[:en]Mr. Shuhei Yoshida
Hometown in Japan: Fukushima
Home University in Japan: University of Tokyo
Major & School Year: Mechanical Engineering, B4
Research Host at Rice: Prof. Bruce Weisman, Chemistry
Research Project Title: Strain Direct Mapping by Using Carbon Nanotube Strain Sensor

Why TOMODACHI STEM @ Rice University?
Many people say that it is comfortable to be in their own country. However, I believe, participation in a science and engineering research internship in the U.S. is significantly beneficial for me to achieve my academic goals, to become a principle investigator.

To begin with, I would like to have a scientific research experience at diverse and collaborative environment, communicating by foreign language in a foreign country. I think very interesting results have never been achieved without collaborations of research groups.

In addition, I want to extend my network to contact toward all over the world. The merit of studying abroad is not only research but making new friends, and it will be a chance to interact with foreign students and to share cultural differences. Hopefully, some of them might be collaborators of my research projects in the future.

Moreover, I would like to study different area apart from research topics on which I have worked in Japan because experiencing different research can be a big advantage to conduct innovative interdisciplinary research. This experience can also make me differentiate in the competitive world of scientific research since I can show my originality, mixing several disciplines.

Finally, I definitely believe that the research internship in the U.S. will be valuable experience to attain my academic goals, and I expect that I will be able to have nice five weeks in the U.S.

Goals

• To know the difference between the U.S. and Japan in terms of education and research in universities.
• To be able to conduct research projects using foreign language in diverse environment.
• To expand my knowledge going through my research project.
• To make new friends and future collaborators in the U.S.

Questions I have About Research or Life in the U.S. – Prior to Departure

1. What are the advantages and disadvantages on studying in the U.S.?
2. What are the good points of the U.S. that we can mimic in Japan?
3. How can we keep ourselves healthy in the U.S.?

Excerpts from Shuhei’s Weekly Reports

• Week 01: Arrival in the U.S.
• Week 02: Critical Incident Analysis
• Week 03: Research in the U.S. vs. Research in Japan
• Week 04: Research Project Presentation
• Week 05: Final Week at Rice University
• Week 06: Science & Technology Policy Study Tour in Washington, DC
• Final Report

Week 01: Arrival in the U.S.

Since it is my 4^th time to come to the U.S., but it is also my first time to come to Texas, I was wondering what it would like. As we arrived, I felt that it was warm and humid, and it was different from the West coast and East coast regions where the weather is cold and dry. After immigration process, we moved to a hotel. Then I realized that the place we are staying is very clean and safe, in comparison to my experience in San Francisco where strangers harassed me. The next day we arrived, we had a campus tour in Rice University. I just remember that I was impressed because the Fondren Library in the campus was clean and large. Moreover, it is open even until midnight, and I thought that if the library in my university were also opened when I was junior, I would spend a lot of time studying there.

One thing different from what I expected was health care systems in the U.S. I have heard that health insurance in the U.S. is very expensive, but I did not know what the health care systems are like. Our hotel, actually, was close to medical center in Houston. Even though hotel fee was more than $200 per night, there were many people who look like patients. This means that it can be cheaper if they stay at the hotel rather than at hospital. I could not believe because, in Japan, hotel fee is normally much cheaper than the case of staying at a hospital. I was wondering what they would do when emergency cases happened. I realized that Japanese health care system was better.

One of my aims to participate this research internship is to compare universities in the U.S. with that in Japan and to find good things that we can imitate in Japan. This week, I encountered a good opportunity to get the information about that. Professor Kono and some Japanese graduate students introduced us how graduate studies in the U.S. are like. Then, I realized that Japanese graduate schools are not so bad, as they are accused in Japan, because there are many advantages and disadvantages of university systems in the U.S. as well as the system in Japan. The most important thing I thought, listening to their presentations, is that we should choose research groups to apply based on faculty members, not on name of universities.

One person who is very helpful in Rice is Ms. Keiko Kato, a first-year graduate student in Materials Science and NanoEngineering, and a student assistant for the TOMODACHI STEM @ Rice University program. Since she graduated high school and university in the U.S., she knows most of things related to living in the U.S., and she always shares us useful information such as foods, sightseeing spots and so on.

