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Sudden Collapse: Supercomputing Spotlight on Gels

Published on May 2, 2016 by Jorge Salazar

Snapshot of gel simulation, periodic cell. With permission from the Journal of Rheology (Zia et al, J. Rheol. , 58(5), 2014).

What exactly is life? A popular idea with some scientists in the early 1800s held that life was constructed of yet-unseen building blocks of a fundamental life force. The success around that time with microscopes in seeing tiny things for the first time helped drive the idea that through an eyepiece one could find the core of life.

Roseanna Zia, Assistant Professor, James C. and Rebecca Q. Morgan Sesquicentennial Faculty Fellow, Chemical and Biomolecular Engineering, Cornell University. Credit: Cornell University.

Botanist Robert Brown saw something in 1827 through the single lens microscope he built that astounded him. Pollen grains suspended in water danced around, animated by what he guessed might be the fundamental life force. Like any good scientist, he tested his hypothesis, by watching the motion of non-living things: floating bits of coal and of glass pulverized into microscopic dust. But they too danced in water, seemingly animated by some unseen source. Life didn't cause the motion – but what did was a mystery. As long as the particles were very small, they moved. Brown published his findings in detail with more questions than answers. His zigzag motion of microscopic floating objects would baffle scientists and survive nearly a century of misinterpretation and argument under its enduring name, Brownian motion.

Explanations of Brown's discovery were key to the discoveries that set much of 20th century science in motion. In 1905 the physicist Albert Einstein used the mathematical description of Brownian motion to help prove the existence atoms and molecules. And thus began a scientific field called microrheology, the study of material properties by tracking the Brownian motion of tiny suspended particles.

First light on a microscale world

Looking beyond just particles dancing in water, much study has since followed, focusing on the flow of fluids in which microscopically small particles, droplets, or bubbles are suspended, so-called "colloids." When microscopic forces between the particles causes them to bond together into a network, they form a solid-like scaffold immersed in a fluid, forming a soft solid – which engineers utilize for building artificial tissue scaffolds and injectable pharmaceuticals, for example. But these materials are everywhere around us in nature and common household products as well, in the form of shampoo, Jell-o, and yogurt, to name a few.

"XSEDE and Stampede high performance computational access was a game-changer... Having a Campus Champion right here made a huge difference for our early success with XSEDE."
Roseanna Zia, Cornell University.
Chemical engineering researcher Roseanna Zia has begun to shed light on the microscale world of colloidal gels — liquids dispersed in a solid medium as a gel. Sometimes the gels act like liquids, and sometimes they act like a solid, such as hair gel that squirts out of a tube like a liquid, but retains its shape sitting in your hand.

"Colloidal gels are actually soft solids, but we can manipulate their structure to produce ‘on-demand' transitions from liquid-like to solid-like behavior that can be reversed many times," Zia said. Zia is an Assistant Professor of Chemical and Biomolecular Engineering at Cornell University.

The tiny solid particles in a colloidal gel — invisible to the naked eye — bind together and form a 3-D scaffolding of filaments throughout the medium. One can break that scaffolding by simply squirting the material or dragging a surface across it. For example, toothpaste or hair gel — after it has been squirted — will sit on a toothbrush like a solid because its scaffold has re-formed on its own due to the attractions between the particles.

Sudden collapse of gel structure

New research by Zia and colleagues is focused on understanding why gel scaffolding can suddenly collapse on its own. "This research grew out of our initial quest to understand why some gels, once they are formed and placed in a container, remain intact but then later they suddenly, seemingly inexplicably, collapse under their own weight. This unpredictable behavior has presented a roadblock in the design of technological materials like tissue scaffolds, injectable pharmaceuticals, petroleum drilling fluids, and even commercial household fluids. It's a very broad and technologically pressing problem," Zia said.

'Resistance is colloidal' in these Borg-like snapshots of gel simulation. (a) One periodic cell. (b) 2x magnification. (c) 4x magnification. Number of contacts per particle indicated by color, ranging from red (few contacts) to blue (many contacts). Gel strands spanning the boundary of the periodic cell shown in section view, revealing high-contact number interior particles. With permission from the Journal of Rheology (Zia et al, J. Rheol. , 58(5), 2014).
A survey of the recent scientific literature surrounding this problem revealed that some basic questions about how colloidal gels behave in general, even without collapse, hadn't yet been answered. Open questions included how does the gel scaffolding evolve over time after it forms, if it evolves at all; what does its structure look like; and do its properties change over time? Even today, the challenges of precisely imaging these microscopic structures as they evolve rapidly over time pushes the limits of imaging techniques.

"This is where dynamic simulations play a really important role in scientific discovery," Zia said. "In these simulations we can track and view changes in the material all the way down to individual particles. There, another challenge computationally is that these gels could form long network-length scales, and the simulation has to capture that. This means that to simulate a realistic system, one must develop a very large simulation."

