Publications

Keita Saito, Shogo Ookubo, and Yasuyuki Kimura*,
Change in collective motion of colloidal particles driven by an optical vortex with driving force and spatial confinement,
Soft Matter 14, 6037-6042 (2018).

[Summary] We studied the change in collective behavior of optically driven colloidal particles on a circular path. The particles are simultaneously driven by the orbital angular momentum of an optical vortex beam generated by holographic optical tweezers. The driving force is controlled by the topological charge l of the vortex. By varying the driving force and spatial confinement, four characteristic collective motions were observed. The collective behavior results from the interplay between the optical interaction, hydrodynamic interaction and spatial confinement. Varying the topological charge of an optical vortex not only induces changes in driving force but also alters the stability of three-dimensional optical trapping. The switch between dynamic clustering and stable clustering was observed in this manner. Decreasing the cell thickness diminishes the velocity of the respective particles and increases the spatial confinement. A jamming-like characteristic collective motion appears when the thickness is small and the topological charge is large. In this regime, a ring of equally-spaced doublets was spontaneously formed in systems composed of an even number of particles.

Mariko Suga, Saori Suda, Masatoshi Ichikawa, and *Yasuyuki Kimura,
Self-propelled motion switching in nematic liquid crystal droplets in aqueous surfactant solutions,
Physical Review E 97, 062703/1-8 (2018).

[Summary] The self-propelled motions of micron-sized nematic liquid crystal droplets in an aqueous surfactant solution have been studied by tracking individual droplets over long time periods. Switching between self-propelled modes is observed as the droplet size decreases at a nearly constant dissolution rate: from random to helical and then straight motion. The velocity of the droplet decreases with its size for straight and helical motions but is independent of size for random motion. The switching between helical and straight motions is found to be governed by the self-propelled velocity, and is confirmed by experiments at various surfactant concentrations. The helical motion appears along with a shifting of a point defect from the self-propelled direction of the droplet. The critical velocity for this shift of the defect position is found to be related with the Ericksen number, which is defined by the ratio of the viscous and elastic stresses. In a thin cell whose thickness is smaller than that of the initial droplet size, the droplets show more complex trajectories, including “figure-8s” and zigzags. The appearance of those characteristic motions is attributed to autochemotaxis of the droplet.

Kenji Nishizawa, Kei Fujiwara, Masahiro Ikenaga, Nobushige Nakajo, Miho Yanagisawa, *Daisuke Mizuno,
Universal glass-forming behavior of in vitro and living cytoplasm,
Scientific Reports 7, 15143 (2017).

Kenji Nishizawa, Marcel Bremerich, Heev Ayade, Christoph F. Schmidt, Takayuki Ariga, Daisuke Mizuno*,
Feedback-tracking microrheology in living cells,
SCIENCE ADVANCES 3, e1700318 (2017).

*Yasuyuki Kimura,
Hydrodynamically Induced Collective Motion of Optically Driven Colloidal Particles on a Circular Path,
Journal of the Physical Society of Japan 86, 101003/1-7 (2017).

[Summary] Among typical active matter such as self-propelled micro-objects, the characteristic collective motion originating from the hydrodynamic interaction between constituents has been observed in both biological and artificial systems. In illustrating such motion of micrometer-size particles in a one-dimensional optically driven system with a low Reynolds number, we highlight the importance of the hydrodynamic interaction. We show the appearance of regular stationary and dynamic arrangements resembling “crystals” or “clusters” observed in the equilibrium state. A transition in the collective motion has been observed by varying the hydrodynamic interaction in a system of two particle sizes and in a spatially confined system. An optical manipulation technique and the related hydrodynamic equations are also discussed.

Takashi Kurihara, Msato Aridome, Heev Ayade, Irwin Zaid, *Daisuke Mizuno,
Non-Gaussian limit fluctuations in active swimmer suspensions,
Physical Review E 95, 030601(R) (2017).

Tomohiro G. Noguchi, *Yasutaka Iwashita, and Yasuyuki Kimura,
Dependence of the Internal Structure on Water/Particle Volume Ratio in an Amphiphilic Janus Particle–Water–Oil Ternary System: From Micelle-like Clusters to Emulsions of Spherical Droplets,
Langmuir 33, 1030-1036 (2017).

[Summary] Amphiphilic Janus particles (AJP), composed of hydrophilic and hydrophobic hemispheres, are one of the simplest anisotropic colloids, and they exhibit higher surface activities than particles with homogeneous surface properties. Consequently, a ternary system of AJP, water, and oil can form extremely stable Pickering emulsions, with internal structures that depend on the Janus structure of the particles and the system composition. However, the detail of these structures has not been fully explored, especially for the composition range where the amount of the minority liquid phase and AJP are comparable, where one would expect the Janus characteristics to be directly reflected. In this study, we varied the volume ratio of the particles and the minority liquid phase, water, by 2 orders of magnitude around the comparable composition range, and observed the resultant structures at the resolution of the individual particle dimensions by optical microscopy. When the volume ratio of water is smaller than that of the Janus particles, capillary interactions between the hydrophilic hemispheres of the particles induce micelle-like clusters in which the hydrophilic sides of the particles face inward. With increasing water content, these clusters grow into a rodlike morphology. When the water volume exceeds that of the particles, the structure transforms into an emulsion state composed of spherical droplets, colloidosomes, because of the surface activity of particles at the liquid–liquid interface. Thus, we found that a change in volume fraction alters the mechanism of structure formation in the ternary system, and large resulting morphological changes in the self-assembled structures reflect the anisotropy of the particles.