Research Project Overview: My research topic is “Carbon nanotube strain sensor”. Monitoring deformation of materials is one of the most important things for structural health care. In conventional methods, we can get information of deformation, so called “strain”, from only specific measurement points for large sensor size. This means local dangerous deformation, where the material is about to fracture, can be missed. To solve this problem, we are developing “carbon nanotube (CNT) strain sensor”. It is well known that optical properties of CNTs, which we can measure by using spectroscopy, can be altered when CNTs are deformed, and the difference of fluorescence is almost proportional to strain. By applying this phenomena, we can measure strain of materials at arbitrary points because CNTs are significantly smaller than conventional strain sensors. My job in this research internship will be making 2D strain map with vector, which describe the direction of deformation, from directly measured data. This map is very important because we can know many things such as stress field, stiffness and so on, but no one successfully made.

First day in the lab: In the beginning of my research, I met my supervisor, Dr. Bruce Weisman, Professor in the Department of Chemistry, and he gently introduced me the laboratory as well as the research topic that I would be working on. Also, I met my mentor Dr. Sergei Bachilo, scientist in the group, and Mr. Patrick Sun, a graduate student who is working on CNT strain sensor project, and we discussed my internship project. As my first job, I studied how to determine direction of strain at our office searching previous research papers. After that, I formulated some algorithms to calculate various parameter.

As mentioned above, my mentors are Dr. Sergei Bachilo, scientist in the group, and Mr. Patrick Sun, a graduate student who is working on CNT strain sensor project. Since Patrick was busy because of preparation for conferences and examinations, I hardly had chances to talk to him apart from exchanging e-mail. Normally I sometimes discussed with Dr. Bachilo to know his opinions. In addition, there were some graduate students and undergraduate students. Sometimes, we talk, drinking coffee, about various topics such as history of Texas and so on.

Question of the Week
How long do graduate students work every day?

• This varies and it depends on many factors.  First, what are the expectations of the student’s research advisor or professor?  What is the culture of the research group or that particular lab? If many students stay late, then it will encourage other students to stay late too. Second, what is required by the research project? If the student needs to stay late in the lab to run their experiment or analyze the data but if they have completed everything they need to do for that day many U.S. Ph.D. students would go home after a normal or typical working day in the U.S. which is ~8 – 10 hours.  Some students may also still be taking classes or they may be teaching or serving as teaching assistants for other classes at Rice as part of their stipend or fellowship (how they are paid) so may need time outside of the lab to prepare for classes or grade.
• An important thing to remember is that in U.S. culture there is a high value placed on efficiency and effectiveness when working.  It is not the number of hours you work in the U.S. that matters as much as what you can do or produce in the time that you work.  A common saying in the U.S is “Work smarter, not harder.”  This means, be more effective with the working hours you have, prioritize what is most important, and when you are done with your tasks for the day you can leave and go home to relax and ‘recharge your batteries’ for the next day.
• However, just as in Japan, the best graduate students (and professors) tend to work very long hours and spend a great deal of time in the research lab or office doing experiments, analyzing data, and writing or reading papers. Often this is because they genuinely like research and love what they do.

Return to Top

Week 02: Critical Incident Analysis

Since I have been working on an interdisciplinary research topic connecting physics, chemistry, mechanical engineering, civil engineering and material engineering in which different cultures exist, sometimes I felt difficulties to estimate how much knowledge people have when I explain about my research. Things which are normal for Japanese students to know are not always “normal” in diverse cultural context.

For example, when I presented the progress of my research in my group meeting, I had to explain about stress and strain. In Japan, I have been in research group related to physics and chemistry even though it is in department of mechanical engineering, and it is normal for me that students understand both mechanical engineering and chemistry. So, I started my presentation without explaining what stress and strain are. However, my professor told me to explain the definition of each because they were not common for ordinary chemistry students. Then I was confused what “common” for them is, but when there was not common things in my presentation, professor always added some information.

Based on this experience, I learned that something “common” for Japanese is not always “common” for people coming from other countries. In addition, I realized again that learning various kinds of realms is quite important to understand what people coming from another research area are saying. I have taken many classes of physics, chemistry, geoscience, material science, mechanical engineering, electrical engineering, computer science, civil engineering and so on, not depending on my department, to be able to work on interdisciplinary research, in which we can get unconventional interesting results, and I decided to continue this way.