In reality, even a small sample of a gel comprises billions of of particles that interact to form the gel. Before Zia and colleagues began their research, the largest prior simulations included only a few thousand gel particles, which did not produce the full range of structural variations in a gel that give it its unique properties. Using supercomputers, she was able to increase that number to over 750,000 and, in so doing, more faithfully capture the way the complex scaffold rearranges over time

The computational microscope

Zia and colleagues utilized over seven million service units on the Stampede supercomputer at the Texas Advanced Computing Center (TACC), awarded through an allocation by XSEDE, the eXtreme Science and Engineering Discovery Environment. "Stampede high performance computational access was a game-changer," Zia said.

"I often say that I'm Stampede's biggest fan, not just in terms of computational access, but also, the technical staff are world-class in terms of making available the best computational resources, training, and help services. They're literally available 24/7 and always solve problems really quickly. I can't say enough good things about the TACC staff."

Zia used Stampede to simulate how gels evolve over time. The particle strands of a gel scaffolding coarsen and grow thicker as they age, leading to changes in the mechanical properties. "The next question was, how, at a microscopic length scale, do gels rearrange themselves?" Zia asked.

Gels coarsen as they age. Simulation shows evolution of particle microstructure over time for (a) top row, 5 kT gel and (b) bottom row, 6 kT gel. The gel in each snapshot is older than the previous image as indicated. Credit: Journal of Rheology.
"It was already known that gels are built from an interconnected network of strands, with each strand made up of many bonded-together particles. What was not known was, how are these particles packed together? Are individual particles all frozen into place like the solid-like macroscopic scaffold?" Zia said. They discovered that the interior of each strand is a frozen, disordered solid, like a glass. "But on the surface of these strands, the particles are actually a lot more mobile. The surface of each strand is covered with weakly bonded particles — the surface is like a liquid." And it turned out that Brownian motion plays a key role in how they evolve.

Brownian motion allows the particles to break free of their bonds, allowing them to crawl along the network, settling into more energetically favorable positions via a process called diffusion.

"They might temporarily detach from the network due to a Brownian fluctuation, then get pulled back down onto the network and get trapped in a cage of other particles," Zia said. These roving particles migrate from cage to cage and eventually sink down into the strand as more particles get deposited on top of them. "We call that ‘cage-trapping,' and once they fall down into these energy wells, they rarely come out. They become trapped there essentially forever, and that's how gels coarsen with age," Zia said.

Understanding the theory behind these transitions can translate to real-world applications, such as helping research why the airway mucus — a colloidal gel — of people with cystic fibrosis can thicken, resist flow and possibly threaten life. It will also be a piece of the puzzle in understanding more rapid transitions in gels, such as sudden collapse. Their current work focuses on the response of such gels to imposed flow and to gravity.

Jell-o is one example of a colloidal gel, a liquid dispersed in a solid. Roseanna Zia and colleagues created the largest computer simulation of colloidal gels yet attempted. Credit: Steven Depolo.

Massive simulation of colloidal gels

The simulated gel aging and coarsening are described in the study, "A micro-mechanical study of coarsening and rheology of colloidal gels: Cage building, cage hopping, and Smoluchowski's ratchet," published in the September/October 2014 issue of The Journal of Rheology. Funding was provided by the National Science Foundation Grant No. CBET-0754078 and the Office of Naval Research Grant No. N00014-14-1-0744.

Study co-author Benjamin Landrum, also at Cornell with the Zia Research Group, created the computer model of nearly a million particles using the simulation package called LAMMPS, the Large-scale Atomic/Molecular Massively Parallel Simulator.

"LAMMPS is optimized to spread out the particles onto many, many processors," Zia said. She explained that the code basically applies a set of rules based on the physics equations that govern the forces acting on the particles, such as attraction, collision, Brownian motion, and so on. Combining LAMMPS and XSEDE was the key to carrying out the massively parallelized large-scale simulations required to accurately model gel behavior.

Jumpstart with XSEDE Campus Champions

Zia's research group got help from the Campus Champions program of XSEDE to get started on using its high performance computational resources. Campus Champions are typically but not limited to campus I.T. faculty and staff that are trained by and maintain close ties to XSEDE. They help researchers access XSEDE resources, get allocations and training, and more.

"We enjoy working with Dr. Zia and other researchers to facilitate their timely access to and efficient use of XSEDE resources."
Susan Mehringer, Cornell University.
"Our campus champion is Susan Mehringer, and she's been a tremendous help," said Zia. "I didn't know anything about XSEDE. She's introduced us to XSEDE and guided me through the process of acquiring user credentials and applying for startup allocations. A champion has their own allocation for researchers to do initial benchmarking, an important step in the proposal process. Most importantly, she gave us guidance for preparing for our first research allocation proposal. Having a campus champion right here made a huge difference for our early success with XSEDE."

