Research

Non-equilibrium soft matter physics, which I specialize in, is a major field of condensed matter physics at the interface between chemistry and biology. In this field, phase transitions from liquid to solid in a broad sense, such as glass transition and crystallization, and their rheology (response to external fields) are central issues. I have been working on structural and kinetic analyses, mainly using large-scale numerical methods such as molecular dynamics. In recent years, the non-equilibrium soft matter research field has grown into an interdisciplinary research field, where methods for studying the ordering of materials and their external field properties have been applied to quantum many-body systems such as many-body electrons and superconducting magnetic flux, and biological systems such as cellular tissues. Based on my research experience in non-equilibrium soft matter, I would like to contribute to the development of a wide range of research fields by carrying out highly universal research from the viewpoint of quantum systems to biological systems, as well as the further development of this field.

1. Discovery of medium-range order in supercooled liquids

In supercooled liquids near the glass transition point, fast and slow dynamical regions coexist, which is called dynamical heterogeneity. This property has attracted much attention as one of the candidates to explain the rapid increase in relaxation time during the glass transition, but we do not have satisfactory knowledge of its origin. On the other hand, most of the previous studies on the glass transition have neglected the effect of crystallization, which occurs universally in liquids. In this study, we performed molecular dynamics calculations to analyze the local structures of various undercooled liquids in detail by introducing bond orientation order variables. Contrary to the conventional wisdom, the static structure of many supercooled liquids is not homogeneous, and there is a localized medium-range order, which may be the origin of dynamic heterogeneity. This is analogous to the criticality phenomenon, which is important for understanding the physical origin of slow dynamics in glasses. In recent years, there have been many reports of the discovery of medium-range order in undercooled (overcompressed) liquids and glasses in experimental systems such as metallic glasses and colloidal dispersion systems.

• T. Kawasaki, T. Araki, and H. Tanaka, Phys. Rev. Lett. 99, 215701 (2007).
• T. Kawasaki and H. Tanaka, Phys. Rev. Lett. 102, 185701 (2009).
• H. Tanaka, T. Kawasaki, H. Shintani, and K. Watanabe, Nature Materials 9, 324 (2010).

2. Discovery of crystal precursors

It was found that many supercooled liquids have characteristic local structures and are not structurally homogeneous at the particle level. We have also investigated in detail the elementary processes involved in the crystallization of supercooled liquids in monodisperse rigid particle systems. In particular, the phase information from the spherical harmonic function was averaged for the first nearest neighbour particle, and the bond orientation order variables were analyzed. In contrast to the conventional wisdom that the order of crystal nuclei is determined solely by the progressive order of the liquid, we have discovered that in undercooled liquids there is a crystal precursor, which is an intermediate state between the liquid and the crystal, and that crystal nuclei are always formed through the crystal precursor. In the initial stage of nucleation, when the precursors are formed, only the bond orientation order grows without any increase in density (translational order). The translational order here is formed as the bond orientation order increases, and is not generated discontinuously from a disordered liquid state. This implies that the interfacial energy of the crystal is reduced when the precursor is taken into account, which is not taken into account in conventional classical nucleation theories that use the energy difference between the liquid and the crystal. In recent years, the existence of crystal precursors similar to those found by the applicants has been directly observed in experimental colloidal dispersion systems.

For more information, please refer to the following papers:

• T. Kawasaki and H. Tanaka, Proc. Natl. Acad. Sci. USA 107, 14036 (2010).

