The team—Yuan Yang, assistant professor of materials science and engineering; Nanfang Yu, associate professor of applied physics; and Jyotirmoy Mandal, lead author of the study and a doctoral student in Yang’s group (all department of applied physics and applied mathematics)—built upon earlier work that demonstrated that simple plastics and polymers, including acrylic, silicone, and PET, are excellent heat radiators and could be used for PDRC. The challenges were how to get these normally transparent polymers to reflect sunlight without using silver mirrors as reflectors and how to make them easily deployable.

They decided to use phase-inversion because it is a simple, solution-based method for making light-scattering air-voids in polymers. Polymers and solvents are already used in paints, and the Columbia Engineering method essentially replaces the pigments in white paint with air voids that reflect all wavelengths of sunlight, from UV to infrared.

“This simple but fundamental modification yields exceptional reflectance and emittance that equal or surpass those of state-of-the-art PDRC designs, but with a convenience that is almost paint-like,” says Mandal.

The researchers found their polymer coating’s high solar reflectance (R > 96%) and high thermal emittance (Ɛ ~ 97%) kept it significantly cooler than its environment under widely different skies, e.g. by 6˚C in the warm, arid desert in Arizona and 3˚C in the foggy, tropical environment of Bangladesh. “The fact that cooling is achieved in both desert and tropical climates, without any thermal protection or shielding, demonstrates the utility of our design wherever cooling is required,” Yang notes.

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The team also created colored polymer coatings with cooling capabilities by adding dyes. “Achieving a superior balance between color and cooling performance over current paints is one of the most important aspects of our work,” Yu notes. “For exterior coatings, the choice of color is often subjective, and paint manufacturers have been trying to make colored coatings, like those for roofs, for decades.”

The group took environmental and operational issues, such as recyclability, bio-compatibility, and high-temperature operability, into consideration, and showed that their technique can be generalized to a range of polymers to achieve these functionalities. “Polymers are an amazingly diverse class of materials, and because this technique is generic, additional desirable properties can be conveniently integrated into our PDRC coatings, if suitable polymers are available,” Mandal adds.

“Nature offers many ways for heating and cooling, some of which are extremely well known and widely studied and others that are poorly known. Radiative cooling—by using the sky as a heat sink—belongs to the latter group, and its potential has been strangely overlooked by materials scientists until a few years ago,” says Uppsala University Physics Professor Claes-Göran Granqvist, a pioneer in the field of radiative cooling, who was not involved with the study. “The publication by Mandal et al. highlights the importance of radiative cooling and represents an important breakthrough by demonstrating that hierarchically porous polymer coatings, which can be prepared cheaply and conveniently, give excellent cooling even in full sunlight.”

Yang, Yu, and Mandal are refining their design in terms of applicability, while exploring possibilities such as the use of completely biocompatible polymers and solvents. They are in talks with industry about next steps.

“Now is a critical time to develop promising solutions for sustainable humanity,” Yang notes. “This year, we witnessed heat waves and record-breaking temperatures in North America, Europe, Asia, and Australia. It is essential that we find solutions to this climate challenge, and we are very excited to be working on this new technology that addresses it.”

Yu adds that he used to think that white was the most unattainable color: “When I studied watercolor painting years ago, white paints were the most expensive. Cremnitz white or lead white was the choice of great masters, including Rembrandt and Lucian Freud. We have now demonstrated that white is in fact the most achievable color. It can be made using nothing more than properly sized air voids embedded in a transparent medium. Air voids are what make snow white and Saharan silver ants silvery.”