Susan Mehringer is the Assistant Director of the Center for Advanced Computing, Cornell University. "As a Campus Champion," said Mehringer, "I'm always eager to introduce faculty to XSEDE capabilities, from advanced computation to educational opportunities for their students. We enjoy working with Dr. Zia and other researchers to facilitate their timely access to and efficient use of XSEDE resources."

Screening in a crowded space

Back in the lab, other work in the Zia Group addresses the difficulties of modeling colloidal dispersions with free-flowing fluid systems that arise from the strong hydrodynamic coupling between particles. In other words, when one particle moves it disturbs and moves other particles, even far away. The coupling decays at a rate of only one over the separation distance, compared to one over the distance squared for say, electrostatic or gravitational interactions. Trying to incorporate those interactions between millions of particles quickly overwhelms even the most powerful supercomputers.

"Sometimes people decide that it's too difficult, and they just throw [the hydrodynamic coupling approximations] away," Zia said. "There are some situations where that's a reasonable approximation. There are other situations where that leads to major inaccuracies in predicted suspension behavior. In those cases, we just have to roll up our sleeves and do the physics."

(a) Robert Brown's microscope, by permission of the Linnean Society of London. (b) Reproduction of Brown's experiment: pollen grains released from pollen sacs (Credit: P. Jones).
Zia and colleagues addressed these ideas in a recent study, "Pair mobility functions for rigid spheres in concentrated colloidal dispersions," published in December 2015 in the Journal of Chemical Physics. In this case she set up a computational scenario to test whether the coupling can be neglected in crowded suspensions. One case where such interactions can be neglected is when particles are fixed in place and block or "screen" the long-distance hydrodynamic interactions. One can think of the particles like the walls of an office cubicle, separating two sides of a fluid allowed to flow around it, a set-up scientists call Brinkmann screening.

"The idea has been proposed that in very crowded systems like the interior of a cell, the dense population leads to a screening effect, and thus one can neglect these difficult-to-model interactions," Zia said. "We know that the more particles you put in a confined space, the less mobile they are. Think of being on a crowded subway. The more people get on, the harder it is to move, and good luck getting out the door when it comes to your stop." So they decided to test whether the restriction of motion from crowding leads to the same "screening" as having fixed particles.

"It turns out that screening does not occur," Zia said. "Crowding does not give the same behavior as the fixed-particle case. Regardless of how dense the suspension is, as long as the spheres are free to move, they're coupled very strongly, over very long distances. And so we do have to carefully model those hydrodynamic interactions."

Those interactions are guiding what's next in research for the Zia Group. "The most important thing is solving that 'collapse problem,'" Zia said. "This is our current top goal, figuring out what's going on with this sudden collapse."

Good things can come from finding out why things go bad quickly for the gel scaffolds. "We have a lot of interest in this, not only from our peers working in this area but also in industry, such as petroleum and personal care products," Zia said.

The 'computation microscope' of supercomputers. XSEDE resource Stampede at the Texas Advanced Computing Center provides the computing power of thousands of desktop computers for open science in the U.S.

Materials of the future

Colloidal gels and other complex fluids are the materials of the future, according to Zia. "People have engineered these materials to chemically control insulin delivery; to control the release of drugs; to stimulate bone formation to repair defects in bones; they're even being used to encapsulate compounds to deliver into cells; to transplant cells; to do tissue repair; and even, if you can imagine post-surgery pain treatments injecting materials that are long-lasting pain treatments, so that a patient doesn't have to take a drug that affects their entire physiology," Zia said. "They're even being sought about for using repair and prevention materials for soldiers in battle to help them with diagnostics in battlefield conditions."

"I'd like people to know that I work with the support of a fantastic research group of my graduate students here at Cornell University, in particular, Ben Landrum, the graduate student who worked closely with me in our initial gel studies and simulations" Zia said, "as well as my collaborator Bill Russel from Princeton University, who played an important role in our first big gel-aging study".

You can find out more about the Zia Group at http://www.icse.cornell.edu/ziagroup/. Funding resources: the National Science Foundation, the Office of Naval Research, and XSEDE.


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Story Highlights

Computer simulations show colloidal gels coarsen as they age, one piece of the puzzle in how their structures suddenly collapse.

Largest simulation yet of colloidal gels captured complex scaffolding rearrangement of over 750,000 particles.

Over seven million service units of XSEDE resource Stampede at TACC utilized by Zia Group research on colloidal gels.

XSEDE Campus Champions program catalyzed advanced computing in fluid dynamics research of colloidal gels.

Engineered complex fluids like colloidal gels considered a 'material of the future,' important for insulin delivery, controlled drug release, bone repair stimulation, artificial tissue scaffolding, and more.


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