Postdoctoral position opening (starting January 2019):
UNIfied multi-scale framework for PLASticity of metals, amorphous and cellular materials
Dislocations in a crystalline foam.
PROJECT:
A fundamental question in materials science is identifying the microscopic origin of plasticity. Why do materials display plastic rather than elastic response and can we predict when this will occur ? An improved understanding of plastic deformation is important in a wide range of amorphous materials, such as metallic and polymeric glasses, foams, granular materials, colloids, and emulsions.
In molecular materials with crystalline order, plastic behavior is understood in terms of defect nucleation and dynamics. For amorphous materials, a description in terms of topological defects is not possible due to the inherent structural disorder. Therefore identifying and characterizing local plastic events in amorphous materials is essential for a complete understanding of their structural and mechanical properties. Experimentally, the challenge is identifying systems for which the microscopic events are directly observable. This is one of the main advantages of mesoscopic systems, such as foams, and the reason for the interest in making connections between mesoscopic systems and molecular materials.
Foams are interesting as a model for atomic or molecular systems with tunable structural disorder (either quenched of evolving on large time scales) and interactions between units (from pairwise to many-body interactions). Foams are also a model for other soft cellular materials such as emulsions or confluent biological tissues: these systems are constituted of highly deformable (yet almost incompressible) units (bubbles, drops, cells,... ). Interfacial energy is the key ingredient in the cohesion and the rigidity of such systems, often constituted solely of fluids.
The proposal aims to bring a unified approach in the understanding of plastic behavior, for both molecular and cellular materials, with various degrees of disorder (from crystalline to amorphous). We will use in conjunction complementary numerical and theoretical models to study the plasticity of two-dimensional foams and compare it with the plasticity of atomic solids, either crystalline or amorphous.
SCIENTIFIC ENVIRONMENT:
The recruited post-doc will benefit from a world-class interdisciplinary environment, both within the MSC and LSPM labs which developed expertise in numerical and theoretical modelling of heterogeneous media.
SKILLS:
We seek motivated researchers, with theoretical and computational expertise. Candidates should have a background in computer simulation, statistical mechanics, or condensed matter. Postdoctoral position is for a period of one year, with estimated starting date January 2019.
CONTACT:
Interested candidates can apply by sending their CV (including publication list) and a short research statement (with plans and motivations) by email to marc.durand@univ-paris-diderot.fr with subject: "Application UNIPLAS".
Download the postdoctoral position opening description in english
A fundamental question in materials science is identifying the microscopic origin of plasticity. Why do materials display plastic rather than elastic response and can we predict when this will occur ? An improved understanding of plastic deformation is important in a wide range of amorphous materials, such as metallic and polymeric glasses, foams, granular materials, colloids, and emulsions.
In molecular materials with crystalline order, plastic behavior is understood in terms of defect nucleation and dynamics. For amorphous materials, a description in terms of topological defects is not possible due to the inherent structural disorder. Therefore identifying and characterizing local plastic events in amorphous materials is essential for a complete understanding of their structural and mechanical properties. Experimentally, the challenge is identifying systems for which the microscopic events are directly observable. This is one of the main advantages of mesoscopic systems, such as foams, and the reason for the interest in making connections between mesoscopic systems and molecular materials.
Foams are interesting as a model for atomic or molecular systems with tunable structural disorder (either quenched of evolving on large time scales) and interactions between units (from pairwise to many-body interactions). Foams are also a model for other soft cellular materials such as emulsions or confluent biological tissues: these systems are constituted of highly deformable (yet almost incompressible) units (bubbles, drops, cells,... ). Interfacial energy is the key ingredient in the cohesion and the rigidity of such systems, often constituted solely of fluids.
The proposal aims to bring a unified approach in the understanding of plastic behavior, for both molecular and cellular materials, with various degrees of disorder (from crystalline to amorphous). We will use in conjunction complementary numerical and theoretical models to study the plasticity of two-dimensional foams and compare it with the plasticity of atomic solids, either crystalline or amorphous.
