Twisted cholesteric liquid crystal patterns are found in the iridescent tessellated cuticles of many insects and a few fruits. Their accurate replication is extremely difficult since discontinuous patterns and colors must coexist in a single layer within continuous structures. We approach the problem of the high-fidelity capture of the structural complexity observed in nature by focusing on patterned, iridescent carapaces of insects with a non-uniform twisted organization of chitin fibers. We use cholesteric liquid-crystalline materials to mimic biological cholesteric liquid crystals, which choice is yet underutilized in the toolbox of researchers and engineers working in the domain of biomimicry. We attempt to reproduce the textural, structural and color properties of biological structures at several length scales. We made optimal use of resources and stages during the fabrication procedure, in the spirit of eco-design.
We focus on the emblematic case of the scarab beetle Chrysina gloriosa. The cuticle of C. gloriosa exhibits green and silver bands with selective and broad light reflections, respectively. Several requirements have to be met: the biomimetic material must be a monolayer; exhibit a disruptive character of green and silver colors; display the continuity of the twisted structures in all directions with, in the upper part of the film, an alternate mix of variable and fixed orientations of the helical axis; include a pitch gradient related to reflection colors ranging from green to near IR; and exhibit pitch variations in similar ranges.
By means of a single sequence based on self-organization, precise control of a single-piece sample structure composed of two different colored patterns with the same unique pitch gradient was enabled. A multicriterion comparison reveals a very high level of biomimicry.
Biomimetic sample with a piece of carapace.
Biomimetic sample with a piece of carapace.
In addition to this set of conceptual advances, we present a concrete use of the material for advanced optical tags in cryptography. The functionality of synthetic and biological materials is relevant to optical communication and camouflage.
The present design involves a high versatility of chiral patterns unreached by the current manufacturing techniques such as metallic layer vacuum deposition, template embossing and various forms of lithography which are limited and often prohibitively expensive.
A. Scarangella, V. Soldan and M. Mitov, Biomimetic design of Iridescent Insect Cuticles with Tailored, Self-Organized Cholesteric Patterns, Nature Communications, 11, 4108 (2020).
From biomimicry to bioinspiration: by designing time-temperature controllers
Involved people: Cécilia Boyon (PhD), Michel Mitov (PI).
Collaborator: Vanessa Soldan (Centre de Biologie Intégrative, CNRS-Univ. Paul-Sabatier, Toulouse-France).
Funding: ANR, projet COLEOPTIX (ANR-17-CE30-0025).
Previously, we have researched the transfer of strategy and biological properties to artificial films: colors, spectral signature, textures, structures up to function. Mimicry is the quest. With a bio-inspiring approach, we start from the natural structure to create a physical property and function unnecessary related to the insect. Cuticles of Chrysina genus suggests us the design of reflectors which in turn inspire the manufacture of tags recording the thermal history of a product.
Intrigued by the observation of Bragg gratings with a depth-dependent periodicity in the carapace of Chrysina beetles, we have determined the experimental conditions leading to their transcription into synthetic materials. We correlate the optical properties of such reflectors with their internal morphology as observed by transmission electron microscopy. With the use of a single parameter, the reflection color is made time-tunable. Different spectral bands, from golden yellow to near-infrared are available and irreversibility of the final color is reached. On the basis of the design concept and these properties, these hybrid chiral–achiral materials inspire us the fabrication of smart reflective labels. When encapsulated in the package of a product to be kept in cold conditions, such a vaccine, the label records the history of the product conservation. Two kinds of information based on spectral changes are in fact recorded: a qualitative information reporting that the product was kept outside of the specified storage temperature and a quantitative information giving an indication of the time elapsed since then.
Structural analysis of samples for different annealing times. Transmission electron microscopy transverse views accompanied by the half pitch (distance between two bright stripes) as a function of the normalized depth (). Scale bar = 2 μm.
Structural analysis of samples for different annealing times. Transmission electron microscopy transverse views accompanied by the half pitch (distance between two bright stripes) as a function of the normalized depth (local depth over the total thickness of the sample). Scale bar = 2 μm.
