Dr. Hunter King, physicist and polymer scientist, draws inspiration from birds and beetles05/27/2020
As part of The University of Akron’s sesquicentennial celebration — honoring 150 years of our people, place and promises — we are hosting a “Celebration of Academic Excellence” to highlight the history and future of our many academic disciplines.
Today, meet Dr. Hunter King, who joined UA in 2016. He’s an experimental soft matter physicist, assistant professor of polymer science and core faculty member in UA’s interdisciplinary Biomimicry Research and Innovation Center (BRIC).
Dr. Hunter King
Your work on the mechanics of birds’ nests was recently featured in the New York Times. What’s the current state of that project — and what do you hope to learn from studying how birds build their nests?
Many birds collect individual skinny elements, and combine them to form a cohesive, sturdy nest. In selecting a given stick, they probably don’t know what role it will play in the structure. They do seem to know what stick properties will make for good nest properties. That means that they have some intuition for how a “nest” material emerges from sticks interacting with each other. Insight into that emergence is what we’re after, broadly speaking. It is the equivalent of looking at an individual molecule and guessing the stiffness, yield stress, etc., of its bulk glassy state — a very polymer “physics-y” question, but at macro-scale, without thermal energy, and also with animal protagonists.
It turns out a simplified jumble of straight sticks, collectively, is already a mechanically complicated material. We are presently working to characterize it experimentally in such a way as to validate results from our collaborators in Illinois, who are performing one-to-one computational experiments. We have some ideas about how a “nest-like” material, for instance, manages to absorb energy while being slowly compressed and released. These ideas can be verified by tracking internal motions of simulated elements, and can have implications for other, fairly ubiquitous systems of tangled fibers.
On the biological side, we have begun collaborating with proper bird people in Scotland for behavioral experiments. With their help, we hope to improve and complement our tests at the Akron Zoo — to challenge birds with pre-selected building materials (for which we can make simple artificial analogs in the lab) to better understand how their decision-making anticipates emergent mechanics.
Your work on termite mounds, which you conducted as a postdoctoral scholar at Harvard University, has been featured in Popular Science and Massive Science. Can you briefly describe that work and what you discovered?
Termites, in a very different application of granular physics, can individually extrude tiny pellets of wet soil for construction. In an intensely coordinated, yet decentralized process with little cognition, they build gigantic, complex structures (mounds) above their subterranean nests and fungus gardens (that they cultivate as nutritional supplement). Our project initially aimed to derive the rules of the swarm construction process, but we quickly got caught up in the question of its physiological function, a subject of existing debate, and inseparable from that of process. Two leading theories described how the mound’s mostly empty, interconnected, internal conduit system facilitated respiratory exchange, but speculated completely different driving forces of internal bulk flow between nest and mound: one cited metabolic heating, which would create buoyancy in the nest air, driving a convective cell; the other cited transients in the wind, which would occasionally churn enough stale air into the mound above. The theories predicted very different (both very slow) flow patterns, but had not directly measured them, so we built our own custom termite-mound-anemometers and traveled to India and Namibia to do so in the field. Neither theory matched the data: wind was largely uncorrelated to internal flow, and the nest was rarely the warmest part of the mound. We instead measured a flow pattern consistent with an oscillating convective cell — one which reversed itself with a reversal in the internal thermal gradients. It became clear that the mound architecture, rather than requiring a directed energy source, takes advantage of the natural day-night oscillation in ambient temperature. By having the conduit network straddle exposed and insulated portions of the mound, the non-uniform heating and cooling of the structure was enough to consistently drive ventilation. We thought it was a novel way to derive passive function, and source of inspiration for sustainable, alternative HVAC solutions.
We have been studying a fog-capturing solution demonstrated by a Namib Desert beetle, and a passive thermoregulation strategy in a Fijian ant species.
A couple species of beetle in the Namib desert are known to stand at the top of dunes in the foggy, morning wind and intercept fog droplets as their primary source of drinking water. As their success at collecting water is so crucial to survivability, it is reasonable to think that the behavior would accompany additional physical adaptation to make it easier, but looking at the beetle, it is not clear if/how it is manifested. A paper in 2001, which has dominated the narrative about this beetle’s solution, pointed to the wettability of its surface, and how it promotes transport of collected water from its back to its mouth. The details of this explanation never made sense to me, and didn’t seem to pan out in subsequent attempts to mimic the solution. We have instead been investigating how surface structure of the beetle affects the primary collection step: fluid dynamics of deposition — or how a fog droplet is coaxed into collision with an intercepting target. We have found that addition of subtle texture can make a big difference in that process, independent of wettability.
The obvious application for this result is in fog collector design — it is typically way easier and less toxic to modify a surface’s morphology than its wettability — but we’ve noticed that the same physics is responsible for the collection efficiency of, for instance, respiratory droplet nuclei on the fine fibers of an N95 mask (this realization, regrettably, came shortly after our last grant proposal submission on the topic.).
The Fijian ant in question is the only known example of an obligate mutualist relationship between ant and plant. The ants sow and maintain a particular epiphytic plant high in the trees, and the plant provides both food and housing to the ant colony. The ants “forgot” how to build their own nests (they don’t even modify the structure of the plant), and the plants have lost the ability to propagate on their own. The relationship effectively couples them, with respect to evolution, as a unit — an adaptation that benefits one had better not harm the other.
