Junsoo Kim
@Junsoo_twit
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Assist. professor @NorthwesternU | Ph.D. @Harvard | #Softmaterials #Fracture #Mechanics #Structures
Evanston, IL
Joined May 2020
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We have reported a new, multi-scale approach that improves the fatigue resistance of particle-filled rubbers, published in Nature.
Our sample (right) was nearly intact after 350,000 cycles.
Look at the thread from Jason, co-first author of the paper.
Our paper is online today in Nature. We study an old material: rubbers reinforced with inorganic particles.
@Junsoo_twit
@ykutsovsky
@zhigangsuo
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I will be joining the Department of Mechanical Engineering at Northwestern University as an assistant professor, starting this July.
@NorthwesternU
@NorthwesternEng
(1/n)
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It took 11 years from entering grad school at SNU in Korea to become a Ph.D. at Harvard. I was so fortunate to have so many talented, kind, and passionate people. Special thanks to my advisor
@zhigangsuo
, wife Younglong, and daughter Ain. It’s an honor to have you…
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I got an email from
@zhigangsuo
that my paper is accepted.
It is the first paper after 4 years at Harvard. Yeah. I had to work without any paper for 4 years. Was I unhappy? No. I didn't have time to cry, as I had a too great advisor, friends, and family.
@MeixuanziS
@GuogaoZhang
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In a gel, some strands bear loads and some do not.
In our new paper, we modify the existing model of gels (Flory-Rehner model) to distinguish load-bearing strands and the rest.
#JMPS
@zhigangsuo
@Tengh_Yin
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I joined the
@JenniferALewis1
lab at Harvard as a postdoctoral researcher in July. I am so excited to interact with talented people there and explore soft mechanics further through advanced manufacturing technologies!
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Our paper is online now!
- Entanglements stiffen elastomers and gels but do not embrittle them.
- With entanglements, polymer chains can be extremely long: toughening without hysteresis, low friction, low wear rate.
@zhigangsuo
@GuogaoZhang
@MeixuanziS
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Leah made our work (my first paper at Harvard)
more accessible to a broad audience. The paper will be published in Science this week.
"Elastic polymer that is both stiff and tough, resolves long-standing quandary"
@zhigangsuo
@GuogaoZhang
@MeixuanziS
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Our paper is online now!
We make a hydrogel elastic and tough by kneading, annealing, and cross-linking a DOUGH :)
@zhigangsuo
@GuodongNian
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My Ph.D. defense is next Monday, 4/25 at 4 PM EST, entitled "Fracture of Highly Entangled Polymer Networks".
In-person: Maxwell-Dworkin G125, Harvard University
Zoom: please DM me for the link.
Anyone interested in the mechanics of soft materials is welcome!
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I am recruiting motivated students and a postdoc, who are interested in the mechanics of soft materials. See the details and Advising Statement at .
Learn more about our fantastic department.
(2/n)
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The first project at Harvard is finally published!
We solve a basic problem in soft pneumatic robots inspired by pit membranes in plants.
@zhigangsuo
@zhe_mination
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Thermosets, elastomers, and gels are polymer network, where polymers are crosslinked. One way to make them sustainable/degradable is to use reversible crosslinks.
In our new paper, we study the effects of entanglements in degradation using hydrogels.
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I share my defense publicly as all the papers in the defense are published.
The past five years were hard but fantastic. It was one of the best decisions my wife and I made to quit the national lab in Korea to study abroad when everything was uncertain.
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I am teaching a mechanics course for undergrads as a TF, and one student told me that she likes my way of teaching/thinking. It was only one student but encourages me so much. The agony that lasted until 3AM yesterday to make the concept straightforward and simple was worth it.
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My first research grant will be a seed grant from MRSEC at NU! I will study the fracture of protein hydrogels. Can proteins be made as durable as synthetic hydrogels? Can they be better? If we design them carefully, can we replace the load-bearing tissues?
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Take a look our PEG hydrogels of 93% water. The toughness is over 1,500 J/m2. Nearly zero hysteresis.
