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Updated: 12 hours 36 min ago

Hi Teng,

Sun, 2018-03-04 22:29

In reply to Very nice review

Hi Teng,

Thanks for the questions!

1) No one has looked at a nanoparticle array with the same composition and geometry in both tension and compression. It's tricky to try to learn about tension/compression asymmetry by comparing pillar compression and nanoindentation into thick films, to membrane deflection (tensile deformation) on monolayer or few-layered membranes because of the differences in pre-strain, and even ligand conformation in the compression and tension samples because the different processing steps to make the samples.

2) Fracture be brittle or ductile depending on the ligands used. The short chain polystyrene ligands that I used in my study are brittle at room temperature so fracture of the nanoparticle thin films is brittle as well. Dodecanethiol capped Au nanoparticle membranes are also brittle (Wang et al., Faraday Discussions (2015)) because the ligands are too short for entanglement. Fracture strengths for this system ranged from 11-15 MPa, and depended on membrane thickness and size of the inorganic nanoparticle (fracture toughness has not been measured). Superlattices formed with longer, less dense polystyrene ligands (polymer brushes) can be brittle or ductile, with fracture toughness that is 0.2-0.9 of the toughness of the bulk polymer (Choi et al., Soft Matter (2012)). 

3) Defects of individual nanoparticles (like vacancies and interstitials) may cause dips or bumps at the surface of film, but I did not see any evidence of this using AFM. The polymer ligands near the defect may stretch or compress to minimize the defect. Ripples are not seen in freestanding nanoparticle membranes because the membranes are under tension. I imagine that ripples could form if the membrane could be made very big, or tension free. 

4. The defects in nanoparticle superlattices definitely affect their mechanical properties. I'm not sure if defects could control topological features in a freestanding structure. One difference between the nanoparticle membranes and a 2D material like graphene is that the "bonding" between nanoparticles is not directional.

Hi Xiaoyan!

Sun, 2018-03-04 20:33

In reply to Inspiring and exciting work!

Hi Xiaoyan!

Thanks for the encouraging comments and interesting questions!

1) The main factor for controlling defect formation and distribution was the rate of self-assembly of the nanoparticles on the fluid interface. During the self-assembly process, nanoparticles are deposited in their native solvent (hexane or toluene) onto a droplet of an immiscible fluid (ethylene glycol). As the solvent dries, the nanoparticles are confined to a smaller and smaller space above the immiscible fluid and nanoparticles try to find an energetically favorable position relative to other nanoparticles. If drying occurs slowly, the nanoparticles are likely to form into a close-packed conformation. If drying occurs quickly, the nanoparticles are frozen into energetically unfavorable positions which results in defects. These two limits are analogous to slow cooling to form an atomic crystal and rapid quenching to form a glass. 


2) Yes, these nanoparticles can form polycrystals with grain boundaries. The size of crystalline domains depends on nucleation vs growth much like in atomic crystals. 

Inspiring and exciting work!

Sun, 2018-03-04 03:43

In reply to Journal Club for March 2018: Colloidal Self-Assembly of Architected Nanomaterials

Hi Wendy,

Thank you very much for your sharing this comprehensive and inspiring review! It is amazing for colloidal nanoparticles to spontaneously assemble into various architectured nanostructures and nanomaterials with unique mechanical properties!

I am very interested in your impressive work about polymer-grafted nanoparticle supper-lattices. I have two questions: (1) It was observed that some defects (such as vacancies, interstitials) formed during self-assembly of nanoparticles. What are the main factors to control and determine the formation and distribution of these defects? (2) During self-assembly of nanoparticles, can these nanoparticles form the polycrystalline configurations with grain boundaries? Thank you very much in advance!

Very nice review

Sat, 2018-03-03 23:33

In reply to Journal Club for March 2018: Colloidal Self-Assembly of Architected Nanomaterials

Hi Wendy,

Thanks a lot for your very nice review. I am particularly interested in the discussion of "Strong and flexible membranes". This is a new filed to me, and I have a few questions

1. For a membrane formed by nanoparticles with polymer or DNA ligands, do you observe asymmetric elatic modulus during tension and compression?

2. What is the typical fracture toughness of such membranes? And is the fracture in membranes brittle or ductile? 

3. For nanoscale thin film structures, topological defects can cause substantial out-of deformation. Examples include nanoscale Cu film [1] and graphene [2-3]. Do you see similar effects in these nanocrystal monolayers/membranes?

4. Working with Prof. Huajian Gao and Prof. Xiaoyan Li, we showed that topological defects can be utilized to design 3D morphlogy of graphene and furture tailor the mechaical properties of graphene [4-5]. I was wondering whether defects can also induce 3D shapes of these membranes?


1. Zhang, Xiaopu, Jian Han, John J. Plombon, Adrian P. Sutton, David J. Srolovitz, and John J. Boland. "Nanocrystalline copper films are never flat." Science 357, no. 6349 (2017): 397-400.

2. Lehtinen, O., S. Kurasch, A. V. Krasheninnikov, and U. Kaiser. "Atomic scale study of the life cycle of a dislocation in graphene from birth to annihilation." Nature communications 4 (2013): 2098.

