Counting on Grains of Sand

Civil engineers have developed a new method that measures the way forces move through granular materials—one that could improve our understanding of everything from how soils bear the weight of buildings to what stresses are at work deep below the surface of the earth.

Granular materials—conglomerations of solid particles larger than a micrometer, such as gravel or coffee grounds—are everywhere in the world around us. The snow on a mountain top is a granular material, as is the grain stored in a silo. Granular materials exhibit distinct and sometimes unusual properties. For example, if you shake an aggregate composed of particles spanning a wide range of sizes, the larger particles rise to the top; this counterintuitive behavior is known as granular size separation, although it is sometimes referred to as the "Brazil nut effect" because large Brazil nuts tend to rise to the top of a packet of mixed nuts.

Because of their ubiquity, granular materials are of significant interest to scientists and engineers. And an important goal of granular material science is to be able to measure how forces move through such materials, says Caltech's Jose Andrade, professor of civil and mechanical engineering in the Division of Engineering and Applied Sciences. "It is the ultimate frontier in granular materials," he says. "Understanding the way they move and carry forces will help us reconstruct their collective behavior."

Andrade and his colleagues used a combination of computed tomography (CT) scanning and X-ray diffraction to measure the deformation of individual grains, in effect turning each particle into a force gauge that shows the direction and intensity of force.

Imagine two identical houses: one sitting on bedrock and the other on a sandy soil. Both are subject to gravity's downward pull. The mass of the house on bedrock, because it pushes on a solid object, generates a force that is easy to model. However, the force generated by the house on sand, although identical, is much more difficult to model because it is dispersed across many millions of sand grains, each with a different shape and orientation and moving with respect to one another and in different directions.

In this example, the house on sand exerts force on the grains that it touches, which exert force on the grains around them, which in turn exert force on the grains below them, and so on. The force is not transmitted straight down, but rather radiates out in asymmetric patterns determined by each grain's shape and interaction with its immediate neighbors.

Prior research measuring forces in granular materials often used grains made of a special material that shines when placed under stress. By observing which grains were shining and which were not, researchers could track the propagation of stress through the material as a whole. Andrade and his colleagues, however, wanted to be able to track the propagation of stress through any granular material. Their new technique is based on the fact that, when under stress, the shape of an individual grain will change slightly, much the way a foam ball deforms to varying degrees based on how hard you squeeze it.

The team used CT scanning to reveal how particles are shaped and oriented and X-ray diffraction to show—at an atomic level—how those particles deform under pressure. This information was then used to calculate how much force each individual grain is under and to quantify the transmission of force through granular materials.

More than just a proof-of-concept, the test revealed a surprising characteristic of granular materials: the more external pressure is placed on them, the more homogenous the substance acts, regardless of how heterogenous the grains may actually be. As the team slowly increased the amount of force on their test particles, they noted that the dispersion of that force grew more equitable throughout the entire material.

The observation led to an unforeseen connection with the social sciences.

Social scientists use a statistical expression called the Gini coefficient to measure income and wealth inequality in societies. For example, a Gini coefficient of 0 signifies a society in which all of the wealth is concentrated in the hands of a single individual, while a coefficient of 1 signifies a society in which everyone has an equal share.

Andrade and his colleagues found that this same coefficient can be used to model the dispersion of force through granular materials. "To draw a correlation, the granular 'societies' in our samples become more equitable as external pressure increases," Andrade says. "We can call this observation granular solidarity."

Being able to quantify granular solidarity means that engineers will know how much external pressure a granular material needs to be under before it behaves like a single, unbroken material—as opposed than a collection of tiny grains. The finding could simplify future engineering calculations about granular materials.

The work is described in a paper, titled "Quantifying Interparticle Forces and Heterogeneity in 3D Granular Materials," published online on July 18 in the journal Physical Review Letters. Coauthors on the paper include Ryan Hurley, formerly of Caltech but now with the Lawrence Livermore National Laboratory; S. A. Hall of Lund University in Sweden; and J. Wright of the European Synchrotron Radiation Facility in France. The research was funded by the U.S. Air Force Office of Scientific Research, the U.S. Defense Threat Reduction Agency, Lawrence Livermore National Laboratory, and the European Commission.