3. Investigation of anomalous transport phenomena in various supercooled liquids

It is known that the Stokes Einstein (SE) law, which holds for homogeneous simple liquids, is universally violated in undercooled liquids, and its relation to the glass transition has been widely discussed. However, its physical origin is poorly understood. In particular, one of the reasons for the delay is that the computational cost of the viscosity (the stress correlation function in the Green-Kubo formula) is extremely high in the undercooled state, and little progress has been made in theoretical studies using conventional computers. In order to solve this problem, we have succeeded in performing large-scale molecular dynamics calculations on a supercomputer in a very efficient manner, and have calculated the viscosity of water and silica in the undercooled state down to the cryogenic region. For the first time, we found that the time scale governing the viscosity is the structural relaxation time, which is characterized by the correlation function of the density, whereas the time scale characterizing the diffusion coefficient is the molecular bond-breaking time (hydrogen-bond breaking time for supercooled water, covalent bond breaking time for supercooled silica). In addition, these timescales were found to be the same as those of the covalent bond breaking times of the molecules. Furthermore, although these timescales tend to be lumped together as structural relaxation times, we found that the former is dominated by the contribution of “slow particles” due to dynamic heterogeneity, while the latter is dominated by the contribution of “fast particles”, and that the separation of these timescales is the origin of SE-side breaking. The results also indicate that such time-scale separation is the origin of the SE break. We have also confirmed that the same concept is applicable to metallic glasses and even to liquids consisting of rigid spheres, and we believe that it is highly universal and applicable to anisotropic particle systems.

For more information, please refer to the following papers:

• T. Kawasaki and K. Kim, Science Advances 3, e1700399 (2017).
• T. Kawasaki and A. Onuki, Phys. Rev. E 87, 012312 (2013).
• T. Kawasaki, K. Kim, and A. Onuki, J. Chem. Phys. 140, 184502 (2014).
• T. Kawasaki and K. Kim, Sci Rep 9, 8118 (2019).

4. Mechanism of nonlinear rheology in suspension systems

The flow curves (shear rate dependence of shear stress and viscosity) of suspensions of colloids and emulsions in solvents show a very complex non-linear behaviour of the viscosity with increasing shear rate. The understanding of shear thickening, in which the viscosity increases with shear rate, is still in its infancy. In this study, in order to clarify the physical origin of shear shear, we propose the simplest model to reproduce the flow curve of a suspension system (controlled for inertia effects, but also considering only tangential frictional forces and thermal oscillatory forces). By using these methods, we have succeeded in reproducing semi-quantitatively almost all the complex flow curves observed in experiments. We believe that this research will contribute to the fundamental understanding of the mechanism of the extremely complex flow characteristics observed in suspension systems, such as shear thickening.

For more details, please refer to the following papers.

• T. Kawasaki , A. Ikeda, and L. Berthier, EPL 107, 28009 (2014).
• T. Kawasaki and L. Berthier, Phys. Rev. E 98, 012609 (2018).

5. Precise measurement of the viscous divergence associated with the jamming transition and elucidation of the mechanism of the yielding transition

When relatively large particles (e.g., powders), which are not subject to thermal fluctuation, are packed in a box, viscous divergence and rigidity, similar to the glass transition, are observed. This is called the jamming transition, and is understood as a transition determined solely by the structural dynamics of the particles, unlike the glass transition, where the contribution of thermal fluctuations is important. In the vicinity of the Jamming transition density, however, critical behaviors that are nontrivial to the structure and dynamics of the particles are observed, and computer simulations play an important role in understanding these behaviors. We have succeeded in performing large-scale particle simulations efficiently under constant pressure, and in precisely measuring the critical exponents of viscous divergence and rigidity down to near the jamming transition.

For details, please refer to the following paper.

• T. Kawasaki , D. Coslovich, A. Ikeda, and L. Berthier, Phys. Rev. E 91, 012203 (2015).

6. Mechanism of various non-equilibrium phase transitions in amorphous solids

When the shear amplitude is increased for a system of particles under periodic shear, the particle orbits change from reversible to irreversible orbits. Particularly in dilute systems, a non-equilibrium phase transition (a kind of absorbing state transition) has been found in which the number of particles exhibiting irreversible orbits continuously increases with respect to the shear amplitude after a critical point. However, the behavior of this transition in high-density systems has been poorly understood. We tackled the above problem in high-density systems using molecular dynamics, and found a very wide range of absorbing state transitions near the jamming transition point, which are related to yielding phenomena and particle structure. It has been reported that the phenomena discovered here share many aspects with the dynamics of quantum vortex threads in Type 2 superconductors [M. Dobroka, et al., New J. Phys. 19, 053023 (2017)], and further spillover effects of the research are expected.

For more information, please refer to the following paper.

• T. Kawasaki and L. Berthier, Phys. Rev. E 94, 022615 (2016).
• K. Nagasawa, K. Miyazaki, and T. Kawasaki, Soft Matter 15, 7557 (2019).