The Research Triangle Materials Research Science and Engineering Center (www.mrsec.duke.edu) is seeking outstanding candidates with expertise in soft matter research for positions on its technical staff. Detailed descriptions for the positions of Executive Director, Research Scientist, and Postdoctoral Fellows are available at:
http://www.pratt.duke.edu/research/mrsec

Applications can be submitted online at:
http://www.pratt.duke.edu/mrsec_technical_staff_positions

Xuanhe Zhao 

Soft Active Materials Laboratory, Duke University, Durham, NC 27708

Journal of the Mechanics and Physics of Solids, In press 

Abstract 

Elastomers and gels can be formed by interpenetrating two polymer networks on a molecular scale. This paper develops a theory to characterize the large deformation and damage of interpenetrating polymer networks. The theory integrates an interpenetrating network model with the network alteration theory. The interpenetration of one network stretches polymer chains in the other network and reduces its chain density, significantly affecting the initial modulus, stiffening and damage properties of the resultant elastomers and gels.

The Duke Soft Active Materials Laboratory directed by Prof Xuanhe Zhao is seeking a highly motivated postdoctoral fellow to study mechanics of polymers and hydrogels with applications in tissue regenerations. The work will be carried out in close collaboration with the Duke Orthopaedic Bioengineering Laboratory directed by Prof Farshid Guilak.

NONEQUILIBRIUM THERMODYNAMICS OF DIELECTRIC ELASTOMERS
Xuanhe Zhao, Soo Jin Adrian Koh, Zhigang Suo

Abstract
This paper describes an approach to construct models of dielectric elastomers undergoing dissipative processes, such as viscoelastic, dielectric and conductive relaxation. The approach is guided by nonequilibrium thermodynamics, characterizing the state of a dielectric elastomer with kinematic variables through which external loads do work, as well as internal variables that describe the dissipative processes.

Faculty position - Mechanical Engineering and Materials Science, Duke University Faculty Position

The Department of Mechanical Engineering and Materials Science at Duke University invites applications for a tenure-track faculty position to begin Septmeber 1, 2011. We anticipate hiring at the tenured Associate or Full Professor level, although truly exceptional candidates may be considered at the level of untenured Assistant or Associate Professor.

Department Chair - Mechanical Engineering and Materials Science

Duke University and the Pratt School of Engineering invite applications and nominations for the position of Chair of the Department of Mechanical Engineering and Materials Science (MEMS). MEMS is one of four departments in the Pratt School of Engineering, within a world-class, top-ranked teaching and research institution.

Theory of dielectric elastomers capable of giant deformation of actuation
Xuanhe Zhao, Zhigang Suo
Physical Review Letters, 104, 178302 (2010)

The deformation of a dielectric induced by voltage is limited by electrical breakdown if the dielectric is stiff, and by electromechanical instability if the dielectric is compliant. The interplay of the two modes of instability is analyzed for a dielectric elastomer, which is compliant at a small stretch, but stiffens steeply. The theory is illustrated with recent experiments of interpenetrating networks, and with a model of swollen elastomers. The theory predicts that, for an elastomer with a stress-stretch curve of a desirable form, the voltage can induce giant deformation.

Faculty Opportunity in Thermal Sciences/Energy Technology, Department of Mechanical Engineering and Materials Science, Pratt School of Engineering, Duke University

http://www.mems.duke.edu/employment

Electromechanical instability in semicrystalline polymers
Xuanhe Zhao , Zhigang Suo
Abstract
When a layer of a semicrystalline polymer is subject to a tensile force in its plane and a voltage through its thickness, the deformation of the layer is initially homogeneous,but then localizes.

As a complement to the current issue of journal club, I would like to bring your attention to our current work on dielectric elastomers. 

Xuanhe Zhao and Zhigang Suo A layer of a dielectric elastomer expands its area when a voltage is applied across its thickness.  The layer can be programmed to deform in three dimensions by using patterned prestretches, electrodes, and stiffeners. 

Xuanhe Zhao and Zhigang Suo  We develop a thermodynamic model of electrostriction for elastic dielectrics capable of large deformation. The model reproduces the classical equations of state for dielectrics at small deformation, but shows that some electrostrictive effects negligible at small deformation may become pronounced at large deformation.

A polymer network can imbibe water from environment and swell to an equilibrium state. If the equilibrium is reached when the network is subject to external mechanical constraint, the deformation of the network is typically anisotropic, and the concentration of water inhomogeneous.  Such an equilibrium state in a network constrained by a hard core is modeled here with a nonlinear differential equation.  The presence of the hard core markedly reduces the concentration of water near the interface and causes high stresses.