SCIENTIFIC ENVIRONMENT:
The recruited post-doc will benefit from a world-class interdisciplinary environment, both within the MSC and LSPM labs which developed expertise in numerical and theoretical modelling of heterogeneous media.
SKILLS:
We seek motivated researchers, with theoretical and computational expertise. Candidates should have a background in computer simulation, statistical mechanics, or condensed matter. Postdoctoral position is for a period of one year, with estimated starting date January 2019.
CONTACT:
Interested candidates can apply by sending their CV (including publication list) and a short research statement (with plans and motivations) by email to marc.durand@univ-paris-diderot.fr with subject: "Application UNIPLAS".
Download the postdoctoral position opening description in english
M2 internship and funded PhD thesis (2018):
Morphogenesis of the vascular network of a biological model (P. Polycephalum):
Growth, self-organization, and optimization
Example of P. Polycephalum in its plasmodium stage.
Like E. Coli, C. Elegans or Drosophila, Physarum Polycephalum is a model organism intensively studied by the scientific community. In its vegetative phase, called plasmodium, this organism is made of thousands of undifferentiated cells fusing in a single, multinuclear cell, and can reach macroscopic sizes (dozens of cm²). This organism then develops a tubular network in which oscillatory flows (with period ~ 1 minute) are generated by the contraction of the membranous layer surrounding the “veins”. The role of this transport network is to supply the diffusive flows which are too slow to efficiently transport oxygen, nutrients, or waste through the whole cell. Physarum Polycephalum is likely the simplest organism with a vascular network.
The second function of this network is to efficiently transport the body mass during the growing stage or displacement of the cell. This requires from this organism, deprived from central nervous system, a local control of the sol-gel transition of the cytoplasm, and a synchronization of the contractile activity in the whole organism in order to generate a contractile wave traveling from one side of the cell to the other.
In spite of its apparent simplicity, the growth of the tubular network shares common features with the development of vascular systems in higher organisms, or with the mechanisms that take place in the irrigation of tumors. In particular, one can clearly identify two stages in the development of the plasmodium: a growing phase during which P. Polycephalum explores its environment covering all the plane with a very dense and ramified tubular network. Then a reorganizing phase during which the organism seems to follow an optimization scheme: the network is less and less reticulated. Eventually, the organism is then simply reduced to a few tubular elements directly connecting the different sources of food. Besides, separate plasmodia of same strain can merge together to create a larger plasmodium. Their networks connect to each other, allowing to exchange cytoplasm, nuclei, vesicles, etc.
From a physicist’s perspective, the plasmodium is an active gel that is able, at short times, to generate and adapt contractile waves along the veins to generate peristaltic flows, and at long times to control actively its sol-gel transition to modify the network architecture.
During this internship, we will study the coupling between the contractile activity and the formation of the tubular network in the early stage of the plasmodium development, called microplasmodium. At this stage (size~200 µm), the plasmodium can be seen as an active drop with an erratic contractile activity at the beginning. This activity generates local heterogeneities of shear strain, leading to sol-gel transition locally within the cytoplasm, which in turn will affect the local contractile activity of the microplasmodium. Progressively, a primitive network of flowing material emerges within the gel.
In practice, we will measure simultaneously the thickness field of the microsplasmodium using transmitted light imaging, and the velocity field within the microplasmodium using standard velocimetry techniques, in order to highlight the feedback role of the flow in the contractile activity and the synchronization phenomena. We will also study how the addition of inhibitors of contractile activity affect the network development.
At long term (PhD), we will identify the mechanisms involved in the network formation and evolution during the reorganization and optimization of the network, and during the merging of the networks of two plasmodia in contact.