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Faraon Receives Naval Research Grant

Andrei Faraon (BS '04) has been awarded a research grant from the Office of Naval Research's Young Investigator program. From a pool of 267 applications, Faraon was among 47 selected for the honor, which is designed to attract young scientists and engineers with exceptional promise both as teachers and researchers. In 2015, Faraon received a Young Investigator grant from the Air Force.

Faraon was selected because of his academic achievements, ability to help strengthen the nation with his work, and the exceptional support provided by Caltech, according to the ONR. Previous awardees of this grant include Austin Minnich, in 2015, and Chiara Daraio, in 2010.

Faraon's lab creates nanoscale photonic devices that operate at the quantum limit of light–matter interaction. His work could one day generate practical optical quantum memory devices, a crucial step in the development of quantum computing machines, as well as systems for quantum communications.

"This ONR grant will enable us to develop devices that perform quantum transduction between optical and microwave photons," says Faraon. "These technologies will enable optical communications between superconducting quantum computers."

Faraon joined the Caltech faculty in 2012 as an assistant professor of applied physics and material science.

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Wednesday, August 24, 2016
JPL

JPL Postdoc Research Day - Poster Session

The Utility of Instability

A team of researchers from Caltech and Harvard has designed and created mechanical chains made of soft matter that can transmit signals across long distances. Because they are flexible, the circuits could be used in machines such as soft robots or lightweight aircraft constructed from pliable, nonmetallic materials.

Unlike hard materials, which transit signals readily, soft materials tend to absorb energy as it passes through them. An analogy is hitting a firm punching bag versus a soft one: with the firm bag, the energy of your punch moves through the bag and sends it swinging, but the soft bag deforms your fist like a lump of dough and therefore will swing less.

To overcome that response, Caltech's Dennis Kochmann, Chiara Daraio, and their colleagues created an unstable, "nonlinear" system. Their findings have appeared in three papers published over the past few months.

"Engineers tend to shy away from instability. Instead, we take advantage of it," says Kochmann, assistant professor of aerospace in the Division of Engineering and Applied Sciences, and one of the lead researchers on the project.

Stable, or "linear," systems are attractive to engineers because they are easy to model and predict. Take, for example, a spring: If you push on a spring, it will respond by pushing back with a force that is linearly proportional to how much force you apply. The response of a nonlinear system to that same push, by comparison, is not proportional, and can include sudden changes in the direction or amplitude of the responsive force.

The nonlinear systems that Kochmann and his colleagues designed rely on bistable elements, or elements that can be stable in two distinct states. The bistable elements that the team developed consist of arches of an elastic material, each a few millimeters in size. The elements can be in either a convex or a concave position—and are stable in either configuration. However, if you push on the element in its convex position, it responds by pushing back against the direction of force until it snaps into a concave position, accompanied by a sudden release of energy in the opposite direction.

"It's an elastic response, and then a snap-through," explains Daraio, professor of aeronautics and applied physics.

Collaborating with Katia Bertoldi, Jennifer Lewis, and Jordan Raney of Harvard University, Kochmann, Daraio, and Caltech graduate student Neel Nadkarni designed chains of the bistable elements, connected to one another by springs. When one link "pops" from the concave to the convex state, its spring tugs at the link that is next downstream in the chain, popping it to a convex position as well. The signal travels unidirectionally down the chain. The energy released by the popping balances out any energy absorbed by the soft material, allowing the process to continue down the chain across long distances and at constant speed.

"We were inspired by the physics of phase transformations. A phase transformation is a process where a switching occurs between two stable states of a system. It is governed by strongly nonlinear mathematical equations that are not very well understood," Nadkarni says.

A proof-of-concept version of the design constructed from 3-D printed elements is described in a paper published August 8, 2016 in the Proceedings of the National Academy of Sciences. This paper was the third in the series of publications outlining the new concept for transmitting signals. It outlined how the design can be used to build mechanical AND and OR logic gates such as those used in computer processors. Logic gates are the building blocks of circuits, allowing signals to be processed.

