Duke Research Blog

Following the people and events that make up the research community at Duke.

Category: Faculty (Page 1 of 13)

Aging and Decision-Making

Who makes riskier decisions, the young or the old? And what matters more in our decisions as we age — friends, health or money? The answers might surprise you.

Kendra Seaman works at the Center for the Study of Aging and Human Development and is interested in decision-making across the lifespan.

Duke postdoctoral fellow Kendra Seaman, Ph.D. uses mathematical models and brain imaging to understand how decision-making changes as we age. In a talk to a group of cognitive neuroscientists at Duke, Seamen explained that we have good reason to be concerned with how older people make decisions.

Statistically, older people in the U.S. have more money, and additionally more expenditures, specifically in healthcare. And by 2030, 20 percent of the US population will be over the age of 65.

One key component to decision-making is subjective value, which is a measure of the importance a reward or outcome has to a specific person at a specific point in time. Seaman used a reward of $20 as an example: it would have a much higher subjective value for a broke college student than for a wealthy retiree. Seaman discussed three factors that influence subjective value: reward, cost, and discount rate, or the determination of the value of future rewards.

Brain imaging research has found that subjective value is represented similarly in the medial prefrontal cortex (MPFC) across all ages. Despite this common network, Seaman and her colleagues have found significant differences in decision-making in older individuals.

The first difference comes in the form of reward. Older individuals are likely to be more invested in the outcome of a task if the reward is social or health-related rather than monetary. Consequently, they are more likely to want these health and social rewards  sooner and with higher certainty than younger individuals are. Understanding the salience of these rewards is crucial to designing future experiments to identify decision-making differences in older adults.

A preference for positive skew becomes more pronounced with age.

Older individuals also differ in their preferences for something called “skewed risks.” In these tasks, positive skew means a high probability of a small loss and a low probability of a large gain, such as buying a lottery ticket. Negative skew means a low probability of a large loss and a high probability of a small gain, such as undergoing a common medical procedure that has a low chance of harmful complications.

Older people tend to prefer positive skew to a greater degree than younger people, and this bias toward positive skew becomes more pronounced with age.

Understanding these tendencies could be vital in understanding why older people fall victim to fraud and decide to undergo risky medical procedures, and additionally be better equipped to motivate an aging population to remain involved in physical and mental activities.

Post by undergraduate blogger Sarah Haurin

Post by undergraduate blogger Sarah Haurin

Combatting the Opioid Epidemic

The opioid epidemic needs to be combatted in and out of the clinic.

In the U.S. 115 people die from opioids every day. The number of opioid overdoses increased fivefold from 1999 to 2016. While increased funding for resources like Narcan has helped — the opioid overdose-reversing drug now carried by emergency responders in cities throughout the country — changes to standard healthcare practices are still sorely needed.

Ashwin A Patkar, MD, medical director of the Duke Addictions Program, spoke to the Duke Center on Addiction and Behavior Change about how opioid addiction is treated.

The weaknesses of the current treatment standards first appear in diagnosis. Heroin and cocaine are currently being contaminated by distributors with fentanyl, an opioid that is 25 to 50 times more potent than heroin and cheaper than either of these drugs. Despite fentanyl’s prevalence in these street drugs, the standard form and interview for addiction patients does not include asking about or testing for the substance.

Patkar has found that 30 percent of opioid addiction patients have fentanyl in their urine and do not disclose it to the doctor. Rather than resulting from the patients’ dishonesty, Patkar believes, in most cases, patients are taking fentanyl without knowing that the drugs they are taking are contaminated.

Because of its potency, fentanyl causes overdoses that may require more Narcan than a standard heroin overdose. Understanding the prevalence of Narcan in patients is vital both for public health and educating patients so they can be adequately prepared.

Patkar also pointed out that, despite a lot of research supporting medication-assisted therapy, only 21 percent of addiction treatment facilities in the U.S. offer this type of treatment. Instead, most facilities rely on detoxification, which has high rates of relapse (greater than 85 percent within a year after detox) and comes with its own drawbacks. Detox lowers the patient’s tolerance to the drug, but care providers often neglect to tell the patients this, resulting in a rate of overdose that is three times higher than before detox.

Another common treatment for opioid addiction involves using methadone, a controlled substance that helps alleviate symptoms from opioid withdrawal. Because retention rate is high and cost of production is low, methadone poses a strong financial incentive. However, methadone itself is addictive, and overdose is possible.