Yuta Tamura and *Yasuyuki Kimura,
Two-dimensional assemblies of nematic colloids in homeotropic cells and their response to electric fields,
Soft Matter 12, 6817-6826 (2016).

[Summary] Micrometer-sized colloidal particles dispersed in nematic liquid crystals interact with each other throughanisotropic interactions induced by orientational deformation of the nematic field. In the case of so-calleddipole nematic colloids, their interaction is of the dipole–dipole type. Two-dimensional, non-close-packedcolloidal assemblies having various characteristics were fabricated using optical tweezers by exploitingthe attraction between anti-parallel dipole nematic colloids in homeotropically aligned nematic cells.Structures comprising polygons, squares, and tetrahedra were built using equal-sized particles, andhexagonal structures were built using particles of two sizes. As the nematic field is sensitive to electricfields, the response of the fabricated assemblies toward an alternating electric field was also studied.All assemblies exhibited homogeneous reversible shrinkage, and their shrinkage rates were dependenton the structure. The maximum shrinkage rate in the linear dimension of the assemblies was over 20%at 5 Vrms for a hexagon comprising tetrahedral units.

Irwin Zaid and *Daisuke Mizuno,
Analytical Limit Distributions from Random Power-Law Interactions,
Physical Review Letters 117, 030602 (2016).

[Summary] Nature is full of power-law interactions, e.g., gravity, electrostatics, and hydrodynamics. When sources of such fields are randomly distributed in space, the superposed interaction, which is what we observe, is naively expected to follow a Gauss or Lévy distribution. Here, we present an analytic expression for the actual distributions that converge to novel limits that are in between these already-known limit distributions, depending on physical parameters, such as the concentration of field sources and the size of the probe used to measure the interactions. By comparing with numerical simulations, the origin of non-Gauss and non-Lévy distributions are theoretically articulated.

Yuta Tamura and *Yasuyuki Kimura,
Fabrication of ring assemblies of nematic colloids and their electric response,
Applied Physics Letters 108, 011903 (2016).

[Summary] Colloidal particles with a limited number of interactive sites are called colloidal molecules, andtheir assemblies have been intensively studied to reveal complex micro-structures. In this study, weexamine colloidal particles in nematic liquid crystals, so-called nematic colloids, as colloidal moleculesand fabricated some non-close-packed assemblies. Micrometer-sized particles with homeotropicsurface anchoring of liquid crystal in a homeotropic cell interact with each other throughdipolar-type anisotropic interactions arising from the elastic deformation of the nematic fieldaround the particles. Using optical tweezers, we have built two-dimensional colloidal assemblieswith low packing densities, including polygon-rings, chains of polygon-rings, and lattices composedof octagon-rings in a hierarchical way from smaller structure units. Because the nematic fieldis sensitive to the electric field, the response of the polygon-rings to an alternative electric field hasbeen studied. They exhibited homogeneous reversible shrink as large as 15%–22% to their originalsizes under several volts.

Shogo Okubo, Shuhei Shibata, Yuriko Sasa Kawamura, Masatoshi Ichikawa and *Yasuyuki Kimura,
Dynamic clustering of driven colloidal particles on a circular path,
Physical Review E 92, 032303 (2015).

[Summary] We studied the collective motion of particles forced to move along a circular path in water by utilizing anoptical vortex. Their collective motion, including the spontaneous formation of clusters and their dissociation,was observed. The observed temporal patterns depend on the number of particles on the path and the variationof their sizes. The addition of particles with different sizes suppresses the dynamic formation and dissociation ofclusters and promotes the formation of specific stationary clusters. These experimental findings are reproducedby numerical simulations that take into account the hydrodynamic interaction between the particles and the radialtrapping force confining the particles to the circular path. A transition between stationary and nonstationaryclustering of the particles was observed by varying their size ratio in the binary-size systems. Our simulationreveals that the transition can be either continuous or discontinuous depending on the number of different-sizeparticles. This result suggests that the size distribution of particles has a significant effect on the collectivebehavior of self-propelled particles in viscous fluids.

Kentaro Takahashi and Yasuyuki Kimura,
Dynamics of colloidal particles in electrohydrodynamic convection of nematic liquid crystal,
Physical Review E 90, 012502/1-5 (2014).