Research Project Update

(1) Formulating physical model based on elastic theory

I derived formula to calculate the magnitude and direction of strain, which should be form of sin or cos curve because of the characteristics of strain tensor. This method can go over conventional principle strain vector determination method, known as “Rosette strain gauge measurement”, in terms of precision, easiness and so on because we can fit the curve with many data points.

(2) Formulating an algorithm to determine principle stress vector.

If we know principle strain vector, we can also know principle stress vector, which most of engineers concern when they design mechanical structures. I derived formula to calculate that from stress strain transformation tensor (the case of anisotropic materials included). Hopefully, we will be able to see principle stress distribution.

(3) Formulating an algorithm to calculate fitting curve fast.

When we make the vector map, we need to calculate with large amount of data, and it seems to take much time. Practically, calculation time have to be shorter. So, by using optimization mathematics and machine learning, I made algorithms to calculate the fitting curve parameters fast.

(4) I made calculation program.

The algorithms described above were realized and confirmed that it works correctly.

(5) Peak fitting program was made.

Since we did not have a program that can do peak deconvolution of large amount of data at same time, I made a program to calculate gaussian curve fitting and extract peak shift from row data.

(6) Test experiment was done.

I and Dr. Bachilo did an experiment with small amount of data to confirm that every programs I made work.

(7) Prototype stress / strain vector maps

Based on the data, we can obtain stress strain maps.

(8) Local work hardening map

In addition to stress / strain map, we could realize new idea, local work hardening map, that I came up with. By looking this map, we can know initial state of dangerous deformation of materials, which we cannot distinguish by naked eyes, even if it happens in small area.

Since the maps are not easy to look at, as you can see, I am going to modify the programs to change their appearance. My next work would be conducting experiment and getting new detailed data.

Question of the Week?
Do graduate students have opportunities to make research proposal to get grants?

• Yes, there are opportunities for graduate students in the U.S. to apply for some grants and fellowships but these are typically to fund their Ph.D. research or for participation in specific educational or research programs.  Some universities also offer small grants or fellowships to graduate students that they must apply to and receive by competitive selection.
• Below are some websites that list an array of funding resources available to both undergraduate and graduate students in science & engineering in the U.S..
  • NSF Funding Opportunities
  • NIH Funding Opportunities
  • Other Related Programs in S&E (For U.S. students)

Return to Top

Week 03: Research in the U.S. vs. Research in Japan

I have studied at 5 different laboratories in Japan, where students are responsible to their research projects and independently conduct their experiments. In the laboratories, when I came up with new ideas, I often suggest it to my supervisors. When the supervisors approved me to use budget, I worked hard to realize the ideas. When I got stuck in my project or found something wrong with others’ project, I often searched previous papers or discussed with faculty members and colleagues. When I had any chance to collaborate with other researchers, I did not hesitate and be involved together with my supervisors. When I found new things in my projects, I was allowed to write research papers or to present in conferences by my supervisors. I was able to do these things in Japan. As for my research lab at Rice, I think the situation is very similar. I have been conducting my project independently. Everyone encourages me when I am trying new things such as new method that I came up with. I feel comfortable like when I was studying in Japan. I think the activities I mentioned above are ideal state for every researcher in the world.

I think the approach to research and education in my lab is not so different from that I experienced in Japan. The way of research is almost same as I explained, and students learn from their experiences, classes, coaching by seniors and so on. However, I found one significant difference in communication style. In the U.S., students do not hesitate to interact with other people coming from different countries. On the other hand, Japanese students normally do not have interaction with foreign students and faculty members, and sometimes I have observed huge divide between them. Although I do not hesitate, the majority of Japanese students do hesitate. It seems to be because Japanese are used to living in racially homogenous society, and the language barrier also accelerates this divide. For example, in my lab at Rice, where students are from several different countries, I often saw everyone talking each other equally. On the contrary, in my lab in Japan, I always feel that students form 2 groups, “Japanese students” and “international students”, and they hardly interact each other. This divide can be an obstacle to conducting collaborative research project between them. Even though there should be some problem related to diverse society in the U.S. such as racial segregation, national extremism and so on, I think US society more tolerant for diversity than Japanese society, and we should be also tolerant as a part of global citizen.

I think the role of research labs in universities is to train young researchers in a practical environment, and this is common throughout the world, both in Japan and the U.S. Every graduate student should value what they have learned from their research experience, which are applicable for their future jobs including researchers.

