A single strand of polymer is a chain of a large number of monomers.  The monomers are joined by covalent bonds, and two bonded monomers may rotate relative to each other.  At a finite temperature, the polymer rapidly changes from one configuration to another.  When the two ends of the polymer are pulled by a force, the distance between the two ends changes.  The polymer is known as an entropic spring.  These notes are developed as part of statistical thermodynamics to supplement the course on advanced elasticity. 

These notes may serve as a reminder of tensor algebra.  The notes supplement the course on advanced elasticity.  Several books are listed at the end of the notes.

In the notes on the general theory of finite deformation, we have left the free energy function unspecified. The notes here describe free energy function commonly used to describe the elasticity of rubber-like materials.  These notes are part of a course on advanced elasticity

These notes are part of a course on advanced elasticity.  The notes recall several phenomena where both elasticity and surface energy are significant, including

• Griffith crack
• Adhesion of flexible structures
• Wafer bonding
• Contraction of a soft elastic sheet 

The notes also contain a formulation of combined surface energy and elasticity of finite deformation.  

You may also wish to follow the ongoing discussion on the topic.

This classic paper is attached

Xuanhe Zhao, of Duke University, has just accepted our invitation to be the next Editor of the iMechanica Journal Club.  The Journal Club was initiated in January 2007, soon after the launch of iMechanica.  A glance at the list of past Themes and Discussion Leaders gives an impression of vibrant topics and dynamic researchers at the frontier of mechanics.  iMechanica is fortunate to have a succession of excellent Editors.  Xuanhe is an exceptionally creative and energetic researcher:   his reserch is deep and wide-ranging.  We look forward to his leadership, and to many more challenging and charming Themes and Discussion Leaders.        

Reading PDF files of papers and textbooks on computers has long been difficult for me.  The resolution of the screens has been too low.  The computers have been too heavy for reading in couches and beds. 

In recent months  iPad 3 has made a difference for me.  The resolution of iPad 3 is so good that color pictures often look better on screen than in print.  iPad lets me read comfortably anywhere.  I’d choose to read a book with iPad even if I have the same book in printed form.  I wish iPad were available when I was young and had a lot of time for reading.  I would have read all these large biology books in bed. 

With the help of my sons, I get things organized by using the following software.

Full announcement of the meeting (in both Chinese and English) 

申请条件. 具有海外博士学历(5年以内)或者2013年即将毕业的博士。或者具有国内博士学历，已在海外工作或学习2-4年。在本学科领域取得突出成绩或在领域内发表有高水平学术论文。具备工作经验或博士后研究经验更佳。通过资格审查者，由主办单位邮件通知具体参会事宜.

申请注册. 2012年7月15日 500-1000字摘 http://202.114.18.186:8888/member/contribute_forumregister.jspx

会议日程. 2012年10月6日 - 10月8日, 华中科技大学

差旅及住宿. 主办单位统一安排食宿(免费), 提供每人1万元人民币的机票补助。

Full announcement of the meeting (in both Chinese and English) 

Sigurd Wagner and Siegfried Bauer have just edited an exciting issue of the MRS Bulletin: Materials for Stretchable Electronics.   All articles contain phenomena related to mechanics.  Here is the abstract of my contribution:  Mechanics of Stretchable Electronics and Soft Machines.  In the emerging field of soft machines, large deformation of soft materials is harnessed to provide functions such as regulating flow in microfluidics, shaping light in adaptive optics, harvesting energy from ocean waves, and stretching electronics to interface with living tissues. Soft materials, however, do not provide all of the requisite functions; rather, soft machines are mostly hybrids of soft and hard materials. In addition to requiring stretchable electronics, soft machines often use soft materials that can deform in response to stimuli other than mechanical forces. Dielectric elastomers deform under a voltage. Hydrogels swell in response to changes in humidity, pH, temperature, and salt concentration. How does mechanics meet geometry, chemistry, and electrostatics to generate large deformation? How do molecular processes affect the functions of transducers? How efficiently can materials convert energy from one form to another?  These questions are stimulating intriguing and useful advances in mechanics. This review highlights the mechanics that enables the creation of soft machines.

