Text | Zhang Qingdan, reporter of China Science News
While most international students choose to go to Europe and the United States for further study, Wang Meng went to Israel, a niche study place. At the Hebrew University of Jerusalem, he became the first Chinese student in the lab of Jay Fineberg, a professor in the Department of Physics.
In this laboratory of only 4 people, Wang Meng did not set himself high goals, but stepped on his own pace and did scientific research step by step without distractions.
After 3 years of hard work, he published his breakthrough research results in Science for the first time as the first author, and experimentally proved that the tensile crack velocity broke through the limits described by classical fracture mechanics. There are only three authors in total, and in addition to the corresponding author Jay Fineberg, the other author, Shi Songlin, is also from China.
The new discovery subverts people's previous understanding of the fracture process and opens a new chapter in the field of fracture.
The three authors of the article, from left to right, are Shi Songlin, Jay Fineberg and Wang Meng.
A very serendipitous discovery
Fracture mechanics is a difficult and hot spot in the field of mechanics, although it has been studied for nearly a hundred years, but people's understanding of fracture problems is still very limited.
"It is difficult to study dynamic cracks at the experimental level because cracks propagate very quickly, especially in brittle materials." Wang Meng said in an interview with China Science News.
For example, if a glass accidentally falls to the ground, we can usually only see it go from an intact state to a broken state. This process is short and difficult to capture because the crack propagates too quickly.
Under the theoretical framework of classical fracture mechanics, when a system is subjected to a large enough external force, cracks will propagate at a speed close to that of Rayleigh waves, and for brittle materials like glass, Rayleigh waves usually travel hundreds to kilometers per second, so it is difficult to capture the moment of breaking.
In Wang Meng's research group, this process was studied by using hydrogels as a medium. Hydrogels have the benefit of having Rayleigh waves with a wave speed of only a few meters per second, which helps to study crack growth through direct observation in experiments.
The hydrogel used in the experiment was not hand-made, but Wang Meng tailored the chemical ratio of the hydrogel to control its mechanical response. Before leaving the laboratory the night before, he will prepare the hydrogel and leave 4~7 hours of reaction time to "produce" the samples to be used in the next day's experiment; Go to the laboratory early the next morning and prepare another batch of hydrogels for the afternoon experiment.
"I prefer hands-on experiments to analyzing data and writing articles, and I enjoy the process."
At first, Wang Meng wanted to study the fracture problem of heterogeneous media through hydrogels. In countless experiments, a very accidental discovery completely opened a new chapter in the field of fracture.
"One day, while I was doing an experiment, I accidentally observed two shallow marks in front of the tip of the crack, and this very unusual phenomenon aroused my great curiosity, and I repeatedly wondered in my mind what these things were, after all, I had never seen them before." Such an unexpected discovery made Wang Meng's heart surging and full of enthusiasm.
He began to calculate the crack speed with doubts, and found that it exceeded the Rayleigh wave speed, which was also beyond their understanding. The classical fracture mechanics theory states that when the material is in a tensile state, the limit of the crack propagation velocity is the Rayleigh wave velocity. So when he told his mentor, Jay Fineberg, about this discovery, the mentor wasn't convinced.
In order to prove that what he saw was true, Wang Meng began to rack his brains and design deeper experiments to characterize this phenomenon.
"After many days of repeated experiments, I determined that the two shallow marks were actually Mach cones, and this is the first time we have observed Mach cones in a solid. Moreover, from my experiments, I found that the hydrogel crack not only exceeds the Rayleigh wave velocity, but also exceeds the shear wave velocity, and even approaches the expansion wave velocity. ”
This study proves the existence of "supershear" tensile cracks that exceed the shear wave velocity, completely subverting the traditional understanding of classical fracture mechanics theory.
"Unexpected discoveries that surprise you are hard to design, you need to keep exploring, you need luck, and you need eyes that can recognize luck. It's a tough process, but it's also fun. For me, that's what really appeals to me. Wang Meng said.
I didn't expect to publish Science, and the initial psychological expectation was to publish a sub-journal
At first, Wang Meng did not think of submitting the article directly to Science, and his expectation at that time was to vote for a sub-journal. Jay Fineberg said: "We can give it a try. ”
Previously, Wang Meng studied for a PhD in the LaMCoS laboratory of the French National Center for Scientific Research, and published many papers in PNAS and international mechanics journals. However, he has no experience in investing in science. Although he didn't have a bottom in his heart, he still chose to follow his mentor's advice.
"At the beginning of the submission, I wanted to send a letter, so I only prepared 4 pictures, and after referring to other articles of the same type, I submitted them to the editor. The editor was optimistic about the study, but he told us that the letter form had been cancelled and that we should revise it according to the form of the article and resubmit it. Wang Meng said.
Add pictures, add subheadings, polish language expressions... The re-"revised" article was re-submitted in early February this year. The process went surprisingly smoothly, and the article was accepted in only 4 months.
From the responses of the three reviewers, it is not difficult to see that they all showed a strong interest in this disruptive finding. The first reviewer directly gave the "green light" and was very supportive of publication.
The second reviewer mentioned that top Chinese scholar Gao Huajian once published a paper in Nature that discovered supersonic crack propagation for the first time through ultra-large-scale molecular dynamics simulations. He felt that the results were similar to the research mechanism of Wang Meng and others.