About the Study

Journal: Science

Title: Hierarchically Porous Polymer Coatings for Highly Efficient Passive Daytime Radiative Cooling

Authors: J. Mandal*, Y. Fu*, A. Overvig*, M. Jia**, K. Sun*, N. Shi*, H. Zhou***, X. Xiao***, N. Yu*, Y. Yang*

Affiliations: *Department of Applied Physics and Applied Mathematics, Columbia Engineering. **Department of Mechanical Engineering, Columbia Engineering ***Advanced Photon Source, Argonne National Laboratory, Lemony, IL 60439, USA

The study was supported by startup funding from Columbia University, the NSF MRSEC program through Columbia University’s Center for Precision Assembly of Superstratic and Superatomic Solids (Y.Y. DMR-1420634), AFOSR MURI (Multidisciplinary University Research Initiative) program (N.Y. grant # FA9550-14-1-0389), AFOSR DURIP (Defense University Research Instrumentation Program) (N.Y. grant # FA9550-16-1-0322), and the National Science Foundation (N.Y. grant # ECCS-1307948). A.C.O. acknowledges support from the NSF IGERT program (# DGE-1069240). We acknowledge support from the Advanced Photon Source in Argonne National Laboratory (under Contract No. DE-AC02-06CH11357) and the Brookhaven National Laboratory (under Contract No. DE-AC02-98CH10886.)

COI: The authors have no competing interests. The authors have filed a provisional patent application with Columbia Technology Ventures for this study.

Columbia Engineering

Columbia Engineering, based in New York City, is one of the top engineering schools in the U.S. and one of the oldest in the nation. Also known as The Fu Foundation School of Engineering and Applied Science, the School expands knowledge and advances technology through the pioneering research of its more than 250 faculty, while educating undergraduate and graduate students in a collaborative environment to become leaders informed by a firm foundation in engineering. The School’s faculty are at the center of the University’s cross-disciplinary research, contributing to the Data Science Institute, Earth Institute, Zuckerman Mind Brain Behavior Institute, Precision Medicine Initiative, and the Columbia Nano Initiative. Guided by its strategic vision, “Columbia Engineering for Humanity,” the School aims to translate ideas into innovations that foster a sustainable, healthy, secure, connected, and creative humanity.

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Pranav Shrestha, Salutatorian

Major: Computer Science

Hometown: Kathmandu, Nepal

Why Columbia Engineering?

Growing up in Nepal, America and an Ivy League education are the proverbial shining city on the hill. A lot of us could not even dream of such an experience—less than 10 people from the entire country are accepted at Ivy League schools. Such an education and the mentorship that entailed was always a distant dream, and the perk of the New York experience put Columbia at the top of the list for me.

Why Computer Science?

I’ve always been fascinated by developing an understanding of how the world works, and computer science offers an entire world governed by simple logical rules and explicit algorithms whose beauty and effectiveness captivate me.

What was your favorite course and professor?

Hands down, Computer Vision with Carl Vondrick. I loved its succinct coverage of everything from the physics involved and traditional algorithms to the state-of-the-art deep learning papers! Professor Vondrick’s teaching style, challenging yet engaging assignments, and straightforward assessments definitely added to the charm. My only regret was not taking it earlier.

How has your education and experience primed you for your career?

Columbia Engineering has played an essential role starting from my very first internship. I was part of the Columbia Center for Career Education’s Startup Internship Program, which was an incredible opportunity as a freshman. This helped me secure my internship with Qualcomm and eventually the research role I’ll be joining full-time.

How have you been spending your time away from Columbia? 

I’ve been staying with my brother in Dallas attempting to stay as occupied as possible—keeping healthy with short jogs and light workouts, catching up with friends, taking time for introspection and exploring interests within and beyond computer science. 

What are your plans for after graduation?

I’ll be joining Qualcomm’s research department working on machine learning compression, primarily the study and deployment of models for Internet of Things devices and low-power chips, and eventually pursuing further studies in a related field. While we’re still a long way away, I’d love to work on projects like the brain-computer interface.

Who are the most inspirational people in your life?

My parents have always been the biggest inspirations for me. The amount of work and sacrifice my parents have had to make for us to be who we are today has been one of my biggest motivators. I also want to thank my brother who has always been my biggest supporter. He was my first teacher, sparking a joy of learning from a very early age, and has always pushed me to pursue new adventures whether it’s hiking a 14er or learning to ski despite my fear of heights. For everything, I’m eternally grateful!

