## Abstract

The objective of this work is to study the electrostatic response of materials accounting for boundary surfaces with their own (electrostatic) constitutive behaviour. The electric response of materials with (electrostatic) energetic boundary surfaces (surfaces that possess material properties and constitutive structures different from those of the bulk) is formulated in a consistent manner using a variational framework. The forces and moments that appear due to bulk and surface electric fields are also expressed in a consistent manner. The theory is accompanied by numerical examples on porous materials using the finite-element method, where the influence of the surface electric permittivity on the electric displacement, the polarization stress and the Maxwell stress is examined.

## 1. Introduction

Boundary surfaces typically present different properties than those of the bulk materials because of to various reasons (surface oxidation, ageing, coating, atoming rearrangement, termination of atomic bonds). Surface triggered phenomena are dominant mechanisms in micro and nanomaterials where the surface-to-volume ratio is large. Phenomenological models that consider surface effects on the mechanical behaviour of materials and introducing surfaces with their own structure have been studied firstly by Gibbs. In more recent contributions, Gurtin & Murdoch [1] described surface effects with the help of tensorial surface stresses. Moeckel [2] developed an alternative approach at the same period, based on the concept of a moving thermomechanical two-sided surface. Later, Daher & Maugin [3] invoked the method of virtual power to endow a surface with its own thermodynamic constituents. For applications of surface elasticity theory in nanomaterials, see e.g. Cammarata [4], Dingreville & Qu [5], He & Lilley [6], Duan *et al.* [7] among many others. An important extension of the surface elasticity model to account for the flexural resistance of the surface was developed by Steigmann & Ogden [8]. Park *et al.* [9], Park & Klein [10] developed an alternative continuum framework based on the surface Cauchy–Born model, an extension of the classical Cauchy–Born model, to include surface stresses.

Our own contributions in this field include [11] whereby the balance laws for a mechanical problem accounting for energetic boundaries were obtained in a variational setting based on potential energies. Later Javili & Steinmann [12] studied the thermomechanical response of materials with energetic surfaces and specified restrictions on the surface constitutive laws arising from the second law of thermodynamics. Javili & Steinmann [13–15] proposed a novel finite-element framework to model the surface elasticity and surface thermoelasticity theories. A unifying review of various approaches accounting for mechanical and/or thermal mechanisms of surface, interface and curve energies was presented in Javili *et al.* [16].

The interfaces between particles and matrix material play a significant role in the relative permittivity [17,18]. It has been shown experimentally that the electric performance of a particle can be improved with surface modification [19–22]. Also it has been observed that the increase in the area-to-volume ratio (i.e. decrease in the particle and/or grain size) can influence substantially the dielectric constant [23,24]. With regard to nanoporous materials, studies have shown that the dielectric constant is overpredicted by the classical micromechanics approaches [25,26]. A possible explanation for such behaviour could be the simplified approximations of the local electric field at the void surface [26]. Moreover, the influence of the independent surface behaviour on piezoelectric materials is studied in Dai *et al.* [27].

The aim of this work is to study, from a phenomenological point of view, the electric response of materials with boundary surfaces that possess their own electrostatic constitutive behaviour. The introduction of an energetic boundary allows to account for the influence of the surface-to-volume ratio. Conceptually speaking, the energetic boundary surface resembles a thin layer of dielectric material with large dielectric constant. To this end, surface electric phenomena are described in the spirit of surface mechanics.

The structure of the manuscript is as follows. First, notation and definitions are briefly introduced. Then in §2 the theoretical aspects of electric behaviour of continua with surface structure is elaborated. The presented theory in §2 is based on a variational approach. This theory is underpinned by the Maxwell equations given in appendix A. That is, it is possible to alternatively establish the same theory using pillbox arguments. This procedure is carefully explained in appendix A. The theory is then elucidated via a series of numerical examples in §3. Section 4 concludes the paper and discusses further extensions of the current work.

