(6fa) From Understanding the Transport of Complex Fluids to Development of Liquid Lenses

My past and current research work involves extensive numerical simulations of a ring-sheared drop where the air-liquid interface of the drop is laden with surfactants. In the ring-sheared drop, a drop is constrained between two rings. One of the rings rotate and the other is held stationary. The surfactants cause the air-liquid interface to behave in a viscous manner. An interfacial flow is created when the rotating ring shears the viscous air-liquid interface. Further, this interfacial flow drives flow in the bulk of the drop. The ring-sheared drop is an experiment being developed by the Hirsa lab at Rensselaer Polytechnic Institute (RPI) in collaboration with NASA, to study the formation of protein amyloid fibrils aboard the International Space Station. Amyloid fibrils are the waxy plaques which destroy the neurons of people with Alzheimer's disease. Shear stress has been shown to significantly accelerate the formation of amyloid fibrils.

A part of my Ph.D. work at RPI involved the study of formation and constraining of drops between two rings using Earth-based and microgravity-based experiments and computations. Finite Element Modeling (FEM) was used to predict the drop shape and the flow field pertaining to drops grown with and without Earth's gravity. The numerical results were benchmarked against experimental data associated with the formation of pendant and sessile drops.

My Ph.D. work also included a numerical study of the flow that can be created in a ring-sheared drop. Application to mixing within drops was also shown. The surface shear viscosity was assumed to be large in this study. A strong flow in the bulk of the drop and the subsequent mixing was demonstrated in this work. Further, during my postdoctoral work at RPI, I implemented a more rigorous modeling of the air-liquid interface in the ring-shear drop. It involved smaller yet finite surface shear viscosities as compared to the large surface shear viscosity considered in the previous work. This work showed that even a small surface shear viscosity can lead to a strong flow in the bulk. Also, the flow field for various non-dimensional ring rotation speeds and surface shear viscosity was studied in detail.

Currently, my colleagues from the Hirsa lab group and I are working on studying the deformation of ring-sheared drops in a simulated microgravity environment using experiments and numerical simulations. Simulated microgravity experiments were performed in the lab using a density matched silicone oil-salt water system. The experimental velocity field at the silicone oil-salt water interface agrees with the numerical prediction. We intend to develop an understanding of how the various parameters i.e. rotation rate of the ring, bulk viscosity of the oil, surface shear viscosity and surface tension, impact the drop deformation in such a simulated microgravity environment. We plan to submit a manuscript on the same very soon.

Future research direction:

Several pharmaceutical drugs contain recombinant proteins which are used to target diseases such as flu, tuberculosis and rabies. These proteins are expressed by microorganisms and are generally formulated in a buffer solution. Proteins are then concentrated using techniques such as tangential flow filtration and centrifugal filtration. However, passing the protein solution through a tangential flow filtration system or subjecting it to centrifugal forces can lead to high shear stresses. Protein molecules subjected to high shear stresses are at a risk of becoming unfolded leading to formation of aggregates or fibrils which results in the loss of functionality of the recombinant protein molecules. However, evaluating the shear stresses in such situations is challenging as the protein solution is a complex fluid and can behave as a non-Newtonian fluid especially at higher concentrations. The effect of protein concentration on the viscosity of the protein solution is not well understood. Increasing concentration can affect the interaction between the molecules. Through numerical simulations performed by my research group and with collaboration with experimentalists, I would like to understand how the interactions between molecules contribute to a change in viscosity of the bulk protein solution. Further, tangential flow filtration utilizes hollow fiber membranes or filter cassettes and the flow physics not only involves flow through a porous channel but also involves additional phenomenon such as cake formation and pore blockage which can be hard to model. Current models are based on empirical correlations. In the future, I would like to develop predictive models which minimize the requirement of performing prior experiments to characterize the cake formation and pore blockage while still representing the fluid mechanics that occurs at the molecular level.

Another area of interest is liquid lenses. Liquid lenses have the ability to provide variable focus with a fast response. Also, their frequency response can be tuned which makes them suitable for several imaging applications including adaptive optics. I worked as a co-principal investigator with Dr. Garrett Milliron from the Mackinac Technology Company, Dr. Amir Hirsa at RPI, Dr. Guoyu Lu at Rochester Institute of Technology to conceptualize a novel bio-inspired liquid lens attachment. This attachment is intended to be used for measuring spatial dimensions from a mobile phone camera. Recently, a proposal on this idea was submitted to the National Science Foundation under the Small Business Innovation Research (SBIR) program and is currently under review.

Teaching Interests:

During my role as a Lecturer at RPI, I have taught undergraduate-level courses on thermodynamics, heat transfer, fluid mechanics, engineering mechanics and linear algebra. Half of these courses were lecture-based courses and the remaining half were laboratory-based courses. In the future, I would like to not only teach similar courses but also develop and teach new courses and curriculum related to transport phenomena and thermal sciences.

Outreach:

I have mentored a few undergraduate students in performing research in the past. In the future, besides working with graduate students in my research group, I would also like to motivate undergraduate students and even high school students to join the research group and contribute to the advancement of science and technology.

Accepted Journal Publications:

1. Gulati, S., Riley, F.P., Lopez, J.M., and Hirsa, A.H. Flow in a containerless liquid system: Ring-sheared drop with finite surface shear viscosity, Phys. Rev. Fluids 4, 044006 (2019).
2. Gulati, S., Riley, F.P., Lopez, J.M., and Hirsa, A.H. Mixing within drops via surface shear viscosity, Int. J. Heat and Mass Transf. 125, 559-568 (2018).
3. Gulati, S., Raghunandan, A., Rasheed, F., McBride, S.A., and Hirsa, A.H. Ring-sheared drop (RSD): Microgravity module for containerless flow studies, Microgravity Sci. Technol. 29, 81-89 (2017).

Manuscript in preparation:

1. McMackin, P. M., Griffin, S. R., Riley, F. P., Gulati, S., Debono, N. E., Raghunandan, A., Lopez, J. M. and Hirsa, A. H. Simulated microgravity in the ring-sheared drop, npj Microgravity (2019).