Materials Horizons Emerging Investigator Series: Dr Wei Zhai, National University of Singapore

Dr Wei Zhai is an Assistant Professor in the Department of Mechanical Engineering at the National University of Singapore (NUS), Singapore. Her current research focuses on multi-material and multi-scale manufacturing of materials with multi-functionalities. She was previously a Research Scientist at the Singapore Institute of Manufacturing Technology (SIMTech). She obtained her PhD from the Department of Engineering/Downing College, the University of Cambridge in 2015, and her bachelor's degree from the School of Materials Science and Engineering, University of Science and Technology Beijing in 2011.

Read Wei Zhai's Emerging Investigator Series article ‘Multifunctional sound-absorbing and mechanical metamaterials via a decoupled mechanism design approach’ ( https://doi.org/10.1039/D2MH00977C ) and read more about her in the interview below:

MH: Your recent Materials Horizons Communication reports a new paradigm in the design of acousto-mechanical multifunctional metamaterials, and you demonstrate your new concept using 3D printing. How has your research evolved from your first article to this most recent article and where do you see your research going in future?

WZ: Our research on acoustic and mechanical metamaterials first originated from our work on metallic foams. Back then, I was working on the template replication fabrication of foams, with a focus on the process–structure–property relationships and mechanical properties. Looking at these porous foams, I was then wondering if they could also be used for sound absorption. I had then collaborated with an acoustic scientist, Dr Yu Xiang, currently an Assistant Professor at the Hong Kong Polytechnic University, to investigate their sound absorption capabilities. To our pleasant surprise, good sound absorption properties were measured. We worked together to develop microstructure-based acoustic models to predict the sound absorption coefficient curves based on the foam morphology. We then published a paper in Materials & Design (2018) on our initial findings. Following this, I also further refined the template replication technology to create unique metallic foams which are functionally graded and with a unique window-like microstructure. Through this development, I was fascinated by how the acoustic behavior of the foams can be altered by simply changing the geometry of the porous structures.

Since joining the National University of Singapore in 2019, I have started to work on 3D-printed multifunctional lattice structures. Defined to be a new class of architectured porous solids, lattice structures are an emerging class of advanced materials. Their properties (such as mechanical, fluid flow, surface area, etc.) derive from their structural design, as opposed to their base material. Enabled by the advent of 3D printing, lattice structures have high design freedoms – where they can consist of features such as struts, plates, shells, or a hybrid of these. This also constitutes a new paradigm for materials design – where it is now based on physics, as opposed to chemistry. Our previous works on foams reveal that sound absorption is highly dependent on the microstructure of the porous material. Being porous, and with the freedom for customization, it is intuitive to wonder about the sound absorption properties of lattice structures. However, little has been worked on in the literature. We could not find our answers directly. Therefore, we decided to venture into this research gap.

With the addition of a brilliant research fellow, Dr Li Xinwei, we have thus quickly set sail on this research track. We first discovered and reported that lattice structures could simultaneously exhibit outstanding sound and mechanical energy absorption properties. Our first work on this related topic is published in Small (2021). The lattice structures exhibited near 100% sound absorption at a few resonant frequencies. Mechanisms are fully elucidated through analytical calculations and they are found to be closely related to that of the multi-layered Helmholtz resonators. However, these lattices lack broadband absorption capabilities. To broaden the sound absorption frequencies, leveraging our previous analytical model, we then designed and optimized heterogeneously porous lattice structures. Indeed, a distinctly different absorption coefficient curve, where it is now broadband in nature, was observed. This work is published in Advanced Materials (2021). Throughout the development of this lattice, we have identified that both the mechanical and acoustic performance of lattice structures are geometrically determined. Is there a design method that allows the separate optimization of the two properties individually but they can yet perfectly synchronize within one lattice structure? This then brings us to our current work published in Materials Horizons. By now, our team had received a visiting PhD student, Mr Li Zhendong, from Prof Wang Zhonggang's group at Central South University, China. Together, we have come up with this decoupled design approach where one can freely design the acoustic and mechanical properties of lattice structures individually, but can yet superimpose their properties without the trade-off of the other. Meanwhile, we have also improved the acoustic properties of the lattice structure, where sound absorption now functions in the low-frequency region as opposed to the high-frequency region in the previous works. This also brings it closer to practical applications.

Following this, there are still many more exciting ideas and concepts generated from our group. Could there be more sound absorption mechanisms to be introduced? Our latest work reveals that hollow-truss lattice structures can harness an additional sound dissipation mechanism based on the acoustic resonance of the hollow cavity (Small, 2022). Can we draw inspiration from Nature for the lattice design? Our latest work reveals the broadband sound absorption capabilities of a cuttlebone inspired microlattice (Advanced Functional Materials, 2022). Are there more new mechanisms and concepts that can be uncovered? Definitely yes – it is up to researchers to find out. You are welcome to follow up on our future research, or even better, join us in developing this exciting research direction.

MH: What aspect of your work are you most excited about at the moment?

WZ: I am fascinated with how nature can create sophisticated functional materials through the hierarchical structural design of basic building blocks of materials. We can draw numerous inspirations from nature for advanced materials designs. Apart from 3D printing, our group also works on other microstructural controllable materials processing technologies, such as freeze casting, emulsion templating, etc. It is interesting to see how the materials' properties can be manipulated by structures, which is in turn related to the fabrication process, to achieve the desired functionalities.

MH: In your opinion, what are the most important questions to be asked/answered in this field of research?

WZ: An understanding of the fundamental relationship between the material structure and properties is essential. They must be identified as soon as possible. Best, analytical or numerical models should be worked out to link these two. These models would be useful to predict, design, and optimize properties. If not, researchers could easily be stuck in an endless pit of repeating experiments with marginal improvement. This would be both time-consuming and not economically efficient.

MH: What do you find most challenging about your research?

WZ: It is exciting to create novel materials with outstanding performances. However, to transfer the technology for economic and societal impacts may be challenging. Thus far, there is still a gap in adopting 3D-printed lattice structures for commercial applications. How to bridge this gap remains a challenge. Some of the main concerns include standardization, safety concerns, cost-efficiency, and the mindsets of designers and engineers. While not every research outcome may be adopted for engineering applications, it is important to bear in mind the potential applications throughout the development.

MH: In which upcoming conferences or events may our readers meet you?

WZ: I will attend the 2023 MRS Spring Meeting in San Francisco, California, the ICMAT 2023 in Singapore, and the Euromat23 in Frankfurt, Germany.

MH: How do you spend your spare time?

WZ: I like to take my kids to explore different parks and playgrounds for ventures. I also like hiking during weekends.

MH: Can you share one piece of career-related advice or wisdom with other early career scientists?

WZ: As a junior PI, it is easy to be overwhelmed by many different and new tasks. We need to stay focused on our main research direction and declutter unnecessary tasks. I think it is also important to stay open-minded to seek collaboration with experts in complementary research domains. This way, we can diversify our research directions, get onto the right track faster, while still focusing on our main research direction. For instance, the work in this published article comes from a successful collaboration with researchers working in acoustics.

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