"These systems could be used as actuators to control robotic limbs, while passively performing simple logic decisions," Daraio says. Actuators use the transfer of energy to perform mechanical work, and in this case, the transfer of energy would occur via a mechanical rather than an electrical system.

The first paper in the series was published in March in the journal Physical Review B, and it described Kochmann's theoretical, mathematical framework for the system.  The second paper was published in Physical Review Letters in June, and it describes Daraio's first experimental model for the system.

While springs can be employed between the bistable elements, the team also demonstrated in the Physical Review Letters paper how magnets could be used to connect the elements—potentially allowing the chain to be reset to its original position with a reversal of polarity.

"Though there are many applications, the fundamental principles that we explore are most exciting to me," Kochmann says. "These nonlinear systems show very similar behavior to materials at the atomic scale but these are difficult to access experimentally or computationally. Now we have built a simple macroscale analogue that mimics how they behave."

The PNAS paper is titled "Stable propogation of mechanical signals in soft media using stored elastic energy." The authors are Nadkarni, Daraio, and Kochmann of Caltech and Jordan Raney, Jennifer Lewis, and Katia Bertoldi of Harvard University. The work was funded by the National Science Foundation.

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Researchers from Caltech and Harvard have designed and created mechanical chains made of soft matter that can transmit signals across long distances.

Caltech's RoboSub Team Takes First Place

After facing off against 47 competitors from around the world, the Caltech Robotics Team won first prize at the 19th annual International RoboSub Competition for autonomous underwater vehicles in San Diego, held from July 25-31.

The team's robot submarine, nicknamed "Dory," successfully navigated an obstacle course with tasks that required it to touch buoys, fire torpedoes at targets, and rescue an object underwater—all autonomously.

"Not only was Caltech's entry widely acclaimed as the most beautiful robot in the competition, it performed impressive feats of autonomy in the underwater race course, including a double barrel roll through the 'style points' navigation gate," says faculty advisory Joel Burdick, the Richard L. and Dorothy M. Hayman Professor of Mechanical Engineering and Bioengineering in the Division of Engineering and Applied Science. 

"It's great to see the culmination of all of our efforts," says Tyler Okamoto, a rising senior and the team's project manager, who was one of 18 representatives from Caltech's team at the competition.

Caltech had the largest student team to show up to participate in the event of any school, despite having the smallest student body, Burdick notes. The team's students worked around the clock as the competition drew nearer, with the software team making tweaks to the hydrophone system just hours before the race that enabled their victory.

"Working with the team during the months and days leading up to the competition reminded me of why I chose a career in academia," Burdick says. "It was such a thrill to be associated with this great group of students."

"We are very proud of the win and the exemplary work this team has done," says Guruswami Ravichandran, John E. Goode, Jr., Professor of Aerospace and Mechanical Engineering; and Otis Booth Leadership Chair, Division of Engineering and Applied Science. "The team was exceptional and represented Caltech extremely well."

2016 was only the third year in which a Caltech team of undergraduates entered this competition. The first year, its submarine placed 7th out of 39 teams, winning the "best newcomer" award. The following year, the team's ranking climbed to 4th out of 37.

The Caltech team chose to redesign its submarine each year from the ground up—even though teams are allowed to re-enter submarines, tweaking them to build on past successes. This year's submarine, which features seven thrusters, two grippers, a hydrophone, and other sensors, took nine months and about $75,000 to build.

One major upgrade for this year's design was a wide-view camera that sits inside a clear plastic bubble and has two axes of rotation that allow the submarine to spot targets without changing orientation; last year's design required the submarine to fire its thrusters and roll and pitch while conducting searches in order to find its targets. The competition's judges noted this as an innovation that had not been seen before at the event.

Another major improvement was the addition of a clear acrylic dome on the top of the robot that allowed for easy access to its electronics, allowing the team to make changes without having to do major disassembly and reassembly.