      This paper studies a gel formed by a network of cross-linked polymers and a species of mobile molecules. The gel is taken to be a dielectric, in which both the polymers and the mobile molecules are nonionic. We formulate a theory of the gel in contact with a solvent made of the mobile molecules, and subject to electromechanical loads. A free-energy function is constructed for an ideal dielectric gel, including contributions from stretching the network, mixing the polymers and the small molecules, and polarizing the gel. We show that the free-energy function is non-convex, leading to instabilities. We also show that mechanical constraint markedly affects the behavior of the gel.

      This letter describes a method to analyze electromechanical stability of dielectric elastomer actuators.  We write the free energy of an actuator using stretches and nominal electric displacement as generalized coordinates, and pre-stresses and voltage as control parameters.  When the Hessian of the free-energy function ceases to be positive-definite, the actuator thins down drastically, often resulting in electrical breakdown.  Our calculation shows that stability of the actuator is markedly enhanced by pre-stresses.

Active polymers are being developed to mimic a salient feature of life: movement in response to stimuli. Large deformation can lead to intriguing phenomena; for example, recent experiments have shown that a voltage can deform a layer of a dielectric elastomer into two coexistent states, one being flat and the other wrinkled. This observation, as well as the needs to analyze large deformation under diverse stimuli, has led us to reexamine the theory of electromechanics. In his classic text, Maxwell showed that electric forces between conductors in a vacuum could be calculated using a field of stress in the vacuum. The Maxwell stress has since been invoked in deformable dielectrics. This practice has been on an insecure theoretical foundation.

I'm Xuanhe Zhao, a first year PhD student working in Suo's Group, at Harvard University. Prior to coming to Harvard, I obtained a Master Degree in Materials Engineering from University of British Columbia.

The mechanical performance of a homogeneous material can be varied by the addition of second-phase particles. In this project, we will model a planar composite under plastic deformation. As shown on the following figure, the composite consists of matrix material and randomly-distributed inclusion particles. The matrix is assumed to be an elastic-plastic material with isotropic or kinematic hardenings, and the inclusion particle pure elastic with a higher Young’s modulus. The stress/strain field throughout the composite will be calculated numerically with finite element method. The effective constitutive behavior of the composite will be evaluated and compared with theoretical and experimental results from literature [1, 2].

Measuring mechanical properties of materials on a very small scale is a difficult, but increasingly important task. There are only a few existing technologies for conducting quantitative measurements of mechanical properties of nanostructures, and nano-indentation is the leading candidate. In this project, we simulate the nano-indentation tests of thin film materials using finite element software ABAQUS. The materials properties and test parameters will be taken from references on nano-indentation experiments [1, 2]. Therefore, the model can be validated by comparing its predictions with experiment results. In addition, we will change 1) the thickness of the thin film and 2) the material of the substrate (for the thin film) in the model, in order to study substrate's effects on nano-indentation tests.

I would like to recommend "Elasticity: Theory, Applications, and Numerics" by Prof. Martin H. Sadd as a reference for ES240. The book, as its name indicated, is mainly focused on elasticity theory and its applications, but also discusses numerical methods such as finite element method and boundary element method.

Prof. Martin H. Sadd, organized the book into two parts: I. foundations, and II Advanced topics. In part I, the book clearly outlines the basic equations of elasticity, i.e. strain/displacement relation, Hooke's law, and equilibrium equation. The other context of part I is devoted to the formulation and solution of two-dimensional problems. This structure matches the progress of our class very well.

The second part of the book begins with the discussion of anisotropic elasticity, thermo-elasticity, and micromechanics. These topics are complementary to the notes of ES240, and helpful in solving homework problems. In its last chapter, the book introduced finite element method and boundary element method.

My name is Xuanhe Zhao, and I'm a first year student in DEAS. Before joining Harvard, I got my Master Degree in Materials Engineering from University of British Columbia, Canand. I have took one course on Computational Mechanics, and read a couple of books on theory of elasticity.

 The major goal for me taking ES 240 is to learn how to understand and solve engineering problems, both familiar and unfamiliar, in a intuitive way. In addition, I will further consolidate my background in solid mechanics.

 University of Michigan, tenure-track faculty positions

The Department of Mechanical Engineering, University of Michigan, Ann Arbor, invites applications for tenure-track faculty positions in various areas of mechanical engineering including design and manufacturing, dynamics, systems and controls, materials and solid mechanics and thermal/fluid sciences. Candidates with research interests in automotive engineering, biotechnology, eco/sustainable systems, energy-systems, manufacturing, and micro/nano systems are particularly encouraged to apply.

Applicants should have an earned Ph.D. in Mechanical Engineering or related fields, a demonstrated record for conducting independent research, and the potential for leadership and impact in teaching and research. Appointments at all levels will be considered. For best consideration, candidates should apply by February 28, 2007, but the positions will remain open until filled.