Patkar points to a resource developed by Julie Bruneau as a reference for the Canadian standard of care for opioid abuse disorder. Rather than recommending detox or methadone as a first line of treatment, Bruneau and her team recommend buprenorphine , and naltrexone as a medication to support abstinence after treatment with buprenorphine.

Buprenorphine is a drug with a similar function as methadone, but with better and safer clinical outcomes. Buprenorphine does not create the same euphoric effect as methadone, and rates of overdose are six times less than in those prescribed methadone.

In addition to prescribing the right medicine, clinicians need to encourage patients to stick with treatment longer. Despite buprenorphine having good outcomes, patients who stop taking it after only 4 to 12 weeks, even with tapering directed by a doctor, exhibit only an 18 percent rate of successful abstinence.

Patkar closed his talk by reminding the audience that opioid addiction is a brain disease. In order to see a real change in the number of people dying from opioids, we need to focus on treating addiction as a disease; no one would question extended medication-based treatment of diseases like diabetes or heart disease, and the same should be said about addiction. Healthcare providers have a responsibility to treat addiction based on available research and best practices, and patients with opioid addiction deserve a standard of care the same as anyone else.

Post by undergraduate blogger Sarah Haurin

Post by undergraduate blogger Sarah Haurin

Becoming the First: Nick Carnes

Editor’s Note: In the “Becoming the First” series,  first-generation college student and Rubenstein Scholar Lydia Goff explores the experiences of Duke researchers who were the first in their families to attend college.

A portrait of Duke Professor Nick Carnes

Nick Carnes

Should we care that we are governed by professionals and millionaires? This is one of the questions Nick Carnes, an assistant professor in the Sanford School of Public Policy, seeks to answer with his research. He explores unequal social class representation in the political process and how it affects policy making. But do any real differences even exist between politicians from lower socioeconomic classes and those from the upper classes? Carnes believes they do, not only because of his research but also because of his personal experiences.

When Carnes entered Princeton University as a political science graduate student, he was the only member of his cohort who had done restaurant, construction or factory work. While obtaining his undergraduate degree from the University of Tulsa, he worked twenty hours a week and during the summer clocked in at sixty to seventy hours a week between two jobs. He considered himself and his classmates “similar on paper,” just like how politicians from a variety of socioeconomic classes can also appear comparable. However, Carnes noticed that he approached some problems differently than his classmates and wondered why. After attributing his distinct approach to his working class background, without the benefits of established college graduate family members (his mother did go to college while he was growing up), he began developing his current research interests.

Carnes considers “challenging the negative stereotypes about working class people” the most important aspect of his research. When he entered college, his first meeting with his advisor was filled with confusion as he tried to decipher what a syllabus was. While his working class status did restrict his knowledge of college norms, he overcame these limitations. He is now a researcher, writer, and professor who considers his job “the best in the world” and whose own story proves that working class individuals can conquer positions more often inhabited by the experienced. As Carnes states, “There’s no good reason to not have working class people in office.” His research seeks to reinforce that.

His biggest challenge is that the data he needs to analyze does not exist in a well-documented manner. Much of his research involves gathering data so that he can generate results. His published book, White-Collar Government: The Hidden Role of Class in Economic Policy Making, and his book coming out in September, The Cash Ceiling: Why Only the Rich Run for Office–and What We Can Do About It, contain the data and results he has produced. Presently, he is beginning a project on transnational governments because “cash ceilings exist in every advanced democracy.” Carnes’ research proves we should care that professionals and millionaires run our government. Through his story, he exemplifies that students who come from families without generations of college graduates can still succeed.    

 

Post by Lydia Goff

 

Becoming the First: Erika Weinthal

Editor’s Note: In the “Becoming the First” series,  first-generation college student and Rubenstein Scholar Lydia Goff explores the experiences of Duke researchers who were the first in their families to attend college.

A portrait of Erika Weinthal

Erika Weinthal

In her corner office with a wall of windows and stuffed bookshelves, Erika Weinthal keeps a photo of her father. He came to the United States from Germany in 1940. And for a German Jew, that was extremely late. According to the family stories, Weinthal’s father left on the second to last boat from Italy. It is no surprise that he was never a big traveler after his arrival to America. As Weinthal describes it, “America…was the country that saved him.” Not only did it protect him, but it also gave his children opportunities that he did not have, such as going to college.