### (a) Notation and definitions

Direct notation is adopted throughout. Occasional use is made of index notation, the summation convention for repeated indices being implied. The scalar product of two vectors **a** and **b** is denoted **a**⋅**b**=[**a**]_{i}[**b**]_{i}. The tensor product of two vectors **a** and **b** is a second-order tensor **D**=**a**⊗**b** with [**D**]_{ij}=[**a**]_{i}[**b**]_{j}. The dot product of a vector **a** with a second-order tensor **A** is given by [**a**⋅**A**]_{j}=[**A**]_{ij}[**a**]_{i}. In the following, ** ε** denotes the third-order permutation tensor with components

*ε*

_{ijk}. Kronecker delta is denoted as

*δ*

_{ij}while

**i**denote the second-order identity tensor, i.e. [

**i**]

_{ij}=

*δ*

_{ij}. The latin letter indices take values 1, 2 or 3, while the greek letter indices take values 1 or 2. The preliminary mathematical tools from the differential geometry essential for this work are contained in table 3 of appendix A. The free space electric permittivity constant is denoted

*ϵ*

_{0}. Unless stated otherwise quantities defined on the surface are differentiated from those in the bulk by a hat placed above the symbol. That is, refers to a variable corresponding to the surface, its counterpart in the bulk being {•}. The basic variables used in this work are introduced in table 1.

Consider a continuum body that occupies the configuration within the free space occupying the open set . The boundary surface of the body is denoted by , while the boundary unit vector is denoted **n** as illustrated in figure 1.

The jump of a bulk quantity {•} across the surface and average of {•} over the surface are identified as
respectively. The terms {•}^{+} and {•}^{−} are quantities in the free space and in the body adjacent to the boundary surface, respectively. The following identities hold for two bulk quantities **a** and **b**:

## 2. Electric behaviour of bodies with energetic surface

The electrostatic behaviour of a body under the action of an electric field is well identified in the electromagnetic theory [28–31]. With regard to an energetic surface attached to a body though, the development of conservation and constitutive laws requires to specify key assumptions on the electric variables that lie on the surface. Thus, we proceed with the following main hypotheses:

(1)

*The boundary surface electric variables (electric field, electric displacement and polarization) have only tangential components*.(2)

*We consider that the energetic nature of the boundary does not contribute to the free space electric energy*.

The first hypothesis states that
2.1and furthermore, it implies that
2.2In appendix A, we show that the non-existence of a normal component to the surface electric field causes the tangential part of the (bulk) electric field to be continuous on the boundary surface, i.e.
2.3and therefore equation (2.2) follows from (2.1). This hypothesis is also supported theoretically by the work of Barham *et al*. [32] in the case of magnetoelasticity, when the magnetic field is connected with a scalar potential and the general three-dimensional problem is reduced to two-dimensions through a membrane approximation.

The second hypothesis (no boundary surface free space electric energy) states that the purely electric part of the free energy is only due to the free space and is not affected by surface electricity, which is exclusively due to the material nature of the boundary surface. Thus, the total internal energy of the energetic surface does not include a free space contribution, as opposed to the bulk material.

### (a) Variational approach

The variational approach that we develop in this section allows us to compute the field equations by identifying stationary points of a total potential energy functional. For better insight on energy formulations of electrostatics and energy formulations of continuum magneto-electro-elasticity for identifying the relationship between the field equations, variational principles and thermodynamics, the interested reader is referred to Liu [33,34].

Based on the electric scalar potential *φ*, one can identify the variational format of the electric problem with (electrostatic) energetic surface. Recalling the usual relation for bulk materials that connects electric field and scalar potential via
2.4we can similarly define the analogous relation for the boundary surface
2.5

Using thermodynamic considerations, we define the total potential energy as the sum of internal and external potential energies including the free space contribution in the body. The total potential energy for the bulk material is expressed as
2.6and for the boundary surface as
2.7We also consider the free space electric energy
2.8According to the second hypothesis, the free space electric energy in the boundary surface is considered zero, i.e.^{1}
2.9The reason for omitting this term is because it is already included in the free space energy (2.8) of the bulk (recall that is the projection of on the boundary according to (2.2)).

Next, considering as independent variables^{2} the potential *φ*, the bulk electric field and the surface electric field , we identify the total potential energy functional^{3}
2.10for which we want to identify a stationary point for all admissible variations of the independent variables, i.e.
2.11This last relation is expressed as
2.12where we considered the following constitutive equations:
2.13From the relations (2.4) and (2.5), the variation can be written only with respect to the electric potential, and renders
2.14

Using the bulk and surface divergence theorems (table 3) and taking into account that (i) is a tangential vector and (ii) the boundary surface is closed, we eventually obtain
Since *δI* vanishes for any arbitrary *δφ*, we obtain the governing equations in local form
2.15Relations (2.15) constitute the general system of electric equations that describe the free space, the bulk material and the boundary surface. By neglecting the surface electric field, equations (2.15)_{3} reduce to the classical boundary conditions in terms of the normal jump of the electric displacement. The same conclusions can be established if alternatively we examine the Maxwell equations using pillbox arguments (for details see appendix A).