3. Warner, Jamie H., Ye Fan, Alex W. Robertson, Kuang He, Euijoon Yoon, and Gun Do Lee. "Rippling graphene at the nanoscale through dislocation addition." Nano letters 13, no. 10 (2013): 4937-4944.

4. Zhang, Teng, Xiaoyan Li, and Huajian Gao. "Defects controlled wrinkling and topological design in graphene." Journal of the Mechanics and Physics of Solids 67 (2014): 2-13.

5. Zhang, Teng, Xiaoyan Li, and Huajian Gao. "Designing graphene structures with controlled distributions of topological defects: A case study of toughness enhancement in graphene ruga." Extreme Mechanics Letters 1 (2014): 3-8.

Reply to Comment "Material Properties Across Size Scales"

Thu, 2018-03-01 19:26

In reply to Morphable 3D mesostructures and microelectronic devices by multistable buckling mechanics

Hello Luke Porisch,

Thanks for your kind words and questions!  In our study, the loading was applied to the thin-film structures through the elastomeric substrate by controlling the biaxial prestretching, which can be understood as a type of displacement loading.  In this condition, the non-linear buckling behavior is not very sensitive to the elastic properties of the materials, if the thin-film structure is relative homogenous.  

In this NAT MATER paper, we considered silicon and polymeric thin films with the thickness ranging from around 1.5 micron to 50 microns.  In this range, the change of material properties induced by the size effect seems not so prominent, which is evidenced by the agreement between the measured 3D configurations and FEA calculations based on homogenous thin-film models with elastic properties equal to their bulk material.  It can be expected that the mechanical properties (e.g., elastic modulus and ductility) might change due to the inhomogeneity, defect, and other effects, as the size goes down further, e.g., below 100 nm.  When the feature size of inhomogeneity is comparable to the lateral (or in-plane) dimensions of the thin-film structures, then I would expect this effect to have an impact on the buckling process.

Yes, the standards in fabrication would be helpful in the development of small scale system, in my opinon.  It might depend on the specific principles of different methods, but each method might generally need its own test properties.  Not sure.

Best regards!


National press ore comments on our petition

Thu, 2018-03-01 07:42

In reply to Corriere della Sera: recruitment in Italian Academia

 Some more coverage on national important newspapers.  Repubblica and  Sole24ore.
  Saluti  Michele Ciavarella

This is convincing

Wed, 2018-02-28 19:45

In reply to about intrinsic fracture fracture energy

Thanks, Shaoting. This is convincing.

interesting point

Wed, 2018-02-28 15:26

In reply to Some numbers

Interesting point. Thanks, Ruobing!

Great points

Tue, 2018-02-27 19:31

In reply to Some numbers

Hi Ruobing,

You made great points and pointed out more evidence. Thanks!

Also, I totally agree with you that hydrogel can be a very good platform to expereimentally and theoretically explore new material design and advance our understanding of relation between the microscale structures and macroscopic mechanical behaviors.



Fracture and Fatigue of soft materials

Tue, 2018-02-27 17:14

In reply to Some numbers

Hi Ruobing,

I highly enjoy reading a set of papers from your group, especially for fatigue fracture. It turns out that the intrinsic fracture energy is the parameter as important as total toughness, which is relavent to long-term performance for practical applications.



Some numbers

Tue, 2018-02-27 16:24

In reply to original double network gels

Dear Lihua,

The fracture toughness of a hydrogel can greatly depend on a lot of things. Start from simple things, for example, the crosslink density, as Shaoting mentioned. For PAAm hydrogels, if the crosslink density is low enough, the toughness is measured to be over 400 J/m2 with pure shear tests. Interesting, right? We frequently mentioned in papers that PAAm gels are brittle or weak, but in fact its toughness is not bad at all! Even for Gong's original DN gel, it was initially only hundreds of J/m2. However, in the fatigue paper by Zhang et. al, the fracture toughness can also be enhanced to thousands of J/m2. 

As a result, 400 J/m2 for intrinsic toughness seems very high, but not quite suprising, depending on how the gel is made. Maybe in the end they are just gels with different compositions, not necessarily any special unknown interactions (but of course it could be important as well).

I think fracture of soft materials in general is still a big topic to explore. From purely scientific view, hydrogels provide a platform, where one can easily tune the compositions and relate them to macroscopic mechanical behaviors. This was hard to imagine at the age of stiff materials like metals.



agree with you

Tue, 2018-02-27 13:28

In reply to original double network gels

Hi Lihua,

I agree with you that it is not fully unerstood why the toughness of the pre-damged PAAm-alginate gel can be higher than PAAm gel. Our model captured the toughness enhancement due to the Mullins effect if we chose the toughness of pre-damaged (nearly to the failure point) as a rerfence. This is the reason we define it as the intrinsic toughness. I think there may be other mechanism contributing the toughness enhacment as well, which calls for a multiscale modeling and experimental study. 