Weinthal, Lee Hill Snowdon Professor of Environmental Policy in Duke’s Nicholas School of the Environment, took this opportunity to become the first in her family to attend college, launching her career researching environmental policy and water security in areas including the former Soviet Union, Middle East, East Africa, India and the United States.

In high school, Weinthal traveled as an exchange student to Germany, a country her relatives could never understand her desire to visit. “As a child of a refugee, you didn’t talk about the war,” she explains as she describes how this silence created her curiosity about what happened. That journey to Bremen marked only the first of many trips around the world. In the Middle East, she examines environmental policy between countries that share water. In India, she has researched the relationship between wildlife and humans near protected areas. “What do you do when protected wildlife destroys crops and threatens livelihoods?” she asks, proving that since her curiosity about the war, she has not stopped asking questions.

However, her specific interest in environmental science and policy came straight from a different war: the Cold War. She became obsessed with everything Russian partly thanks to a high school teacher who agreed to teach her Russian one-on-one. The teacher introduced Weinthal to Russian literature and poetry. While her parents, like many parents, would have loved for her to become a doctor or a lawyer, they still trusted her when she enrolled in Oberlin College intent on studying Soviet politics. A class on Soviet environment politics further increased her interest in water security.

Currently, her work contends that water should be viewed as a basic human need separate from the political conflicts in Palestine and Israel. She has studied how protracted conflict in the region has led to the deterioration of water quality in the Gaza Strip, creating a situation in which water is now unfit for human consumption. Weinthal argues that these regions should not view water as property to be secured but rather as a human right they should guarantee.

Erika Weinthal’s father in 1940

As a child of a refugee and a first-generation college student, Weinthal says “you grow up essentially so grateful for what others have sacrificed for you.” Her dad believed in giving back to the next generation. He accomplished that goal and, in the process, gave the world a researcher who’s invested in environmental policy and human rights.

Post by Lydia Goff

 

Stretchable, Twistable Wires for Wearable Electronics

A new conductive “felt” carries electricity even when twisted, bent and stretched. Credit: Matthew Catenacci

The exercise-tracking power of a Fitbit may soon jump from your wrist and into your clothing.

Researchers are seeking to embed electronics such as fitness trackers and health monitors into our shirts, hats, and shoes. But no one wants stiff copper wires or silicon transistors deforming their clothing or poking into their skin.

Scientists in Benjamin Wiley’s lab at Duke have created new conductive “felt” that can be easily patterned onto fabrics to create flexible wires. The felt, composed of silver-coated copper nanowires and silicon rubber, carries electricity even when bent, stretched and twisted, over and over again.

“We wanted to create wiring that is stretchable on the body,” said Matthew Catenacci, a graduate student in Wiley’s group.

The conductive felt is made of stacks of interwoven silver-coated copper nanotubes filled with a stretchable silicone rubber (left). When stretched, felt made from more pliable rubber is more resilient to small tears and holes than felts made of stiffer rubber (middle). These tears can be seen in small cavities in the felt (right). Credit: Matthew Catenacci

To create a flexible wire, the team first sucks a solution of copper nanowires and water through a stencil, creating a stack of interwoven nanowires in the desired shape. The material is similar to the interwoven fibers that comprise fabric felt, but on a much smaller scale, said Wiley, an associate professor of chemistry at Duke.

“The way I think about the wires are like tiny sticks of uncooked spaghetti,” Wiley said. “The water passes through, and then you end up with this pile of sticks with a high porosity.”

The interwoven nanowires are heated to 300 F to melt the contacts together, and then silicone rubber is added to fill in the gaps between the wires.

To show the pliability of their new material, Catenacci patterned the nanowire felt into a variety of squiggly, snaking patterns. Stretching and twisting the wires up to 300 times did not degrade the conductivity.

The material maintains its conductivity when twisted and stretched. Credit: Matthew Catenacci

“On a larger scale you could take a whole shirt, put it over a vacuum filter, and with a stencil you could create whatever wire pattern you want,” Catenacci said. “After you add the silicone, so you will just have a patch of fabric that is able to stretch.”

Their felt is not the first conductive material that displays the agility of a gymnast. Flexible wires made of silver microflakes also exhibit this unique set of properties. But the new material has the best performance of any other material so far, and at a much lower cost.

