Spring 2018

Mar 15

Inauguration

Mar 22

Locomotion Strategies in Granular Media
by Dr. Baptiste Darbois-Texier – Universidad de Santiago de Chile (Usach)

Mar 29

Programmable Multistable Mechanisms
by Mohamed Zanaty – EPFL STI IMT INSTANT-LAB
Watch the video here!

Apr 12

Modeling of Snow and Avalanche Mechanics Using the Material Point Method
by Dr. Johan Gaume – EPFL ENAC IIE Laboratory of Cryospheric Sciences (CRYOS)

Apr 19

Physics of Bubble-Propelled Microrockets
by Giacomo Gallino – EPFL STI IGM LFMI
Watch the video here!

Rayleight-Taylor Instability Under Curved Substrates
by Gioele Balestra  – EPFL STI IGM LFMI
Watch the video here!

Marangoni Bursting
by  Dr. Ludovic Keiser – EPFL STI IGM LFMI
Watch the video here!

Apr 26

Robust Elastic Metamaterial Design for Additive Fabrication
by Dr. Julian Panetta – EPFL IC IINFCOM LGG
Watch the video here!

An Interlocking Method for 3D Assembly Design and Fabrication
by Dr. Peng Song – EPFL IC IINFCOM LGG
Watch the video here!

May 3

Dynamic Stall
by Dr. Julien Deparday – EPFL STI IGM UNFOLD
Watch the video here!

May 17

Generation of Double Emulsions Drops Through Microfluidics as Template for Encapsulation
by Antoine Vian – EPFL STI IMX Soft Materials Laboratory (SMAL)
Watch the video here!

Studying the Early Stages of CaCO3 Formation Using a Microfluidic Spray Dryer
by Huachuan Du – EPFL STI IMX Soft Materials Laboratory (SMAL)
Watch the video here!

May 24

The latest from flexLab: Mechanics of Slender Structures and Fluid-Structure Interactions
by Dr. Matteo Pezzulla, Dr. Dong Yan and Dr. Paul Grandgeorge – EPFL STI IGM flexLab

May 31

Efficient FFT-based Homogenisation without Linear Reference Medium
by Dr. Till Junge – EPFL STI IGM LAMMM
Watch the video here!


Efficient FFT-based Homogenisation without Linear Reference Medium
By Dr. Till Junge – EPFL STI IGM Laboratory for Multiscale Mechanics Modeling (LAMMM)​

Thursday 31 May 2018 16:15 – 17:15 MED 0 1418 EPFL

Abstract: In the field of computational homogenisation of periodic representative volume elements (RVE), over the last two decades, fast Fourier transform (FFT)-based spectral solvers have emerged as a promising alternative to the finite element method (FE).

Most spectral methods are based on work of Moulinec and Suquet [1] and split an RVE’s response into that of a linear reference medium and a periodic fluctuation due to heterogeneities. The main advantage of this formulation over FE is that it can be both significantly faster and memory-saving. The two main problems are 1) the choice of the reference medium, which is typically based on heuristics, non-trivial and has a strong impact on the method’s convergence (A bad choice can render the method non-convergent), and 2) convergence is not uniform. Numerous studies have suggested mitigations to both of these problems (e.g. [2]), but they have remained substantial disadvantages compared to the more expensive, but also more robust FE.

Recent work by Zeman et al. [3] proposes a new formulation for spectral solvers which dispenses with the linear reference problem and converges unconditionally. We present µSpectre, an open implementation of this novel method and use it to show that the new approach is more computationally efficient than its linear reference medium-based predecessors, converges in the presence of arbitrary phase contrast – including porosity – and eliminates or drastically reduces Gibbs ringing.

