iMechanica - hydrogel - Comments

Re:Re: A very good topic

Chang-Guo Xue — Sat, 24 May 2008 00:48:19 -0400

Thanks!Your answer give me prompting.The "onion structure of natural hydrogels was a promising work.I am very interested in it.But this time , I must do the research on microcantilever biosensor ,so I only study the hydrogels in my spare time . In the next step, I will fabricate a simple onion using Chitosan in plan.I prepare to learn more Knowledge for my research on the hydrogels.If you have any idea or good articles, Please give me in the email. I wish I can do some work in this field

Re: A very good topic

Zhigang Suo — Thu, 22 May 2008 12:36:59 -0400

Dear Chang-Guo: Thank you very much for your interest. My group has only started working on polymers recently. These lectures mainly described what we have learnt so far, from reading papers, running calculations, and talking to experts in the field. Our recent papers have been posted on iMechanica, filed with the tag Suo Group Research. It would be great that people like you, with more background in polymers, are interested in this line of work. We hope to hear more feedback form you. Incidentally, we also liked the "onion paper" in Nature.

thanks a lot

zhan-sheng guo — Thu, 22 May 2008 00:20:56 -0400

gone with the wind

A very good topic

Chang-Guo Xue — Wed, 21 May 2008 22:31:26 -0400

Thanks Dear Zhigang

These were very useful to me. I have download it .

In the past three years, I have research on the microcantilever base sensor and this research also will be continued in the next two years for my Ph .D degree. The mechanic was not my original specialty . I graduated from polymer science Qiqihaer University and in the following years was a teacher as polymer material of Anhui University Science and Technology.

I want to find a point between the polymer science and mechanics. In the past few years I have been interesting in the natural polymer. A article about Multi-membrane hydrogels (Se´bastien Ladet NATURE arch 2008) give more interesting in polyelectrolyte gels .

I wish I can do some work in this direction in the future.

Best Regards

Changguo

No videos available for my lectures.

Zhigang Suo — Wed, 21 May 2008 14:42:28 -0400

Dear Teng: Thank you very much for your interest. I have just asked the IT person at UCSB, and he told me that there is no permanent record of my lectures. Sorry about it. Best, Zhigang

Videos of your lectures on soft active materials

Teng Li — Wed, 21 May 2008 14:02:09 -0400

Zhigang,

Thanks for posting your lecture slides. Since the lecture was sent live to Los Alamos, I'm just wondering, if it's possible to make the archived video of your lecture open to public. I'm quite sure the video will make the learning process of this new field more effective and fun.

hydrogel experiments

Rui Huang — Thu, 17 Jan 2008 11:05:37 -0500

Dear Wei and Zhigang,

Thank you for your responses and for pointing to your recent works on hydrogels. Indeed, I have been following your works (quietly so far). I have several experiments in mind and try to develop models, with little success so far. One particular experiment involves patterned hydrogel lines constrained by a substrate. The swelling and deformation in this case is highly inhomogeneous and anisotropic. What caught my attention at the beginning is that these lines buckle into wavy structures. The question here is how to relate the buckling phenomenon to the material and geometry properties of the hydrogel lines.

Relate the theory of hydrogels to experimental observations

Zhigang Suo — Thu, 17 Jan 2008 09:23:12 -0500

Dear Rui: Thank you very much for your interest, and for going through the calculation. We have since made a number of applications, which have been posted at

Working through these specific problems, we are trying to learn about applications of hydrogels, and to connect the theory to experimental observations. The experimental literature on hydrogels is huge, and will take many theoreticians many years to sort out.

They are just typos

Wei Hong — Wed, 16 Jan 2008 21:17:43 -0500

Hi Rui,

Thank you for reading our paper so carefully and pointing out the errors.

Actually these were just typos we had on the first version of our manuscript. We have corrected them on the later versions.

Besides the two places you have identified, there are more:
1) the chi number we used should be 0.2 instead of 0.1
2) the lambda value sould be 3.215 instead of 3.125

I am uploading the new version. Please take a look at this version and sorry for the misleading typos.
You can also check on the final version on JMPS website at http://dx.doi.org/10.1016/j.jmps.2007.11.010

Thanks,

Wei

a question for Wei Hong

Rui Huang — Wed, 16 Jan 2008 18:01:08 -0500

Hi Wei,

I am reading your paper, "A theory of coupled diffusion and large deformation in polymeric gels", which I like very much. I am trying to do some simple calculations myself, which leads to a minor question here. For the uniaxial creep problem, I reached an equation similar to Eq. (32) in your paper, but different for the second last term on the right hand side. Instead of 1, I have 1/lambda3. Solving this nonlinear equation for s = 0 with Matlab, I could not get the same stretch, lambda = 3.125. With your equation, I got 1.294, and with my equation I got 3.390. I wonder if I have missed something somewhere. I would appreciate it if you can check your equation and solution to let me know.

Thanks.

