iMechanica - electrostriction - Comments

New items on deformable dielectrics

Zhigang Suo — Sun, 02 Sep 2007 07:35:20 -0400

I posted a theory of polyelectrolyte gels.
Xuanhe Zhao has just posted a new paper from my group on dielectric gels.
A thread of discusion on Maxwell stress and electrostriction has just started.

Papers on nonlinear deformation of dielectric elastomers

Zhigang Suo — Wed, 30 May 2007 10:25:51 -0400

Dear Aman: In addition to this paper, members of my group have posted the following papers on iMechanica:

These papers have shown how the basic formulation may be used to analyze some of the intriguing experimental observations reported recently. Please do let us know if you have comments.

Accepted: A nonlinear field theory of deformable dielectrics

Aman Haque — Wed, 30 May 2007 09:54:12 -0400

Dr. Suo,

Reading the manuscript, I couldnt curb my curiosity..can I have copies of the papers Zhao et al. and Zhou et al.?

Many thanks

Aman

Van de Ven's book

Temesgen Markos — Sun, 20 May 2007 13:20:54 -0400

I finished my master's thesis under Fons Van de Ven roughly half a year ago. I remember seeing him with the just published second edition of his book. If you are interested, I will talk to him to find out how his book is being distributed.

"I am always ready to learn, although I do not always like to be taught." Winston Churchill

Accepted: A nonlinear field theory of deformable dielectrics

Zhigang Suo — Sat, 19 May 2007 17:45:17 -0400

I'm delighted that this paper is now accepted for publication in JMPS. The following is the comments of the reviewers and our responses. Both shed addition light on the topic. I have also attached the revised manuscript.

Ms. Ref. No.: JMPS-D-07-00003

Title: A nonlinear field theory of deformable dielectrics

Journal of the Mechanics and Physics of Solids

Dear Huajian:

Thank you very much your message dated 6 May 2007, and for finding very perceptive reviewers. As you know, the problem we study has a long history, and the existing literature is complex and difficult. We have worked hard to simplify our presentation, and are delighted that both reviewers found our paper transparent.

Reviewer #1 stated that "this is a very interesting paper," but is not sure if the paper is worthy of publication in JMPS. Reviewer #2 stated that "The paper clarifies a number of concepts that are currently not clearly understood in the literature", and recommended the paper be published in JMPS.

We believe that this paper makes the following contributions:

1. A new formation of the mechanics of deformable dielectrics that removes superfluous postulates, which one can find many in the literature. Section 4 looks extremely simple and familiar. Its significance should be appreciated by comparing it to the classic papers of Toupin, Eringen, Maugin, and Tiersten, as well as with more recent papers by McMeeking and Landis, Rinaldi and Brenner, and Dorfmann and Ogden.

2. Central to our formulation are the weak statements (4.2) and (4.6). We have not found these weak statements in the literature. In a follow-up work (Zhou et al., 2007), we have used the weak statements to implement a finite element method to solve problems of large deformation.

3. In the limiting case of a fluid dielectric, the theory recovers the Maxwell stress. However, in general, we show that the stress depends on electric field and deformation in a coupled way, so that Maxwell stress cannot be identified. Our theory does not rely on the notion of the Maxwell stress.

4. A formulation of problems involving small perturbations around a given state. The given state can be fully nonlinear. Problems include bifurcation and localization, which are known to be highly nonlinear problem. In terms of the nominal quantities, all field equations are linear; see Section 4. Nonlinearity comes in through the free-energy function. In a follow-up work (Zhao et al. available online), we have used the new theory to give the first theoretical interpretation of an experimentally observed instability reported in 2006.

In what follows, we will respond to the
comments of the reviews in detail. In
the revised manuscript, we have added the modifications described below,
corrected typos, updated references, and reworked paragraphs to improve
clarity.

Best wishes,

Zhigang

Reviewer #1

Comments. This is a very interesting paper. In the absence of the large volume of prior work in this area I would recommend that this manuscript be accepted without significant revision. However, upon reading this I get the feeling that I am already aware of what is being presented, this has already been done. In addition to the general formulations by Toupin, Eringen, Maugin and others, the additional idea that the Maxwell stress cannot be separated from the total stress is a point that has also been emphasized by other authors.

