Kuniyasu Saitoh

Associate Professor, Department of Physics, Kyoto Sangyo University *Profile is at the time of the award.

2021Inamori Research GrantsBiology & Life sciences

Research topics
Anomalous viscoelastic and elastoplastic properties of poly-dispersed particles and mathematical physics for their hierarchical structures of sizes
Soft particulate systems, e.g. emulsions, foams, colloidal suspensions, and granular materials, are ubiquitous in nature and a better understanding of their rheological and mechanical properties is crucial to engineering science. Generally, soft particles are macroscopic in size such that their motions are not affected by thermal fluctuations. Instead, they can dissipate kinetic energy by dissipative forces such as inelastic interactions and viscous damping. Moreover, structures of soft particle packings are mostly disordered so that their mechanical responses to global deformations are heterogeneous in space. Because of these athermal, dissipative, and heterogeneous nature, soft particulate systems have been the subject of non-equilibrium physics including statistical mechanics and soft matter physics. There have been many theoretical, numerical, and experimental studies of soft particulate systems. However, much less attention has been paid to “particle polydispersity”. For instance, size distributions of grains in seismic fault are given by power-law distribution functions which cause unexpectedly small resistance to shear and thus trigger serous earthquakes. This research aims to unveil the impact of particle polydispersity on the rheology and mechanics of soft particulate systems. By using molecular dynamics simulations, we try to reveal how the size distributions alter the macroscopic behavior of soft particles.


I believe that there are still many new approaches for research on soft particles.While conducting physics and numerical simulations, I look forward to interpreting these results from different perspectives and ideas every day.

Outline of Research Achievments

n this study, we have clarified how tangential forces between soft particles in contacts affect mechanical responses of the system to simple shear deformations. To demonstrate elastic responses, we introduced dynamical matrix of the particles, where each element is defined as second derivatives of elastic energy. Employing molecular dynamics (MD) simulations, we generated disordered configurations of the particles. We used the disordered configurations to calculate each element of the dynamical matrix. If the shear strain applied to the system is infinitesimal, shear modulus can be predicted by the eigenvalues and eigen-vectors of the dynamical matrix. We found that the shear modulus given by the dynamical matrix well agrees with numerical results of MD simulations and its dependence on packing fraction of the particles is significantly different from that of “frictionless” soft particles. We also confirmed that stress-strain curves in a steady state are well predicted by the dynamical matrix if the system does not exhibit slip avalanches. Furthermore, we have investigated statistics of the slip avalanches and found that the tangential forces or “friction” between the particles in contact is crucial to the scaling of avalanche size distributions.


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Biology & Life sciences