2D Soft Particle Clogging: A Hard Problem

Mia Morrell is a sophomore majoring in Physics. She was awarded a Spring 2018 Conference Grant which she used to attend the American Physical Society Meeting in Los Angeles.

When shaking parmesan cheese on your pizza, have you mused in frustration why the cheese always seems to clog up in the holes of its container? Have you ever clogged a toilet and wondered what led to this most unfortunate form of humiliation? Or on a more serious note, have you contemplated the arterial clotting of human blood cells in the early phases of a stroke?

Our plexiglass hopper chamber displaying hydrogel particles in a clogged state.

These questions have fascinated me ever since I was introduced to soft matter physics upon my freshman year at Emory. During my time in Emory University’s Weeks lab, which focuses on soft matter and complex systems, I have studied soft particle clogging in two dimensions, devising experiments which can be applied to the previously cited situations of blood clots, parmesan cheese flow, and toilet clogs.

The clogging behavior of hard particles such as sand has been a popular subject of study for the past sixty years.[1] Simulations of pedestrians (modeled as hard particles) leaving a room through a single door have shown that increasing the speed at which people rush towards the door leads to increased clogging frequency (that is, people get stuck in the door!).[2] This celebrated phenomenon is known as the “faster is slower” principle. People are usually modeled as hard particles, because any “deformation” of a person will result in injury.

Our plexiglass chamber, which can be angled to vary the speed at which the hydrogel particles rush towards the chamber exit slit

I wondered whether forcing a system of soft particles such as cheese, blood cells or any other deformable material through a small opening lead to increased or decreased clogging. Would soft particles display the same “faster is slower” principle? Would allowing soft particles to wiggle around slightly once a clog was formed dislodge the clog?

A hydrogel bead. We place 200 of these soft, deformable polyacrylamide spheres into our hopper chamber.

To answer these questions, my PI Eric Weeks and I devised an ongoing series of experiments using a plexiglass chamber in which we placed soft polyacrylamide hydrogel beads. The chamber has a small exit slit of adjustable width through which we allow the hydrogels to flow. We can adjust the speed at which the hydrogel particles rush towards the exit slit by angling the hopper, therefore varying the driving force of gravity upon the hydrogel particles. The chamber is also outfitted with an eccentric rotating mass motor which can apply vibrations to the hopper system to test whether we can dislodge hydrogel clogs by causing the particles to wiggle around.

The eccentric rotating mass (ERM) motor used to make the hydrogel particles wiggle during clog formation.

With this experimental setup, we found that when the hydrogels flow faster towards the exit slit, clogging occurs less frequently. This result contradicts the “faster is slower” phenomenon and is most likely due to the soft hydrogels’ ability to deform and squeeze through the exit slit, as opposed to hard particles, which cannot be deformed. We also found that allowing the hydrogels to wiggle around during clog formation does not dislodge clogs but contributes to increased clogging frequency. We are currently investigating the roots of this phenomenon.

I recently presented these findings at the 2018 APS March Meeting in Los Angeles, CA. I attended the Jamming and Clogging session, where I was able to share my research with experts in the field including Dr. Bob Behringer of Duke University and Dr. Douglas Durian of UPenn, both whose work I deeply admire. I received plenty of ideas concerning new directions for my investigation of soft particle clogging, including high-speed video analysis, which I am currently applying. I am so grateful for Emory University’s continual support of my research and cannot emphasize the importance of Emory’s role in fostering my intellectual and professional development enough. Thank you, Emory!

Visit the Undergraduate Research Programs website to learn more about applying for Conference Grants. 

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[1]  W. A. Beverloo, H. A. Leniger, and J. van de Velde, The flow of granular solids through orifices, Chem. Eng. Sci., 15, 260–269 (1961).

[2] D. Helbing, I. Farkas, and T. Vicsek, Simulating dynamical features of escape panic, Nature, 407, 487–490 (2000).