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In reply to Journal Club for March 2019: Fatigue of hydrogels

Hi Ruobing,

This is very nice contructive and inspiring summary. I like it a lot.

I am particualrly interested in poroleastic fatigue and viscoelastic fatigue. The time effect of poroelasticity and viscoelasticity on the total toughness is understood I think. I am trying to think through the molecular picture on the effect of poroelasticity and viscoelasticity on fatigue threshold. Can you give an example to give a more intutive explanation on both effects on fatigue threshold?

Best,

Shaoting

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In reply to Journal Club for March 2019: Fatigue of hydrogels

Dear Shengqiang,

Thank you for the comment. Regarding the cyclic-fatigue threshold, we think the Lake-Thomas model is the molecular picture that happens at the crack front in a hydrogel with a long-chain solid-like primary network (such as polyacrylamide). As an analogy to ductile metal, during cyclic loading, the bond breaking of tougheners (either solid-like or liquid-like) accumulates around the crack tip, and the tougheners fail to toughen the hydrogel after many cycles of loads. The Lake-Thomas model basically says that by fracturing a single layer of polymer chains, the crack can grow, and the energy release rate required to do so is ~ 10-100 J/m2 for most hydrogels and elastomers. This is a small number compared to the fracture energy of many tough hydrogels.

For a hydrogel crosslinked by some weaker noncovalent bonds (e.g., Ca-alginate, gelatin, etc.), we also proposed a modified Lake-Thomas model in the review. The idea is still breaking a single layer of polymer chains, but now the polymer chains are modeled as entropic springs stretched to a force equivalent to the bond strength of the noncovalent crosslinks, instead of being stretched to their own bond strength (C-C bond in most cases) in the initial Lake-Thomas picture.

For hydrogels with no tougheners (e.g., polyacrylamide), the cyclic-fatigue threshold is still one order of magnitude smaller than the bulk toughness. This remains to be a really interesting question to be addressed. One thought is that poroelasticity (water migration) may have some toughening effect, especially when there is a crack, the stress field is inhomogeneous, so is the chemical potential field. However on the other hand, the period of each loading cycle is rather short in experiments (1-10 s), which may hardly lead to any effective diffusion and toughening. If you have thoughts on this, I would love to hear.

After all the Lake-Thomas model is qualitative, though some quatitative comparison seems to be okay (Fig. 7) in some range. A good quantitative theoretical model may be worth investigating here. 

Best,

Ruobing

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In reply to Journal Club for March 2019: Fatigue of hydrogels

Hi Ruobing, thanks for the review, really informative. Zhigang mentioned some of the results during his visit to UCSD.  While I am still trying to understand all your discoveries, I have a small question. You mentioned the cyclic fatigue toughness of hydrogel with solid-like toughner can be much less than its static fatigue toughness. Any explainations? For ductile metal, I believe it is understood. During cyclic loading, dislocation accumulates around the crack tip, which results in crack growth.  

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In reply to Postdoctoral Research Fellow Position at the University of São Paulo, Brazil.

Marcelo

Does it need to be proficient in ABAQUS or ANSYS would do?

Regards,

J

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In reply to Extreme enhancement of interfacial adhesion by bulk patterning of sacrificial cuts

Dear Frederick, thank you for your valuable feedback. My student brought your recent Soft Matter paper to my attention a couple of weeks ago and we found it very intriguing. Thanks for sharing the Advanced Material paper here. I think you have found a very robust way towards realizing these sacrificial systems and that could significantly broaden their practical application. I look forward to further discussions with you on this topic.

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In reply to A preliminary document on my fresh new approach to QM

For the sake of ... no other word but ``idiots'', esp. the American borns / intellectual goons:

https://ajitjadhav.wordpress.com/2017/12/12/yes-i-know-it-part-2/

https://ajitjadhav.wordpress.com/2017/12/11/yes-i-know-it/

Often the issue also applies to many (but not all (but excepting for only a very small insignificant minority of)) the past Indian immigrants to the USA.

Sincerely,

--Ajit

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In reply to interesting model, Emilio

Thank you Mike. You are right, returning to the square root singularity is indeed an interesting finding. 

