** Post Doctoral Associates **

## the Zia Research Group |

** Post Doctoral Associates **

Dr. Gaddiel Ouaknin

I am a postdoctoral research associate working with Prof. Roseanna Zia in the School of Chemical and Biomolecular Engineering. My research interests are in computational algorithms, complex fluids and polymers.
Prior to joining her group, I worked on developing shape optimization algorithms with level set methods for direct and inverse geometric problems arising in self-assembled block copolymers at equilibrium with applications to nano lithography.
My current research is developing algorithms for parallelizing Accelerated Stokesian Dynamics (ASD) and implement it to simulate large-scale complex fluids. The current ASD algorithm on one core is effectively limited to O(100) of particles. My research is to develop strategies, algorithms and data structures to parallelize ASD based on Message Passing Interface (MPI). This in turn will allow us to study multi-scale complex fluid systems with O(10^{6}) particles spanning different physical length scales which is not possible with a single core. It will enable the discovery of the properties of large complex fluids system where the particle-fluid interactions are taken into account exactly.
**gyo3@cornell.edu**

Dr. Poornima Padmanabhan

I am a postdoctoral research associate working with Prof. Roseanna Zia in the School of Chemical and Biomolecular Engineering. My broad research interests are in understanding structure-property relationships and to strategize methods to tune the microstructure that ultimately affects material properties. Prior to joining her group, I worked on obtaining phase diagrams of ordered networks self-assembled by block copolymers. My current research involves looking at colloidal gel networks to understand and characterize their behavior under external fields. In particular I’m looking at the sedimentation behavior of the gel under the influence of gravity. I will study in detail the factors that influence this phenomenon such as the strength of gravity, the dominant length of initial gel microstructure and the volume fraction of colloids that form the gel. Detailed analysis of the microstructure will follow to track the height of the collapsing gel, and heterogeneities in density during the compaction procedure.
**pp335@cornell.edu**

** Graduate Students **

Christian Aponte-Rivera

I am a Ph.D. student in chemical engineering, where my work focuses on transport phenomena and microscale flows. My interests are in the area of micro-particle transport in non-equilibrium suspensions, and in particular on hydrodynamically interacting colloids which are fully enclosed within a microscopic domain. This work is of interest from both a fundamental standpoint, in understanding how fully 3D micro-confinement affects transport properties, and also with a view toward applied science: this model can be used to understand the mechanical motion of particles inside the crowded, watery interior of eukaryotic cells. Knowledge of such behavior will be used not only to answer fundamental questions about the impact of mechanical transport on overall cell behavior, but may also be used in the rational design of cellular-level disease treatments such as gene therapy. We approach the problem theoretically from the perspective of non-equilibrium statistical mechanics and hydrodynamics, and computationally using Accelerated Stokesian Dynamics to account for many-body hydrodynamic (and other) interactions. We plan to compare our results to experimental studies done by collaborators.
**caa225@cornell.edu**

Henry C.W. Chu

I am a Ph.D. student in the Sibley School of Mechanical and Aerospace Engineering at Cornell University. I have a broad interest in fluid physics, particularly in complex fluids dynamics, electrokinetics and the electro-hydrodynamics of lipid bilayers. In my current research I am focused on extending our Non-Equilibrium Stokes-Einstein Relation / Equation of State for colloidal suspensions to include hydrodynamic interactions between particles, with a dual aim. First, to enlarge on our current model that allows measurement of suspension viscosity, flow-induced diffusion, and the full stress tensor by tracking the motion of one “probe” particle driven through the material. This active, nonlinear micro-rheology theoretical framework has proven a powerful technique for interrogating microscopically small soft-matter systems. We are completing the comparison of our phenomenological model to solutions obtained via statistical mechanics and Accelerated Stokesian Dynamics simulations, with excellent agreement, suggesting that full rheological characterization of hydrodynamically interacting suspensions can be obtained by tracking the motion of one externally force probe particle.
**hc776@cornell.edu**