Response. As explained in the Introduction of the general formulations by Toupin, Eringen, Maugin invoked various many additional postulates and quantities. Given the broad applications of deformable dielectrics, and recent interest in dielectrics capable of large deformation, it seems to us that this paper makes a significant contribution by placing the subject on a secure theoretical foundation. Although qualms about the Maxwell stress in deformable solids have been expressed by several authors, notably by McMeeking and Landis (2005), all existing formulations start with some notion of Maxwell stress. By contrast, we show that one can formulate a theory without invoking the notion of the Maxwell stress. We have added the following to the manuscript.

p.7 "We will first show that a procedure to define nominal stress in continuum mechanics is still applicable for deformable dielectrics. This procedure makes additional postulates in the literature of deformable dielectrics superfluous. We will then apply the same procedure to define the nominal electric displacement."

Comments. The significant contributions of the present manuscript are its presentation of these ideas in a very elegant and convincing manner and in its formulation in the reference configuration instead of the current configuration (although I think that this has been done as well). Given that the primary premise of the paper has appeared before, I would have liked to have seen the authors make some critical evaluations of the two approaches, reference versus current. Granted this may be a matter of taste, but I would imagine that there are some situations where one approach might be preferred over the other. For example, the mapping of a closed crack onto and open crack is singular and solving such a problem in the reference configuration might not be straightforward (I think this may not even be possible, but I could be wrong).

Response. We are delighted that the reviewer thanks that our formulation is a significant contribution. However, formulation in reference configuration is not new, as pointed out by the reviewer. Our paper regards both current and reference states as well understood settings to formulate continuum theories. What is new in this paper, we believe, is our removal all superfluous postulates. This removal is done in both current and reference states. We have reworked a paragraph as follows:

p. 6. "There is considerable flexibility in choosing measures of stress, strain, electric field, and electric displacement. So long as a theory relates measurable quantities, all alternative definitions are equally valid, and are related by transformations. However, a given boundary value problem may be easier to solve in terms of one set of variables than in terms of another. To avoid confusion, we will develop one set of measures (the nominal quantities) in the body of the paper and discuss alternatives in Appendix A and B."

Comments. The examples that are given, on small deformation and small electric displacement, and infinitesimal perturbations are interesting but don't really exercise the need for a finite deformation electrostatics theory.

Response. In section 8, we described the approach to a class of problems involving small perturbations from a given state. The given state is arbitrarily nonlinear. Among examples given are bifurcation and localization. These problems can only be formulated with a fully nonlinear theory, so that one can ask whether the tensor of tangent modulus is positive-definite or not.

Comment. Another issue in using one formulation versus another is the measurement of the material properties. If one just assumes a certain constitutive relationship then it is likely that there will be differences in the derivation of the stresses of the order of E*D which are in effect the Maxwell stresses. So while the separate identification of Maxwell stresses is not

measurable, the subtle effects of these forces must still be measured.

On the more philosophical issue of Maxwell stresses, I certainly agree that experimentally it is not possible to separate the Maxwell stress from the total stress, but can this be done in a model? Certainly, when we model a fluid as incompressible (or even compressible with zero bulk modulus) then an unambiguous formula for the Maxwell stress results. What if we model a dielectric solid as a simple cubic array of charges (or dipoles if you prefer). At each lattice point there are equal positive and negative charges. The electrostatic interaction between these charges is ignored (to avoid the infinity) but there is a linear "spring" connecting them such that the separation of these charges requires energy (or the dipole is initially zero and requires energy to be created). This model accounts for linear dielectric behavior. Next, link each of the pairs of charges to their nearest neighbors with a linear spring in order to account for elasticity (perhaps we need diagonal springs as well to stabilize the lattice). Now, we can apply electric fields to this lattice and compute the electrical forces on the dipoles and the forces due to the elasticity springs. Aren't the electrical forces related to the Maxwell stress? In such a model is it fair to claim that the identification of the Maxwell stress is "without merit"? The problem with measuring these different contributions in a real system is that the elastic springs are due to electromagnetic forces as well and ultimately an experimental distinction is impossible.

Response. We believe that the reviewer has addressed his/her concerns in the last sentence above. We wish to point to the following in our original manuscript:

p.12 "By contrast, for compressible fluids, and for solid dielectrics discussed below, the stress (6.7) depends on deformation and electric displacement in a coupled way. Any attempt to separate them and call part of the stress the Maxwell stress must be arbitrary. The practice may provide temporary mental comfort, but on close examination is without merit."

Comments. Ultimately, the derivation in here is correct and likely to be worthy of publication. My only concern is that I am not sure that it is worthy of publication in JMPS. I tend to think that papers in JMPS should present new theories, which this does not, or they should significantly enhance the state of the art, which I don't think this does in its present form. I am suggesting a major revision so that the authors can perhaps address a problem where finite deformation is important. Another course of action is for the authors to withdraw the manuscript from JMPS and submit to another relevant journal.

