View count: 807

Abstract

Tetsuya Hiraiwa-Dynamic self-organization of active matter and living systems
Materials working under far-from-equilibrium conditions by consumption of chemical energy are generically called active matter. Living systems, as well as their elements like living cells and their cytoskeletons containing molecular motors, are representative of soft and active material systems. Emergence of dynamic structures and coherent dynamics as active matter is one of the key processes to achieve complex structures and functions for living organisms. For example, living cells exhibit collective migration and dynamic organization by relying on their complex intercellular communication. Studying such dynamic self-organization processes of living systems enables us to extend our knowledge of out-of-equilibrium and softmatter physics. In this presentation, firstly, I will explain the concept of collective motion of selfpropelled particles, where the constituent self-propelled particles are active matter elements which move according to its intrinsic polarity. Along this line, our studies on dynamic self-organization in an active material system, called gliding assay of microfilaments on molecular motors, will be introduced [1-3]. Secondly, I will address theoretically how contact communication between cells influence dynamic self-organization of migrating cells. For this purpose, by combining the collective motion concept to a model of single-cell migratory behavior [4,5], an individual cellbased model to study dynamics of migrating cells in population will be introduced [6-9]. It will be shown, by numerical simulations, that varieties of dynamic self-organization pattens are induced by two types of contact communications between migrating cells [8]. These studies may pave the way toward revealing a principle for living organisms to organize dynamic structures and ordered dynamics in their bodies as well as new design and control principles of artificial active material systems expressing desired dynamic emergent features.

[1] S. Tanida, …, T. Hiraiwa, …, M. Sano, Phys. Rev. E 101, 032607 (2020). [2] F. Afroze, D. Inoue, …, T. Hiraiwa, …, A. Kakugo. Biochem. Biophys. Res. Comm. 563, 73 (2021). [3] T. Hiraiwa, R. Akiyama, D. Inoue, A. Kabir, A. Kakugo. Phys. Chem. Chem. Phys. 24, 28782 (2022). [4] T. Hiraiwa, A. Nagamatsu, N. Akuzawa, M. Nishikawa, T. Shibata, Phys. Biol. 11, 056002 (2014). [5] T. Hiraiwa, A. Baba, T. Shibata, Euro. Phys. J. E 36, 32 (2013). [6] T. Hiraiwa, Phys. Rev. E 99, 012614 (2019). [7] M. Hayakawa, T. Hiraiwa, Y. Wada, H. Kuwayama, T. Shibata, eLife 9, e53609 (2020). [8] T. Hiraiwa, Phys. Rev. Lett. 125, 268104 (2020). [9] T. Hiraiwa, Euro. Phys. J. E (Topical Collection “Tissue mechanics”) 45, 16 (2022).

Guan-Rong Huang-Investigate the structural evolution of soft matter immersed in water solution using contrast-variation neutron scattering
The structural richness of soft matter solutions under different thermodynamic conditions has led to various applications in many fields, such as biomedicine and energy materials fabrication. To control and optimize new materials with desirable properties, understanding the physical mechanisms behind the structural changes and stability is inevitably essential. Generally, the soft matter structure is constantly balanced by the competition between the intra-molecular and inter-particle interaction, thermal entropy effect, and solute-solvent interaction.

In this lecture, I will introduce a general way to reveal critical structural information, including the density profile of invasive water profile and soft matter, molecular aggregation, and geometric shape, in response to different thermodynamic variables by using contrast-variation neutron scattering (CVNS). The fundamental principle of neutron scattering cross-section for various elements, such as Born approximation and Fermi pseudo potential, will be briefly lectured. And the corresponding data analysis of scattering spectra for interacting and non-interacting soft matter aggregates, such as micelles, emulsions, and colloids, will be mentioned. The advantages of neutron scattering compared to photon scattering for investigating soft matter systems will be simply explained. Two examples of our neutron scattering research, including charged surfactant aggregation with different salt concentrations and thermal responsive polymer with different temperatures, will serve as a demonstration for the validity of exploring the structural mechanism by CVNS. Finally, the analysis results will be explained based on the inter/intra-molecular interaction and thermal entropy effect.