Research Project Update:

(1) Program debugging

Sometimes I found strange behaviors of my program, and I fixed everything. I spent most of time on this problem this week.

(2) Modifying peak position extraction algorithms

I noticed that my program sometimes produced strange fitting curves. So, I started to apply parabolic function to extract peak position of spectra, which can be solved analytically.

(3) Applying annealing method to physical model fitting algorithm

Also, I found similar problem in physical model fitting. So, I decided to add new algorithms so called annealing method, in which we can deal with the problem by using theory of probability.

(4) Modifying map appearances

Previously, the maps generated by my program were not so easy to look at, and I added scale color bar and so on.

(5) To be able to output map data

The best way to make good graph is to use professional graph software. Then we need numerical data for plotting. So, I modified my program to be able to output numerical data of the results.

(6) Theoretical model

I came up with new idea to reduce fitting parameter in physical model, in which the most insensitive parameter can be determined from the data extracted from the peak shapes of experimental results.

(7) Experiment with lager number of data points

I conducted new experiment with lager number of data points than the previous experiment. However, something unusual happened, and 1/3 of the experimental data disappeared. Thinking positively, I can calculate with remained 2/3 of the data.

(8) Making strain maps

With the remained 2/3 of the experimental data, I made strain maps. It looked pretty good, and very similar to the predicted appearance. These were the maps we expected.

Question of the Week
How often do US students discuss with other people in other laboratories?

• If you are a graduate student in the U.S., typically you will not officially join a research lab until your second year of graduate study (though some students may join a lab earlier).  For the first year of a Ph.D. program most students are paid their graduate student stipend and receive a tuition remission from their academic department.  You focus on classes and meet with many professors and may do trial projects to see which professor will offer you a job as a research assistant in their group from year two forward.  This means that during your first year of graduate school you probably will become friends with students who may end up in different labs from yours.  So, therefore, you will know people in a number of different labs on campus.
• However, graduate students in the U.S. spend a lot of time in their research lab and often become close friends with their research group members.  Even when socializing, such as at Valhalla at Rice on a Friday night, you will see groups of professors and graduate students from the same lab typically standing and talking with each other.  They may walk around to visit with friends in other lab groups, but they’ll typically return to socializing with their own lab group.  (This is also because graduate students really like talking about their research projects and your own lab group members probably know/understand your research project better than anyone else.)
• Most universities in the U.S. have graduate student associations and clubs that students are encouraged to join. Participating in these associations, clubs, or organizations can be a way to meet people from many different academic fields and develop a more diverse group of friends outside of your research lab.  It depends on how active and involved the individual student wants to be with other student groups but as long as it doesn’t negatively impact your research, as a graduate student  you are allowed to and encouraged to be involved on campus too.  This is an example how of universities in the U.S. try to promote a good “work-life balance” for graduate students.
• For example, here are some of the groups that students can join at Rice.
  • Rice University Graduate Student Association
  • Graduate Student Association Clubs at Rice
  • Graduate Student Resources and Life at Rice
  • Intramural Sports at Rice

Return to Top

Week 04: Research Project Presentation

On Friday, March 18 all TOMODACHI STEM students gave a research project presentation at Rice University.

Research Host at Rice
Prof. Bruce Weisman, Chemistry

Research Project Title
Strain Direct Mapping by Using Carbon Nanotube Strain Sensor

Return to Top

Week 05: Final Week at Rice University

I spent much of my time the final to realize my new idea to extract data, which I came up with last week, as well with preparations for the final presentation. I modified my programs, and finally, I made it! The problem of the previous program was that the resolution for some parameters obtained from data was not enough to attain my project goal, making principle strain maps, but the situation was drastically improved in my new program. I obtained the world’s first results, and we could be able to extract strain data from simple experiments. On the final day, I gave a presentation about my experiences in this internship. After the presentation, I talk with my professor and Dr. Bachilo about my project overall

What I will miss about my research experience at Rice would be coffee time with Dr. Bachilo, who always supported my project. He is very knowledgeable and very unique, and he often taught me some interesting facts during coffee time such as history of Texas, his experiences in Soviet Union and so on. Sometimes, I came up new ideas based on something we dealt with in the coffee time. I enjoyed that every week.

