Communication Flexible transparent aerogels as window retrofitting films and optical elements with tunable birefringence

Highlights • We develop aerogels with thermal super-insulation and mechanical flexibility. • Potential applications range from single pane window retrofitting to smart fabrics. • These transparent aerogels exhibit liquid crystallinity and tunable birefringence. • Fabrication utilizes self-assembly of nanocellulose and polysiloxane cross-linking. Abstract Experimental realization of optically transparent, mechanically robust and flexible aerogels has been a longstanding challenge, which limits their practical applications in energy-saving devices, such as thermally insulating films for enhancing energy efficiency of windows. The poor transparency precluded even hypothetical consideration of the possibility of birefringent aerogels. We develop birefringent and optically isotropic aerogels that combine properties of thermal super-insulation, mechanical robustness and flexibility, and transparency to visible-spectrum light. This unusual combination of physical properties is achieved by combining liquid crystalline self-organization of cellulose nanofibers with polysiloxane cross-linking and control of the nanoscale porosity to form hybrid organic-inorganic mesostructured aerogels. Potential applications of these inexpensive materials range from single pane window retrofitting to smart fabrics. Graphical abstract

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Keywords Aerogel Cellulose nanofiber Liquid crystal Thermal insulation Transparent

Qingkun Liu obtained his BS degree in Optics from the University of Electronic Science and Technology of China (2007) and Ph.D. degree in Optical Engineering from Zhejiang University (2013). He joined in Department of Physics of the University of Colorado-Boulder as a research associate in 2013, where he also was a visiting scholar since 2008. His current research focuses on nanostructured and biological soft matter, including nanocellulose, liquid crystals, colloids, nanomaterials, etc.

Allister W. Frazier obtained his BS degree in Physics from the University of Nevada, Reno (2016). He is currently a Ph.D. student in Materials Science and Engineering at the University of Colorado Boulder. His research focuses on the design and fabrication of cellulose-based aerogels and production of nanocellulose from bacterial cellulose using sustainable resources.

Xinpeng Zhao received his BS degree at Northwestern Polytechnical University in 2012 and MS degree at Xi’an Jiaotong University in 2015. He is currently a Ph.D. student in Mechanical Engineering at University of Colorado Boulder. His research focuses on modeling, simulation and experimental characterization of thermal transport in thermal insulation materials such as aerogels and nanowoods.

Joshua A. De La Cruz obtained his BS degree in Materials Science and Engineering from the University of Arizona (2014) and MS degree in Materials Science and Engineering from the University of Colorado Boulder (2016). He is currently a Materials Science and Engineering Ph.D. student at the University of Colorado and his research focuses on cellulose-based biomimetic photonic structures used for energy-saving technologies.

Andrew J. Hess obtained his BS degree in Physics from the Department of Physics and Astronomy at Calvin College (2013), a MS degree in Physics, and a Renewable Energy Certificate from the University of Colorado Boulder (2016). He is currently a Physics Ph.D. candidate at the University of Colorado Boulder and his research focuses on the study of liquid crystals, including liquid crystalline polymers, colloids, solitons and gels.

Ronggui Yang obtained a BS in Thermal Engineering from Xi’an Jiaotong University (1996), a MS in Thermal Engineering from Tsinghua University (1999), a MS in Micro-Electro-Mechanical Systems from The University of California, Los Angeles (2001) and a Ph.D. in Mechanical Engineering from MIT (2006). He started his faculty career as an Assistant Professor at the University of Colorado Boulder (CU-Boulder) in January 2006, was promoted to Associate Professor in 2011 and to Full Professor in 2016. He currently directs the Nano-enabled Energy Conversion and Thermal Management Systems group (NEXT) at CU-Boulder. His research interests are on the fundamentals of nanoscale transport phenomena (thermal, electrical, thermoelectric, phase-change) and applications of micro/nanotechnologies to thermal, energy and water systems. Dr. Ronggui Yang has published ~ 140 journal papers, with an h-Index of 44, a total citation > 9000 times per Google Scholar. He is an elected ASME Fellow.

Ivan I. Smalyukh obtained BS and MS degrees from Lviv Polytechnic National University (1995) and Ph.D. from Kent State University (2003). He was a postdoctoral research associate at the University of Illinois Urbana-Champaign. He joined the Department of Physics at CU-Boulder as an Assistant Professor in 2007, was promoted to Associate Professor in 2014 and Full Professor in 2017. He is a fellow of Renewable Sustainable Energy Institute and Materials Science Engineering Program and directs the Soft Matter Physics Group at CU-Boulder. His research focuses on soft matter and biological systems, including liquid crystals, colloids, polymers, bacteria, gels, biomaterials and their photonic, electro-optic and energy-related applications. He published ~160 papers and has an h-index of 44. He is a fellow of the American Physical Society (APS). He received many awards, including the Bessel and Glenn Brown Awards, the PECASE Award from the White House and the GSoft Award from APS.

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