Download the thesis/internship opportunity in english
The second function of this network is to efficiently transport the body mass during the growing stage or displacement of the cell. This requires from this organism, deprived from central nervous system, a local control of the sol-gel transition of the cytoplasm, and a synchronization of the contractile activity in the whole organism in order to generate a contractile wave traveling from one side of the cell to the other.
In spite of its apparent simplicity, the growth of the tubular network shares common features with the development of vascular systems in higher organisms, or with the mechanisms that take place in the irrigation of tumors. In particular, one can clearly identify two stages in the development of the plasmodium: a growing phase during which P. Polycephalum explores its environment covering all the plane with a very dense and ramified tubular network. Then a reorganizing phase during which the organism seems to follow an optimization scheme: the network is less and less reticulated. Eventually, the organism is then simply reduced to a few tubular elements directly connecting the different sources of food. Besides, separate plasmodia of same strain can merge together to create a larger plasmodium. Their networks connect to each other, allowing to exchange cytoplasm, nuclei, vesicles, etc.
From a physicist’s perspective, the plasmodium is an active gel that is able, at short times, to generate and adapt contractile waves along the veins to generate peristaltic flows, and at long times to control actively its sol-gel transition to modify the network architecture.
During this internship, we will study the coupling between the contractile activity and the formation of the tubular network in the early stage of the plasmodium development, called microplasmodium. At this stage (size~200 µm), the plasmodium can be seen as an active drop with an erratic contractile activity at the beginning. This activity generates local heterogeneities of shear strain, leading to sol-gel transition locally within the cytoplasm, which in turn will affect the local contractile activity of the microplasmodium. Progressively, a primitive network of flowing material emerges within the gel.
In practice, we will measure simultaneously the thickness field of the microsplasmodium using transmitted light imaging, and the velocity field within the microplasmodium using standard velocimetry techniques, in order to highlight the feedback role of the flow in the contractile activity and the synchronization phenomena. We will also study how the addition of inhibitors of contractile activity affect the network development.
At long term (PhD), we will identify the mechanisms involved in the network formation and evolution during the reorganization and optimization of the network, and during the merging of the networks of two plasmodia in contact.
Download the thesis/internship opportunity in english
Thesis / Internship Opportunity (2018):
Uncovering shuffling mechanisms
Figure 1: Example of cellular pattern obtained using extended Potts model. Here, every color represents a different domain.
During at least four centuries, cellular patterns ranging from foams to biological tissues have attracted scientists and artists. The fascination they exert lies in the balance between small-scale regularities and global disorder. Mathematical quantitative descriptions of these patterns have alternatively emphasised the cell size, curvatures and angles (geometry) or the cell number of sides and neighbour relationships (topology), trying to establish correlations.
We propose to use statistical physics to investigate physical mechanisms underlying the observed disorders. We will build upon our two recent successes. First, analytically, within a mean field approximation (top-down approach), we have reproduced without any adjusting parameter the correlation between cell size and number of sides in a shuffled foam [1]. Second, numerically, we have developed a computer simulation (bottom-up approach) of shuffled cellular patterns which is now both realistic (see Figures 1 and 2) and efficient [2]. By combining both approaches, we are now able to investigate in detail the correlations in such systems.
We propose to use statistical physics to investigate physical mechanisms underlying the observed disorders. We will build upon our two recent successes. First, analytically, within a mean field approximation (top-down approach), we have reproduced without any adjusting parameter the correlation between cell size and number of sides in a shuffled foam [1]. Second, numerically, we have developed a computer simulation (bottom-up approach) of shuffled cellular patterns which is now both realistic (see Figures 1 and 2) and efficient [2]. By combining both approaches, we are now able to investigate in detail the correlations in such systems.
Figure 2: Mixing large and small cells. From left to right: an increasing ratio of large to small size yields an increasing disorder. Colors label the cell side numbers: yellow: 6-sided domains; green: 5-sided domains; light gray: 7-sided domains; dark gray: 8-sided domains.