic forces.

Small planktonic organisms ubiquitously display unsteady or impulsive motion to attack a prey or escape a predator in natural environments. Despite this, the role of unsteady forces such as history and added mass forces on the low Reynolds number propulsion of small organisms, e.g. Paramecium, is poorly understood. In this paper, we derive the fundamental equation of motion for an organism swimming by the means of surface distortion in a nonuniform flow at a low Reynolds number regime. We show that the history and added mass forces are important as the product of Reynolds number and Strouhal number increases above unity.

  • 2- S. Wang, A.M. Ardekani “Unsteady swimming of small organisms,” Journal of Fluid Mechanics, 2012.

Bio-locomotion in stratified fluids

Funded by NSF grant No. CBET-1066545

Flow field generated by small organisms is
markedly affected by stratification.

Swimming of microorganisms, a topic of long-standing interest for both biologists and physicists, has been mostly studied in homogeneous fluids. However, many aquatic environments, including oceans, lakes and the interstitial fluid in sea ice, are routinely stratified, due to salt- or temperature-induced variations in fluid density. We have analytically showed that stratification dramatically alters the flow field induced by the organism, generating recirculation cells in the velocity field. The size of these cells scales with Ra-1/4 where the Rayleigh number, Ra, measures the relative importance of buoyancy and diffusion. For stronger stratification, cells are more compressed and the velocity field decays faster with distance from the organism. These less conspicuous flow fields could affect those predator-prey interactions based on sensing of hydromechanical signals, potentially acting as a ‘silencer’ for swimming prey or a ‘stealth’ mechanism for approaching predators. We use a combination of methods that range from simple scaling laws to detailed computational models and sophisticated experimental equipments to explore the effects of stratification on swimming of small organisms.

  • A. Doostmohammadi, R. Stocker, A.M. Ardekani “Low Reynolds number swimming at pycnoclines,” Proceedings of the National Academy of Sciences, Volume 109, 3856-3861, 2012.
  • A.M. Ardekani, R. Stocker “Stratlets: low Reynolds number point-force solutions in a stratified fluid” Physical Review Letters, 105, 084502, 2010.
  • A.M. Ardekani, R. Stocker “Swimming at low Reynolds number in a stratified fluid” 16th US National Congress of Theoretical and Applied Mechanics, June 17-July2, 2010, State College, PA.

Instability and breakup of viscoelastic jets

Funded by Schlumberger

Jet Extensional Rheometry enables evaluation of
transient extensional rheological properties for even
very weakly elastic fluids.

Understanding the instability and breakup of polymeric jets is important for a wide variety of applications including inkjet printing, and spraying of fertilizers and paint. Such fluids are typically only weakly viscoelastic and the jetting/breakup process involves a delicate interplay of capillary, viscous, inertial and elastic stresses. The initial growth of disturbances can be predicted using linear instability analysis for small perturbations. A viscoelastic jet is initially more unstable when compared to a Newtonian fluid of the same viscosity and inertia. As the radius of the jet thins under the action of surface tension, elastic stresses grow and become comparable to the capillary pressure, leading to formation of a uniform thread connecting two primary drops. This beads-on-a-string structure can be captured by the Oldroyd-B model, and the radius of the thin cylindrical ligament connecting the beads necks down exponentially in time. The finite time breakup of the jet observed experimentally can be captured using the nonlinear Giesekus model. We show that by understanding the physical processes that control each phase of the temporal evolution in the jet profile it is possible to extract transient extensional viscosity information even for very low viscosity and weakly-elastic liquids. This is especially useful since filament-stretching and capillary breakup elongational rheometers face challenges for low-viscosity elastic polymer solutions.

  • A.M. Ardekani, V. Sharma, G.H. McKinley, “Dynamics of Bead Formation, Filament Thinning, and Breakup in Weakly Viscoelastic Jets,” Journal of Fluid Mechanics, Volume 665, 46-56, 2010.
  • V. Sharma, A.M. Ardekani, G.H. McKinley “‘Beads on a String’ Structures and Extensional Rheometry using Jet Break-up” 5th Pacific Rim Conference on Rheology, August 1-6, 2010, Japan.
  • A.M. Ardekani, V. Sharma, G.H. McKinley “Jetting and breakup of weakly viscoelastic liquids” 16th US National Congress of Theoretical and Applied Mechanics, June 17-July2, 2010, State College, PA.

Past Projects

Particles Interaction, Deformation, and Collision in Viscous and Viscoelastic Fluids, funded by NSF grant No. CBET-0828104

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