Publications

*Natsuhiko Yoshinaga and Tanniemola B. Liverpool,
From hydrodynamic lubrication to many-body interactions in dense suspensions of active swimmers,
European Physical Journal E 41, 76 (2018).

[Summary] We study how hydrodynamic interactions affect the collective behaviour of active particles suspended in a fluid at high concentrations, with particular attention to lubrication forces which appear when the particles are very close to one another.We compute exactly the limiting behaviour of the hydrodynamic interactions between two spherical (circular) active swimmers in very close proximity to one another in the general setting in both three and (two) dimensions. Combining this with far-field interactions, we develop a novel numerical scheme which allows us to study the collective behaviour of large numbers of active particles with accurate hydrodynamic interactions when close to one another. We study active swimmers whose intrinsic flow fields are characterised by force dipoles and quadrupoles. Using this scheme, we are able to show that lubrication forces when the particles are very close to each other can play as important a role as long-range hydrodynamic interactions in determining their many-body behaviour. We find that when the swimmer force dipole is large, finite clusters and open gel-like clusters appear rather than complete phase separation. This suppression is due to near-field lubrication interactions. For swimmers with smallforce dipoles, we find surprisingly that a globally polar-ordered phase appears because near-field lubrication rather than long-range hydrodynamics dominates the alignment mechanism. Polar order is present for very large system sizes and is stable to fluctuations with a finite noise amplitude. We explain the emergence of polar order using a minimal model in which only the leading rotational effect of the near-field interaction is included. These phenomena are also reproduced in two dimensions.

*Hiroyuki Kitahata and Natsuhiko Yoshinaga,
Effective diffusion coefficient including the Marangoni effect,
Journal of Chemical Physics 148, 134906 (2018).

[Summary] Surface-active molecules supplied from a particle fixed at the water surface create a spatial gradient of the molecule concentration, resulting in Marangoni convection. Convective flow transports the molecules far from the particle, enhancing diffusion. We analytically derive the effective diffusion coefficient associated with the Marangoni convection rolls. The resulting estimated effective diffusion coefficient is consistent with our numerical results and the apparent diffusion coefficient measured in experiments.

Kyongwan Kim, Natsuhiko Yoshinaga, Sanjib Bhattacharyya, Hikaru Nakazawa, Mitsuo Umetsu, and *Winfried Teizer,
Large-scale chirality in an active layer of microtubules and kinesin motor proteins,
Soft Matter 14, 3221-3231 (2018).

[Summary] During the early developmental process of organisms, the formation of the left-right laterality requires a subtle mechanism, as it is associated with other principal body axes. Any inherent chiral feature in an egg cell can in principal trigger this spontaneous breaking of chiral symmetry. Individual microtubules, major cytoskeletal filaments, are known as chiral objects. However, to date there lacks convincing evidence of a hierarchical connection of the molecular nature of microtubules to large-scale chirality, particularly at the length scale of an entire cell. Here we assemble an in-vitro active layer, consisting of microtubules and kinesin motor proteins, on a glass surface. Upon inclusion of methyl cellulose, the layered system exhibits a long-range active nematic phase, characterized by the global alignment of gliding MTs. This nematic order spans over the entire system size in the millimeter range and, remarkably, allows hidden collective chirality to emerge as counterclockwise global rotation of the active MT layer. The analysis based on our theoretical model suggests that the emerging global nematic order results from the local alignment of MTs, stabilized by methyl cellulose. It also suggests that the global rotation arises from the MTs’ intrinsic curvature, leading to preferential handedness. Given its flexibility, this layered in-vitro cytoskeletal system enables the study of membranous protein behavior responsible for important cellular developmental processes.

＊Natsuhiko Yoshinaga and Tanniemola B. Liverpool,
Hydrodynamic interactions in dense active suspensions: From polar order to dynamical clusters,
Physical Review E Rapid Communications 96, 020603(R) (2017).

[Summary] We study the role of hydrodynamic interactions in the collective behavior of collections of microscopic activeparticles suspended in a fluid. We introduce a calculational framework that allows us to separate the differentcontributions to their collective dynamics from hydrodynamic interactions on different length scales. Hence weare able to systematically show that lubrication forces when the particles are very close to each other play asimportant a role as long-range hydrodynamic interactions in determining their many-body behavior.We find thatmotility-induced phase separation is suppressed by near-field interactions, leading to open gel-like clusters ratherthan dense clusters. Interestingly, we find a globally polar ordered phase appears for neutral swimmers with noforce dipole that is enhanced by near-field lubrication forces in which the collision process rather than long-rangeinteraction dominates the alignment mechanism.

Shunsuke Yabunaka, Natsuhiko Yoshinaga,
Collision between chemically-driven self-propelled drops,
Journal of Fluid Mechanics 809, 205-233 (2016).

[Summary] We use analytical and numerical approaches to investigate head-on collisions between two self-propelled drops described as a phase separated binary mixture. Each drop is driven by chemical reactions that isotropically produce or consume the concentration of a third chemical component, which affects the surface tension of the drop. The isotropic distribution of the concentration field is destabilized by motion of the drop, which is created by the Marangoni flow from the concentration-dependent surface tension. This symmetry-breaking self-propulsion is distinct from other self-propulsion mechanisms due to its intrinsic polarity of squirmers and self-phoretic motion; there is a bifurcation point below which the drop is stationary and above which it moves spontaneously. When two drops are moving in the opposite direction along the same axis, their interactions arise from hydrodynamics and concentration overlap. We found that two drops exhibit either an elastic collision or fusion, depending on the distance from their bifurcation point, which may be controlled, for example, by viscosity. An elastic collision occurs when there is a balance between dissipation and the injection of energy by chemical reactions. We derive the reduced equations for the collision between two drops and analyse the contributions from the two interactions. The concentration-mediated interaction is found to dominate the hydrodynamic interaction for a head-on collision.