[1] H. Moulinec and P. Suquet. Comput. Methods Appl. Mech. Eng. (1998)
[2] M. Kabel, et al. Comput. Mech. (2014)
[3] J. Zeman, et al. Int. J. Num. Meth. Eng. (2016).
Bio: Till Junge is an Ambizione Fellow in EPFL’s laboratory for multiscale mechanics modeling (LAMMM) and the PI for project µSpectre (https://c4science.ch/u/muspectre), an open-source platform for efficient FFT-based continuum mesoscale modelling. Till graduated from EPFL with a PhD in computational mechanics in the civil engineering department in 2014 and subsequently worked at the University of Lausanne and the Karlsruher Institut für Technologie before coming back to EPFL for his Ambizione Fellowship.

He researches novel computational methods in the fields of concurrently coupled atomistic-continuum multiscale mechanics, FFT-based contact mechanics, machine-learning for interatomic potentials and efficient spectral methods for continuum mesoscale modelling.

Contact:

Till Junge


Deformation of perforated elastic sheets due to the hydrodynamic loading by a viscous fluid
By Matteo Pezzulla – EPFL STI IGM Flexible Structures Laboratory (flexLab)

Defect-controlled buckling of depressurized elastic spherical shells
By Dong Yan – EPFL STI IGM Flexible Structures Laboratory (flexLab)

Unravelling the tight interplay between geometry and mechanical response of elastic knots
By Paul Grandgeorge – EPFL STI IGM Flexible Structures Laboratory (flexLab)

Thursday 24 May 2018 16:15 – 17:15 MED 0 1418 EPFL

Abstract: Deformation of perforated elastic sheets due to the hydrodynamic loading by a viscous fluid by Matteo Pezzulla
From spider webs and insect wings, to wire fences and parachutes, Nature and technology provide us with vast examples of perforated flexible structures that undergo elastic deformation due to fluid flow. Whereas fluid flow through porous media has been studied extensively, the fluid-structure interactions of a perforated slender elastic object that undergoes large deformations due to the loading of a surrounding viscous fluid has received much less attention. I will talk about our ongoing research, where we use precision desktop experiments to focus on the prototypical problem of a perforated elastic plate moving through a viscous fluid, at low to moderate Reynolds number. Via a reduced theoretical model based on Kirchhoff-Euler beam theory coupled with a low Reynolds description for the fluid forcing, we seek to provide a predictive framework for the deformation of perforated plates due to hydrodynamic loading. We hope that our findings may lead to a better understanding of fluid-structure interactions between porous slender structures and viscous flows, across biological and technological applications.


Defect-controlled buckling of depressurized elastic spherical shells by Dong Yan 
Spherical shells are ubiquitous in nature and engineering structures, across a wide range of length-scales. Small geometric imperfections can substantially decrease the buckling strength of shells. In this talk, I will present our ongoing research on the buckling of spherical shells containing a thickness defect. Through a customizable coating technique, a thickness defect with controllable geometric properties is fabricated by blowing the polymer layer during curing. We vary the amplitude of thickness variation and study the buckling behavior of our spherical shells. We quantify the effect of a thickness defect on the buckling strength. The experimental results are then contrasted against finite element modeling (FEM) simulations. Upon validation of the numerics, we then use FEM to perform a broader exploration of the parameter space. Our results lead to a better understanding of imperfection sensitivity, which is a concrete foundation for accurately predicting the buckling pressure of shell structures in engineering.


Unravelling the tight interplay between geometry and mechanical response of elastic knots by Paul Grandgeorge 
May they be functional for shoelaces, undesirable when tangling up cables, or even aesthetically appealing in Celtic decoration art, knots are dealt with on a daily basis. Even for medical purposes, surgeons have employed knots to secure sutures for millennia. Even though they come across in such a wide variety of fields, the predictive understanding of knot topology and mechanics remains challenging. Past studies have primarily focused on mathematically ideal knots, discarding most physical properties of the knotted rod such as cross-section diameter, bending stiffness or frictional behavior along self-contact regions. In this talk, I will present our ongoing experimental research on elastic knots, performed in collaboration with Prof. John Maddocks. In particular, I will focus on the orthogonal clasp, a simple yet rich configuration (2 fibers crossing, see picture), to present our 3D-imaging and image-processing strategy. We hope the gained physical insight will pave the road for future material and configuration optimizations of functional knots in the areas of surgery and other engineering applications.
Bio: Matteo Pezzulla is a postdoc in the Flexible Structures Laboratory (flexLab) at EPFL. He is interested in the fluid-structure interactions (FSI) of perforated slender structures and viscous fluids, at low Reynolds numbers, and in the geometric mechanics of slender structures such as shells, possibly coupled with swelling.