I enjoyed reading Gong's work on hydrogels. Depending on the fabrication process and material combination, double netwrok gels may have different microstructures. We may be able to link some of some designed materials to the pre-damaged PAAm-alginate gel and even directly observe detailed distributions of the hard phases in the double network hydrogels. The research of soft tough materials is under rapid development, and I hope we can achieve better understanding on these puzzles from micro- and nao-scales.




about intrinsic fracture fracture energy

Tue, 2018-02-27 13:14

In reply to intrinsic fracture energy

Dear Lihua,

Thanks for raising this question. In addition to Teng's explanation,I would like add a few more points.

1. When the network is highly stretchable, the intrinsic fracture energy can reach the order of 100J/m^2. I measure the fracture energy of single network of PAAm with 15 wt% polymer concetration and 0.01 wt% crosslinker. The fracture energy was measured to be 180J/m^2. 

2. As teng mentioned, the PAAm-alginate system is not just a mixture of PAAm netwrok with alginate moleculars. The interaction between alginate and PAAm may give the additional reinforcement to the single PAAm network. Prof. Suo's group recently proposed to perform fatigue test to identify the threshold, which is corresponding to the intrinsic fracture energy. Zhang, Wenlei, et al. "Fatigue of double-network hydrogels." Engineering Fracture Mechanics (2017) measured the critical threshold fracture energy of fatigue for PAMPS/PAAM double network (two interpenetrating covalent network). The threshold is measured to be 400J/m^2 for the system with PAAm of low crosslinker density and 200J/m^2 for the system with PAAM of high crosslink density. The number is also larger than the intrinsic fracture energy of each single network, which might be due to the interaction between the two networks as well.

3. Lake Thomas picture give quanlitative prediction on the intrinsic fracture energy of a polymer chain network. Sakai (Akagi, Yuki, et al. "Fracture energy of polymer gels with controlled network structures." The Journal of chemical physics 139.14 (2013): 144905.) measured fracture energy of ideal network, which shows that there is a perfactor of ~3 for the quantitative calculation of intrinsic fracture energy.

Indeed, How to quantitative calculate and measure the intrinsic fracture network of hydrogel needs effort to be elucidated. 




original double network gels

Tue, 2018-02-27 12:58

In reply to Great question

Teng, thanks for the explanation. Although I am not fully convinced, I can't think of other reasons. However, Gong's original double network gels have a toughness a few hundred J/m2. 

Material Properties Across Size Scales

Tue, 2018-02-27 11:30

In reply to Morphable 3D mesostructures and microelectronic devices by multistable buckling mechanics



This is very interesting research.  What are your observations on the impact of material properties as various size scales on elastic performance, and especially related to non-linear buckling criteria?  I am interested your thoughts on challenges/opportunities based on changes in homogeneity, defects, grain size, etc..., as scale changes.

In general, do you feel standards in fabrication would help to accelerate small scale system development?  Does each method need its own test properties? 

Thank you for your time.


Luke Porisch

St. Ansgar, IA

Great question

Tue, 2018-02-27 09:46

In reply to intrinsic fracture energy

Hi Lihua,

Thanks for your great question. The 400J/m2 is indeed higher than the normal PAAm hydrogel. It also puzzles us, and we think it is very likely related to the microstructures of a double network hydrogel (e.g., PAAm-alginate) after pre-damaged deformation. 

You can see from our EML paper that the toughness of PAAm-alginate gel indeed exhibited a plateu as we increased the pre-stretch before introducing a main crack into the sample. The plateu was taken as the intrinsic fracture energy of PAAm-alginate, and gave the value of 400J/m2 for the gel we used in Fig. 5. Our guess is that the broken brittle components (alginate and/or bonding between alginate and PAAm) in the double network hydrogel may be still larger than the small alginate moleculars. They may form some meso-scale stiff fillments to reinforce the PAAm matrix. So, we should still think the pre-damaged PAAm-alginate gel as a composite of soft and hard phases, not just a mixture of PAAm netwrok with alginate moleculars. 

This picture definitely needs more experimental and modeling efforts to validate and is worthwhile for more in-depth studies. Also, it seems the PAAm hydrogel can be tuned (e.g., chain length and crosslinkers) to have a little higher toughness, such as higher than 10 J/m2. But definitely, the toughness of PAAm hydrogel itself we used in our paper is much lower than 400 J/m2. Shaoting did the experiments, and he may have more information. 





intrinsic fracture energy

Tue, 2018-02-27 00:11

In reply to ABAQUS files of coupled Mullins effect and cohesive zone model for fracture and adhesion of soft tough materials

Teng, thanks for sharing the files. I have one question about the intrinsic fracture energy. You experimentally measured T0=400J/m2, which is much higher than that of normal hydrogels. How to understand this?

Thanks, Kejie!

Mon, 2018-02-26 20:08

In reply to nice work!

Thanks, Kejie!

nice work!

Sun, 2018-02-25 22:55

In reply to Molecular mechanism of superior resilience and dissipation in thermoplastic polyurethanes at large tensile deformation

Nice work, Shuzhe!  congratulations on the new publication.


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