My research spans classical fluid dynamics theory and experimental microfluidics involving multi-physics heat and mass transfer, with an emphasis on numerical simulations. The applications of my research range from modeling and predicting biophysical flow conditions such as those that could possibly lead to aneurysms and designing novel devices for measuring the electrokinetic potentials of particles and surfaces to engineering new systems for efficient evaporative cooling and/or water purification.
Vortex dynamics in branching and bending flows
I use experiments and numerical simulations to study the nonlinear vortex breakdown phenomenon. We recently discovered recirculation zones in simple branching junctions that have all the classical signatures of vortex breakdown. I use simulations to study how these features originate and evolve as flow properties change.
I also use experiments to study how these vortex breakdown recirculation zones interact with bubbles and particles in multi-phase flows. I especially study the effects of geometry on the dynamical flow features and how flow conditions can be used to maximize or eliminate particle/bubble capture in these zones (Video
). I am also interested in the practical applications of this new capture mechanisms and recently demonstrated a shear-induced rapid technique for producing large lipid vesicles using this vortex trapping.
- Ault, J. T., Fani, A., Chen, K. K., Shin. S., Gallaire, F., and Stone, H. A. Vortex-breakdown-induced particle capture in branching junctions. Physical Review Letters (2016), vol. 117, no. 8, pp. 1-5.
- Shin., S.*, Ault, J. T.*, and Stone, H. A. Flow-driven rapid vesicle fusion via vortex trapping. Langmuir (2015), vol. 31, pp. 7178-7182. *The authors contributed equally to this work.
Classical fluid dynamics in pipe flows
Part of my research involves the theoretical modeling and simulation of developing flows in the entrance and exit regions of a curved pipe. In particular, I have developed asymptotic analytical solutions for the developing flow in a curved pipe and for the transitioning flow downstream of a curved pipe. More recently, I am working to develop an analytical solution governing the spatial decay of any general 3D pipe flow perturbation.
- Ault, J. T., Chen, K. K., and Stone, H. A. Downstream decay of fully developed Dean flow. Journal of Fluid Mechanics (2015), vol. 777, pp. 219-244.
- Ault, J. T., Rallabandi, B., Shardt, O., Chen, K. K., and Stone, H. A. Entry and exit flow in curved pipes. Accepted for publication in Journal of Fluid Mechanics.
- Ault, J. T., Rallabandi, B., and Stone, H. A. Axial decay of perturbed steady state pipe flows. In preparation.
Diffusiophoretic particle motions in dead-end pores
My research also involves the study and simulation of complex flows experiencing multiple simultaneous transport processes occurring over a wide range of scales, such as the diffusiophoretic motion of particles in dead-end pores with concentration gradients. Recently we used experiments and simulations to show that the penetration of particles into dead-end pores can be driven and controlled using solute gradients.
- Shin, S., Um, E., Sabass, B., Ault, J. T., Rahimi, M., Warren, P. B., and Stone, H. A. Size-dependent control of colloid transport via solute gradients in dead-end channels. Proceedings of the National Academy of Sciences (2016), vol. 113, no. 2, pp. 257-261.
- Shin, S., Ault, J. T., Feng, J., Warren, P. B., and Stone, H. A. Low-cost zeta potentiometry using solute gradients. Under review for publication in Nature Materials.
Evaporative cooling and water purification using thin-film flows over superhydrophilic surfaces
One issue that I am especially passionate about is water scarcity. According to the World Health Organization, nearly 1 billion people do not have reliable access to safe drinking water. The water scarcity index reported by the United Nations Environment Programme
shows that many millions of people live in regions where water use exceeds minimum recharge levels. One project that I have been working on is to redesign a simple solar still to enhance efficiency. Our specific approach involves using superhydrophilic Boehmite-coated aluminum wire meshes to create thin film continuous flows which can potentially enhance evaporation rates and therefore improve efficiency and production rates of solar stills.
Dynamics of bubble bouncing at a liquid/liquid/gas interface
Previously, I also studied the dynamics of an air bubble bouncing at a compound liquid/liquid/gas interface. The presence of an oil layer affects the interfacial properties, and thus modifies the entire bubble bouncing/bursting process. Specifically, we experimentally studied the bubble/fluid motions during bubble bouncing for a range of oil layer viscosities, thickness, and surface tensions. We also explored the influence of bubble size and approach velocity. I specifically developed reduced-order coupled mass-spring-damper models to describe bubble bouncing at a compound interface which accurately predicts both the contact time and coefficient of restitution of the first bubble bounce.
- Feng, J., Muradoglu, M., Kim, H., Ault, J. T., and Stone, H. A. Dynamics of a bubble bouncing at a liquid/liquid/gas interface. Accepted for publication in Journal of Fluid Mechanics.
Point-source imbibition into dry aqueous foams
Another project that I worked on involved using experiments, modeling, and numerical simulations to study imbibition from a point source into a dry homogeneous aqueous foam. The dynamics is driven by the capillary pressure in the liquid microchannels of the foam (Plateau borders) and is resisted by both viscous and gravitational forces. Specifically, we showed that the imbibition front tends to flatten out over time due to gravitational effects, an effect which can be quantified through the use of the Bond number, which compares the gravitational effects to the capillary pressure. We concluded that both the ratio between the oil-water and air-water interfacial tensions and the Bond number together determine the imbibition efficiency.
- Mensire, R., Ault, J. T., Lorenceau, E., and Stone, H. A. Point-source imbibition into dry aqueous foams. Europhysics Letters (2016), vol. 113, no. 4, 44002.
Diffusive behaviors of circle-swimming nanomotors
Another project that I have worked on involves the locomotion of catalytic nanomotors. Synthetic nanomotors are frequently being developed in an attempt to better understand and/or mimic nanoscale biomotors that are present in biological systems. Bimetallic nanomotors have been created which can swim many body lengths per second as well as pick up, manipulate, and drop off microscale cargo under the action of applied magnetic fields. We studied the short- and long-time diffusivities of gold/platinum coated bimetallic nanomotors, and compared the motors' behaviors with Brownian dynamics simulations and analytical theory.
- Marine, N. A., Wheat, P. M., Ault, J. T., and Posner, J. D. Diffusive behaviors of circle-swimming motors. Physical Review E (2013), 87(5), 052305.