“This material retains its conductivity after stretching better than any other material with this high of an initial conductivity. That is what separates it,” Wiley said.

Stretchable Conductive Composites from Cu-Ag Nanowire Felt,” Matthew J. Catenacci, Christopher Reyes, Mutya A. Cruz and Benjamin J. Wiley. ACS Nano, March 14, 2018. DOI: 10.1021/acsnano.8b00887

Post by Kara Manke

MRI Tags Stick to Molecules with Chemical “Velcro®”

An extremely close-up view of Velcro

In the new technique, MRI chemical tags attach to a target molecule and nothing else – kind of like how Velcro only sticks to itself. Credit: tanakawho, via Flickr.

Imagine attaching a beacon to a drug molecule and following its journey through our winding innards, tracking just where and how it interacts with the chemicals in our bodies to help treat illnesses.

Duke scientists may be closer to doing just that. They have developed a chemical tag that can be attached to molecules to make them light up under magnetic resonance imaging (MRI).

This tag or “lightbulb” changes its frequency when the molecule interacts with another molecule, potentially allowing researchers to both locate the molecule in the body and see how it is metabolized.

“MRI methods are very sensitive to small changes in the chemical structure, so you can actually use these tags to directly image chemical transformations,” said Thomas Theis, an assistant research professor in the chemistry department at Duke.

Chemical tags that light up under MRI are not new. In 2016, the Duke team of Warren S. Warren’s lab and Qiu Wang’s lab created molecular lightbulbs for MRI that burn brighter and longer than any previously discovered.

A photo of graduate students Junu Bae and Zijian Zhou in front of a bookshelf.

Junu Bae and Zijian Zhou, the co-first authors of the paper. Credit: Qiu Wang, Duke University.

In a study published March 9 in Science Advances, the researchers report a new method for attaching tags to molecules, allowing them to tag molecules indirectly to a broader scope of molecules than they could before.

“The tags are like lightbulbs covered in Velcro,” said Junu Bae, a graduate student in Qiu Wang’s lab at Duke. “We attach the other side of the Velcro to the target molecule, and once they find each other they stick.”

This reaction is what researchers call bioorthogonal, which means that the tag will only stick to the molecular target and won’t react with any other molecules.

And the reaction was designed with another important feature in mind — it generates a rare form of nitrogen gas that also lights up under MRI.

“One could dream up a lot of potential applications for the nitrogen gas, but one that we have been thinking about is lung imaging,” Theis said.

Currently the best way to image the lungs is with xenon gas, but this method has the downside of putting patients to sleep. “Nitrogen gas would be perfectly safe to inhale because it is what you inhale in the air anyways,” Theis said.

A stylized chemical diagram of the hyperpolarization process

In the new technique, a type of molecule called a tetrazine is hyperpolarized, making it “light up” under MRI (illustrated on the left). It is then tagged to a target molecule through a what is called a bioorthogonal reaction. The reaction also generates a rare form of nitrogen gas that can be spotted under MRI (illustrated on the right). Credit: Junu Bae and Seoyoung Cho, Duke University.

Other applications could include watching how air flows through porous materials or studying the nitrogen fixation process in plants.

One downside to the new tags is that they don’t shine as long or as brightly as other MRI molecular lightbulbs, said Zijian Zhou, a graduate student in  Warren’s lab at Duke.

The team is tinkering with the formula for polarizing, or lighting up, the molecule tags to increase their lifetime and brilliance, and to make them more compatible with chemical conditions in the human body.

“We are now developing new techniques and new procedures which may be helpful for driving the polarization levels even higher, so we can have even better signal for these applications,” Zhou said.

15N4-1,2,4,5-tetrazines as potential molecular tags: Integrating bioorthogonal chemistry with hyperpolarization and unearthing para-N2,” Junu Bae, Zijian Zhou, Thomas Theis, Warren S. Warren and Qiu Wang. Science Advances, March 9, 2018. DOI: 10.1126/sciadv.aar2978

Post by Kara Manke

How Earth’s Earliest Lifeforms Protected Their Genes

A colorful hot spring in Yellowstone National Park

Heat-loving thermophile bacteria may have been some of the earliest lifeforms on Earth. Researchers are studying their great great great grandchildren, like those living in Yellowstone’s Grand Prismatic Spring, to understand how these early bacteria repaired their DNA.