From biomimicry to bioinspiration: by playing with the surface tension anisotropy
Involved people: Cécilia Boyon (PhD), Michel Mitov (PI).
Collaborator: Vanessa Soldan (Centre de Biologie Intégrative, CNRS-Univ. Paul-Sabatier, Toulouse-France).
Funding: ANR, projet COLEOPTIX (ANR-17-CE30-0025).
By acting on the surface anisotropy of the film from top to bottom, different structural periodicities and colors are created in a cholesteric layer. By acting on the surface tension anisotropy of the film from top to bottom, different structural periodicities and colors are created in a cholesteric layer.
Cholesteric colored patterns are ubiquitous in the body of insects and fish. Intelligent coatings for optical communication, signaling, and camouflage would be among the bio-inspired materials. A local and transverse surface tension anisotropy is produced in a one-piece cholesteric polymer film to provide distinct structural colors. During the twisted self-assembly of a cholesteric oligomer film in confined geometry, we show that local surface tension anisotropy may allow various colors to coexist in a monolayer film. Customized layers of a transparent, isotropic surfactant on the free side of a cholesteric film with a pitch gradient are used to tailor the wavelength of reflection. Transmission electron microscopy of cross-sections has shown that structural colors are related to internal structure at the nanoscale. Aside from their conceptual uniqueness, these structures with changing surface tension anisotropy would be of interest to optical self-guiding systems for autonomous cars, functional facades and anti-counterfeiting labels.
Involved people: Chloé Bayon (PhD), Gonzague Agez (Senior lecturer UPS), Michel Mitov (PI).
Cholesteric liquid crystals are omnipresent in living matter under both in vivo and in vitro conditions by concerning the major types of molecules essential to life. In many insect carapaces, chitin fibers organize into a cholesteric structure which confers to insects iridescent colors producing visual displays. A great number of insects utilize tessellated carapaces, containing arrays in the form of bumps, pits, stripes, pixels or other patterns. Understanding the sense of these physical variations may help to understand the role of optical information in the evolution of insects. It may also inspire the design of novel biomimetic materials to be used in nanophotonics, micro-optics, thermal regulation and camouflage.
We show that the optical properties of the two-band carapace of the scarab beetle Chrysina gloriosa are issue from twisted structures with a variable orientation of the helicoidal axis coupled to a pitch gradient. Over the visible and the near IR spectra, polygonal cells in tessellated green stripes behave as multiwavelength selective micro-mirrors and the silver stripes as non-textured specular broadband mirrors. The conclusions of the literature supporting the similarity of the structures and optical behaviors for synthetic and biological materials have to be revised. Biomimetic materials could be used in the domain of wavelength-selective light modulators and packet switching for the routing of optical information.
Involved people: Michel Mitov (PI). Collaborators: Vanessa Soldan and Stéphanie Balor (Centre de Biologie Intégrative, CNRS-Univ. Paul-Sabatier, Toulouse-France).
Studies on wax layer in insect cuticles have mainly focused on its chemical composition revealing complex mixtures of lipids. In the absence of information on its physical organization, the wax layer has been considered as isotropic. However we show in the case of the scarab beetle Chrysina gloriosa that it may exhibit an internal anisotropic striped texture with a mean periodicity of 28 nm. Re-examining past investigations of cuticular structures with a major focus on the wax seems to be necessary. It could give the impetus to search for anisotropic textures in the wax layer of other scarab beetles, insects or arthropods in general.
Physics of complex twisted liquid crystal structures
Cholesteric liquid crystals offer a multitude of possibilities for shaping light, many of which are still unexplored. Shaping of ultrafast laser pulses (20 fs), optical materials whose the function depends on the length scale for light-matter interactions, broadening of the reflection bandgap, solutions for the interlacing of opposite helicity senses when a single sense is the current situation: such a selection of contributions is summarized in this section.
Funding: ANR, projet COLEOPTIX (ANR-17-CE30-0025).
