£2.3M boost to revolutionize optical and mechanical metamaterials

Researchers from the University of Cambridge and the University of Warwick have secured £2.3M in UKRI funding to create materials with radically new optical and mechanical properties, that can be produced at scale and low cost.

The new funding will allow the multi-disciplinary team to push the boundaries of what is possible with metamaterials.

Optical metamaterials are special materials designed to control light waves in ways that natural materials cannot. These materials can bend, absorb, or reflect light in unusual ways, making them useful for a variety of applications.

With the Reconfigurable Nano-Opto-Mechanical Metamaterials (RENOMM) project, researchers aim to create metamaterials using nanoscale building blocks that can be self-assembled and disassembled for reuse at the end of their life.

This involves developing a new synthesis and scalable processing platform for 3D sub-micron sustainable materials. These materials will combine mechanical, optical, optomechanical, and other functionalities, opening up new possibilities for metamaterial applications. These include new sensors for health biomarkers, thermal switching films, or mechanical information processing.

The project seeks to disrupt existing approaches and capitalise on the UK’s strengths in 3D self-assembly. The goal is to develop 3D metamaterials (3DMMs) that can be produced at scale and low cost, enabling a wide range of new applications.

One of the key challenges for RENOMM will be to incorporate, for the first time, both mechanical bistability and nonlinearity into these new materials. It will also tackle fundamental questions about how to assemble and disassemble these structures at the nanoscale, and seek to unlock the new properties that emerge from this process.

“Mechanical metamaterials often break our intuition about what is possible with mechanics, but have mostly been at explored using large, centimetre-scale patterns. Making these patterns at the nanoscale is an exciting new frontier,“ according to Anton Souslov, Associate Professor in the Cavendish Laboratory and one of the Co-Investigators of RENOMM.

The project will involve collaboration between experts in physics, chemistry, and engineering, going beyond previous work that has focused only on macroscale mechanical systems and nanoscale 2D microfabricated optical devices.

All 3DMMs will be evaluated based on their materials’ sustainability and compatibility with low-emission, cost-efficient, and ideally circular manufacturing processes.

The core of the project is a close collaboration between researchers at the Cavendish Laboratory – (Professor Jeremy Baumberg, Professor Ulrich Keyser and Dr Anton Souslov – as well as the Department of Engineering with Professor Michael de Volder) and researchers at the University of Warwick’s Chemistry Department from the team of prof. dr. ir. Stefan Bon.

Prof. dr. ir. Stefan Bon, lead scientist from the University of Warwick, says: “The RENOMM project is a fantastic initiative that will bring together the forefronts of supracolloidal science, roll-to-roll processing, and bespoke bottom-up metamaterials design to mass-produce submicron-scale products with fascinating optical and mechanical 3DMMs features.”

RENOMM is an initiative sparking from MetaHUB, a pioneering new research collective, designed to spearhead the UK’s world leading, cutting-edge 3D nanoscale metamaterials science.

Original source URL: https://www.phy.cam.ac.uk/news/cambridge-team-receives-major-boost-to-revolutionise-optical-metamaterials/

BonLab designs a stick-on-demand adhesive for linerless labels

Labels are big business. A typical label has multiple layers: a topcoat for protection, the face stock, which contains the message in the form of text and/or images, a pressure-sensitive adhesive, and a release liner, which often has a release coating. The release liner and coating are only there to protect the label from sticking to things you do not wish it would stick to. You remove the liner when you wish to apply the label onto your substrate of choice, for example, a bottle containing a drink.

Imagine a label without a release liner and coating, imagine a label that could be activated at the moment you want it to stick to a substrate, a stick-on-demand linerless label.

BonLab has designed and developed a concept and prototype for a sustainable solution: a mesh reinforced pressure-sensitive adhesive for linerless label design. The idea was worked out by Emily Brogden and prof. dr. ir. Stefan Bon, in collaboration with UPM Raflatac Oy, a global supplier of label materials for branding and promotion, information and functional labelling (patent application: WO2023105120A1). The complete study, which was done at the University of Warwick, is now published in the new journal RSC Applied Polymers.

A water-based pressure sensitive adhesive (PSA) is contained in a hard 3D mesh structure. This replaces the standard PSA layer of a label. It provides structural support upon storage and prevents the adhesive layer from sticking. In adhesive jargon: it shows excellent blocking resistance.

Upon short heat treatment, for example, upon contact with a hot sterilized glass bottle, the mesh will soften, allowing for adhesion to occur.