As a final remark we note that, even if we consider that there is a normal component of the electric displacement, this component will not participate in equation (2.15) (see appendix A, or in the case of thin films, eqn (223) of [36]).

### (b) Polarization

The relationship between the polarization, the electric field and the electric displacement inside the bulk is written as
2.16For the boundary surface, we consider that the polarization, the electric field and the electric displacement are connected through the relation^{4}
2.17which means that the surface electric displacement is equal to the surface polarization. The motivation for equation (2.17) arises from the tangential continuity of the electric field across the boundary surface. Using equation (A12) of appendix A in conjunction with (2.16), we have that
which states that the tangential parts of the bulk electric displacement and the bulk polarization across the surface should be equal.

### (c) Resultant bound charge and electric moment

Following classical approaches [31], the total charge density inside a body is considered as the sum of the bound charge density, which appears due to the material, and the free charge density which is due to moving charges. Thus, using equations (2.15)–(2.17), we have
2.18where the subscript *d* denotes the jump of a bulk quantity and the subscript *s* denotes the contribution of the energetic surface. Using the divergence theorems, the equations (2.18) and noticing that (i) the polarization has a meaning only inside the material, i.e. , (ii) the boundary surface polarization is a tangential vector and (iii) the boundary is a closed surface, we can show that:

The presence of a boundary surface with different constitutive behaviour than the material does not influence the general requirement that the total bound charge in an isolated body vanishes, i.e. it holds 2.19

The material nature of the boundary influences the electric moment, since it includes the contribution of the bulk polarization and the surface polarization , i.e. 2.20

### (d) Ponderomotive force and moment densities

The ponderomotive body forces **b**^{pon} in the bulk material and the tractions (or alternatively surface forces) at the boundary surface satisfy
2.21The term in the second integral of the left-hand side represents the contribution of the boundary as a surface with discontinuity in the tangential part of the electric field [37].^{5} After proper calculations, the resultant ponderomotive force densities are expressible as
2.22Note that in expression (2.22)_{2} the boundary surface ponderomotive force depends not on the total boundary surface free charge density , but only on its part that arises from the normal discontinuity of across the boundary.

The ponderomotive body force is connected with a ponderomotive stress tensor, which consists of two parts, the Maxwell stress and the stress due to the polarization. Thus, we can express the bulk ponderomotive stress tensor in the form 2.23Using the expressions (2.23), we obtain

Following the same approach, the ponderomotive moment densities satisfy
2.24where **c**^{pon} and denote the ponderomotive bulk and boundary surface moments, respectively. Thus,
leading to
2.25

A summary of the equations and expressions that are related to both the bulk and the boundary surface is given in table 2.

Before we close the theoretical section we would like to point out that, from a thermodynamic point of view, our theory assumes that the energetic surface supplies electrical power without any mechanical effect, as opposed to the bulk material, in which we have also mechanical power due to the polarization. A proper thermodynamic treatment though of the current problem would require to write our equations taking into account large deformation formulations, i.e. identifying the deformation gradient and its Jacobian for connecting spatial and material description. This would exceed the scope of the current contribution.

## 3. Numerical examples

In order to carry out a finite-element implementation of the proposed theory, one would need to employ the weak form of balance equations (2.15). The procedure of obtaining the weak form from a strong form is standard in the context of the finite-element method [38]. Here, the weak form is obtained by multiplying the strong form (2.15) by appropriate test functions of proper Sobolev spaces in the bulk and on the surface and applying the (bulk) divergence theorem in the bulk and surface divergence theorem on the surface. This procedure would lead to a weak form resembling the variational form (2.14) and therefore, we do not repeat it. The numerical implementation of the problem accounting for the surface conservation laws and constitutive equations is similar to the one developed in the case of thermoelastic solids [39]. Thus in this work, we do not describe the finite-element formulation.