Benjamin E. Dolata

I am a Ph.D. student in chemical engineering with a primary research interest in complex fluids. My current project focuses on understanding depletion interactions in non-equilibrium soft matter. Equilibrium depletion interactions are well understood, but real systems often operate far from equilibrium, due to imposed flows or kinetic arrest, for example. I will determine the depletion force between two particles in a range of flows and orientational geometries including shear flow and the translation of a pair along and normal to their line of centers, and at arbitrary orientations between these two limits in the presence of hydrodynamic interactions. The force will be calculated in the dilute limit utilizing numerical and analytical solutions of the Smoluchowski equation, and by Stokesian dynamics computer simulation for more concentrated suspensions.
**bed53@cornell.edu**

Emma Gonzalez

I am a Ph.D. student in the Robert Frederick Smith School of Chemical and Biomolecular Engineering. My research interests include colloidal systems, complex fluids, and fluid mechanics. My current project aims to improve understanding of intracellular mechanical transport utilizing our group’s newly developed and evolving computational model, where confinement and particle shape, size, and interactions are aimed toward rigorous modeling of the key transport processes in a model eukaryotic cell. My study focuses on the dynamics of a confined polydisperse suspension where particles interact via many-body hydrodynamic and lubrication interactions, as well as via attractive and repulsive interactions, as they undergo colloidal-scale transport driven by thermal fluctuations and by deterministic forces (as would be generated by the towing of molecular motors). Our approach combines the non-equilibrium statistical mechanics and low-Reynolds number hydrodynamics theory with Confined Stokesian Dynamics simulations. My goal is to understand the changes in particle configuration, motion, phase behavior, and self-organization that can arise as a result of the interplay between microscopic forces, changes in concentration, confinement, and active motion.
**eg563@cornell.edu**

Derek E. Huang

I am a first-year Ph.D. student in the School of Chemical and Biomolecular Engineering at Cornell University. My primary research interests are in complex fluids and polymers, and the focus of my graduate research is on microrheology of sticky colloidal suspensions. I am studying the viscosity, diffusivity, and normal stresses in concentrated, hydrodynamically interacting colloidal dispersions undergoing forcing by a probe particle that exerts an attractive force on other nearby particles. In microrheology the position of the forced probe is tracked and its motion used to infer material properties of the embedding medium. Interrogation by a single, microscopic probe provides an alternative to traditional bulk, macroscopic approaches such as shear rheometry, where the latter requires large samples of material and smears out microscopic variations in material properties. Our model will be useful for understanding the rheological properties of sticky suspensions which exist only in small quantities, such as rare biological fluids, and provide a theoretical stepping stone to our models for colloidal gels.
**deh265@cornell.edu**

Lilian C. Johnson

I am a Ph.D. student in chemical engineering with research interests in complex fluids, polymers, and advanced materials. The focus of my graduate research is to develop a computational model including hydrodynamic interactions for the stability of gels that will predict and elucidate the underlying mechanisms of their sudden and currently unpredictable collapse. My focus is on reversible colloidal gels formed by arrested phase separation, in which attractions between the particles are on the order of just a few *kT*. Such gels evolve microstructurally and rheologically over time, and it turns out that they have a limited lifetime, often ended by the collapse of the network structure, and re-consolidation into a new, stronger gel. It is difficult to model the behavior of colloidal gels primarily because they are in a state of non-equilibrium, arrested phase separation and little predictive theory exists. Recently, the Zia group carried out the largest and longest aging simulation of colloidal gels to date, revealing insight into the structure, dynamics, and rheology of such gels. In this study, I will extend the model to include hydrodynamic interactions and qualitatively describe the ‘when’ and ‘why’ of colloidal gel collapse.
**lcj33@cornell.edu**

Ritesh P. Mohanty

I am a PhD student in the School of Chemical and Biomolecular Engineering at Cornell University. My broad research interests are complex fluids and applied mathematics . My current project in the Zia Group involves studying the interplay between hydrodynamics and Brownian motion in colloidal suspensions. I am primarily focused on transient study of these systems using microrheology. This technique gives the scope to predict time evolving behavior and the local heterogeneity of the microstructure of a colloid under both passive and active (with external force applied) regimes using a Brownian probe. We use continuum models for the fluid and statistical mechanics for the suspension particles. I ultimately aim to find if and how the macro and micro scale properties relate to each other. I am an avid cricket fan and also enjoy watching television shows.
**rpm235@cornell.edu**