The International RoboSub Competition is hosted annually by the Association for Unmanned Vehicle Systems International (AUVSI), a non-profit organization dedicated to advancing the robotics community. In addition to completing the obstacle course, participating teams must generate a technical paper about their submarine, create a video, build a website to track progress, and give a presentation to a panel of judges.  

For its first-place finish, Caltech's team was awarded the top prize of $6,000. A video of the event can be viewed online at http://www.robonation.org/competition/robosub.

The team will reconvene in October to begin planning the next year's robotic submarine.

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Thursday, August 11, 2016
Center for Student Services 360 (Workshop Space) – Center for Student Services

Teaching Statement Workshop 2: Peer Review

Richard Murray Named to DOD Panel on Innovation

Secretary of Defense Ash Carter named Caltech's Richard Murray to the Defense Innovation Advisory Board, a 15‑member panel established in March to advise the Department of Defense (DOD) on innovation.

Murray (BS '85), is the Thomas E. and Doris Everhart Professor of Control and Dynamical Systems and Bioengineering in the Division of Engineering and Applied Science. He joined the Caltech faculty in 1991, and served as the EAS division chair from 2000 to 2005.

The Defense Innovation Advisory Board includes scholars and innovators who focus on new technologies and organizational behavior and culture. Secretary Carter has asked the board to identify technology and practices from the private sector that could be used by the DOD.

The board, chaired by Eric Schmidt, executive chairman of Alphabet (the parent company of Google), also includes Jeff Bezos of Amazon Inc., and astrophysicist Neil deGrasse Tyson, among others.

The board members, "represent some of the most innovative minds in America," Secretary Carter said in a press release. "I appreciate their willingness to join this effort and keep the Department of Defense on the cutting edge." He announced the new appointments at the formal opening of the new Defense Innovation Unit Experimental (DIUx) office in Boston on July 26.

Murray's research focuses on control theory and networked systems, with applications in biology, robotics, and aerospace engineering. He received the IEEE Control Systems Award in June 2016 "for contributions to the theory and applications of nonlinear and networked control systems." He was also recently acknowledged by Thomson Reuters for belonging to the top one percent of most cited researchers during the period of 2002 through 2012.

The Defense Innovation Advisory Board will begin work this summer, and will provide initial recommendations to Secretary Carter by October.

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Thursday, August 4, 2016

Data Visualization Summer Internship Program Final Presentations

Improving Computer Graphics with Quantum Mechanics

Caltech applied scientists have developed a new way to simulate large-scale motion numerically using the mathematics that govern the universe at the quantum level.

The new technique, presented at the International Conference and Exhibition on Computer Graphics & Interactive Techniques (SIGGRAPH), held in Anaheim, California, from July 24-28, allows computers to more accurately simulate vorticity, the spinning motion of a flowing fluid.

A smoke ring, which seems to turn itself inside out endlessly as it floats along, is a complex demonstration of vorticity, and is incredibly difficult to simulate accurately, says Peter Schröder, Shaler Arthur Hanisch Professor of Computer Science and Applied and Computational Mathematics in the Division of Engineering and Applied Science.

"Since we are computer graphics folks, we are interested in methods that capture the visual variety and drama of fluids well," says Schröder. "What's unique about our method is that we took a page from the quantum mechanics's 'playbook.'"

The Schrödinger equation, the basic description of quantum mechanical behavior, can be used to describe the motion of superfluids, which are fluids supercooled to temperatures near absolute zero that behave as though they are without viscosity. Viscosity is a fluid's resistance to deformation.  

"Caltech's Richard Feynman was one of the first to recognize that superfluids are governed by so-called vortex filaments, which are basically long strings of pure vorticity," Schröder says. "While we are not interested in quantum mechanics, we realized that the Schrödinger equation—with some tweaks—can also approximate fluids at the macroscopic level, from smoke gently rising from a flame to the concentrated vorticity of a tornadic storm."

When asked why the Schrödinger equation, usually reserved for effects at the atomic level, does so well for fluids at the macroscopic level, Schröder says, "The Schrödinger equation, as we use it, is a close relative of the non-linear Schrödinger equation which is used for the description of superfluids. Their vorticity behavior is in many ways very similar to the behavior we can also observe in the macroscopic world."

