iMechanica - suo group research - Comments

Method to analyze deformation of dielectric elastomer

Zhigang Suo — Fri, 07 May 2010 11:47:33 -0400

This paper is in print:

Xuanhe Zhao, Zhigang Suo, Method to analyze programmable deformation of dielectric elastomer layers . Applied Physics Letters, 93, 251902 (2008).

Hi Lianhua, It is

Yuhang Hu — Thu, 25 Mar 2010 21:05:50 -0400

Hi Lianhua,

It is really one of the key points in this paper that indentation can be an effective way to differentiate viscoelasticity and poroelasticity. Time dependent behavior of gels can be due to the viscous relaxation of polymer network or the diffusion of solvent molecules in it. Viscous time is size independent while diffusive time scales with length square. In this indentation problem, h is the only length scale. So if the indentation depth is large, diffusive time can be much much larger than viscous relaxation time. In this case, if our observation time is long enough, we will mainly observe diffusive effect.

You are right at the point that the relaxation behavior can also be induced by viscoelasticity. But under mm length scale (as shown in my experiment), poroelasticity prevails. If we scale down the indentation depth to micrometer or nanometer, it is highly possible that we can observe both viscoelastic and poroelastic effects. I am also willing to capture it in my future work.

You can take the following references for your information:

C. Y. Hui, Y. Y. Lin, F. C. Chuang, K. R. Shull, and W. C. Ling, J. Polymer Sci B: Polymer Phys. 43, 359 (2006).

M. Galli and M. L. Oyen, CMES 48, 241 (2009).

Hi Li Han, Thank you

Yuhang Hu — Thu, 25 Mar 2010 18:15:00 -0400

Hi Li Han,

Thank you for your valuable comment.

Indeed more experiments need to be done to further validate this method.

Comparison with fluorescence measurement is a good approach.

Alginate gel is self-degradable.

Its further relaxation may be due to it.

In another upcoming work, I will show much better result on PDMS gels.

Yuhang, nice work! Just my

Li Han — Thu, 25 Mar 2010 09:39:22 -0400

Yuhang, nice work! Just my two cents. Maybe it is worthwhile to compare the diffusivity derived from your indentation measurement with a direct fluorescence measurement just to see how accurate the method is. Meanwhile, any idea why the numerical simulation seems to deviate more and more from your experimental data with time?

Again, congratulations.

Li Han

An interesting work

Lianhua Ma — Thu, 25 Mar 2010 03:36:36 -0400

Dear Yuhang,

I am reading your interesting paper. As stated in your paper, combining indentation and analytical formula is surely an effective approach to characterize material parameters of gels.

1. I have a question regarding poroelastic relaxation and viscoelastic relaxation of gels. As you pointed out ,
"our indentation experiment shows that the relaxation behavior of the covalently crosslinked alginate hydrogel cannot be explained by viscoelasticity, but is consistent with poroelasticity",

Could you explain this in more detail? I am not familar with viscoelasticity, in Fig.4(b), F/h^2 has the dimension of stress, and the relationship between F/h^2 and t represents apparent relaxation behaviour, why can't this relaxation curve be explained by viscoelasticity? just because the viscoelastic relaxation time depends on the size of h?

2. Could you recommend some papers regarding the poroelastic relaxation time(which is quadratic in the radius of the contact)?

3. The FE simulation is also performed to simulate the indentation in this paper, Can we obtain the poroelastic properties of gels only by FE simulation?

Thank you
Lianhua

modelling the Buckling of thin films on substrate

Abraham — Wed, 10 Mar 2010 04:46:56 -0500

Hi Nan shu,

I am working in buckling analysis of thin film bonded to the elastomeric sabstrate, as part of my thesis.However, i do have some difficulties in modelling the structure in Abaqus for analyis. So i kindly ask you to help if you have some helpful materials on this regard.

thanks in advance

Thanks for the materials

changyongcao — Fri, 22 Jan 2010 06:00:09 -0500

Hi, Prof.Suo,

Thank you for sharing your good lectures to community.