Yu Su

I am a Ph.D. student in the School of Chemical and Biomolecular Engineering at Cornell University. My primary interests include fluid mechanics, applied mathematics, and scientific computing. My current research is on simulating hydrodynamically interacting suspensions using Accelerated Stokesian Dynamics (ASD). I am currently working on three main projects. In the first, I will parallelize ASD and implement it to simulate large-scale colloidal suspensions, gels, and glasses. The current ASD algorithm is effectively limited to order hundreds of particles because it is primarily a single-thread program. By parallelizing ASD based on Message Passing Interface (MPI), a standardized and portable message-passing system, my aim is to study multi-length scale systems, such as glasses and gels, which necessitates simulating hundreds of thousands of particles. For the second project, we have developed new algorithms to accelerate simulations of concentrated suspensions. In our previous work (Zia, Swan & Su, 2015, in review), we showed that long-range hydrodynamic interactions are not screened in crowded suspensions. This emphasizes the importance of the role of these interactions, even in the most crowded suspensions, motivating evaluation of a new set of concentrated mobility functions in this work. I will apply these functions to ASD to further accelerate the simulations. Finally, we recently developed a new Stokesian Dynamics model to simulate suspensions in a fully confined 3D spherical cavity. Incorporating this model and the parallelization scheme, my goal is to develop a new algorithm to simulate hydrodynamically interacting Brownian suspensions of arbitrary concentrations in unbounded to fully-confined domains.
**ys576@cornell.edu.edu**

Galen Wang

I am a Ph.D. student in the Sibley School of Mechanical and Aerospace Engineering at Cornell University. I have a broad interest in fluid dynamics, especially in complex fluids and their rheology. My focus is on the non-equilibrium behaviors of colloidal glasses around the jamming transition. It is well known that molecular liquids will form a glassy state without crystallization upon sufficiently rapid cooling. However, the underlying physics of this process remains unclear. Meanwhile, recent developments in microscopy and thermo-responsive microgels have shed light on the physics of the colloidal glass transition, which is often assumed to serve as a good model system for the molecular glass transition. Analogous to the temperature quench in the molecular glass transition, the colloidal glass transition can be triggered by a concentration quench, if a jump in colloid concentration is sufficiently rapid. In my work, I am developing several algorithms for carrying out this concentration quench utilizing large-scale dynamic simulation. With the tools recently developed by Zia group, I am able to measure structural, dynamical and rheological quantities via dynamic simulation during the concentration jumps and the following aging processes, and my goal is to reveal insight about the glass transition, in both colloidal and molecular glasses. We collaborate with the McKenna Group at TTU, who carry out the laboratory experiments that realize the colloidal glass transition utilizing thermo-responsive particles.
**jw2277@cornell.edu**

** Undergraduate Students **

Madalyn Baehre

I am a senior majoring in Chemical and Biomolecular Engineering, from Ohio. Last semester I worked with Ritesh Mohanty, developing a code to model the evolving microstructure around a probe particle that moves in a dilute suspension under an externally applied force. This semester I am working with Lilian Johnson studying colloidal gels.
**mmb296@cornell.edu**

Justin Love

I am a senior majoring in Chemical and Biomolecular Engineering, from New Jersey. My research interests include fluid mechanics and biochemistry. In the Zia Group, I work with Lilian Johnson in the computational study of colloidal gels.
**jsl358@cornell.edu**

Paola Torres

I am a senior and McNair Scholar in Chemical and Biomolecular Engineering, from New Jersey. I enjoy studying fluid dynamics and rheology. In the Zia Group, I work with Ben Dolata on Dual-Probe Active microrheology, and am developing Brownian dynamics simulation code to model this behavior.
**pct32@cornell.edu**

** Past Undergraduate Students **

**Ruhani Arya**, currently at Alcoa; Management, Consulting and Strategy Division**Eric Burkholder**, currently PhD student at Caltech**Chris Canova**, currently graduate student at MIT**Bari Grossman**, currently engineer at Gallo Winery**Iyore Olaye**, currently engineer at Walker International, San Francisco**Grace Tan**, currently PhD student at the University of Michigan