Think your life is hard? Imagine being a tiny bacterium trying to get a foothold on a young and desolate Earth. The earliest lifeforms on our planet endured searing heat, ultraviolet radiation and an atmosphere devoid of oxygen.

Benjamin Rousseau, a research technician in David Beratan’s lab at Duke, studies one of the molecular machines that helped these bacteria survive their harsh environment. This molecule, called photolyase, fixes DNA damaged by ultraviolet (UV) radiation — the same wavelengths of sunlight that give us sunburn and put us at greater risk of skin cancer.

“Anything under the sun — in both meanings of the phrase — has to have ways to repair itself, and photolyase proteins are one of them,” Rousseau said. “They are one of the most ancient repair proteins.”

Though these proteins have been around for billions of years, scientists are still not quite sure exactly how they work. In a new study, Rousseau and coworkers, working with Professor David Beratan and Assistant Research Professor Agostino Migliore, used computer simulations to study photolyase in thermophiles, the great great great great grandchildren of Earth’s original bacterial pioneers.

The study appeared in the Feb. 28 issue of the Journal of the American Chemical Society.

DNA is built of chains of bases — A, C, G and T — whose order encodes our genetic information. UV light can trigger two adjacent bases to react and latch onto one other, rendering these genetic instructions unreadable.

Photolyase uses a molecular antenna to capture light from the sun and convert it into an electron. It then hands the electron over to the DNA strand, sparking a reaction that splits the two bases apart and restores the genetic information.

A ribbon diagram of a photolyase protein

Photolyase proteins use a molecular antenna (green, blue and red structure on the right) to harvest light and convert it into an electron. The adenine-containing structure in the middle hands the electron to the DNA strand, splitting apart DNA bases. Credit: Benjamin Rousseau, courtesy of the Journal of the American Chemical Society.

Rousseau studied the role of a molecule called adenine in shuttling the electron  from the molecular antenna to the DNA strand. He looked at photolyase in both the heat-loving ancestors of ancient bacteria, called thermophiles, and more modern bacteria like E. Coli that thrive at moderate temperatures, called mesophiles.

He found that in thermophiles, adenine played a role in transferring the electron to the DNA. But in E. coli, the adenine was in a different position, providing mainly structural support.

The results “strongly suggest that mesophiles and thermophiles fundamentally differ in their use of adenine for this electron transfer repair mechanism,” Rousseau said.

He also found that when he cooled E. Coli down to 20 degrees Celsius — about 68 degrees Fahrenheit — the adenine shifted back in place, resuming its transport function.

“It’s like a temperature-controlled switch,” Rousseau said.

Though humans no longer use photolyase for DNA repair, the protein persists in life as diverse as bacteria, fungi and plants — and is even being studied as an ingredient in sunscreens to help repair UV-damaged skin.

Understanding exactly how photolyase works may also help researchers design proteins with a variety of new functions, Rousseau said.

“Photolyase does all of the work on its own — it harvests the light, it transfers the electron over a huge distance to the other site, and then it cleaves the DNA bases,” Rousseau said. “Proteins with that kind of plethora of functions tend to be an attractive target for protein engineering.”

Post by Kara Manke

How A Zebrafish’s Squiggly Cartilage Transforms into a Strong Spine

A column of green cartilage cells divides into an alternating pattern of green cartilage and red vertebra

Our spines begin as a flexible column called the notochord. Over time, cells on the notochord surface divide into alternating segments that go on to form cartilage and vertebrae.

In the womb, our strong spines start as nothing more than a rope of rubbery tissue. As our bodies develop, this flexible cord, called the notochord, morphs into a column of bone and cartilage sturdy enough to hold up our heavy upper bodies.

Graduate student Susan Wopat and her colleagues in Michel Bagnat’s lab at Duke are studying the notochords of the humble zebrafish to learn how this cartilage-like rope grows into a mature spine.

In a new paper, they detail the cellular messaging that directs this transformation.

It all comes down to Notch receptors on the notochord surface, they found. Notch receptors are a special type of protein that sits astride cell membranes. When two cells touch, these Notch receptors link up, forming channels that allow messages to rapidly travel between large groups of cells.

Notch receptors divide the outer notochord cells into two alternating groups – one group is told to grow into bone, while the other is told to grow into cartilage. Over time, bone starts to form on the surface of the notochord and works its way inward, eventually forming mature vertebrae.