In the sequel, we present numerical examples for a two-dimensional toy example. Clearly, the theory is not limited to two dimensions and holds for arbitrary geometries in three dimensions. This simple example is chosen to illustrate key features of the proposed theory. Consider a (linear) electric body as shown in figure 2 with square shape of length ℓ that includes a circular void at its centre. The surface of the void is considered to present linear electric behaviour. The (thermodynamically consistent) constitutive relations that are used for both the bulk and the surface of the void are expressed as
3.1In the above expressions, *ϵ*_{0} is the electric permittivity of free space, while *ϵ* and denote the relative dielectric constants of the body and the energetic surface, or rather interface, respectively. In the numerical analyses, we consider a solid material with *ϵ*=10, i.e. 10 times the free space. The dielectric constant of the void is clearly the same as the free space, i.e. equal to one. The surface relative dielectric constant varies from 0 to 100. In terms of boundary conditions, we choose zero normal electric displacement on the left and right side, while on the top and the bottom side we apply 0 and 10 V electric potential, respectively. The domain is discretized using 2000 bilinear quadrilateral elements. The mesh quality of the finite-element method and details about the boundary conditions are presented in figure 2.

In order to better understand the influence of the surface electric behaviour on the overall response of the body, in a series of numerical examples, distributions of the quantities are illustrated in figures 3–11.

Note that has the dimension of length, i.e. reducing the size ℓ by a factor *f* is equivalent to increase the by the same factor *f*. More importantly, for all stresses the results depend on the length squared ℓ^{2} which indicates a strong size effect. The results here are given for ℓ=1 and normalized by multiplying with ℓ^{2} for stresses and with ℓ for electric fields due to similarity reasons. Also, all stress values are divided by *ϵ*_{0}.

The results in figures 3–11 are carefully produced in a self-explanatory manner. Here, for more clarity some of the key features are explained. In figure 3, we observe that the distribution of the scalar potential in the area close to the void depends strongly on the surface dielectric constant. When the surface of the void does not have independent material behaviour () the potential lines tend to enter the void. In all the following results, this (top-left) illustration shall be understood as the classical response of the example of interest with no additional surface constitutive behaviour. Increasing the surface dielectric constant varies the distributions of electric potential lines. For , one observes a very strong surface effect such that the potential lines tend to move away from the void.^{6} We also see that for the void becomes (almost) invisible to the scalar potential, leading to an (almost) homogeneous behaviour (see also figures 4 and 5). Thus, electrostatic energetic surfaces allow to create ‘invisible’ metamaterials.

Figure 4 demonstrates the distribution of the electric field in the direction normal to the applied electric potential difference. The presence of a material void surface alters significantly the close to the void, since for to the electric field tends to become homogeneous and for to it becomes again inhomogeneous close to the void with opposite signs compared to . Similar behaviour is observed for the electric field in the direction parallel to the applied electric potential difference in figure 5. Here, we also observe that for the electric field inside the void almost vanishes.

Regarding the Maxwell stress distributions, a strong void surface in terms of electric properties causes almost disappearance of stress inside the void and a very high stress concentration close to the surface. This phenomenon holds for both normal and parallel directions to the applied electric potential difference (figures 6 and 7).

The distribution of the polarization stress follows a similar pattern with the distribution of the electric fields according to figures 8 and 9. The distribution of the ponderomotive stress, i.e. the sum of the Maxwell and the polarization stress, is shown in figures 10 and 11, where we observe that for there is a significant stress concentration around the void.

## 4. Conclusion

In this paper, we have identified the governing equations that describe the behaviour of a body with (electrostatic) energetic boundary surface. The assumption of surface electric quantities that are tangential to the surface has the following advantages: (i) it allows the surface electric field to be described in terms of the usual electric scalar potential and thus to connect it with the bulk electric field, (ii) it provides a correct representation of the surface bound charges satisfying the classical principle in electrostatics that the total bound charge in an isolated body vanishes and (iii) the surface polarization represents electric moment per unit area. In the developed framework, we also observe that the surface electric variables do not provide additional Maxwell and polarization surface forces.

An important application of the proposed framework is to study the overall response of micro- and nanoporous materials whereby the surface plays a significant role due to a more dominant surface-to-volume ratio. In order to do so, one needs to set up a micro-to-macro transition framework to calculate the overall response from the microscopic behaviour. This extension will be the subject of our further contributions to this field.

## Funding statement

The support of this work by the ERC Advanced Grant MOCOPOLY is gratefully acknowledged. The first author would like to acknowledge the support by the King Abdullah University of Science and Technology (KAUST) project NumPor.