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In this paper, we built multiple fatigue testers with ~$500/ea. It is good enough for soft materials where the load is low. All parts are from either Amazon or McMaster-Carr, controlled by Arduino. This allowed us to measure the fatigue threshold as a function of two variables!
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Dear friends and colleagues, thanks for all coming to my defense both in-person and on zoom. I am so blessed to have you all.
This is just a start. I look forward to the science and innovation we will discover, together.
Thank you
@zhigangsuo
, my role model.
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Boulder workshop on soft & active matter mechanics.
4/28-29 8-11AM, 3-6PM.
What an amazing line up.
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In undergrad thermodynamics course, I had to memorize many equations of energy, entropy, pressure, volume, exergy(!), and so on. I had to agree with the three laws although I was not convinced. After the final exam, I forgot literally all.
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Tomorrow, I will go over the Basic Algorithm of Thermodynamics (BAT) in my class.
The BAT totally changed my perception of thermodynamics. If you cannot explain what is entropy right away, I strongly recommend reading
In the 2019 fall,
@zhigangsuo
taught a thermodynamics course for undergrads, and I did TF. He wrote his own text during the summer. Unlike conventional engineering textbooks, he emphsized basic things, such as entropy, temperature, etc.
#thermofact
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Thanks for sharing the beautiful story of love and life. My wife and I saw ourselves in Denian and you, and reaffirmed the power of love. May she be with you forever in love.
My wife, Denian, died at home, on Sunday, 30 July 2023.
She was diagnosed with glioblastoma in March 2018.
The last five years have been fulfilling. She saw births of two more grandchildren. She attended wedding of our second son. She pointed to me pretty spots on our walks.
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New paper in PNAS! We show a stiff, strong, and tough hydrogel. The underlying mechanism in the synthesis is very interesting.
Multi-scale design of polymer structure is the key for the exceptional properties.
It’s been so much fun to work with
@GuogaoZhang
!
Our paper is online today in PNAS!
-We fabricate a nanocomposite with a hydrogel and a polymer glass by self-assembly.
-The nanocomposite has high water content and load-bearing capacity comparable to that of PE!
@zhigangsuo
@Junsoo_twit
@SammyHassan_
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The first recorded talk on our Science paper. It is a short talk (~25 mins) and prepared for a relatively general audience.
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Apple needs to hire more fracture mechanicians. I think that is a fatigue crack growth. Mine is halfway through.
@SammyHassan_
I’m the VP of Hardware at Apple ($12.4M TC) responsible for making this part that cracks on every charger that we’ve produced over the last 15 years.
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In the 2019 fall,
@zhigangsuo
taught a thermodynamics course for undergrads, and I did TF. He wrote his own text during the summer. Unlike conventional engineering textbooks, he emphsized basic things, such as entropy, temperature, etc.
#thermofact
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Soft microstructures often form clusters. Once the structures cluster, it is hard to de-cluster.
We show a reversible clustering with thermo-responsive hydrogels. The suggested model predicts the condition of clustering transition.
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Mooney-Rivlin model is introduced by Mooney and Rivlin.
The Gent model is introduced by Gent.
Neo-Hookean model is introduced by?
It seems Treloar. A neo-Hooke's law.
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"only one type of comparison is really a fair one: a comparison between my career progress now and my career standing in the past."
Although I couldn't publish any paper for 3 years at Harvard, I believe what I'm doing now is the best for my career.
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I got a Certificate of Distinction in Teaching for doing TF last semester! I am so glad that the students liked what I did.
Thanks to our amazing teaching crew: Buse,
@jasongsteck
, Jason M., and Tony.
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An open peer review is going on (after publication). I admire the ambitious goal of the paper, as well as the responsible criticism from inorganic chemists.
It reminds me of what I am responsible for, as a author and reviewer.
This exciting paper shows AI design of materials, robotic synthesis. 10s of new compounds in 17 days.
But did they?
This paper has very serious problems in materials characterisation. In my view it should never have got near publication. Hold on tight let's take a look 😱
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The Preview that
@jasongsteck
and I wrote is published in
@Matter_CP
. We discuss the paper from
@CretonCost
and Prof. Lopez, where the fatigue of double network elastomers is studied by mechanofluorescence.