X-ray images of four zebrafish spines

Meddling with cellular signaling on the notochord surface caused zebrafish spines to develop deformities. The first and third image show healthy spines, and the second and fourth image show deformed spines.

When the team tinkered with the Notch signaling on the surface cells, they found that the spinal vertebrae came out deformed – too big, too small, or the wrong shape.

“These results demonstrate that the notochord plays a critical role in guiding spine development,” Wopat said. “Further investigation into these findings may help us better understand the origin of spinal defects in humans.”

Spine patterning is guided by segmentation of the notochord sheath,” Susan Wopat, Jennifer Bagwell, Kaelyn D. Sumigray, Amy L. Dickson, Leonie F. Huitema, Kenneth D. Poss, Stefan Schulte-Merker, Michel Bagnat. Cell, February 20, 2018. DOI: 10.1016/j.celrep.2018.01.084

Post by Kara Manke

Duke Scholars Bridge Disciplines to Tackle Big Questions

A visualization showing faculty as dots that are connected by lines

This visualization, created by James Moody and the team at the Duke Network Analysis Center, links faculty from across schools and departments who serve together on Ph.D. committees. An interactive version is available here.

When the next big breakthrough in cancer treatment is announced, no one will ask whether the researchers are pharmacologists, oncologists or cellular biologists – and chances are, the team will represent all three.

In the second annual Scholars@Duke Visualization Challenge, Duke students explored how scholars across campus are drawing from multiple academic disciplines to tackle big research questions.

“I’m often amazed at how gifted Duke faculty are and how they can have expertise in multiple fields, sometimes even fields that don’t seem to overlap,” said Julia Trimmer, Director of Faculty Data Systems and Analysis at Duke.

In last year’s challenge, students dug into Scholars@Duke publication data to explore how Duke researchers collaborate across campus. This year, they were provided with additional data on Ph.D. dissertation committees and asked to focus on how graduate education and scholarship are reaching across departmental boundaries.

“The idea was to see if certain units or disciplines contributed faculty committee members across disciplines or if there’s a lot of discipline ‘overlap.’” Trimmer said.

The winning visualization, created by graduate student Matthew Epland, examines how Ph.D. committees span different fields. In this interactive plot, each marker represents an academic department. The closer together markers are, the more likely it is that a faculty member from one department will serve on the committee of a student in the other department.

Epland says he was intrigued to see the tight-knit community of neuroscience-focused departments that span different schools, including psychology and neuroscience, neurobiology, neurology and psychiatry and behavioral Sciences. Not surprisingly, many of the faculty in these departments are members of the Duke Institute for Brain Sciences (DIBS).

Duke schools appear as dots and are connected by lines of different thicknesses

Aghil Abed Zadeh and Varda F. Hagh analyzed publication data to visualize the extent to which faculty at different Duke schools collaborate with one another. The size of each dot represents the number of publications from each school, and thickness of each line represents the number of faculty collaborations between the connected schools.

Sociology Professor James Moody and the team at the Duke Network Analysis Center took a similar approach, creating a network of individual faculty members who are linked by shared students. Faculty who sit on committees in only one field are bunched together, highlighting researchers who bridge different disciplines. The size of each marker represents the extent to which each researcher sits “between” two fields.

The map shows a set of strong ties within the natural sciences and within the humanities, but few links between the two groups. Moody points out that philosophy is a surprising exception to this rule, lying closer to the natural sciences cluster than to the humanities cluster.

“At Duke, the strong emphasis on philosophy of science creates a natural link between philosophy and the natural sciences,” Moody said.

Duke graduate student Aghil Abed Zadeh teamed up with Varda F. Hagh, a student at Arizona State University, to create elegant maps linking schools and departments by shared authorship. The size of each marker represents the number of publications in that school or department, and the thickness of the connecting lines indicate the number of shared authorships.

“It is interesting to see how connected law school and public policy school are. They collaborate with many of the sciences as well, which is a surprising fact,” Zadeh said. “On the other hand, we see Divinity school, one the oldest at Duke, which is isolated and not connected to others at all.”

The teams presented their visualizations Jan. 20 at the Duke Research Computing Symposium.

