## A04 Proposed Research Projects (2014-2015)

### Paper | Original Paper

#### 2016

*John J. Molina, Kotaro Otomura, Hayato Shiba, Hideki Kobayashi, Masaki Sano, and Ryoichi Yamamoto,
Rheological evaluation of colloidal dispersions using the smoothed profile method: formulation and applications,
Journal of Fluid Mechanics 792, 590-619 (2016).

[Summary] The smoothed profile method is extended to study the rheological behaviour of colloidal dispersions under shear flow by using the Lees–Edwards boundary conditions. We start with a reformulation of the smoothed profile method, a direct numerical simulation method for colloidal dispersions, so that it can be used with the Lees–Edwards boundary condition, under steady or oscillatory-shear flow. By this reformulation, all the resultant physical quantities, including local and total shear stresses, become available through direct calculation. Three simple rheological simulations are then performed for (1) a spherical particle, (2) a rigid bead chain and (3) a collision of two spherical particles under shear flow. Quantitative validity of these simulations is examined by comparing the viscosity with that obtained from theory and Stokesian dynamics calculations. Finally, we consider the shear-thinning behaviour of concentrated colloidal dispersions.

Shugo Yasuda and Ryoichi Yamamoto,
Synchronized molecular-dynamics simulation for the thermal lubrication of a polymeric liquid between parallel plates,
Computers & Fluids 124, 185 (2016).

#### 2015

*Satoshi Yui, Kazuya Fujimoto, Makoto Tsubota ,
Logarithmic velocity profile of quantum turbulence of superfluid 4He,
Physical Review B 92, 224513/1-5 (2015).

[Summary] The logarithmic velocity profile is the most important statistical law of classical turbulence affected by channelwalls. This paper demonstrates numerically that the logarithmic velocity profile of a superfluid flow appears inquantum turbulence under pure normal flow in a channel. We investigated the configuration and dynamics of aninhomogeneous vortex tangle affected by the walls, and found the characteristic behavior of the log-law.

Michiel Laleman, Marco Baiesi, Boris P. Belotserkovskii, Takahiro Sakaue, Jean-Charles Walter,and *Enrico Carlon,
Torque-Induced Rotational Dynamics in Polymers: Torsional Blobs and Thinning,
Macromolecules 49, 405-414 (2015).

[Summary] By using the blob theory and computer simulations, we investigate the properties of a linear polymer performing a stationary rotational motion around a long impenetrable rod. In particular, in the simulations the rotation is induced by a torque applied to the end of the polymer that is tethered to the rod. Three different regimes are found, in close analogy with the case of polymers pulled by a constant force at one end. For low torques the polymer rotates maintaining its equilibrium conformation. At intermediate torques the polymer assumes a trumpet shape, being composed by blobs of increasing size. At even larger torques the polymer is partially wrapped around the rod. We derive several scaling relations between various quantities as angular velocity, elongation, and torque. The analytical predictions match the simulation data well. Interestingly, we find a “thinning” regime where the torque has a very weak (logarithmic) dependence on the angular velocity. We discuss the origin of this behavior, which has no counterpart in polymers pulled by an applied force.

Kristopher Erickson, Xiaowei He, A. Alec Talin, Bernice Mills, Robert H. Hauge, Takashi Iguchi, Naoki Fujimura, Yukio Kawano, *Junichiro Kono, *François Léonard,
Figure of Merit for Carbon Nanotube Photothermoelectric Detectors,
ACS Nano 9, 11618–11627 (2015).

[Summary] Carbon nanotubes (CNTs) have emerged as promising materials for visible, infrared, and terahertz photodetectors. Further development of these photodetectors requires a fundamental understanding of the mechanisms that govern their behavior as well as the establishment of figures of merit for technology applications. Recently, a number of CNT detectors have been shown to operate based on the photothermoelectric effect. Here we present a figure of merit for these detectors, which includes the properties of the material and the device. In addition, we use a suite of experimental characterization methods for the thorough analysis of the electrical, thermoelectric, electrothermal, and photothermal properties of the CNT thin-film devices. Our measurements determine the quantities that enter the figure of merit and allow us to establish a path toward future performance improvements.

*Takuya Saito, *Takahiro Sakaue,
Driven anomalous diffusion: An example from polymer stretching,
Physical Review E 92, 012601/1-13 (2015).