Thanks to
@CranfordMATTER
for the invitation:)
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When a physicist says that curiosity is important, I say that impact is also important. When an engineer says that impact is important, I say that curiosity is also important. Like a flip-flopper. Maybe I want to be someone who bridges the two worlds. A physineer?
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My second paper at Harvard is online at JMPS!
It was such a pleasure to work with
@SammyHassan_
and
@zhigangsuo
!
In this new paper with
@SammyHassan_
@Junsoo_twit
, experimental data make us modify the Late-Thomas model.
A hydrogel has rate-independent deformation, but rate-dependent toughness.
@CretonCost
@ProfZhaoMIT
@NPeppas
@DRudolfKing
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A snap-through between buckling modes can be a powerful actuator.
The authors trigger snap-through by drying the gel.
Using chemical potential, we can store enormous energy.
Is there a more practical power source that can trigger?
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The 3D version of stereolithography.
It is also a layer-by-layer process, requiring sweeping. However, here, the moving part is information, not the structure.
The dual-color photoinitiator seems the key.
Q. How about a triple-color photoinitiator?
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Evaluate with impact, not impact factor.
Be the first, not the best.
Move on when crowded.
Study what can help people.
Think in the long term. Study something would inevitably happen.
Remember that our neighbors are paying for the grant.
Help students to be productive citizens.
The video of Dave Weitz’s
@EMLwebinar
is online:
George Whitesides came for the after-lecture discussion.
Look for the conversations between George Whitesides and Dave Weitz.
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I will focus on improving the mechanical properties of soft materials by designing both molecular and macro structures. I am also interested in advancing 3D printing technology. I wish I can discover something meaningful and impactful.
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The problematic oxide layer of the liquid metal has been doped.
Now, it is deformable and conductive. Not a problem anymore.
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Flat + Flat = Good contact. ex) PDMS on wafer.
Flat + Rough = Bad contact. ex) PDMS on skin.
Then, how about Rough + Rough?
Please find our strategy for better contact on rough surfaces. We have covered skin, leaf, and hair.
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Why energy dissipation increases the toughness?🧵
To understand it, let’s revisit the basics of fracture mechanics. Consider two bodies, at a fixed stretch, with different crack areas A. As the crack grows, the elastic energy U decreases.
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I present our recent work on how entanglements affect fracture, fatigue, and friction of elastomers and gels at the APS March meeting.
Session info:
I'd be happy to hear from you guys on this topic.
#apsmarch
#APS2022
@APSMeetings
@ApsDsoft
@APS_DPOLY
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Theory is like grammar.
As grammar formulates language, theory formulates nature. Theory is imperfect and often boring like grammar. We can learn a new language without grammar but it is essential to be fluent.
Unfortunately, none of us is a native speaker of nature.
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Great ideas are often trivial, and therefore, universal.
Before these trivial ideas, they were not trivial.
-In an undergrad course taught by
@zhigangsuo
.
Creativity might be recognizing novelty from trivial things.
What am I missing from what I know?
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The CEE and ME departments at Northwestern University invite applications for the newly established Jan and Marcia Achenbach Professorship (at the tenured full professor level).
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A mechanical memory made of soft materials.
A small block operated by a magnetic field switches the instability, analogous to triggering I-V curves by the gate voltage.
It is not just to invent a memory.
It is about multimodal metamaterials.
Can be >2.
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Tactile sensors are getting smart and unrecognizable!
The next question I have is the interface.
Note that there are massive interconnections behind the photos.
Can it be wireless? Is there a fundamental limitation or it is just an engineering problem?
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It worked to me only if physics and engineering are merged.
Engineering without physics made me forget things like light. House of cards.
Physics without engineering seems not relevant to problems. Boring.
Maybe these words "physics" and "engineering" make the boundaries.
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The mechanical properties of PVA hydrogels are greatly improved.
The key is to form a network of PVA nanofibrils.