Schröder hopes his work will have an impact on computer-generated graphics, and may also be used to model real-world phenomena, such as the curling motion of a hurricane.

Schröder's paper, entitled "Schrödinger's Smoke," was presented on July 26. His coauthors include Albert Chern, a graduate student at Caltech; Felix Knöppel and Ulrich Pinkall of Technische Universität Berlin; and Steffen Weißmann of Google. This research was supported by the German Research Foundation, the Office of Naval Research, and the German Academic Exchange Service.

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Bhattacharya Named Vice Provost

On July 15, Kaushik Bhattacharya, the Howell N. Tyson, Sr., Professor of Mechanics and Materials Science, will become one of Caltech's two vice provosts. He takes on the role filled for the last six years by Mory Gharib, the Hans W. Liepmann Professor of Aeronautics and Bioinspired Engineering. Gharib will continue to serve as director of the Graduate Aerospace Laboratories (GALCIT) and the recently established Center for Autonomous Systems and Technology.

Bhattacharya joined the Division of Engineering and Applied Science (EAS) faculty in 1993. His research group studies the mechanical behavior of solids and uses theory to guide the development of new materials. He has made contributions on a wide array of topics, ranging from the fundamental mechanics of materials, to active materials and devices, to multi-scale and multi-physics scale simulation of materials. Though trained as a theoretician, he is well known for live demonstrations of shape-memory materials in action. 

Bhattacharya was executive officer of the mechanical and civil engineering department from 2007 to 2015, overseeing the department's academic program and the renovation of the Charles C. Gates Jr.–Franklin Thomas Laboratory.  

As vice provost, he will focus on overseeing sponsored research policies and proposal authorizations, human subject policies and procedures, the technology transfer and corporate relations program, and research compliance. Caltech's other vice provost, Cindy Weinstein, professor of English, focuses on academic matters. "I have great confidence in Kaushik and am very pleased that he has agreed to take on the job of vice provost," says Ed Stolper, Caltech's provost and the William E. Leonhard Professor of Geology and Carl and Shirley Larson Provostial Chair. "I am confident that he, Cindy Weinstein, and I will function as an effective team carrying out the diverse tasks of the provost's office."

"Caltech is a special place and I look forward to the opportunity to assist my colleagues in their pursuit of excellence in research and innovation," says Bhattacharya.

"Kaushik's technical strength, deep knowledge of the Institute, energy, and enthusiasm will serve him and us well as he takes on this important role," says Guruswami (Ravi) Ravichandran, the John E. Goode, Jr., Professor of Aerospace and Mechanical Engineering and Otis Booth Leadership Chair of the EAS division.

Bhattacharya received his PhD from the University of Minnesota in 1991 and was a postdoctoral scholar at the Courant Institute for Mathematical Sciences from 1991 to 1993. He is a recipient of several honors and awards, including the Warner T. Koiter Medal of the American Society of Mechanical Engineering, the Young Investigator Prize from the Society of Engineering Science, the Special Achievements Award in Applied Mechanics from the American Society of Mechanical Engineers, and the National Science Foundation Young Investigator Award. In 2013, Bhattacharya received Caltech's annual Graduate Student Council Teaching and Mentoring Award. He served as editor of the Journal of Mechanics and Physics of Solids from 2005 to 2015.

"I want to thank Mory Gharib for his six years of service as vice provost," Stolper says. "In my opinion, Mory's deep knowledge of Caltech; his instincts in interacting with his fellow faculty members regarding sometimes contentious issues; his understanding of issues associated with technology transfer and interactions with industry based on his many successful experiences in this realm; his leadership in the establishment of the Linde Institute of Economic and Management Sciences; and his intuition for resolving potentially difficult conflicts of interest have all resulted in unique and lasting contributions to the Caltech community for which we all, and I in particular, owe him our thanks."

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Kaushik Bhattacharya will replace Mory Gharib as Caltech vice provost.

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