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A03 YOSHINAGA, Natsuhito |Proposed Research Projects (2016-2017)

Paper | Original Paper

2017

*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.

2016

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.