Smart Guy

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Summer

2014

Singularities - Profiles in Physics

Smart Guy

SPSer Wins Physics Award for Undergraduate Research

By:

Guy Marcus

Johns Hopkins University

Guy MarcusSPS alum Guy Marcus, now a graduate student at Johns Hopkins University in Baltimore, Maryland, recently received the American Physical Society’s LeRoy Apker Award. The award recognizes “outstanding achievements in physics by undergraduate students” and comes with a certificate as well as $5,000 for travel to an APS meeting. We sat down with Marcus, who previously published in the Journal of Undergraduate Physics (JURP), to talk about his prize-winning work.

"Receiving this award means a great deal to me, but the achievement is not just my own. I would like to provide my advisor at Wesleyan University, Professor Greg Voth, with the recognition he deserves for the formative role he played in my scientific development.

Voth’s lab studies the dynamics of anisotropic particles in turbulent fluid flows using high-speed video imaging. For my senior thesis, the main project cited in this award, I helped to develop a new approach for doing this using 3D-printing technology.

The simplest particle shapes are ellipsoids—rods, spheres, disks, and everything in-between. Understanding the dynamics of ellipsoids in turbulent flows has important applications. Ice crystals found in clouds that produce lightning as well as small biological organisms such as biodiesel-producing algae can be modeled as ellipsoids. But it has proven difficult to measure the tumbling rates of ellipsoidal particles.

We printed particles that are not ellipsoids but behave just like them and are much easier to measure. Crosses and jacks rotate like ellipsoidal disks and spheres, respectively, but lack the symmetries that make their round counterparts difficult to measure. We also designed new image analysis algorithms to determine the orientation and solid-body rotation of our particles from experimental data.

The rotation of the sphere-like jacks in a turbulent flow proved to be governed solely by the vorticity in the fluid flow, the amount of swirling in the fluid (more precisely, the curl of the velocity vector field at any given point). Our experiments on jacks demonstrate a new way to measure vorticity using standard tools put to new use.

Our measurements of crosses are the first direct experimental observation of alignment by turbulence. If you have ever dropped blades of grass into a stream, you may have noticed that the stream eventually aligns the blades in the direction of the flow. By measuring tens of thousands of tracks, we showed that crosses in a turbulent flow align in a particular direction, on average, which tells us that they rotate like spun quarters rather than tossed Frisbees.

The Apker Award marks neither the beginning nor the end of my contributions to physics. It is rather an important reminder for me of the simple benefits of hard work." //

the trajectory of a cross. Different colors label the arms to help visualize the rotation.the trajectory of a jack. Images courtesy of Guy Marcus.

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