Regarding the number of parameters. In its simplest version, the model requires two elastic constants (Young's modulus and Poisson's ratio), two conventional plastic properties (yield stress and hardening exponent), two fracture parameters (cohesive strength and fracture energy) and one material length scale for strain gradient plasticity. All of them can be measured experimentally except for the cohesive strength, which can be chosen on physical grounds (e.g., theoretical lattice strength if we are modelling atomic decohesion). I agree with your vision and I try to keep it a simple as possible (Occam's razor principle) but sometimes the physical picture is complicated and we need to capture those mechanisms if we want to be predictive. As you say, it is a trade-off at the end of the day.

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In reply to Thank you for sharing. They

Thank you Liu. If you encounter any problems while using the codes do not hesitate to drop an e-mail.

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In reply to The role of plastic strain gradients in the crack growth resistance of metals [code included]

...particularly the fact that with SGP we return to the square root singularity.  However, a limitation perhaps is that one needs material constants for hardening (there are models with dozens of them), material constants for SGP (one or more length scales), material constants for the cohesive model (although perhaps the fracture energy is what matters most).   With all these material constants, on one hand it is possible to fit any behaviour, on the other hand, we loose the simplicity and the convenience of modelling at all. Personally, I never liked HRR and elasto-plastic fracture mechanics, like anything that I don't find easy to explain to undergraduate students.

I like mostly Griffith brittle fracture, and full plastic model, and in between, some sort of fitting (or "asymptotic matching").

One hopes to keep a good trade-off, how many constants you have introduced?

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In reply to The role of plastic strain gradients in the crack growth resistance of metals [code included]

Thank you for sharing. They are very helpful

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In reply to The role of plastic strain gradients in the crack growth resistance of metals [code included]

Thank you, Shailendra. I hope that you find the code useful.

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In reply to The role of plastic strain gradients in the crack growth resistance of metals [code included]

Emilio, 

Very nice work!

It is a wonderful initiative on your part to make the codes accessible to the public. This will accelerate future investigations and provide interesting directions. 

Thanks,

Shailendra 

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In reply to Fully funded Marie-Curie PhD positions (UK and Denmark)

The link in my previous blog was incorrect, now corrected. The deadline is 27th February. Applications must be made via the link and not CVs sent directly by emails!

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In reply to Extreme enhancement of interfacial adhesion by bulk patterning of sacrificial cuts

Very interesting! Thanks for sharing. I will definitely have a more careful look at these papers. We are also interested in the sacrificial bond and hidden length mechanism. We use a fluid mechanical instability to create microstructured fibers with these sacrificial bonds. We play with the instability parameters to tune the mechanical properties of the fibres accordingly.

Cheers

1. Zou, S., Therriault, D., Gosselin, F.P. “Failure mechanisms of coiling fibers with sacrificial bonds made by instability-assisted fused deposition modeling” Soft Matter, 2018, 14 (48), 9777-9785
2. Passieux, R., Guthrie, L., Hosseini Rad, S., Lévesque, M., Therriault, D., Gosselin, F.P. “Instability-Assisted Direct Writing of Micro-Structured Fibers featuring Sacrificial Bonds” Advanced Materials, doi:10.1002/adma.201500603 

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In reply to Extreme enhancement of interfacial adhesion by bulk patterning of sacrificial cuts

Very interesting! Thanks for sharing. I will definitely have a more careful look at these papers. We are also interested in the sacrificial bond and hidden length mechanism. We use a fluid mechanical instability to create microstructured fibers with these sacrificial bonds. We play with the instability parameters to tune the mechanical properties of the fibres accordingly.

Cheers

1. Zou, S., Therriault, D., Gosselin, F.P. “Failure mechanisms of coiling fibers with sacrificial bonds made by instability-assisted fused deposition modeling” Soft Matter, 2018, 14 (48), 9777-9785
2. Passieux, R., Guthrie, L., Hosseini Rad, S., Lévesque, M., Therriault, D., Gosselin, F.P. “Instability-Assisted Direct Writing of Micro-Structured Fibers featuring Sacrificial Bonds” Advanced Materials, doi:10.1002/adma.201500603 

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In reply to Great work!

Thanks for the comments!  I'm glad to hear that our bonding work is finding use outside our lab!  I've pasted in your questions with responses below.