## Appendix A. Maxwell laws

It is enlightening to show how the governing equations (2.15) can be alternatively obtained by examining the Maxwell equations using pillbox arguments. Table 3 summarizes certain key definitions and properties of differential geometry necessary in the following derivations (for further details, see [16] and references therein).

Let *x*^{q} denote the Cartesian coordinates, while denotes the curvilinear surface coordinates. Moreover, **g**_{q} and **g**^{q} denote Cartesian covariant and contravariant base vectors in the bulk, while and denote curvilinear covariant and contravariant base vectors on the surface, respectively. Some of the vectors used in the table are shown in figure 12.

**(a) First Maxwell equation**

We examine the following cases for the first Maxwell equation.

We consider the control region of figure 12

*a*. The canonical control volume region is defined as one which includes a part of the material, i.e. . has a smooth boundary with unit normal vector**m**. In the case of the electrostatic problem, the second Maxwell equation reads A1From the volume divergence theorem, we obtain A2which upon localization leads to the local equation A3In order to identify the continuity condition between the bulk and the boundary surface, we follow the procedure described by Kovetz [31]. We consider a small cylinder passing through the boundary surface and including both bulk material and free space (figure 12

*b*). The cylinder crosses the boundary on the surface , bounded by the curve . The curve has normal unit vector (tangent to the surface) and tangent unit vector . We apply the second Maxwell equation on this cylinder and we consider the limit as it collapses onto . In this case, we have A4We note that a normal part of , even if it exists, does not contribute to the last relation, since the term at the top face of cancels out with the term at the bottom face of . Using the surface divergence theorem, we can rewrite the last equation as A5The second integral vanishes since is tangential, thus the arbitrariness of allows us to identify the local format as A6The final result dictates that the boundary surface free charge density can be split in two parts: the first part is due to the normal discontinuity of across the boundary, while the second part is due to the independent behaviour of the boundary surface.

**(b) Second Maxwell equation**

We examine the following cases for the second Maxwell equation.

We assume a surface with boundary curve and normal unit vector

**m**_{n}, exclusively embedded in the bulk material. The boundary curve of the bulk has tangent unit vector**m**_{t}(figure 12*c*). Then the second Maxwell equation in the case of electrostatics reads A7which, by using the Stokes’ theorem, rewrites as A8Since is an arbitrary surface in the bulk material and**m**_{n}is an arbitrary vector, the local form of (A8) leads to A9In order to identify the continuity condition between the bulk and the boundary surface, we follow the procedure described by Kovetz [31]. We consider a plane perpendicular to (figure 12

*d*). The orientation of the plane is defined by its normal , which is tangential to the surface. We apply the (first) Maxwell equation on this plane and we consider the limit as it collapses onto in a single curve with tangent unit vector and boundary points . In this case, we have A10In the above relation, we note that in a closed loop around the term above the curve is cancelled with the term below the curve. Since is tangential, the second term of equation (A10) vanishes and the equation reads A11Since is arbitrary, we derive the jump condition in local form as A12The last equation states that the tangential part of the electric field is not affected by the boundary surface.

## Footnotes

↵1 If one would like to account for an independent free space electric energy in the boundary, then it could be expressed in a similar manner to (2.8) as where should be a surface parameter different than

*ϵ*_{0}. In such formalism, our second hypothesis implies that the parameter is zero.↵2 Mathematically speaking, one could think of

*φ*as the only purely independent variable and refine the derivations here, too.↵3 In a similar manner as in Kankanala & Triantafyllidis [35] approach for magnetostatics, we split the electric displacement in the free space (and consequently the electric field and the Maxwell stress) in two parts, the applied electric displacement and the perturbed electric displacement owing to the presence of the solid. The perturbed electric quantities in the free space are assumed to decay and eventually vanish far away from the solid. In the sequel, we will omit the terms that are related to the applied electric field at .

↵4 Strictly speaking, the analogous relation of (2.16) for surfaces is written as where is a surface parameter different than

*ϵ*_{0}. As we already mentioned, the second hypothesis implies that is zero, thus the last relation leads to (2.17).↵5 Since the contribution of the boundary surface to the ponderomotive force is already included in equation (2.21).

↵6 The surface dielectric constant is given in terms of the bulk matrix dielectric constant, i.e. corresponds to .

- Received September 23, 2013.
- Accepted January 10, 2014.

- © 2014 The Author(s) Published by the Royal Society. All rights reserved.