[Summary] We have studied the dynamics of micrometer-sized colloidal particles in electrohydrodynamic convection of nematic liquid crystal. Above the onset voltage of electroconvection, the parallel array of convection rolls appears to be perpendicular to the nematic field at first. The particles are forced to rotate by convection flow and are trapped within a single roll in this voltage regime. The frequency of rotational motion increases with the applied voltage. Under a much larger voltage where the roll axis temporally fluctuates, the particles occasionally hop to the neighbor rolls. The motion perpendicular to the roll axis exhibits diffusion behavior at a long time period. The effective diffusion constant is 10^3–10^4 times larger than the molecular one. The observed behavior is compared with the result obtained by a simple stochastic model for the transport of the particles in convection. The enhancement of diffusion can be quantitatively described well by the rotation frequency in a roll, the width of the roll, and the hopping probability to the neighbor rolls.

Sayuri Tanaka, Yuma Oki, and Yasuyuki Kimura*,
Melting process of a single finite-sized two-dimensional colloidal crystal,
Physical Review E 89, 052305/1-9 (2014).

[Summary] We have studied the melting process of a finite-sized two-dimensional colloidal crystal by video microscopy. The local area fraction φ and the local hexatic orientational order parameter ψ6 have been evaluated for respective Voronoi cells in the crystal. The histogram of φ exhibits a peak and the peak φ continuously decreases with the time elapsed. The histogram of |ψ6| shows an abrupt broadening for φ < 0.65. This critical value of φ is the transition point between the hexatic and dense liquid phases in finite crystal. We have also evaluated φ and|ψ6| as a function of the distance from the center of the crystal r. φ(r) is almost constant within the crystal and monotonously decreases with the time elapsed. |ψ6(r)| gradually decreases with r but there is the core with |ψ6|=1 at earlier time stage. We have also studied the melting of a finite-sized crystal composed of soft-core particles by Brownian dynamics simulation and verified the finite-size effect on the melting process. The simulated behavior is qualitatively in good agreement with the experimental results.

David A. Head, Emi Ikebe, Akiko Nakamasu, Peijuan Zhang, Lara Gay Villaruz, Suguru Kinoshita, Shoji Ando, and *Daisuke Mizuno,
High-frequency affine mechanics and nonaffine relaxation in a model cytoskeleton,
Physical Review E 89, 042711/1-5 (2014).

[Summary] The cytoskeleton is a network of crosslinked, semiflexible filaments, and it has been suggested that it has properties of a glassy state. Here we employ optical-trap-based microrheology to apply forces to a modelcytoskeleton and measure the high-bandwidth response at an anterior point. Simulating the highly nonlinear andanisotropic stress-strain propagation assuming affinity, we found that theoretical predictions for the quasistaticresponse of semiflexible polymers are only realized at high frequencies inaccessible to conventional rheometers.We give a theoretical basis for determining the frequency when both affinity and quasistaticity are valid, and we discuss with experimental evidence that the relaxations at lower frequencies can be characterized by the experimentally obtained nonaffinity parameter.

Kuniyoshi Izaki and *Yasuyuki Kimura,
Hydrodynamic effects in the measurement of interparticle forces in nematic colloids,
Physical Review E 88, 54501 (2013).

[Summary] We propose improved measurement methods of interparticle force between nematic colloids. Although variousmethods have been utilized for the force measurement, the comparison between the forces obtained by differentmethods has not been reported. In the frequently used method called the “free-release” method, the hydrodynamicinteraction between moving particles has a serious influence on the measurement. In this study we modifiedthose measurement methods by taking the long-ranged hydrodynamic interaction into account. The evaluatedforces have been compared with that obtained by the dual beam “optical trap” method, which is free from thehydrodynamic effect. The agreement between them is quantitatively fairly good.

*Yasutaka Iwashita and Yasuyuki Kimura,
Stable cluster phase of Janus particles in two dimensions,
Soft Matter 9, 10694-10698 (2013).

[Summary] A Janus colloidal particle with an attractive hemisphere has three valences, i.e. it can establish three bonds on the hemisphere in two dimensions. With experiments and simulations, we study how the stable cluster phase of the colloids depends on this attraction. With weak attraction, small micellar clusters form, and with strong　attraction, these aggregate into linear chains; i.e., a micellar cluster behaves as a unit structure with two valences. Such hierarchical clustering plays a crucial role in the collective behavior of low-valence particles, in particular, for short-range interactions.

*David A. Head and Daisuke Mizuno,
Local mechanical response in semiflexible polymer networks subjected to an axisymmetric prestress,
Physical Review E 88, 022717/1-10 (2013).

[Summary] Analytical and numerical calculations are presented for the mechanical response of fiber networks in a stateof axisymmetric prestress, in the limit where geometric nonlinearities such as fiber rotation are negligible. Thisallows us to focus on the anisotropy deriving purely from the nonlinear force-extension curves of individualfibers. The number of independent elastic coefficients for isotropic, axisymmetric, and fully anisotropic networksare enumerated before deriving expressions for the response to a locally applied force that can be tested against, e.g., microrheology experiments. Localized forces can generate anisotropy away from the point of application, so numerical integration of nonlinear continuum equations is employed to determine the stress field, and induced mechanical anisotropy, at points located directly behind and in front of a force monopole. Results are presented for the wormlike chain model in normalized forms, allowing them to be easily mapped to a range of systems. Finally, the relevance of these findings to naturally occurring systems and directions for future investigation are discussed.