Chirped pulse amplification is a technique for amplifying an ultrashort laser pulse with the laser pulse being stretched out temporally and spectrally prior to amplification. Then, the amplified pulse is recompressed back to the original pulse width through reversal of the process of stretching. There are several ways to construct stretchers and compressors. Using cholesteric liquid crystals with a constant pitch or a pitch gradient is a solution that has just appeared in the research domain.
We present a novel statistical approach conducted on a large number of samples that reveals the existence of different groups (clusters) in the optical response. This quantitative approach highlights the possibility of stretching or compressing ultra-short pulses. We show that the profile of the Bragg band allows tuning the dispersion of 20 fs pulses. The negative or positive sign of the Group Velocity Dispersion (GVD) — leading to compression or stretching of the pulses — depends on the relative position of the pulse spectrum versus the Bragg band. A pitch-gradient cholesteric makes it possible to minimize the third order of the spectral phase that adds adverse temporal satellites to the pulse (pre-pulses and post-pulses). Novel and promising opportunities to shape ultrashort (sub-100 fs) pulses are offered.
Ongoing research on cholesteric liquid crystals takes advantage of the peculiar behavior of twisted structures subject to curvature. In tilted or oblique cholesterics, the orientation of the helix axis is not everywhere perpendicular to the film surfaces. The tilted structure may periodically repeat by giving rise to specific deformed twists, like in the polygonal texture. The tunability of color and polarization properties is made available, which is not possible in regular planar textures. Such a variability of optical properties is reached by changing the parameters of the material design, like the surface anchoring energy, without applying any external field.
By hyperspectral imaging we visualize the transmission and the reflection of tilted cholesteric structures with a spectral resolution of 6 nm over 400-1000 nm against a few tens of nm that would be achieved by the available techniques. A correlation between spectral shifts and spatial twists is made possible.
(a) Polygonal textures observed by optical microscopy in transmission (unpolarized light) and reflection (crossed polarizers) modes. (b) 3D structure of the polygonal texture by combining different microscopy methods: atomic force microscopy (AFM), optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM).
Involved people: Chloé Bayon (PhD), Gonzague Agez (Associate professor), Michel Mitov (PI).
Here are multifunctional optical films whose function depends on the scale at which the interaction with light occurs. At the macroscopic scale, the progressive inclination of the helical axis with the annealing time provides a new scenario for adjusting the color of the cholesteric by the fine control of the surface tension. At the micrometer and nanometer scales, the polygonal texture of the film is an array of microlenses that focus and guide the light with a pattern (spot or ring) depending on the incident wavelength. The peculiar structure of this chiral, Bragg lens, elucidated by means of complementary microscopies, is responsible for this behavior.
Applications of chiral microlenses are in the field of wavelength-tunable chiro-optical devices and lab-on-a-chip optical systems that offer the combined benefits of multiple light manipulation capabilities, seamless integration, and mechanical stability.
Highlighted in Liquid Crystals Today and Nature Photonics
Mechanical origin of the broadening the reflection bandgap in cholesteric gels
Model of coil spring with a gradient of stiffness as inspired by cross-sectional views of the polymer network elucidated by TEM. Left column: The network appears dark (respectively, light grey). (a) The network concentration is homogeneously distributed in the bulk (symmetrical irradiation conditions). (b) A network concentration gradient is evident (asymmetrical conditions). (c) In this peculiar case, the network detached from one side of the cell and relaxed into the volume of the liquid crystal (asymmetrical conditions). A small fraction of polymer is visible close to the left substrate. Right column: Model composed of N subsprings placed end to end, which represent the helical network. (a) All the subsprings have the same spring constant ki and the same length. (b) The more concentrated the network, the larger the constant of the local subspring becomes. Due to the network that models the liquid crystal structure, the pitch of the gel is proportionally distorted. (c) The subsprings relaxed and attained their unstressed configuration.
Cholesterics with a broadened bandgap of a few hundreds of nm against a tens of nm address fundamental questions about the fine tuning of the mesoscopic structural chirality with the help of the physical parameters related to the design procedure. Potential applications are smart windows to regulate the solar light and heating or reflective polarizer-free displays operating in a low light ambiance.