1) Relating to the assembled PS structures - are their mechanical properties fairly uniform, or do you notice weakness at the bonded interfaces? Also, have you tried a multi-step process for bonding these structures (with sequel stretching in various directions) in order to create two-dimensional structures? 

We looked at the strength of the weld in some detail and found it approached that of the PS islands, but that indeed that was the weak site of our structures (i.e., where they would break).  This was covered in the SI (pg. S15, Figure S6).  The take home message: the Young's modulus for the weld was 19.8 MPa where the films maxed out at 36.6 MPa.  For the second part (sequential processing), yes, we did try that and it works.  We have not yet published these results, but stay tuned!

2) I can envision using your chemical templates to create microscale electrostatic actuators. In particular, we work on HASEL actuators, which are simply a polymer pouch filled with liquid dielectric, and electrodes on the outside. It seems like your process could be used to create tunable microscale HASEL actuators by precisely defining regions wetted by the liquid dielectric. Haven't figured out how you bond another polymer layer over that yet =). Anyway, has your group ever tried making electrostatic actuators using this process, or have you only explored humidity-sensitive actuators thus far?

We have (and have a manuscript close to submission describing this) looked at actuators that are light responsive, magnetic responsive, and humidity responsive, but nothing using electric fields or electrostatics.  Your suggestion is very interesting!  I know we have successfully created "capsules" using our approach, so that could, in principle, be a way of containing the liquid dielectric, and we have also deposited metals (gold, silver, copper) on these surfaces, so that would be the electrode.  I'd love to discuss further as it seems the answer is yes, micro-HASEL actuators may be possible following these strategies.

Thanks again!

Best,

Steve

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In reply to Great work!

Thanks for the comments!  I'm glad to hear that our bonding work is finding use outside our lab!  I've pasted in your questions with responses below.

1) Relating to the assembled PS structures - are their mechanical properties fairly uniform, or do you notice weakness at the bonded interfaces? Also, have you tried a multi-step process for bonding these structures (with sequel stretching in various directions) in order to create two-dimensional structures? 

We looked at the strength of the weld in some detail and found it approached that of the PS islands, but that indeed that was the weak site of our structures (i.e., where they would break).  This was covered in the SI (pg. S15, Figure S6).  The take home message: the Young's modulus for the weld was 19.8 MPa where the films maxed out at 36.6 MPa.  For the second part (sequential processing), yes, we did try that and it works.  We have not yet published these results, but stay tuned!

2) I can envision using your chemical templates to create microscale electrostatic actuators. In particular, we work on HASEL actuators, which are simply a polymer pouch filled with liquid dielectric, and electrodes on the outside. It seems like your process could be used to create tunable microscale HASEL actuators by precisely defining regions wetted by the liquid dielectric. Haven't figured out how you bond another polymer layer over that yet =). Anyway, has your group ever tried making electrostatic actuators using this process, or have you only explored humidity-sensitive actuators thus far?

We have (and have a manuscript close to submission describing this) looked at actuators that are light responsive, magnetic responsive, and humidity responsive, but nothing using electric fields or electrostatics.  Your suggestion is very interesting!  I know we have successfully created "capsules" using our approach, so that could, in principle, be a way of containing the liquid dielectric, and we have also deposited metals (gold, silver, copper) on these surfaces, so that would be the electrode.  I'd love to discuss further as it seems the answer is yes, micro-HASEL actuators may be possible following these strategies.

Thanks again!

Best,

Steve

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In reply to Dear Philipp,

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In reply to Dear Philipp,

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In reply to Dear Steve,

Dear Philipp,

Thanks for the comment!  We have done some work with 3D (wrinkled) structures using an approach similar to what you've described (Taylor et al. Adv. Mater. 2016, 28, 2595-2600).  The interesting result here was that, by controlling the thin-film microstructure of the hard overlayer using strain, you could access mechano-optical effects.  Namely the transition from reflective to scattering states.  Using this concept we created "skins" with locally switchable reflectance (see Fig. 4 and Video S5).  I think the TOC nicely captures this concept.

We are continuing to build on this concept to manipulate polymer micromaterials.

Thanks again Philipp,

Best,

Steve

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