Response. We believe that this paper makes the following contributions:

1. A new formation of the mechanics of deformable dielectrics that removes superfluous postulates in the literature. Section 4 looks extremely simple and familiar. Its significance should be appreciated by comparing it to the classic papers of Toupin, Eringen, Maugin, and Tiersten, as well as with more recent papers by McMeeking and Landis, Rinaldi and Brenner, and Dorfmann and Ogden.

2. Central to our formulation are the weak statements (4.2) and (4.6). In a followup work (Zhou et al., 2007), we have used the weak statements to implement a finite element method to solve problems of large deformation.

4. A formulation of problems involving small perturbations around arbitrary given state. Problems include bifurcation and localization, which are known to be highly nonlinear problem. Note that in terms of the nominal quantities, all field equations are linear; see Section 4. Nonlinearity comes in through the free-energy function. In a followup work (Zhao et al. available online), we have used the new theory to give the first theoretical interpretation of an experimentally observed instability reported in 2006.

Reviewer #2:

Comments. This is a nice paper. It is very well written and easy to read. The paper clarifies a number of concepts that are currently not clearly understood in the literature. The paper can be published in JMPS after the following comments are addressed:

1) The authors should compare the results from the theory with experimental data. By selecting one or two problems where there is experimental data the authors should compare theory with experiments. This will convince the reader that the theory accounts for all the physical mechanisms that are present in deformable dielectrics.

2) Different dielectrics behave differently under electrical potentials e.g. silicon dioxide and silicon nitride behave differently under electric field. It is not clear how the theory accounts for the differences in various dielectrics.

Response. This paper focuses on fundamental theory. Comparison with experimental observations is planed in follow up work. Our initial attempts are documented in Zhao et al. (available online), and Zhou et al. (available upon request). Different materials are differentiated by the free energy function, as described in Sections 5,6,7

Thesis by Van de Ven

ericmock — Sat, 10 Feb 2007 14:20:14 -0500

There is a nice book on the subject by Hutter and Van de Ven that was published in the late 1970's and seems to have very recently been revised and is back in print. Information about the second edition can be found here.

The original book was essentially a reproduction of Van de Ven's thesis (poor typesetting and all) that can be found at here.

Have not yet seen the second edition (I discovered its existence while doing some background research for this post) but am excited to take a look soon.

Dear Prof. Suo, Thank you

Jie Wang — Tue, 16 Jan 2007 22:14:53 -0500

Dear Prof. Suo,

Thank you very much for your reply. I will read the Feynman Lectures on Physics you mentioned for details.

Jie

Re: traction force of the opposite charges on two electrodes

Zhigang Suo — Thu, 11 Jan 2007 07:54:19 -0500

Dear Jie: Thank you very much for reading our paper. (2.7) is a result in many textbooks (e.g., The Feynman Lectures on Physics, Vol. II, p.8-2). We used this result in our paper to illustrate the Maxwell stress in vacuum. Because the charge is distributed on two plates, rather than concentrated at two points, the expression of the force looks different. Like Feynman and many others, we reached this result by calculating work done by the voltage and the weight. I'm sure you can obtain the same result by integrating forces due to point charges. Hope that this note helps.

On the traction force of the opposite charges on two electrodes

Jie Wang — Thu, 11 Jan 2007 07:47:21 -0500

Dear Prof. Suo,

I am reading your nonlinear field theory of deformable dielectric. I am not very familiar with the nonlinear continuum theory. I have a question on the traction force of the opposite charges on the two electrodes. You illustrate a parallel-plate capacitor, loaded by both the voltage and the weight. The two electrodes are separated by a vacuum. The traction force of the opposite charges on the two electrodes is given by Eq.(2.7), which is balanced by the weight. In general, the interaction force of charges depends on the distance between them, i.e. the force is inversely proportional to the square of the distance if the charges are constant. However, Eq.(2.7) shows that the traction force is independent of the distance between the two electrodes. I don't understand the difference between your traction force and the interaction force of two separated charged particles. Maybe I have no deep understanding to your theory.

Jie

A nonlinear field theory of deformable dielectrics

Luis Dorfmann — Tue, 09 Jan 2007 16:16:48 -0500

Hi Zhigang, Xuanhe and William,

This paper provides physical insight into electro-mechanical interactions and derives the governing equations in an elegant manner.

A number of different (equivalent) equilibrium formulations for electro-sensitive materials are available in the literature. These involve, in general, different definitions of stress tensor and electric body force and may provide potential for confusion and misinterpretation.