The combined strategy involves one notable, conflicting goal: exposure to sun by planting high in the canopy gives more sugar, but elevated temperatures can harm ant broods. We have been trying to understand how the plant’s shape and structure has responded to selective pressure favoring the ants’ thermoregulation concerns. We compared the morphological adaptation in the obligate mutualist plants to their nearest ant-independent relatives with respect to heat transfer function, and measured their performance with our own deployable sensors in the field. Preliminary data suggested a counterintuitive result: that the plants adapted for ants seem to promote variability and extremes in internal temperature. Our working hypothesis is that this might be the nature, rather than a failure, of the solution — that providing a broad range of temperature in a large internal structure gives the ants options to constantly reposition their eggs to comfortable locations, rather than maintain a constant interior temperature that could fall outside of the acceptable range. If true, it would represent another elegant example of natural selection solving a problem by posing it in a way we might not have thought of.
What drew you to “animal engineering” and biomimicry — and, ultimately, to the Department of Polymer Science and BRIC at UA?
I have to admit I wouldn’t have anticipated my association with any of those things a few years ago, but they have come together serendipitously. My deep connection is permanently with physics, as the framework to make sense of everything. As a student, I was drawn to problems that challenged my basic understandings, rather than those calling for further refinement of what we already basically know. I found them in phenomena that were confusing or complex despite their familiarity or dependence on only simple parts, and they tended to have mechanical and statistical explanations that reinforce a certain, satisfying worldview.
My exposure to biology during the postdoc made me realize that I don’t need to understand a cell or a gene to play a role in discovering important lessons from biological systems, because the biological world is in some sense a great big playground of applied physics, often of the sort I’ve always enjoyed. A very similar statement can be made of polymer physics: from seemingly straightforward elements — stringy molecules whose chemical details we may not need to carefully understand — emerges a spectacular richness of mechanical and phase behavior to challenge one’s physical intuition, and, it almost goes without saying, find application everywhere.
I like to think that my lab is part of the biomimicry ecosystem by providing a bridge between bioinspiration and application. That bridge, where we’re concerned, is in recognizing and framing the physical principle at the crux of a biological solution so that the inspired engineer might make use of it. The dynamic research environment of both the College of Polymer Science and Polymer Engineering and BRIC have been invaluable in maintaining the diverse and interdisciplinary research program we’ve only started to put together.
I think I’ve always been concerned (but not formally very well-informed) about environmental issues, but had never managed to steer professional interests specifically toward them. Now, nearly everything we do has some connection — whether it relates to water scarcity or sustainable building.
The HAB project allowed my lab to play a useful role in a solution way outside our area of familiarity (another testament to the crucial interdisciplinary dynamic to be found at UA), and as a recent transplant to the area, I was eager to engage with problems that are relevant here.
Do you have any other projects in progress, or in the queue?
We are presently following two broad ideas seriously. One is passive, sorbent-based vapor harvesting — another extreme solution for water scarcity — which we are attempting with electrospun fiber mats and microwave radiation, and which I think might be a yet-to-be-identified function of termite mounds.
The other regards how surfaces manage to come in contact, at the last moments, at the tiny scale between touching and not touching, when the evacuation of surrounding fluids (air, water) plays a big role. Dynamics there underlies the droplet collision related to fog capture, and also the efficacy of bioinspired underwater adhesion research. For this we have developed an imaging method to track the evolution of near-contact between a soft surface and glass prism, with precision down to a few tens of nanometers.
What problem(s) do you most hope to solve, over the course of your career, through your research? What are your long-term goals?
It feels like admitting a sort of heresy, but my long-term hopes are not so much organized around specific goals or problems. My main satisfaction comes from the day-to-day engagement with science, learning and problem solving with students, in the lab and in the field. I hope to always be engaged in worthy and stimulating endeavors, and always be able to make meaningful contributions such as to deserve the participating role.
Outside the classroom and lab, what do you enjoy doing in your spare time?
As an untenured professor with a baby and a preschooler, the concept is almost hard to imagine. I remember reading books for pleasure — that was important at one point, and I’d like to have it back one day. Between meltdowns and crises at home during the pandemic, I’ve enjoyed some of the improvised projects with my daughter. The best has been in disassembling and reassembling a big fallen maple in the back yard — building a bird-nest/wicker igloo, various forts, bows-and-arrows.
Looking back on your own time in college, what advice do you have for UA students?
Especially as a not-very-mature undergrad, I don’t think I ever had a clear idea of the path I was on — how my actions were actually related to my future career. Things mostly worked out OK (though it easily could have not), but I spent a lot of time unnecessarily worrying about some things, and not conjuring enough effort for others, mostly because I didn’t know how to seek the right advice early on. Looking back from the perspective of the faculty, I see there were people available to help, and that I needn’t have been reluctant.
I was very fortunate to find my way into a summer research position in my third year. That hands-on experience and interaction with my advisor/group really shaped my aspirations at a crucial moment.
So — don’t hesitate to seek career advice; take advantage of experiential learning opportunities.