Sheng-Ting Hung-A Photophysical Perspective on the Development of Fluorescent Proteins
Fluorescent proteins (FPs) have become an essential tool for life sciences owing to their ability to respond to light modulation and genetic encodability, in addition to other advantages of optical imaging methods such as quantitative, non-invasive and sensitive detection. Photophysics of FPs is not only the foundation of the development and applications of FPs, but also central to the development of many super-resolution microscopy techniques that break the diffraction limit in conventional light/fluorescence microscopy. I highlight some important progress in the development of FPs and focus the discussion on some important photophysics and challenges in FP development including brightness, photostability and photoswitching of the green-FP-like superfamily.

Chong-Wai Io-The collective motion of thermophoretic type active particle suspension: from open space to spatial-temporal modulated confinement
In general, active particle is an object which has the capability to convert internal chemical energy or ambient field energies into  mechanical energy for persistent movement. Examples are extended from molecular motors or bacteria at the microscale to the folk of  birds or fishes at meter scale. Nevertheless, there are some intrinsic difficulties of biological systems hindering the long-term steady-state statistical measurement and constructing a generic theory model such as: individual diversity (non-uniform propulsion strength), dead and birth (non-conserved particle number), nutrition supply (unnecessary background flow), and biological signaling, etc. The  hemisphere Au-coated silica particle is a kind of artificial active particle, in which the propulsion strength is controlled by the excitation laser intensity through thermophoretic force induced by the local absorption of the Au thin film. The suspension of such  particles with controllable self-propulsion strength is a good platform for investigating non-equilibrium physics. It bridges the  equilibrium Brownian motion to far-from-equilibrium heterogeneous motion by increasing the particle propulsion strength. In this course, we are going to investigate the collective behavior of this particle suspension from open space (without boundary effect) to static  spatial confinement, and then the spatial-temporal modulated confinement.
Keywords: active particle, thermophoretic force, micro-channel, spatial-temporal confinement

Fu-Lai Wen-Expansion of supracellular contractile rings induces cell sheet folding
The folding of epithelial cell sheets is a fundamental process that sculpts tissues and organs into their proper shapes required for normal physiological functions. In the absence of detailed biochemical regulations, the epithelial sheet folding may simply proceed through buckling due to mechanical compression arising extrinsically from the surroundings or intrinsically within the sheets. Previous studies hypothesized that formation of an expanding supracellular
actomyosin ring within epithelial sheets could result in compression that ultimately leads to epithelial folding during tracheal development in the Drosophila (fruit fly) embryo. However, the exact mechanism remains unclear. Using a vertex-based mechanical model, here I show that expansion of supracellular contractile rings can induce a buckling instability via lowering of the buckling threshold, leading to a robust fold formation against fluctuations in the ring properties such as ring numbers and tensions. In particular, while elevation of the contractile strength is not necessary for fold formation, the qualitative behavior of the folding process is found to depend on the contractile strength of rings. These findings provide fruitful insights into how expanding supracellular actomyosin rings in epithelial sheets mediate the entire sheet morphogenesis.
Keywords: buckling instability, morphogenesis, mechanobiology 

Yuan-Nan Young-The many behaviors of deformable active droplets
Active fluids consume fuel at the microscopic scale, converting this energy into forces that can drive macroscopic motions over scales far larger than their microscopic constituents. In some cases, the mechanisms that give rise to this phenomenon have been well characterized, and can explain experimentally observed behaviors in both bulk fluids and those confined in simple stationary geometries. More recently, active fluids have been encapsulated in viscous drops or elastic shells so as to interact with an outer environment or a deformable boundary. Such systems are not as well understood. In this work, we examine the behavior of droplets of an active nematic fluid. We study their linear stability about the isotropic equilibrium over a wide range of parameters, identifying regions in which different modes of instability dominate. Simulations of their full dynamics are used to identify their nonlinear behavior within each region. When a single mode dominates, the droplets behave simply: as rotors, swimmers, or extensors. When parameters are tuned so that multiple modes have nearly the same growth rate, a pantheon of modes appears, including zigzaggers, washing machines, wanderers, and pulsators. Finally we show how hydrodynamic interactions between active droplets lead to even more exotic collective behaviour, such as flocking and synchronization of many active droplets. This is a collaboration with Michael J. Shelley (NYU/FI) and David Stein (FI).