It is interesting and nice.

Hello Zhigang, Thank you

Jung W. Hong — Fri, 22 Jan 2010 00:06:43 -0500

Hello Zhigang,

Thank you for the interesting information and nice presentation materials.

Dielectric elastomers look really promising. Compared with Piezoelectric materials, still there are some tradeoffs regarding the maximum generated force although elastomer can make much larger deformation. However, elastomers are under development by many manufacturers. So maybe the material will be very versatile for the engineering use in the near future!

I already found the answer.

Mario Juha — Thu, 10 Dec 2009 15:16:16 -0500

I already found the answer. Thanks I was measuring lambda respect to the free-swelling state and not from the dry network state. Now I have same results. But if you could send the info, I would appreciate it.

Thanks,

Mario

Material model implementation

Mario Juha — Thu, 10 Dec 2009 10:34:08 -0500

Dear Dr. Hong.

I have implemented your material model in a non-linear finite element program that I wrote. I am using a Continuum Mechanics Total Lagrangian formulation to deal with nonlinearities, and my measure for stress and strain are Second Piola-Kirchhoff stress tensor and Green-Lagrange strain tensor, respectively. In order to test the program, I would like to reproduce your results. I have followed what you nicely explained in the paper, but I feel I need an extra help.

If it is appropiate, could you give me extra information about the finite element model?, that is, what exactly boundary conditions you used and how many substeps?.

To use your model and doing the following:

1) Specify the initial chemical potential and then calculate lambda_0 using Eq.(19). This value of lambda_0 is what I am going to use in Eq.(23)

2) Increment linearly the current chemical potential using the following relation:

current_mu = mu_0 + delta * ( current_mu - mu_0 ), where delta is the increment corresponding to the load step.

current_mu is what I am going to use in Eq.(23)

3) Solve nonlinear equations using Newton-Raphson.

Thanks in advance,

Mario

Thanks for the descriptive notes

Hari — Mon, 09 Nov 2009 14:36:43 -0500

Dr. Suo,

Thanks very much for your notes. I personally prefer to start with class/lecture notes before reading papers on a new topic.

Hari

Re: Future work

Wei Hong — Wed, 30 Sep 2009 09:46:33 -0400

Dear Mario,

Sure you can! Please let me know if you need more information than those on the paper.

Wei

Future work...

Mario Juha — Tue, 29 Sep 2009 16:47:13 -0400

Dear Dr. Hong.

I am really interested in this work. I have been studied it for weeks and I would like to extend your results. In section 8.3, last paragraph, you said something about instabilities that you can not solve currently with ABAQUS. I would like to know if I can collaborate with you and or your students.

thank you,

Mario

Re: Molecular incompressibility

Wei Hong — Tue, 01 Sep 2009 16:35:06 -0400

Dear Mario,

It is actually similar to the constraint you are familiar with, the only difference being the volume of solvent. When solvent migrate into a polymer network, the network expands. Here we assume the total volume to be a constant.

Your understanding of C(X) is correct.

Wei

Molecular incompressibility and the free-energy function

Mario Juha — Tue, 01 Sep 2009 15:26:00 -0400

Dear Dr. Hong. I have been busy studying your papers and I have more questions than answer (to me that is good). In: W.Hong, X. Zhao, J. Zhou, Z. Suo. A theory of coupled diffusion and large deformation in polymeric gels. Journal of the mechanics and physics 56, 1779 - 1793 (2008), section 3, you specify the molecular incompressibility as:

1 + vC = det(F)

I am familiar with the incompressibility constraint of det(F) = 1, but not with the above equation. Could you give more insight about it? Probably it is an standard constraint in gel theory, but I am not familiar with it.

Other question that I have is related with C(X), distribution of solvent molecules in the gel. Do I need to understand this notation as C(X) = C(phi^-1(x))? , where phi(x)^-1 is the inverse of the motion, lower x is the spatial coordinate and capital X is the reference coordinate. In general terms, what I have understood is that C is an inhomogeneous field, is it correct?

cordially,

Mario J. Juha