testing! research to go here.

In middle school, I began to learn to play chinese yoyo, which is a juggling prop consisting of a pair of handsticks, a string connecting the two, and the yoyo itself. I’ve long held a fascination with the motion of the strings, as well as impossible ‘magic knots’, such as the trick known as ‘金蝉脱壳’. This has fed into my academic interests and projects, which revolve around colloidal physics and rheology, spanning from experimental single fiber dynamics and topology, to Brownian dynamics and hydrodynamics simulations.

For my undergraduate senior thesis, I worked on a computational study of long, flexible fibers, and whether or (k)not they could self-entangle in simple fluid flows. For my doctoral work, I studied the dynamics of a nearly-equivalent experimental system, using colloidal particle chains. At present, we have determined a state diagram of the various modes that come about from different forcing conditions, as well as a bevy of rich and detailed dynamics.

As a postdoctoral fellow at NIST, I am exploring the same themes in a bulk fashion. Currently, I am studying the effects of flow on microstructural changes in a fluid, and how this may change the bulk rheology. To do so, I am working with solutions of Fd-bacteriophage, which acts as a model rod system, and am studying their rheological response using high-shear rheometers and SANS.

Knotting in Flow

I began working in the single fiber dynamics field during my undergraduate senior thesis at Princeton University, in Princeton, New Jersey. With the guidance of my advisor, Professor Howard A. Stone, I performed numerical simulations of flexible, high aspect ratio bead-spring chains in various simple flows, using the HYDROMULTIPOLE algorithm pioneered by our collaborators, Dr. Cichocki and Dr. Ekiel-Jezewska. We sought to answer the question - ‘can long, flexible fibers under shear flow be induced to self-entangle?’ Single fiber dynamics were thus analyzed with particular emphasis on fiber orientation, shape, and topology.

Ultimately, we found that it was possible for a shear flow to induce self-entanglement in a flexible fiber; we found multiple unknotting-knotting transitions, as well as evidence of slipknot formation. The formation of knots appears to be a transient effect, and whether they are linked to chaotic trajectories, and how they might affect bulk rheological properties, remain open questions. Details can be found in the paper, ‘Dynamics and Topology of Flexible Chains in Steady Shear Flows’, in New Journal of Physics (2015), by Kuei et al.

Rotational dynamics of colloidal chains

I joined the Soft Matter Laboratory at Rice University, in Houston, Texas, for my graduate work, where I earned my ph.D working with Professor Sibani Lisa Biswal. Using magnetic fields, we assemble paramagnetic colloidal beads into linked particle chains with tunable length and persistencelength, and actuate them with externally controlled magnetic fields. Our experimental observations and theory are complemented with Brownian Dynamics simulations.

We have found that when a rotational force field is applied to semiflexible chains in a quasi-planar environment, there are four distinctly different rotational regimes. At two extremes are rigid body rotation, reminiscent of Jeffery’s shear orbits, and folded structures, akin to buckled Euler beams. In between is a wagging regime, where fibers deform and relax in a periodic manner, and a coiling regime, where chains bundle into tight coils. Thus far, we have created a state diagram, which predicts the observed rotational regime as a function of chain length and applied force conditions, and matches analytical predictions closely. We are in the process of detailing the precise dynamics of the wagging and coiling regimes, with particular focus on asymmetrical dynamics and potentially chaotic dynamics. Details can be found in the paper, ‘From strings to coils: rotational dynamics of DNA-linked colloidal chains’, in Physical Review Fluids (2017) by Kuei et al.

Rheology of rod-like fluids

From 2019 to 2022, I worked in the Fluids, Suspensions, and Emulsions group at the National Institute of Standards and Technology, alongside Dr. Steven D. Hudson and Dr. Paul Salipante. We used semi-dilute suspensions of filamentous fd bacteriophage as a model rod-like fluid, and investigated its properties.

At low shear, these suspensions are strongly shear thinning. However, as shear induces alignment of rods within the fluid, excluded volume drops until the fluid acts as if dilute.