Post by Kara Manke

 

Glitter and Jell-O Reveal the Science of Oobleck

A black and white image showing a circular disk dropping into a container of oobleck

Mixing black glitter with oobleck allowed researchers to track the movement of individual cornstarch particles after a sudden impact. A computer program locked onto pieces of glitter and illustrated their motion. Credit: Melody Lim.

What do gelatin and glitter have to do with serious science? For some experiments, a lot! Duke alumna Melody Lim used jiggly Jell-O and a just a pinch of glitter to solve a scientific mystery about the curious goo many like to call oobleck.

To the uninitiated, oobleck is almost magic. The simple mixture of cornstarch and water feels solid if you squeeze it, but moments later runs through your fingers like water. You can dance across a bathtub full of oobleck, but stand still for too long and you will be sucked into a goopy mess. Not surprisingly, the stuff is a Youtube favorite.

Oobleck is an example of what scientists call a non-Newtonian fluid, a liquid whose viscosity – how easily it changes shape and flows – depends upon the force that is applied. But exactly how it is that this material switches from solid to liquid and back again has remained a mystery to scientists.

A piece of gelatin being squeezed viewed through a circular polarizer

This blogger mixed up a batch of jello to see the photoelastic effect for herself. When viewed with polarized light – from an iPhone screen and a circular polarizer – the jello changes color when squeezed.

“Water is simple to understand, and so is cornstarch,” said Lim, ’16, who is currently a graduate student at the University of Chicago. “However, a combination of the two produces this ‘liquid’ that ripples and flows, solidifies beneath your feet if you run on it, then turns back into a liquid if you stop running and stand still. I wanted to know why.”

The question beguiling scientists was whether sudden impact causes the cornstarch particles to “jam” into a solid like cement, or whether the suspension remains liquid but simply moves too slowly for its liquid-like properties to be apparent — similar to what happens if you skip a rock off the surface of a lake.

“There are these two opposing pictures,” said Robert Behringer, James B. Duke Professor of Physics at Duke. “Either you squish the material and turn it into cement temporarily, or you simply transmit the stress from the impactor straight to the boundary.”

Lim did two sets of experiments to find out which way oobleck works. In one experiment, she mixed black glitter into a transparent channel filled with oobleck, and then used a high-speed camera to watch how the material responded to the impact. The glitter let her track the motion of individual particles after the disc hit.

A piece of gelatin changes color when you squeeze it.

The photoelastic effect in gelatin.

Her video shows that the particles near the impact site jam and become solid, forming what the researchers call a “mass shock” wave that travels slowly through the suspension.

In a second set of experiments, Lim placed the oobleck in a container lined with gelatin, the main ingredient in Jell-O – besides sugar and food dye, of course. Gelatin is what is called a photoelastic material, which means that applying pressure bends light that travels through it, like a prism.

“Next time you eat Jell-O, get out your sunglasses and get somebody else’s sunglasses and look between them,” Behringer said. “Because if you give it a shake you should see all these stress patterns bouncing around.”

After the metal disc hit the oobleck, the gelatin let Lim see how fast the resulting pressure wave traveled through the material and reached the boundary.

A black and white image showing pressure waves traveling through a transparent material after impact

The researchers poured oobleck into a clear container lined with gelatin, a material that bends light when a pressure is applied to it. They saw that the force of a sudden impact is rapidly transmitted through the oobleck and to the boundary with the gelatin. Credit: Melody Lim.

They found that when the impact is sudden, the pressure wave traveled to the gelatin boundary faster than the “mass shock” wave. This means that the reason oobleck appears solid after a sudden impact is because the force of the collision is quickly transmitted to a solid boundary.

“If you are running across the water, that actually puts you into an impact velocity range where the pressure wave is significantly faster than the mass shock,” Behringer said. “Whereas if you try to walk across it, the impact speeds are slow, and the system actually doesn’t have the ability to transport the momentum quickly through the material and so you just sink in.”

“If you’d told me when I started that I would line a narrow container with Jell-o, add cornstarch, water, and black glitter, drop a piece of metal on it, then publish a paper on the results, I would have laughed at you,” Lim said.

CITATION: “Force and Mass Dynamics in Non-Newtonian Suspensions,” Melody X. Lim, Jonathan Barés, Hu Zheng and Robert P. Behringer. Physical Review Letters, Nov. 3, 2017. DOI: 10.1103/PhysRevLett.119.184501

Post by Kara Manke

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