Dong Yan is a postdoc in the Flexible Structures Laboratory (flexLab) at EPFL. He is interested in the mechanics of slender structures. He is currently working on the buckling of spherical shells.

Paul Grandgeorge is a postdoc in the Flexible Structures Laboratory (flexLab) at EPFL. He is interested in the mechanical behavior of slender structures undergoing non-linear deformations. More specifically, he is currently working on the mechanics of elastic knots.

Contact:

Dong Yan


Generation of Double Emulsions Drops Through Microfluidics as Template for Encapsulation
By Antoine Vian – EPFL STI IMX Soft Materials Laboratory (SMAL)

Studying the Early Stages of CaCO3 Formation Using a Microfluidic Spray Dryer
By Huachuan Du – EPFL STI IMX Soft Materials Laboratory (SMAL)

Thursday 17 May 2018 16:15 – 17:15 MED 0 1418 EPFL

Abstract: Generation of Double Emulsions Drops Through Microfluidics as Template for Encapsulation by Antoine Vian
Double emulsions drops are small drops contained in larger drops. They can be used as picoliter-sized vessels to conduct chemical or biochemical reactions or to conduct high throughput screening assays. Key to a successful application of these drops is a good stability against coalescence and rupture. Previous studies have shown that the mechanical stability of double emulsion drops increases if their shell is reduced below the µm scale. Unfortunately, the fabrication of double emulsion drops with such thin shells at high throughputs is still challenging. Here, we present a new microfluidic device that reduces the thickness of double emulsion shells to values as low as 250 nm. We demonstrate that the reduction of the shell thickness of double emulsions improves their mechanical stability and lowers their permeability. Moreover those drops can be used as templates for capsule production.


Studying the Early Stages of CaCO3 Formation Using a Microfluidic Spray Dryer by Huachuan Du 
Calcium carbonate (CaCO3) is one of the most abundant biominerals that nature often uses as a structural material because of its excellent mechanical properties. Inspired by nature, a lot of work has been devoted to produce CaCO3-based materials. The key to gain a better control over the mechanical properties of CaCO3-based materials is to have a better understanding of formation mechanism of CaCO3. Here, we produce amorphous CaCO3 (ACC) particles using a microfluidic spray-drier that quenches the formation of CaCO3 within 100 ms to 10s. This fast quenching enables studying the evolution of the structure and composition of ACC particles that form during early stages without the need for any external quenching agents. We show that the amount of mobile water of ACC particles increases with increasing formation time and hence with increasing particle size. As a result, larger particles are less stable than smaller counterparts. These insights into the influence of the formation time of ACC on the quantity of mobile water contained in them and their stability might open up new opportunities to fabricate biomimetic CaCO3-based materials with tunable structures.
Bio: Antoine Vian is studying his PhD in Prof. Esther Amstad’s group where he uses microfluidics as a tool to control encapsulation processes. He entered ESPCI ParisTech (Ecole Superieure de Physique et de Chimie Industrielle) after prep class in Stanislas Paris. He did part of his master thesis in Weitz lab in Harvard where he had Esther as his supervisor. He started in September 2014 and he helped her mounting the lab. He also holds a MBA from college des ingenieurs in Paris.

