Helical Ribbon Sedimentation & Mobility Tensors

I've been a research assistant and senior thesis writer in the Voth Group (Fluid Dynamics and Soft Matter Lab) since January 2024. I design, 3D print, and analyze helical particles sinking through both turbulent, high Reynolds number flow and slow, Stokes flow, and measure how their geometry changes the way they fall.

Mobility tensors

A mobility tensor ties a particle's motion to the forces on it. For the helical ribbons I work with, the tensors are simple and diagonal, so each has 3 unknowns instead of 9, which makes them possible to find experimentally.

Tracking

I built a high-accuracy particle tracking workflow that made our group the first able to measure the full set of mobility tensors for a complex sedimenting particle by experiment. Plotting tilt against spin as a phase-space diagram reveals a lot about how the particle behaves.

When the predictions were off

The fits missed in a consistent way, and I worked through three hypotheses: viscosity changes in the tank, transient behavior before terminal velocity, and mass measurement errors. Because printers can quietly change a particle's mass, the updated model accounts for the mass of the beads and attributes the rest to deposited plastic, so only the buoyancy force changes.

Posters

The first poster, "Falling For Us Is Complicated: Mobility Tensor Fitting to Sedimenting Chiral Geometries," won first place. The second, "How We Learn Our a, b, c's: Designing Dynamics by Designing Forms," explains how shapes can be engineered to move in chosen ways, using helical ribbons and three-bladed propellers whose tensors share one eigenframe.

Recognition

This work won the Maureen Snow Summer Poster Prize for communicating research to a broad audience. My image "Phase Space of the Sedimenting Chaotic Helical Ribbon" won the CIS Research in the Sciences Image Prize. The tracking code is a custom MATLAB codebase that uses reverse point projection. Related projects include mobility tensor fitting for chiral geometries at low Reynolds number, chiral and achiral particles in turbulent flows, and chaotic sedimentation of the helical ribbon.