In cholesteric liquid-crystalline gels or Polymer-Stabilized Cholesteric Liquid Crystals, the mechanical role of the polymer network over the structure of the whole gel was ignored. We show that the stress gradient exerted by the network over the helical structure drives the broadening of the optical band gap, in the absence of a gradient in chiral species. Model calculations and finite-difference time-domain simulations show that the network acts as a spring with a stiffness gradient. The present results indicate a revision to the common understanding of the physical properties of liquid-crystalline gels is necessary when a concentration gradient in a polymer network is present.
Sensitive Matter - Foams, Gels, Liquid Crystals, and Other Miracles, Michel Mitov, Harvard University Press, 2012.
Summary
Life would not exist without sensitive, or soft, matter. All biological structures depend on it, including red blood globules, lung fluid, and membranes. So do industrial emulsions, gels, plastics, liquid crystals, and granular materials. What makes sensitive matter so fascinating is its inherent versatility. Shape-shifting at the slightest provocation, whether a change in composition or environment, it leads a fugitive existence.
Physicist Michel Mitov brings drama to molecular gastronomy (as when two irreconcilable materials are mixed to achieve the miracle of mayonnaise) and offers answers to everyday questions, such as how does paint dry on canvas, why does shampoo foam better when you “repeat,” and what allows for the controlled release of drugs? Along the way we meet a futurist cook, a scientist with a runaway imagination, and a penniless inventor named Goodyear who added sulfur to latex, quite possibly by accident, and created durable rubber.
As Mitov demonstrates, even religious ritual is a lesson in the surprising science of sensitive matter. Thrice yearly, the reliquary of St. Januarius is carried down cobblestone streets from the Cathedral to the Church of St. Clare in Naples. If all goes as hoped―and since 1389 it often has―the dried blood contained in the reliquary’s largest vial liquefies on reaching its destination, and Neapolitans are given a reaffirming symbol of renewal.
• Mitov has a light touch, writing like the hip, pop-culture–loving, corduroy-jacket–wearing chemistry teacher that you always wanted but never had.— The Washington Post
• Recommended. Readers come away from the book with a renewed appreciation for the complexity of such everyday substances as champagne, rubber and toothpaste.— Scientific American
• This book is a delight. With grace, poise and precision, Michel Mitov makes the case that there is as much wonder and challenging science in the behavior of everyday substances—egg white, toothpaste, sand, soap foam—as there is in the most esoteric experiments in particle physics. Sensitive matter could wish for no more sensitive, no more responsive and intelligent, a champion. — Philip Ball, essayist
• Champagne bubbles, mayonnaise, rubber, blood and sand are part of the eclectic mix of materials explored in Sensitive Matter. Mitov applies his vivid imagination to explaining how soft materials respond to disturbances. It is a quirky and fun introduction to this practical, complex field.— Nature
• Michel Mitov’s delightful book is well-written, easy to understand and a lot of fun. It will appeal even to those who have just a fleeting interest in science.—Current Science India
• This book is part of the joyous science.— France Cultureradio channel
• A must-read.— Radio France Internationale
• Making use of anecdotes or historical facts and not disdaining humor, the author has managed to make this book not only informative but also and especially captivating. It is rare for a researcher to write such an informative book that reads like a novel!— Roberval Prize Jury
• An inventory of the little wonders of this world where we come out less stupid than before.— NouvelObs.com
• Here is a little book that devours itself and not only because it is about mayonnaise, champagne or jelly meat.— Sciences et Avenir magazine
• Attractive and well written. The contents are entirely understandable by everyone. - Bulletin of the Union of Teachers of Physics and Chemistry, France
• A work that gives back its letters of nobility to the topic, in its various forms. — Claudie Haigneré, physician and astronaut
• It is an excellent book that is really nice to read.— Henri Broch, physicist, writer
• I recommend the super-stimulating reading of these captivating chapters, and the quality of the style when Mitov relates us his Italian stays to the heights of the most picturesque supernatural. A book debunking taboos, because too many subjects deserve a rational look like that of the author.— Yves Bouligand, biologist
• Beautiful educational work! — Michel Chevalet, science journalist
Matière sensible. Mousses, gels, cristaux liquides et autres miracles, Michel Mitov, Seuil, 2010.