Assumptions for isotropic tension/reversibility/cotinuum

Wei Hong — Wed, 02 Jan 2008 11:17:39 -0500

Hi Dr. Li,

Let me try to answer your questions:

(1) The gel is assumed to be bonded to both the rods and the substrate. Constrained in all directions, the gel is just like in a cage, and it simply can not deform (except for tilting which involves additional thredshold as described in the paper). When dried, the gel has an natural tendency to reduce in volume. If without the constraints, the gel would shrink freely. However, it is now constrained in a cage. The only thing it can do before tilting is building up an internal stress against shrinking. As both the initial condition and the constained are assumed to be isotropic, the tensile stress within can only be isotropic.

(2) The process is reversible is that it can shrink when dried or swell back when hydrated. Not as the "reversible process" in thermodynamics terminology, such a process does dissipates energy. Zhigang, shall we consider another word here?

(3) I think the assumption necessary is that the size of the nanostructure is still larger than the microstructure of the material itself. In this case, for example, the assumption should be that the nanorods are still way bigger than the polymer molecules.

Finite deformation Biot references

Sanjay Govindjee — Wed, 02 Jan 2008 09:17:36 -0500

Just to add more completeness to the referencing, here are two interesting citations of Biot's which deal with finite deformation.

M. Biot, Theory of Finite Deformations of Pourous Solids, Indiana Univ. Math. J. 21 No. 7 (1972), 597–620

M. A. Biot, Variational Lagrangian-thermodynamics of nonisothermal finite strain mechanics of porous solids and thermomolecular diffusion, International Journal of Solids and StructuresVolume 13, Issue 6, , 1977, Pages 579-597.

Prof. Dr. Sanjay Govindjee
University of California, Berkeley

diffusivity versus diffusion

Sanjay Govindjee — Wed, 02 Jan 2008 08:58:22 -0500

Ahh, a point of mis-understanding on my part...I agree stress will likely have little to no effect on the molecular mobility M, as in j = -M grad[mu], concentration yes, but not stress. Stress affects the chemical potential (gradient) is what I was trying to say.

I will have to look up the paper by Gibbs as I have not seen it. Biot of course should also gets a lot of credit; somehow I did not see/appreciate those papers when I first saw them because (if I recall correctly) I was blinded by my drive toward finite deformation polymeric systems and the first paper of Biot I had encountered was only linear.

One other side note on Gibbs since I think you are also interested in Statistical Mechanics. I taught a course on Statistical Mechanics in Elasticity last year. I found Gibbs book Elementary Principles of Statistical Mechanics (1902 reprinted 1981) quite good for teaching purposes.

-sanjay

Prof. Dr. Sanjay Govindjee
University of California, Berkeley

Re: Molecular diffusion in polymers

Zhigang Suo — Wed, 02 Jan 2008 08:13:13 -0500

Dear Sanjay: Thank you so much for relating your experience. They are indeed very helpful. Wei is studying your papers, and will explain to me in some detail. As you can probably tell from these conversations, he is a very special researcher. I think we all agree on the following:

If a network imbibes a large quantity of solvent molecules (often greater than 90% by volume for a hydrogel), the volume change is mainly the sum of the volumes of the individual molecules. Molecular compressibility due to stress is very small by comparison. Volumetric change due to molecular interaction (your point 7) is also small by comparison. This is the picture for hydrogels, a situation that my group is focusing on.
However, if the uptake is not that large, then all the above effects can become important. From your comments, this situation seems to be the one that you are focusing on. I'll learn more when I read your papers.
The stress may (or may not) affect diffusivity. It depends on the level of stress and material. I had experience with this issue mostly in the area of electromigration in aluminum and copper. There, the main effect of stress is through its gradient as a driving force to motivate diffusion. The effect of stress on diffusivity itself is a small effect even when the stress is as high as several hundred MPa. I suspect that the effect of stress on diffusivity in a hydrogel is also small, just because the level of stress is not very high compared to molecular compressibility. But again, I totally agree with you that one should check numbers for a given material.

My group has no prior experience with gels. We are learning from the literature, and by talking to people. Thank you very much for pointing out new papers. The continuum theory seemed to be first formulated by Gibbs (187x). His theory is for large deformation, using deformation gradient, nominal stress and chemical potential. He characterized material law using a free-energy function, but he did not give any specific form of this function. His theory is limited to equilibrium. Biot (1941) is commonly credited to add kinetics into the theory. Of course, following Biot, a huge literature now exists on poroelasticity.

It appears that the continuum theory is basically set by Gibbs and Biot. As you pointed out, the devil is in the details of specifying the free-energy function and kinetic laws. The details are extraordinarily rich, and are essential in connecting the theory to experiments, and in discovering new phenomena. Also, experiments in recent decades have shown that hydrogels are responsive to diverse stimuli, further enriching the details. The field is wide open.

Assumptions

Hua Li — Wed, 02 Jan 2008 03:01:45 -0500

Hi, Zhigang, Sorry for the delay. The paper you mentioned is really interesting as it has opened another window for me. However, I still have several questions to be clarified. (1) As it is assumed that the hydrogel is bonded to the rods of silicon (see page 3), how to understand the hydrogel developing a state of isotropic tension near the rods along the rod direction? (2) How to understand that " the process is reversible" (see the line 6 from bottom of Page 2) from the formulations? (3) do you think it is necessary to make some assumptions to use the continuum theory for the present nanoscale domain?