In Japan, there are many books and articles describing that universities in the U.S. are perfect, but I was skeptical about that idea. Going through this research internship program, I found that the idea is almost wrong. Now, I am definitely sure that quality of research does not depend on nationalities but the boss of research groups, at least when we compare between the U.S. and Japan. Some research groups in the U.S. and Japan have many better researchers, many better students, much grants, many new facilities and better education systems, but other groups do not. I believe this difference coming from leadership and philosophy of the principal investigators (PIs). Good research groups have good bosses. Fortunately, the professors who have guided me were very nice PIs, and they have provided me many opportunities to learn important things for my carrier. That is one of the most important things I have found in this internship program.

The experiences at Rice gave me confidence to conduct a research project in international context. I do not think everything was perfect, particularly, explaining very complex ideas in English, but I achieved new developments in my research project cooperating with people in the foreign research group. Based on this experience, I want to be a PI who is not afraid of conducting international collaborative research projects, which many Japanese researchers are not good at.

Apart from research, we had some special lectures. Particularly, the lecture of Dr. Neal Lane was very interesting and helpful for me in terms of understanding of science and technology policy. His talk was clear, and he gave us essential knowledge about science and technology policy even though we knew almost nothing about that before. In addition, sometimes he talked about funny episodes when whe was working with the previous president, Mr. Clinton, in the Office of Science & Technology Policy at the White House. I felt that the lecture was unique and cannot be given other than Dr. Lane.

Question of the Week?
What are the attitudes of students in the U.S. towards studying abroad?

• American students study abroad for a number of reasons.  First, many believe it is a “once in a lifetime opportunity” and they may have always dreamed of living in Japan, for example, since they were a young child and first watched anime.  Second, many students believe it will provide them with language and intercultural communication skills that will make their resume more competitive and attractive to future employers.  For more on alumni impact of study abroad see this new study released by IES, a study abroad program provider. American students also study abroad because they want to gain fluency in a language or want to study a language that is not offered by their home university.  However, it is important to realize that only a very small percentage of U.S. students do in fact study abroad, currently about 10% of U.S. graduates overall. At some universities the percentage of students who study abroad is higher, but overall it is similar to Japan where only a few students choose to spend a summer, semester or academic year abroad as undergraduates.

Return to Top

Week 06: Science & Technology Policy Study Tour in Washington, DC

The most interesting place we visited this week was Howard University. On that day, we met some undergraduate students majoring physics, and we shared our experiences with each other, such as what the student life in the university is like, what kinds of class they have, what kinds of research they are interested in and so on. It was a good opportunity to know some of the differences of education between Howard University and my university. One thing which I thought interesting and Japanese university should imitate is that even sophomore and junior undergraduate students are involved in research projects at universities and companies. I think such internship programs provide us actual working experiences, and there should have a positive effect on our professional development. However, the duration of internship in Japan is not so long as that in the U.S., and many of them are not aimed at education, just focusing on job hunting. This problem has to be solved for better education in Japan.

As a part of the science and technology policy study tour, we visited JSPS Washington, DC office and George Washington University Center for International Science & Technology Policy, and we had some lectures there. Then we were able to know what they are doing. Since we could not visit National Science Foundation owing to transportation trouble, (the Metro line going to the NSF unexpectedly shut down that day) we were not able to learn the details about science related funding system in the U.S., but the special lecture of Dr. Neal Lane in Houston had already provided us a basic introduction of science and technology policy and funding.

On Thursday, we participated a joing program with the TOMODACHI Met Life Women’s Leadership Program. At the time, we met many female students coming from the TOMODACHI Women’s Leadership program, and we could share the experience each other. In addition, we were able to listen to some female business workers’ talk, and that was very interesting for me because I have had little opportunity to know such kinds of things.

In our free time, we visited many famous places such as national museums, monuments and so on. Particularly, The National Museum of Natural History, the place where the movie “Night Museum” was taken, was very interesting for me.

Question of the Week
What is important research, qualified to be funded?