Words to live by?

“Time wasted enjoying is not time wasted.” For over a decade, it’s been a reminder to never feel guilty about the ways I spend my time; to be kind to myself and learn to enjoy life. It’s also a subtle nudge to seek out things I’d enjoy doing.

What does engineering for humanity mean to you?

For me, it’s the ability to look beyond our current obligations and projects to consider our impact on society. Whether it’s on a global scale like tackling misinformation or for smaller communities, engineering for humanity orients us towards our visions of a better society.

New York, NY—January 28, 2020—A new study from Columbia Engineering and Harvard University identified the critical physiological importance of suitable temperatures for butterfly wings to function properly, and discovered that the insects exquisitely regulate their wing temperatures through both structural and behavioral adaptations.

Contrary to common belief that butterfly wings consist primarily of lifeless membranes, the new study demonstrated that they contain a network of living cells whose function requires a constrained range of temperatures for optimal performance. Given their small thermal capacity, wings can overheat rapidly in the sun when butterflies cease flight, and they can cool down too much during flight in a cold environment. The study, published online today by Nature Communications, is the first to explore the implications of temperature in shaping the wing structure and behavior of butterflies.

“Butterfly wings are essentially vector light-detecting panels by which butterflies can accurately determine the intensity and direction of sunlight, and do this swiftly without using their eyes,” says Nanfang Yu, associate professor of applied physics at Columbia Engineering and co-PI of the study.

The team, which was co-led by Naomi E. Pierce, Hessel Professor of Biology in the department of organismic and evolutionary biology, and Curator of Lepidoptera at the Museum of Comparative Zoology, Harvard, used their expertise in biology and optics to make a number of significant discoveries. By carefully removing the wing scales to enable them to peer into the interior of the wings, and by staining the neurons found within the wing, they found that butterfly wings are loaded with a network of mechanical and temperature sensors. The living tissues in the wings are actively supplied by circulatory and tracheal systems throughout the adult lifetime—in the case of painted lady butterflies, for more than three weeks.

They also discovered a “wing heart” that beats a few dozen times per minute to facilitate the directional flow of insect blood, or hemolymph, through a “scent pad” or an androconial organ located on the wings of some species of butterflies.

“Most of the research on butterfly wings has focused on colors used in signaling between individuals,” says Pierce. “This work shows that we should reconceptualize the butterfly wing as a dynamic, living structure rather than as a relatively inert membrane. Patterns observed on the wing may also be shaped in important ways by the need to modulate temperatures of living parts of the wing.”

Yu’s lab designed a noninvasive technique based on infrared hyperspectral imaging, with each pixel of an image representing one infrared spectrum, that enabled them to make—for the first time—accurate measurements of the temperature distributions over butterfly wings. “This has been difficult to do until now,” Pierce notes, “because of the thinness and delicacy of butterfly wings.”

“This imaging technique enables us to examine physical adaptations that decouple the wing’s visible appearance from its thermodynamic properties,” Yu adds. “We discovered that diverse scale nanostructures and non-uniform cuticle thicknesses create a heterogeneous distribution of radiative cooling—heat dissipation through thermal radiation—that selectively reduces the temperature of living structures such as wing veins and scent pads.”

The effect of this regional and selective enhancement of thermal radiation was amply demonstrated in the team’s thermodynamic experiments on butterfly wings. Experimental conditions that mimic the butterflies’ natural environment were created in Yu’s lab, and allowed the researchers to quantify the relative contributions of several environmental factors to the wing temperature. These included the intensity of sunlight, the temperature of the terrestrial environment, and the “coldness” of the sky, which can serve as an efficient heat sink of thermal radiation from heated wings. The team found that in all simulated environmental conditions, despite diverse visible colors and patterns, the areas of butterfly wings that contain live cells (wing veins and scent pads) are always cooler than the “lifeless” regions of the wing due to enhanced radiative cooling.