A network of nanofibrils is formed when a frozen PVA is submerged in the sodium citrate solution.
@jasongsteck
@lin_shaoting
@RuobingB
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The Lake-Thomas model predicts the fatigue threshold. However, it has been mostly used as a scaling since the prefactor has been in a quandary. This paper shows me the physics of the prefactor. That is, we have another opportunity to engineer the fatigue threshold.
"Nonlocal Intrinsic Fracture Energy of Polymerlike Networks" appears in
@PhysRevLett
. We show that the intrinsic fracture energy of polymer-like networks is due to nonlocal dissipation, different from the classical Lake-Thomas model.
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Dr. Will Mars gave a fantastic talk today morning about fatigue of rubbers. There will be another talk in Dec. Look forward to it!
@EnduricaWill
@EnduricaLLC
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This is truly mindblowing. A simple but impactful idea. Inspiring. A beautiful combination between gel and manufacturing.
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As car tires wear, they break down into micro-particles, polluting the environment. The toxic chemical preventing damage from ozone is detected in salmon.
Can tires be made of eco-friendly rubber and additives? It seems extremely challenging though.
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@jasongsteck
This award is for those who got a Q score >4.5 out of 5 from the students. Yeah. It is not a super objective and credible award. But! I am so proud of it because we really did our best to teach the subject without compromising for fun.
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The polymer chains where the stress is concentrated are lengthened by breaking weak crosslinks, leading to a better fracture resistance.
Stretchy, super-tough materials could make car tires less polluting. The secret to their toughness: Weak bonds that actually make them stronger.
@CraigGroup_Duke
@CciMonet
@DukeU
@johnsonchem
@ShuWang_1111
@MIT
#NSFfunded
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An essay from a post-doc who tried to write a solo review paper.
"I have learnt to use the good suggestions and shrug off perceived harshness in the tone."
A must-have attitude for researchers, including me.
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This is one of the most valuable things I did during my PhD. The puzzle put together! Now, I see thermodynamics in daily life. I see atoms dancing around in polymers. I talk about entropy without fear. I cannot forget it. I just can't.
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In the EML seminar, typically there is a 2-hour discussion after a seminar.
There are also valuable advice and comments from both the speaker and the panels! In the EML seminar, they are all my advisors.
I'm so motivated again. I'll attend the next seminar as well.
@EmlWebinar
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Soft robots for applications in which the bulk modulus should be considered.
Rigid systems are vulnerable to a mismatch of bulk moduli. For soft systems, the small strain by the mismatch is a piece of cake.
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I would like to thank my Ph.D. advisor
@zhigangsuo
for his strong support and thoughtful guidance. I’ve been a happy researcher for 6 yrs in his group thanks to the ideal academic environments he provided. One of my visions is to provide a similar experience to more people. (4/n)
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At the journal club last year,
@jasongsteck
presented this paper.
A crack is a corner of 0°. 180° means there is no crack. Then, how about 90°? Is this a crack?
In his recent paper with
@SammyHassan_
@zhigangsuo
, they discuss the toughness of a corner.
How do we predict failure of a soft material near a corner? In our new paper, we investigate this question experimentally and theoretically:
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Magnetic particles are attached to ion channels to stimulate them remotely.
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A 50% of strain-induced crystallization is impressive. Half of the elastomer becomes crystal by mechanical loads and is reversible. Their polymer network topology has two features: low chain length distribution and sparse entanglements.
On the cover of
@ScienceAdvances
, we report an elastomer with over 50% strain-induced crystallization, much higher than the 20% level of state-of-the-art rubbers.
* The new elastomer is achieved by deswelling a tetra-arm “ideal” polymer network gel.…
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Here’s the key points.
1) Use long polymers.
2) Mix them with a small amount of water (becomes a dough!)
3) Kneading and annealing the dough to densely entangle the polymers.
4) Crosslink them to fasten the entanglements.
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Look forward to seeing all the panels at the fishbowl discussion!
Also, I will present my recent work at the poster session. Please stop by if you are there!
Join SES tomorrow, Tuesday, October 19, for Day 4: Soft matter of the SES Virtual Month!