The use of the ‘total stress tensor’ in equation (5.6) allows the equilibrium equation to be written in a very simple form. In this formulation the influence of the electric field on the deforming continuum is incorporated through the stress tensor and not through body force terms.

Congratulations on this nicely written paper.

This is what iMechanica for!

Konstantin Volokh — Sat, 06 Jan 2007 04:12:19 -0500

This is what iMechanica for!

Dorfmann is an iMechanician now!

Zhigang Suo — Fri, 05 Jan 2007 16:19:17 -0500

Kosta: Thank you very much for introducing this paper to me. I followed it up, not only located the paper and read it, and also located one of its authors: Luis Dorfmann. He has joined the faculty of Tufts University, a few miles from Harvard. Today we had a lunch together and spent some time comparing our approaches. He has now posted a more recent version of his work with Ogden. Amazing. It has just been a few days. I posted our paper on line on 1 January 2007, Monday. Today is Friday.

Thank you for bringing us together.

Rui,

Wei Hong — Fri, 05 Jan 2007 11:59:49 -0500

Rui,

I had exactly the same concern as you have.

In the variational methods we see in traditional mechanics books, the variables with the delta are called "virtual" and a virtual displacement (potential) is associated with the virtual work, while a virtual force (charge) should result in the "virtual complimentary work".

On a surface on which the displacement (potential, e.g. the ground or an ideal battery) is given, we can not have change in the displacement (potential), and only change in the force (virtual force) is allowed, so that we can only write the complementary virtual work on it. (The virtual work on a displacement/potential boundary is simply 0.) The result would be a displacement (potential) boundary condition. On the other hand, when the traction (charge) is given, we can only have change in the displacement (potential). This will result in a force (charge) boundary condition.

But all these are just a way of expressing the idea. We can simply place the delta infront of the force/potential, and change the virtual work into complementary virtual work, and we will get the same result without any problem. (Although the "battery" might not be a good example, as people usually don't give the charge boundary condition on a battery, at least in textbooks.)

Helmholtz vs. Gibbs, Battery vs. Weight, Virtual vs. actual

Zhigang Suo — Fri, 05 Jan 2007 09:07:40 -0500

1. Helmholtz vs. Gibbs free energy

They are connected simply by a well known relation, i.e., equation (5.12) in our paper. You can use either one, depending on whether you would like to use E or D as the independent variable in your material law. As mentioned in the paragraph above (5.12), you wish to use E as the independent variable because it matches well with the structure of the field equations. However, if you'd like to study ferroelectric phase transition, for example, you will like to use D as independent variable. Because for a given E, you may get multiple values of D.

Physically, E is a force-like quantity, D is a deformation-like quantity (measuring the displacement of charges in dielectrics). It is interesting that the structure of the electrical field equations looks opposite from the mechanical field equations. Well, such is life.

2. Battery vs. Weight

A wight is just as complex as a battery, if you watch all the coming and going of molecules inside the weight. However, both are very simple if all you care is how much work they do. For the weight, the work is force x displacement. For the battery, the work is voltage x charge. So long as we agree what is work, displacement and charge, the above statements define force and voltage.

An empirical fact is this: there is an external mechanism that can do work through displacement, and there is an external mechanism that can do work through flow of change. If you don't like the word weight and battery, you can call them whatever names you wish.

3. Virtual vs. actual

Virtual work is not actual work. We use (4.1) to define stress, and (4.5) to define electric displacement. Since we endow these quantities no more properties than what are implied by their definitions, we can choose whatever we please to vary. In the way we use it, the virtual displacement has no relation to actual displacement, and need not even have the unit of displacement. Nor does the virtual displacement have to be small. It is merely a test function to define stress. Similar remarks go for the virtual voltage.

As a consequence of how we write (4.1), when the virtual displacement is replaced by the increment of actual displacement, the virtual mechanical work becomes actual mechanical work. As a consequence of how we write (4.5), when the virtual voltage is replaced by the actual voltage, the virtual electrical work becomes actual electrical work. These observations lead to (5.3) for conservative materials.

Perhaps the teaching of mechanics has added too much mystery to the idea of virtual displacement. In my class I have begun to call it fake displacement. Perhaps test function is a more respectable name.

Zhigang, By Maxwell stress I

Konstantin Volokh — Fri, 05 Jan 2007 09:04:19 -0500

Zhigang,

By Maxwell stress I define the stress appearing under the electric potential in the absence of mechanical loads. The non-Maxwellian stress appears under the mechanical loading in the absence of the electric field.

I agree that it can be difficult to formalize this notion in the nonlinear case...