Huachuan Du is studying his PhD in Prof. Esther Amstad’s group where he uses the microfluidic spray-drying technique to study the crystallization process. He obtained his master degree in the Materials Science and Engineering section at EPFL in 2015. Prior to that, he did his bachelor in Chemical Engineering at Harbin Institute of Technology in China and Pohang University of Science and Technology in South Korea.

Contact:

Hervé Elettro



Critical Evolution of Leading Edge Suction During Dynamic Stall

Thursday 3 May 2018 16:15 – 17:15 MED 0 1418

Speaker: Dr. Julien Deparday – EPFL STI IGM Unsteady Flow Diagnostics Laboratory (UNFOLD)
Abstract: To improve wind turbine design, low order models that accurately predict the aerodynamic performance are desirable. Dynamic stall dominates the aerodynamic performance, the robustness, and the wake dynamics of vertical axis wind turbines. To better assess the dynamic stall onset and its associated unsteady effects, this talk will present experimental measurements on the leading edge suction on a sinusoidal pitching airfoil with also time-resolved particle image velocimetry. During the dynamic stall stage, we linked the flow dynamics with the evolution of the leading edge suction.
This oral presentation has been accepted at the seventh edition of the international conference “The Science of Making Torque from Wind” (TORQUE 2018) which will take place in June 20-22, 2018 at Politecnico di Milano. The related paperhas also been accepted for publication in the “Journal of Physics: Conference Series (JPCS)”.
Bio: Julien Deparday is interested in experimental studies on unsteady aerodynamics and fluid-structure interaction. During his PhD, he conducted full-scale experimental studies of fluid-structure interaction on downwind sails at the Naval Academy Research Institute, France. He is now a post-doctoral researcher at UNFoLD and works on unsteady aerodynamics.

Contact:

Anna Lee


Robust Elastic Metamaterial Design for Additive Fabrication
By Dr. Julian Panetta – EPFL IC IINFCOM LGG

An Interlocking Method for 3D Assembly Design and Fabrication
By Dr. Peng Song – EPFL IC IINFCOM LGG

Thursday 26 April 2018 16:15 – 17:15 MED 0 1418 EPFL

Abstract: Robust Elastic Metamaterial Design for Additive Fabrication by Julian Panetta
Additive fabrication technologies like 3D printing can efficiently manufacture geometry of arbitrary complexity but offer only limited control over the elastic material properties of the printed part. In this talk, I will present my work on designing elastic metamaterials for 3D printing: periodic microstructures that are tuned to emulate a large space of elastic materials. Since microstructures typically contain thin features that concentrate stress, they are prone to plastic deformation and fracture even under mild deformations of the macro-scale object. A key goal of my work is to design microstructures that minimize these stress concentrations, improving the metamaterials’ robustness in practice. The design algorithm I developed is based on a efficient, exact solution to the worst-case stress analysis problem for periodic microstructures which supports several failure criteria (e.g., maximum principal stress or von Mises stress); it produces microstructures that minimize worst-case stresses while achieving a particular target elasticity tensor and satisfying fabrication constraints. I also will present a design tool to optimally apply these metamaterials to achieve high-level deformation goals.


An Interlocking Method for 3D Assembly Design and Fabrication by Peng Song
3D assemblies involve multiple component parts. Rather than relying on additional fasteners such as nails and screws to connect the parts, component parts can be interlocked into a steady assembly based on their own geometric arrangements. This talk revisits the notion of 3D interlocking, and explores the governing mechanics of general 3D interlocking assemblies. From this, constructive approaches are developed for computational design of various new interlocking assemblies such as puzzles, 3D printed objects and furniture. These interlocking assemblies are ready for fabrication and their steadiness have been validated in our experiments.
Bio: Julian Panetta is a post-doctoral researcher in the LGG group at EPFL. He is interested in simulation and optimal design problems, specifically focusing on applications for 3D printing.