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My lab will study the mechanics of soft materials theoretically and experimentally.
We will envision that soft materials can be made as durable as hard materials, which would innovate our lives and make polymers sustainable. (3/n)
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I heard that these concepts are carefully taught in thermodynamic courses at the physics department. Instead, applications are taught less. Zhigang's class was initially like a physics course and covered engineering problems later. I spent a lot of time to study the basics.
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Denian wanted Zhigang to work as usual as possible in this difficult time. Zhigang had advised me and all group members flawlessly, and I was able to defend my PhD last year. We all are grateful for her support.
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"... not all entanglements contribute equally in a polymer glass."
In other words, making all entanglements contribute equally is important for strong, tough, and cost-efficient polymers.
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"I traded job security and high pay for doing things I was passionate about. Out of over 20 job offers I received upon graduation from college, I chose the lowest paying one by far because I thought by doing so, I could potentially improve the health of patients."
The Modest Moderna Cofounder And Multibillionaire Robert Langer Shares His Secrets To Success
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This polyethylene-like plastic has reversible bonds in chains.
Recycle it by breaking the reversible bond, and bond again.
The properties do not degrade after recycling.
Simple idea, but phenomenal impact.
@zhigangsuo
@AtteKadoma
@SammyHassan_
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Swelling has been used as an automatic soft valve but stayed in fluidic channels.
We have taken out them to air-water interface, and used them to regulate sweat: artificial perspiration.
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Given peer pressure, competitive job market, and uncertainties, we often lose the fun of research.
To me, curiosity has been a cure.
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@zhigangsuo
@GuogaoZhang
@MeixuanziS
Then I thought about Zhigang and our group. I am at the lab leading the fracture mechanics of hydrogels! I could come up with the main idea of our paper immediately after I combine these two words: entanglements and toughness. 3 years of study on fracture made me recognize it.
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Edible, Ultrastrong, and Microplastic‐Free Bacterial Cellulose‐Based Straws by Biosynthesis
The wet flexural strength of the presented straw is ~2 times higher than that of a paper straw after 2 hours in water. (Figure 4d)
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I've seen this behavior in the lab so often, but never thought in this way. Awesome. Such a great idea!
Excited to share our recent effort in applying hydrogel technologies to combat surface fogging on diverse transparent materials. Please check our recent Research Article (The article can be accessed via ) and Video Abstract ()
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The reason why we need to know “toughness” correctly.
For silica, R = 1 nm. The condition R < f is readily satisfied in experiments. Thus, Wc is not a material constant, and varies enormously from sample to sample.
This old paper reports a lot of data on strength of silica.
@jasongsteck
@AtteKadoma
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Zhigang reviews the document used to arrest a MIT professor, Gang Chen.
And, ironically, I realize that Prof. Chen is just an amazing and exceptional scholar, who are excellent at both science and management.
Thank you,
@BobbyChesney
, for pointing to this affidavit against Professor Gang Chen, of MIT, prepared by the Special Agent Matthew McCarthy, of the Department of Homeland Security.
In this thread, this document will be called
The McCarthy Document.
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The fracture strength of most high-performance TE materials is poor.
"Is it possible to raise the toughness and fracture strength of the new high-ZT materials or to mitigate the risk of cracking during operation via innovative device design?"
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In semicrystalline, chains rupture at the amorphous phase rather than the crystal phase.
Crystals in semicrystalline are defectless.
Found it in this book.
@zhigangsuo
@GuogaoZhang
@GuodongNian
@SammyHassan_
@AtteKadoma
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Importantly, I did not know what entropy is but used it to take exams. Memorizing the equations was the only way to use the entropy. Someone proudly told me that entropy is "randomness". It confused me more.
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This paper is published together with our paper, and is also about the mechanics of polymer networks.
They use an extendable monomer. When a chain is about to break, it extends with energy dissipation, which toughens the polymer.
@CciMonet
@johnsonchem
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@zhigangsuo
@GuogaoZhang
@MeixuanziS
In 2019, I was trying to propose a new thermodynamic model of hydrogels but it was not easy as the entanglements and chemical crosslinks were indistinguishable in my model. This is the moment when entanglements came to my mind.