John Hutchinson has just pointed out to me the website, shellbuckling.com.  The site is devoted to the mechanics of buckled shells, with downloadable photos, slides, papers, and computer codes.  The site also has a section on buckling people.  The site is created by a veteran buckling person, Dr. David Bushnell, formerly of Lockheed Martin.  Check the site out, and enjoy. 

（本招聘广告有效期至2012年12月31日）PDF of this file

为了更好地聚集海外优秀人才、培养国际一流的青年学者、及借此加快建设世界高水平大学的进程，西安交通大学建立了一个新型的、采用美国先进的用人和科研体制管理的研究中心---“国际应用力学中心” International Center for Applied Mechanics (ICAM)。该中心的目标是聚集和培养一流人才，产出一流学术成果，打造国际一流的力学学科。ICAM以独立的tenure-track PI为本，将重点研究国际力学前沿问题及国家重大需求中的关键力学问题，承担国家级的重点和重大项目，并开展高层次国际合作与交流。

ICAM是西安交通大学的一个独立的学术机构，参照美国常春藤盟校的高效和高度扁平化的科研管理体制。中心的学术带头人为国际著名的中青年力学家锁志刚、高华健、陈曦、刘子顺等西安交通大学杰出校友。中心的国际学术顾问包括Hutchinson，Willis，Needleman等世界力学大师。在他们的指导和引领下，ICAM将汇集和激励青年学者成长为有国际影响力的国家级人才。作为西安交通大学机械结构强度与振动国家重点实验室的一个“学术特区”，中心的人才招聘、薪酬、考核、评估、管理与运行等均参照美国的先进体制进行。ICAM实行独立的tenure-track PI制度，使得青年科研人员能够最大限度地发挥其创造力和主导作用，在国际著名学者的指引和激励下快速成长为具有国际能见度的一流学者（且不存在成长的上限）。ICAM将给每位tenure-track PI配备相应的独立的实验室空间和科研平台建设费，研究生招生和国际交流计划单列（不受传统名额限制）。中心的运行模式、人员岗位、人才培养和经费预算等均具有高度灵活性，可为每位tenure-track PI量身定制其发展轨迹。中心充分鼓励有共同兴趣的研究组在中心内部开展全面合作，并大力支持PI和校内外的其它学科（如机械、能动、电子电气、材料、航空航天、土木、生物生医、物理、化学等）的国际国内学者开展广泛的交叉研究。在学校的鼎力支持和中心全体人员的共同努力下，力争在较短时间内将ICAM建设成一个具有鲜明学科交叉特色、拥有世界一流研究水准和较高国际影响力的力学研究中心、成为国际力学学科前沿的领头羊之一。

Sukumar pointed out a Google service to retrieve your papers.  Here are a few examples:

• Wei Hong
• Teng Li
• Harold Park
• N. Sukumar
• Xuanhe Zhao 

I have divided the old notes on temperature into three parts:

• Principle of the conservation of energy
• Empirical observations of temperature (this part)
• Energy and fundamental postulate 

Our feeling of hotness comes from everyday experiences. These experiences indicate that many levels of hotness exist, and that all levels of hotness can be mapped to a real variable.

The world consists of moving parts: stars, planets, animals, electrons, protons, photons, etc. Their movements and interactions carry energy. Energy is a fundamental concept. We do not know how to define energy in terms of more fundamental concepts. We do know many ways to keep track of energy. For example, we know how to calculate the kinetic energy of a flying bullet, and the gravitational energy of a weight. We can measure the electrostatic energy in a capacitor, and the elastic energy in a spring. We can look up the values of energy in all kinds of food. We can find similar numbers for fuels—coal, oil, gas. Energy in foods and fuels are stored in chemical bonds.

(Class notes for ES 181 Engineering Thermodynamics.  Also part of my notes on thermodynamics)  In 1824, Sadi Carnot (1796-1832) published a short book, Reflections on the Motive Power of Fire. (The book is now free online. You should try it out.) To construct an engine, Carnot noted, at least two reservoirs of energy of different temperatures are needed. He further noted that the engine loses efficiency whenever the working fluid of the engine exchanges energy with the rest of the world across a finite difference in temperature. To avoid such exchange of energy, he described a specific cycle—later known as the Carnot cycle—consisting of isothermal and adiabatic processes. Whenever the working fluid exchanges energy with either reservoir, the temperature of the working fluid is kept the same as that of the reservoir. Whenever the temperature of working fluid differs from the temperatures of the reservoirs, the working fluid is thermally insulated. He argued that this cycle is the most efficient of all cycles that convert heat to work by operating between two constant-temperature reservoirs of energy.