• Someone who knew the exact answer to that question would be paid a very good salary by universities or professors to help them apply for their research grants. 🙂
• It really varies by the agency or foundation you are applying to and what their funding priorities are for that year or that particular grant program.   It is important to carefully read the Request for Proposal (RFP) or Call for Proposal that a funding agency puts out before you begin your grant application to ensure that what you want to apply for is a) eligible to be funded under that program and b) falls within any preferences or priority areas that the sponsor has outlined.  You can also often call or email the program officers at the funding agency or foundation to speak with them or ask specific questions and get their guidance on whether this is a good program to apply to or if your idea would be better submitted somewhere else.  Some grant programs require that you submit a White Paper or Concept paper first and only after that has been reviewed will they invite you to submit a full proposal.
• Research funding is highly competitive and there are many more highly qualified important/good research proposals than what can be funded.  Ultimately, you need to work together with your fellow co-PIs or researchers, your home university, the funding agency, and any mentors/senior professors who can give you advice to always put forward a compelling and well-written proposal to have the best possible chance of being awarded.  Professors often apply multiple times for important/good projects before they finally receive funding.
• For more on Research Funding see Funding of Research in the U.S. & Japan.

Return to Top

Final Report

The most important reason why I participated the internship program was that I wanted to confirm if the research environment in the U.S. is perfect. The results based on my observation was that nothing was perfect, but I also found that there are many things to learn from the given environment. Going through the TOMODACHI STEM @ Rice University program, I have learned the following things, and I want tell them to others, such as professors, friends, family members and so on.

First, I learned the importance of tolerance for diversity. In the U.S., there are many international students coming from different countries, and they are cooperating each other. This diverse research environment is quite important because diverse people can come up with new, creative and nice ideas. However, in the case of Japan, we often see the situation like “Japanese students” and “international students” are segregated, and they hardly interact each other. This is because Japanese society is conservative and racially homogeneous. In addition, language barrier can accelerate the situation. Recently, the situation has been gradually improved because the number of foreign people has increased, and Japanese people are becoming used to interact with diverse people. In spite of that, I often find many things which seem to be inconvenient for international students at my universituy. Therefore, more effort will be needed to improve our environment. Although I do not think that the diversity in the U.S. is perfect, and there are many kinds of discrimination there, we, Japanese, can learn from the most diverse country in the world, United States of America.

Second, I learned that how to choose a supervisor is more important than how to choose the countries where you study. During the research internship, I found that there is little difference between the U.S. and Japan in terms of research style, quality of facilities and so on, and as far as I am concerned, the difference mainly depends on the research group and the supervisor. Research environment of research groups can be made by a principal investigator, and this means how to choose your supervisor is significantly important if you want to study in a better laboratory. In Japan, many books describe that universities in the U.S. are much better than Japanese universities, in terms of research and education, have been published lately, but my experience proved that Japanese universities are not as bad as many Japanese people expected, at least the case of materials science. Of course, there are disadvantages in Japanese system, but there are also disadvantages in American system. I think the situation can be improved by our effort, importing good system from foreign countries.

As a result of the internship program I will behave differently in terms of the following things.

First, I would think that I am proud of studying in Japanese universities. Sometimes we see the people who are less confident just because they have been studying in Japan. However, I learned that the research and education environment in Japan is not so bad. We have one of the best research and education environment in the world, and there is no other way without taking advantage of this.

Second, I will be more kind to international students. Going through the internship, I understood how international students feel during their studying abroad, such as difficulties of understanding language including slang, loneliness, importance of friends and so on. Then, I realized that we should provide more support for international students. As I mentioned above, Japanese universities still need more effort to improve the situation in which international students feel inconvenience. Although we need more budgets, I think some parts can be solved by our individual effort, such as talking to international students every day, giving some advices for them and so on.

Third, I will not be afraid of conducting collaborative research projects with researchers coming from different countries. During my undergraduate study and the internship, I had many chances to collaborate with people from different setting in terms of nationality, speciality, ethnicity and so on, and I could successfully get new results. Such experiences gave me confidence to be involved in international collaborative research. In addition, since I could meet good friends in both of Japan and the U.S., I would like to keep in touch with them, and I am looking forward to working together in the future.　Fortunately, I will join an international research project involving more than 3 different countries in my graduate study at Kyoto University, and I want to be able to contribute based on my experiences in the U.S.

In the future, I want to be a principal investigator in the materials science research field and also want to be able to take leadership in big research projects. Now, I definitely believe that all of the experiences I had in TOMODACHI STEM @ Rice University program will help and encourage me to become a leader connecting different countries including Japan and the U.S.

Return to Top[:]