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@zhigangsuo
@GuogaoZhang
@MeixuanziS
With our fantastic co-authors, we got all data during the COVID. There are so many stories and difficulties but we went through them with Zhigang's help. I really wish to be like Zhigang once I mentor someone, and have friends like Guogao and Xuanzi once I leave Harvard.
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The detailed procedure is in the Method.
The entire Arduino code is in the Supplementary Information.
An undergraduate student can build this.
One can improve it with a load cell.
*There is one typo in the code: directionPin = 8 -> 9; stepPin = 9 -> 8.
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It is so educational and inspiring. Please read it especially if you are a young scientist. I learned a lot.
The story of Howard Stone, told by himself at an interview. His office used to be next to mine. But I learned much about him from this interview.
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1. PLA needs a special composting infrastructure to be degraded in days~few months. Otherwise, it takes 80 years.
2. Composting: Oxygen+High temp+Microorganism. Not many.
3. There can be 40% additives in PLA filaments, which is toxic.
Be careful. Could be a greenwashing.
@zhigangsuo
@MeixuanziS
@QuanJiao
@GuogaoZhang
@Junsoo_twit
@GuodongNian
@jasongsteck
@SammyHassan_
@ShiQingWang4
@Baohong_Chen
@AtteKadoma
Some recent criticism of PLA.
Actually nearly all biodegradable polymers may encounter similar problems.
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I am also thankful for my current lab mates and former lab mates, friends, and family. I have been privileged to work with so talented and kind people, where I learn not only research but also life. (5/n)
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It wouldn’t be possible without unconditional support from my wife. We have just decided to stay in the US and my daughter will be grown up as a Korean-American. We are at the biggest life-changing moment and we have always been together in this journey. (6/n)
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“What's the difference between a scientist and an engineer?
The answer: Surprisingly little.”
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Three-dimensional nanoprinting via charged aerosol jets.
The basic concept is published in 2006.
Congratulations to my former PI, prof. Mansoo Choi, and his colleagues. Intriguing results after 15 years of persistent study.
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Special thanks to my co-first authors.
@GuogaoZhang
@GuodongNian
@SammyHassan_
@jasongsteck
@MeixuanziS
All the works presented in the defense are co-first author papers! Without you, I wouldn't have been able to get here.
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In our 2021 Science paper, we densely entangled long polymer chains for better properties, by concentrating the monomer in the precursor.
However, how about pre-existing polymers, such as PEG and natural polymers? Concentrating long polymers is not easy!
Our paper is online now!
- Entanglements stiffen elastomers and gels but do not embrittle them.
- With entanglements, polymer chains can be extremely long: toughening without hysteresis, low friction, low wear rate.
@zhigangsuo
@GuogaoZhang
@MeixuanziS
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This is the overview how we build the model.
- Define an isolated system.
- Calculate the entropy of the system as a function of stretch.
- Find the stretch that maximizes the entropy.
- Get equations of states.
It is also known as "minimize the free energy".
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After I got the model, I investigated all synthesis parameters to see if M > N in all cases. One of the parameters was the monomer concentration, and it was the moment that I got the motive of my Science paper.
Our paper is online now!
- Entanglements stiffen elastomers and gels but do not embrittle them.
- With entanglements, polymer chains can be extremely long: toughening without hysteresis, low friction, low wear rate.
@zhigangsuo
@GuogaoZhang
@MeixuanziS
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Special thanks to the co-first author
@GuodongNian
and
@zhigangsuo
for for being patient and persistent despite so many revisions, which results in >20 supporting figures.
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The paper claims two simple points.
First, highly entangled polymer networks with degradable crosslinks have outstanding mechanical properties, as shown in my first paper about entanglements: .
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A history of solution theory by J. H. Hildebrand (1981).
"The development of solution theory has resembled the flow of the Gulf Stream across the Atlantic— a meandering course with eddies that disappear."
Am I the eddies or the course? We will see.
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