The third reviewer suggested that the author add some text content and pictures to make the article fuller and richer.
In order to let the reviewers have a more intuitive understanding, they added a few more pictures to the attachment, showed the crack field exceeding the wave velocity through pictures, and made a comparative analysis with and without Mach cone.
"In fact, the response of the first and third reviewers was relatively easy, and our pressure mainly came from the opinions of the second reviewer, and we also spent a lot of energy and added a lot of evidence that can fully prove the universality of our phenomenon, before finally passing the customs." Wang Meng said.
The article was accepted in early June this year and published online by Science on July 27. In this regard, Wang Meng's reaction tended to be calm. "Publishing in the top journal is an affirmation of our work, but it doesn't mean anything, my personal thoughts on scientific research are more in the scientific research itself, I just want to explore more interesting phenomena quickly."
Do scientific research should not be ambitious, but focus on the present
Wang Meng
Wang Meng, born in 1993, also had a period of confusion. After graduating with a PhD in 2019, he repeatedly asked himself whether he wanted to continue on the path of scientific research.
"I've been doing basic research since I entered academia, but I actually wanted to do applied research. After graduating with a PhD, I was always vacillating and struggling with whether to work in the corporate world. At that time, the new crown epidemic broke out, and the idea of going to the company was frustrated. Wang Meng said.
He had been studying dynamic brittle fracture since his doctorate, and began to look for laboratories in the same field, but there are not many international research groups that do this research mainly experimentally. After much investigation, he set his sights on the famous physicist Jay Fineberg. Jay Fineberg is highly academic, having published more than 100 research papers in the fields of soft condensed matter, fracture mechanics, and seismology, 16 of which were published in Nature or Science.
Israeli universities have traditionally placed little emphasis on Asian admissions. The country has been labeled as extreme, unstable, insecure, desert everywhere, etc., which also discourages everyone. But Wang Meng, who was bent on going to his mentor, didn't care about these prejudices.
In order to get acquainted with his favorite mentor, Wang Meng signed up for Jay Fineberg's summer school held by the Ecole Normale Supérieure in Lyon, France, "I read many high-impact articles published by him when I was a Ph.D., and I seized this opportunity to ask him several questions to give him a preliminary impression of me." ”
Jay Fineberg is very dedicated and has a strong sense of responsibility. Since becoming dean of science at Hebrew University in 2016, he has devoted himself to his work without recruiting a single student, saying: "I don't have time to accompany and guide students, and I don't want to delay the future of any student." ”
It wasn't until he stepped down as dean in 2020 that the 64-year-old Jay Fineberg re-enrolled the lab's first doctoral student. Wang Meng, who just graduated with a doctorate, took the opportunity to apply to Jay Fineberg, successfully joined the laboratory with his excellent scientific research level, and became Jay Fineberg's first Chinese student.
What attracted Wang Meng to study in the Middle East was not only the main reason for his mentor, but also the aura of the Hebrew University. It is Israel's No. 1 institution of higher learning, leading in basic sciences, medicine and humanities, and has trained 8 Nobel Prize winners and is known as the "Harvard" of the Middle East.
Moreover, the level of scientific research in Israel is very high. This "high" does not refer to how high-end instruments and equipment they have, on the contrary, their experimental conditions may not be as good as some domestic research groups, but their ideas and creativity in scientific research emerge endlessly, and the open and free scientific research atmosphere has given birth to many outstanding results.
Just like Jay Fineberg will not ask Wang Meng to do the project he set, but give students full freedom, encourage brave innovation, let students in the vast scientific research world, he only gives students the greatest help and support when they need it.
"Our lab doesn't have a lot of space, and the highest-end equipment in the lab is a high-speed camera, and many of the observation methods have to be designed by ourselves. The instructor is a 'expert' who is very good at building experimental platforms and equipment, and he provides a lot of useful ideas and assistance in building the observation equipment we need. Wang Meng said.
Jay Fineberg never interferes with students' schedules, and he doesn't like to make doing science a rigid clock-in. In a relaxed environment, Wang Meng gradually found his interest and enthusiasm for scientific research.
Looking back at the original decision now, Wang Meng still has no regrets. During his two and a half years of studying and living in Israel, he revised many stereotypes about the country. "Israeli security is very strict, and you will be questioned in detail and carefully checked your luggage before flying here. Jerusalem is still safe. ”
Jerusalem is a holy city where people can not only broaden their horizons, but also come into contact with the collision of different cultures. But Wang Meng, who is dedicated to scientific research, is not much interested in local religious culture and ancient buildings, and he prefers to stay in a small laboratory with experimental equipment.
Wang Meng is experimenting
"Research is not a sprint, it's a marathon, and you need to have a plan and find your own pace. You don't know when the marathon will reach the end, so you have to adjust your pace all the time. Instead of being ambitious, it is better to focus on the present and stick to it step by step in order to maintain a good attitude. Wang Meng said.
Recently, Wang Meng has been actively learning about the work in China and hopes to return to the motherland to continue his scientific research career.
*All images in this article were provided by interviewees
Reference Links:
https://www.science.org/doi/10.1126/science.adg7693