The 3D mesh structure was generated using a binary mixture of water-based polymer dispersions. One component is a standard PSA latex, and the other is a polystyrene polymer colloid. Film formation of the blend followed by a short annealing time led to the desired phase-separated mesh structure, as displayed in the figure below.

Visualization of the mesh structure in an acetone etched PVAc-PS model film (Film 1) a-c. a) SEM image of the top surface, b) a tilt adjusted image of the cross section after cutting away a section using FIB SEM with a film height of approximately 20 𝜇𝑚, and c) micro-CT 3D reconstruction of the etched film. Visualization of a similar mesh structure in a PSA-PS film (Film 2) d-f. d) SEM image of the top surface of Film 2, annealed for 60 𝑚𝑖𝑛 e) SEM image of the top surface (prepared similiarly to Film 2 but with only a 19 𝑚𝑖𝑛 annealing time) with a clean crack after freezing in liquid nitrogen and f) a cross section of Film 2, where the PS is tagged with hostasol methacrylate (green), taken using confocal microscopy (the original top surface of the cast film is at the top of the image).

Prof. dr. ir. Stefan Bon says: “I am immensely proud of the hard work done by all people involved to get the idea from a sketch on a piece of paper to a working prototype. A big thank you to PhD researcher Emily Brodgen, the paper's first author, who has been the driving force behind turning a concept into reality and has worked tremendously hard on the project for the last couple of years. The work has received excellent feedback from the academic and industrial scientific communities, with Emily winning multiple prizes for best talk and scientific poster.”

The paper can be accessed from the RSC Applied Polymers website:

https://doi.org/10.1039/D3LP00224A

A promotional video will be made available soon.

Replacing titanium dioxide as opacifier: consider a shape change

A fresh lick of paint breathes new life into a tired looking place. Ever wondered how a thin layer of paint is so effective in hiding what lies underneath from vision? Beside colour pigments, and a binder that makes it stick, paints contain microscopic particles that are great at scattering light and turning that thin layer of paint opaque. The golden standard for these opacifiers are small titanium dioxide particles, of dimensions considerably smaller than one micron. Their use is not without controversy, as they are a big environmental burden, with a large carbon footprint and a questionable impact on human health. The reason why titanium dioxide particles are great at scattering light is that they have a high refractive index compared to the other paint ingredients, so when distributed throughout the dried paint film their hiding power of the underlying surface is fantastic. When no coloured pigments are used, the coated surface appears then whiter than white.

Ideally though, titanium dioxide should be replaced, but the list of safe high refractive materials is very limited. This makes you wonder if there is another handle, beside refractive index? Can we design efficient scattering enhancers from materials of lower refractive index?. Inspiration came from the white Cyphochilus beetle, native to southeast Asia. The scales of the beetle are not made of high refractive index materials, but they thank their white appearance to an intricate anisotropic porous microstructure, resembling the bare branches of a dense bush.

We at BonLab formed a team where researchers dr. Brooke Longbottom and dr. Chris Parkins together with dr. Gianni Jacucci and prof. Silvia Vignolini at the University of Cambridge (UK) designed a simplified mimic in the form of tiny rodlike silica particles and compared their scattering performance with spherical silica particles.

Our work published in the Journal of Materials Chemistry C from the Royal Society of Chemistry is part of their HOT paper collection and shows that the anisotropic silica particle outperform their spherical counterparts, and show excellent scattering performances across the visible electromagnetic spectrum when casted as a film.

SEM images and optical characterization of white silica supraparticles. a) low magnification SEM image of supracolloidal balls, b) higher magnification image of single supracolloidal balls, c) supracolloidal ball assembled in the presence of 0.01 M calcium chloride. Scale bars: a) = 15μm b&c) = 10μm. d) Reflectance spectra comparing the scattering properties of supraparticles with films of silica rod particles of similar size (thickness of 25μm). Supraparticles show performance comparable to the corresponding films. Increasing the disorder reduces the scattering efficiency. The reflectance spectra for the supraparticles were measured using a microscope, while for the film they were retrieved from the total transmission data.

We did not stop there, and went a step further to develop a prototype of a new class of micron-sized hiding pigment. We took these rodlike silica particles and assembled and sintered them into stable porous supracolloidal microspheres, as can be seen in the image above.

Prof. dr. ir. Stefan Bon says: “This work has been a number of years in the making. It was an absolute pleasure to work with prof. Silvia Vignolini and her team. We are very happy with the end result. We hope that this new type of hiding pigment provides inspiration to those who wish to replace titanium dioxide. After all, there is more to opacifiers than refractive index.”

The paper can be accessed from here:

https://doi.org/10.1039/D1TC00072A