I gave a seminar at Xian Jiaotong University on 27 October 2009.  I recently found the video of the seminar online.  The seminar was in Chinese, but the slides were in English.

• Video of the seminar
• Slides of the seminar  

If the subject interests you, the following papers will lead you to the literature.

When I learned chemistry in college, the subject was presented to me with equations of chemical reactions.  It took me some time to realize a couple of simple points:  reactants need to meet to produce a product, and compounds take space.

The connection between chemistry and mechanics is made vivid to me in recent years in studying lithium batteries.  As an example, here is a recent paper when chemistry is linked with plasticity, mass transport, and fracture—essential ingredients of solid mechanics.

Kejie Zhao, Matt Pharr, Joost J. Vlassak and Zhigang Suo. Inelastic hosts as electrodes for high-capacity lithium-ion batteries. Journal of Applied Physics 109, 016110 (2011).

George Whitesides has published over 1,100 papers.  In 2004 he published a three-page essay “Whitesides’ Group:  Writing a Paper”.  I have been asking all my students to study this essay when they begin to work with me.  Now you can watch Whitesides on video explaining his approach to publishing papers.    

I’ve just come back from EuroEAP 2011, the First International Conference on Electromechanically Active Polymer (EAP) Transducers & Artificial Muscles.  The technical program was exciting.  The meeting was chaired by Federico Carpi, and took place in Pisa, Italy.  The weather was cool, and air fresh. 

Also in the air was optimism for the new technology of dielectric elastomer transducers.   The potential of soft dielectrics as a broad-ranging technology was first brought into focus by a SRI team in a paper published in Science in 2000.  The technology is based on an extremely robust electromechanical coupling.  A membrane of a dielectric elastomer is sandwiched between two compliant electrodes, such as those made of carbon grease.  When a voltage is applied between the two electrodes, one electrode becomes positively charged, and the other becomes negatively charged.  The opposite charges cause the membrane to reduce thickness and expand area.  Linear strains beyond 100% have been achieved.  Many videos of dielectric elastomer transducers are available on YouTube.

A long polymer consists of many monomers. The monomers are covalently bonded, and two bonded monomers may rotate relative to each other. Consequently, the polymer may be modeled as a chain of many links, each link representing a monomer. At a finite temperature, the polymer rapidly changes from one configuration to another.

A large number of long, flexible polymers can be crosslinked by covalent bonds to form a three-dimensional network. Subject to forces, the network undergoes large elastic deformation. The network is commonly called an elastomer.

In teaching the elements of thermodynamics in the graduate course on soft active materials, I have followed this sequence:

• Isolated system:  a system capable of no variation. Entropy, S.
• Temperature: a system capable of one independent variation. Entropy is a function of energy, S(U).
• Pressure:  a system capable of two independent variations. Entropy is a function of energy and volume, S(U,V).
• Chemical potential: a system capable of three independent variations. Entropy is a function of energy, volume and the number of water molecules, S(U,V,N).

For the third time I am teaching the graduate course on soft active materials.  This course is called Advanced Elasticity in the Catalog of Courses.  In the last several years, I have dropped several traditional topics, and focused on thermodynamics and finite deformation.  I have added several topics where both thermodynamics and finite deformation play significant roles, such as elastomeric gels and dielectric elastomers.

The Harvard School of Engineering and Applied Sciences (HSEAS) seeks applicants for an appointment at the level of tenured professor in the field of computational mechanical and materials engineering. The ideal candidate will have high expertise in computation, and will also have a demonstrated commitment to significant and innovative applications in mechanical engineering and/or materials engineering.

Successful candidates will have enthusiasm for teaching both graduate and undergraduate courses in engineering and applied computational science, and work with HSEAS faculty to develop a curriculum for our ABET-accredited engineering program.