Peng Song is a post-doctoral researcher in the LGG group at EPFL, working with Prof. Mark Pauly since October 2017. He received his B.S. degree from Harbin Institute of Technology in 2007, M.S. degree from Harbin Institute of Technology (Shenzhen) in 2009, and Ph.D. from Nanyang Technological University, Singapore in 2013. His research interests lie in computer graphics and computational fabrication. His research works could be found at his research website.

Contact:

Dong Yan



The latest from LFMI: some stories of Fluid Mechanics and Instabilities

Thursday 19 April 2018 16:15 – 17:15 MED 0 1418

Speakers: Giacomo Gallino, Gioele Balestra, and Ludovic Keiser – EPFL STI IGM Laboratory of Fluid Mechanics and Instabilities (LFMI)
Abstract: Physics of bubble-propelled microrockets by Giacomo Gallino
Self-propelled artificial micro-motors have attracted much attention both as fundamental examples of active matter and for their potential biomedical applications (e.g. drug delivery, cell sorting). A popular design exploits the catalytic decomposition of a fuel (e.g. hydrogen peroxide) on the active surface of the motor to produce oxygen bubbles that propel the swimmer, effectively converting chemical energy into swimming motion. We focus here on a conical shape swimmer with chemically-active inner surfaces. Using numerical simulations of the chemical problem and viscous hydrodynamics, we analyse the formation, growth and motion of the bubbles inside the micro-motor and the resulting swimming motion. Our results shed light on the fundamental hydrodynamics of the propulsion of conical swimmers and may help to improve the efficiency of these machines.


Rayleight-Taylor instability under curved subtrates by Gioele Balestra
A liquid film coated on the underside of a planar substrate is subject to the Rayleigh-Taylor instability so that its interface deforms into waves that lead to the formation of dripping droplets. When the substrate is curved, gravity not only acts as the destabilizing force at the origin of the instability, but also as a stabilizing force originating in the progressive drainage of the film. As a consequence, a two-dimensional thin-film in a circular geometry is asymptotically stable to infinitesimal perturbations. Nevertheless, we have found that the system acts as a transient amplifier and may from dripping droplets. Here, we consider coatings inside cylindrical and spherical substrates and demonstrate that additional instability patterns arise. A linear stability analysis based on lubrication equations is performed and the results are found to be in good agreement with experiments and numerical simulations.


Marangoni bursting by Ludovic Keiser
At the surface of a sunflower oil bath, a drop of water adopts a lenticular shape. Conversely, alcohol totally wets the oil and spreads. Depositing a mixture of water and alcohol reveals a spectacular fragmentation phenomenon. If it contains enough alcohol, the drop spontaneously spreads and fragments into a myriad of minute droplets whose size strongly depends on the initial mixture composition. Marangoni flows resulting from the differential evaporation of alcohol and water play a key role in this self-emulsification process. The intricate coupling of hydrodynamics, wetting and evaporation is well captured by analytical scaling laws that predict the characteristic radius and timescale of spreading. Other combinations of liquids also lead to this fascinating phenomenon and further confirm our scenario.​
Bio: All speakers are active members of the LFMI group.
Giacomo Gallino is a last year PhD student, focusing on numerical studies of two phase systems.
Gioele Balestra is a last year PhD student, with interest in hydrodynamic instabilities of thin flows.
Ludovic Keiser is a fresh post-doctoral researcher. He studies experimentally a variety of capillary phenomena.

Contact:

Hervé Elettro



Unified Modeling of Snow and Avalanche Mechanics Using the Material Point Method

Thursday 12 April 2018 16:15 – 17:15 MED 0 1418

Speaker: Dr. Johan Gaume – EPFL ENAC IIE Laboratory of Cryospheric Sciences (CRYOS)
Abstract: Snow slab avalanches start with the failure of a weak snow layer buried below a cohesive snow slab. After failure, the very porous character of the weak layer leads to its volumetric collapse and thus closing of crack faces due to the weight of the overlaying slab. This complex process, generally referred to as anticrack, explains why avalanches can be remotely triggered from flat terrain. On the basis of a new elastoplasticity model for porous cohesive materials and the Material Point Method (MPM), we accurately reproduce the propagation dynamics of anticracks observed in snow fracture experiments as well as the subsequent detachment of the slab and the flow of the avalanche. In particular, we performed 3D slope scale simulations of both the release and flow of slab avalanches triggered either directly or remotely.
Bio: Dr. Johan Gaume is a research and teaching associate in the CRYOS lab. of EPFL and scientist at the SLF in Davos. His research is focused on the numerical modeling of snow fracture and avalanche mechanics in order to improve avalanche forecasting. Last summer, he was visiting scholar in the Department of Mathematics of UCLA where he worked on a new snow avalanche model in the research group of Joseph Teran who contributed to the snow simulations in the Disney movie “Frozen”.

Contact:

Lorenzo Benedetti



Programmable Multistable Mechanisms

Thursday 29 March 2018 16:15 – 17:15 MED 0 1418

Speaker: Mohamed Zanaty – EPFL STI IMT INSTANT-LAB
Abstract: Multistable mechanisms have a wide range of applications such as mechanical switches and energy harvesters. I introduce programmable multistable mechanisms, where the number of stable states can be modified by programming inputs. Stability programming opens a whole host of applications from medical devices to mechanical computation. The qualitative stability behavior of a generic programmable multistable mechanism, the T-mechanism, is established. An analytical model based on Euler-Bernoulli beam theory is used and standard stiffness methodology is utilized to approximate mechanism strain energy as an eighth degree polynomial, where the coefficients are functions of the programming inputs. The roots of this polynomial determine the qualitative stability and stiffness behaviors. The results are validated using finite element analysis and experimental measurements. This method can be generalized to more complicated designs. Stability programming is applied to surgical tools to improve the treatment of the retinal vein occlusion.
Bio: Mohamed Zanaty is a doctoral student at Instant-lab, Institute of Microengineering at EPFL. His research consists of the design, modeling, simulation and characterization of compliant mechanisms and their applications to medical devices.

Contact:

Anna Lee



Locomotion Strategies in Granular Media

Thursday 22 March 2018 16:15 – 17:15 MED 0 1418

Speaker: Dr. Baptiste Darbois-Texier – Universidad de Santiago de Chile (Usach)
Abstract: Desert is a hard place to live. However, some species of snakes, lizards and bugs are very adapted to this hostile environment. For exemple, the fringe lizard burrows under the hot surface of sand to escape predators and heat, the sandfish lizard swims in sand as a fish and the rattlesnake lifts alternatively his body in order to climb steep dunes. In this talk, we will explore the mechanics that underlies these strategies of locomotion and see how they interact with the special rheology of granular media. This approach will lead us to imagine new modes of locomotion operating in granular materials and that may be useful to develop robots for these environments.
Bio: Baptiste Darbois-Texier leads multidisciplinary researches on fluid and granular mechanics. He conducted his PhD on the trajectories of sport balls and a first postdoc on fluid flows generated by phase changes. He is now a postdoctoral fellowship at the University of Santiago in Chile and works on locomotion in granular media.

Contact:

Hervé Elettro


 


Inaugural event of the MEGA.Seminar

Thursday 15 March 2018 16:15 – 17:15 MED 0 1418


Speakers:

Event:

The inaugural event of the MEGA.Seminar will take place on Thursday, March 15th 16:15-17:15 in the MED Palaz room (MED 0 1418). A series of sound-bite talks will present the various labs at EPFL with interests in Mechanics (fluids & solids). Straight after the inaugural event, there will be an informal reception (apéritif) in the lobby of the MED building.

Contact:

MEGA.Seminar Organizing Committee (Anna Lee, Lorenzo Benedetti, Hervé Elettro, Till Junge, Francesco Maresca, Roozbeh Rezakhani, Dong Yan, Noelia Simone)