“The nanostructures found in the wing scales could inspire the design of radiative-cooling materials to cope with excessive heat conditions,” says Cheng-Chia Tsai, a PhD student in Yu’s group who was lead author of the study.

The researchers conducted a series of behavioral studies of living butterflies from six of the seven recognized butterfly families, to investigate responses to simulated sunlight applied to the wings. The team discovered that the insects use their wings to sense the direction and intensity of sunlight—the main source of warmth or overheating—and to respond with specialized behaviors to prevent overheating or overcooling of their wings. For example, all species studied exhibited a relatively constant “trigger” temperature of approximately 40oC (104oF), turning within a few seconds to avoid overheating of wings from a small light spot shone upon them.

Yu and Pierce are now conducting a large-scale systematic optical study of the lepidopteran collections in Harvard’s Museum of Comparative Zoology. These include thousands of individual specimens of hundreds of butterfly species across the entire phylogenetic tree, each specimen with full hyperspectral imaging data taken from the ultraviolet to the mid-infrared.

In 1863, Henry Walter Bates, an English naturalist and explorer, wrote about butterfly wings in his book The Naturalist on the River Amazons, “On these expanded membranes Nature writes, as on a tablet, the story of the modifications of species …” Just like deciphering enigmatic symbols on a tablet, the team hopes to gain a comprehensive understanding of the wing coloration and pattern, which are the results of many (and often conflicting) biological and physical factors: sexual selection, warning coloration, mimicry, camouflage, and thermoregulation.

“Each wing of a butterfly is equipped with a few dozen mechanical sensors that provide real-time feedback to enable complex flying patterns,” Yu says. “This is an inspiration for designing the wings of flying machines: perhaps wing design should not be solely based on considerations of flight dynamics, and wings designed as an integrated sensory-mechanical system could enable flying machines to perform better in complex aerodynamic conditions.”

About the Study

Journal: Nature Communications

Title: Physical and Behavioral Adaptations to Prevent Overheating of the Living Wings of Butterflies

Authors: Cheng-Chia Tsai 1; Richard A. Childers 2; Norman Nan Shi 1§; Crystal Ren 1; Julianne N. Pelaez 2†; Gary D. Bernard 3; Naomi E. Pierce 2, 4; and Nanfang Yu 1.

1. Department of Applied Physics and Applied Mathematics, Columbia Engineering
2. Department of Organismic and Evolutionary Biology, Harvard University
3. Department of Electrical Engineering, University of Washington
4. Museum of Comparative Zoology, Harvard University

† Currently at Department of Integrative Biology, University of California, Berkeley

§ Currently at Western Digital

The study was supported by the National Science Foundation (no. PHY-1411445 awarded to N. Yu and N. Pierce, no. DEB-0447242 awarded to N. Pierce), and the Air Force Office of Scientific Research (no. FA9550-14-1-0389 through the Multidisciplinary University Research Initiative program and no. FA9550-16-1-0322 through the Defense University Research Instrumentation Program awarded to N. Yu). R. A. Childers was supported by the Graduate Research Fellowship Program of the National Science Foundation. Measurements were carried out in part at the Center for Functional Nanomaterials, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under contract DE-SC0012704.

COI: The authors declare no competing interests.

Columbia Engineering

Columbia Engineering, based in New York City, is one of the top engineering schools in the U.S. and one of the oldest in the nation. Also known as The Fu Foundation School of Engineering and Applied Science, the School expands knowledge and advances technology through the pioneering research of its more than 220 faculty, while educating undergraduate and graduate students in a collaborative environment to become leaders informed by a firm foundation in engineering. The School’s faculty are at the center of the University’s cross-disciplinary research, contributing to the Data Science Institute, Earth Institute, Zuckerman Mind Brain Behavior Institute, Precision Medicine Initiative, and the Columbia Nano Initiative. Guided by its strategic vision, “Columbia Engineering for Humanity,” the School aims to translate ideas into innovations that foster a sustainable, healthy, secure, connected, and creative humanity.