Les Cristaux Liquides, Michel Mitov, Presses Universitaires de France, Que Sais-Je ?, n°1296, 2000. [Out-of-print].
More reviews:
Liquid Crystals Today, book review by L. Lejcek, 20, 36 (2011).
Nature : Soft-Matter miracles, book review by D. Quéré, 465, 1011 (2010).
NouvelObs.com : Les juteux miracles de la matière molle par F. Gruhier, 08.03.10.
Bulletin de l’Union des Physiciens, 924, 640 (210) par B. Courant.
Journal du CNRS, 244, 39 (2010).
Sciences et Avenir, n°103 (juin 2010) par D. Larousserie.
La Recherche, 442, 90 (2010).
La Nouvelle Quinzaine Littéraire, n°1011 (2010).
Pour la Science, 394, 96 (2010).
Scientific American, book review by A. Kuchment, 80 (April 2012).
The Washington Post, book review by A. Leitko, April 10, 2012.
PLOP ! Review (blog), book review by A. Mead (30.03.12).
Soft Matter World Newsletter, book review by L. Hirst, 41, 3 (May 2012).
The Andersen Library Blog (USA), book review by Kyle, July 10, 2012.
Bradly Alicea blog (USA), Michigan State Univ., book review by B. Alicea (2012).
Current Science (India), book review by R. Bandyopadhyay. Vol. 103, n°10, pp. 1217-1218 (2012).
Literatura científica a la francesa, Institut français d’Argentine, mediante Diego Golombek, biólogo en la Universidad Nacional de Quilmes e investigador del CONICET.
Blog hablandodeciencia.com (Portugal). "Hay tensioactivos en mis pulmones ?" by E. Castro, Fisico de Materiales, Univ. do Minho (Portugal). 2012
Blog joyfulpublicspeaking (USA). "The joy of understanding: how ink works" by R. I. Garber, ACS. Dec. 13, 2012.
Blog joyfulpublicspeaking (USA). "Joy and wonder - the science behind mayonnaise and other things" by R. I. Garber, ACS. Dec. 6, 2012.
Blog Desmitificador (Argentina), El milagro de la sangre de San Jenaro, A. Rojo (25 de mayo de 2012).
Blog American Library Association (USA), Review by H. Giesche, Alfred Univ. (2012).
DiarioVeloz.com (Argentina), El Papa Francisco y el ‘milagro a medias’ de la licuefaccion de la sangre de San Gennaro, Por E. Marquez (01.04.2015).
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Previously, we have researched the transfer of strategy and biological properties to artificial films: colors, spectral signature, textures, structures up to function. Mimicry is the quest. With a bio-inspiring approach, we start from the natural structure to create a physical property and function unnecessary related to the insect. Cuticles of Chrysina genus suggests us the design of reflectors which in turn inspire the manufacture of tags recording the thermal history of a product.
C. Bayon, G. Agez and M. Mitov, Wavelength-tunable light shaping with cholesteric liquid crystal microlenses, Lab Chip,14, 2063-2071 (2014). C. Bayon, G. Agez and M. Mitov, Size-effect of oligomeric cholesteric liquid-crystal microlenses on the optical specifications, Optics Lett., 40, 4763-4766 (2015).
h a gradient of stiffness as inspired by cross-sectional views of the polymer network elucidated by TEM. Left column: The network appears dark (respectively, light grey). (a) The network concentration is homogeneously distributed in the bulk (symmetrical irradiation conditions). (b) A network concentration gradient is evident (asymmetrical conditions). (c) In this peculiar case, the network detached from one side of the cell and relaxed into the volume of the liquid crystal (asymmetrical conditions). A small fraction of polymer is visible close to the left substrate. Right column: Model composed of N subsprings placed end to end, which represent the helical network. (a) All the subsprings have the same spring constant ki and the same length. (b) The more concentrated the network, the larger the constant of the local subspring becomes. Due to the network that models the liquid crystal structure, the pitch of the gel is proportionally distorted. (c) The subsprings relaxed and attained their unstressed configuration.