[Summary] The way tension propagates along a chain is a key to govern many anomalous dynamics in macromolecular systems. After introducing the weak and the strong force regimes of the tension propagation, we focus on the latter, in which the dynamical fluctuations of a segment in a long polymer during its stretching process is investigated. We show that the response, i.e., average drift, is anomalous, which is characterized by the nonlinear memory kernel, and its relation to the fluctuation is nontrivial. These features are discussed on the basis of the generalized Langevin equation, in which the role of the temporal change in spring constant due to the stress hardening is pinpointed. We carried out the molecular dynamics simulation, which supports our theory.

Chunyu Shih, John J. Molina, and Ryoichi Yamamoto,
Dynamic polarisation of a charged colloid in an oscillating electric field,
Molecular Physics 113, 2511 (2015).

C. Patrick Royall, Jens Eggers, *Akira Furukawa, and Hajime Tanaka,
Probing colloidal gels at multiple length scales: The role of hydrodynamics,
Physical Review Letters 114, 258302 (2015).

[Summary] Comparing experiments to numerical simulations, with hydrodynamic interactions switched off, we demonstrate the crucial role of the solvent for gelation. Hydrodynamic interactions suppress the formation of larger local equilibrium structures of closed geometry, and instead lead to the formation of highly anisotropic threads, which promote an open gel network. We confirm these results with simulations which include hydrodynamics. Based on three-point correlations, we propose a scale-resolved quantitative measure for the anisotropy of the gel structure. We find a strong discrepancy for interparticle distances just under twice the particle diameter between systems with and without hydrodynamics, quantifying the role of hydrodynamics from a structural point of view.

Gregory Lecrivain, Giacomo Petrucci, *Uwe Hampel and Ryoichi Yamamoto,
Attachment of solid elongated particles on the surface of a stationary gas bubble,
International Journal of Multiphase Flow 71, 83-93 (2015).

[Summary] The flotation process essentially relies on the attachment of solid particles on the surface of gas bubbles immersed inwater. The present study seeks to investigate the effect of the particle shape on the attachment mechanism. Using an in-house optical micro-bubble sensor the approach, the sliding and the adhesion of micron milled glass fibres on the surface of a stationary air bubble immersed in stagnant water is thoroughly investigated. The translational and rotational velocities were measured for fibres of various aspect ratios. The results are compared with a theoretical model and with experimental data obtained with spherical glass beads.

*Kazuya Fujimoto and Makoto Tsubota,
Bogoliubov-wave turbulence in Bose-Einstein condensates,
Physical Review A 91, 053620/1-12 (2015).

[Summary] We theoretically and numerically study Bogoliubov-wave turbulence in three-dimensional atomic Bose-Einstein condensates with the Gross-Pitaevskii equation, investigating three spectra for the macroscopic wavefunction, the density distribution, and the Bogoliubov-wave distribution. In this turbulence, Bogoliubov wavesplay an important role in the behavior of these spectra, so that we call it Bogoliubov-wave turbulence. In aprevious study [Proment et al., Phys. Rev. A 80, 051603(R) (2009)], a −3/2 power law in the spectrum forthe macroscopic wave function was suggested by using weak wave turbulence theory, but we find that another−7/2 power law appears in both theoretical and numerical calculations.

*Satoshi Yui and Makoto Tsubota,
Counterflow quantum turbulence of He-II in a square channel: numerical analysis with nonuniform flows of the normal fluid,
Physical Review B 91, 184504/1-12 (2015).

[Summary] We perform a numerical analysis of counterflow quantum turbulence of superfluid 4He with nonuniform flows by the vortex filament model. The resulting tangle of quantized vortices makes a characteristic nonlinear oscillation to form inhomogeneous quantum turbulence. We investigated the detail of the state and compare them with some typical experimental results.

Elisa Zemma, Makoto Tsubota, and Javier Luzuriaga,
Possible Visualization of a Superfluid Vortex Loop Attached to an Oscillating Beam,
Journal of Low Temperature Physics 179, 310-319 (2015).

[Summary] Visualization using tracer particles is a relatively new tool available for thestudy of superfluid turbulence and flow, which is applied here to oscillating objectssubmerged in the liquid. We report observations of a structure seen in videos takenfrom outside a cryostat filled with superfluid helium at 2 K, which is possibly a vortexloop attached to an oscillator. The feature, which has the shape of an incomplete arch,is visualized due to the presence of solid H2 tracer particles and is attached to a beamoscillating at 38 Hz in the liquid. It has been recorded in videos taken at 240 frames persecond, fast enough to take ∼6 images per period. This makes it possible to follow thestructure, and to see that it is not rigid. It moves with respect to the oscillator, and itsdisplacement is in phase with the velocity of the moving beam. Analyzing the motion,we come to the conclusion that we may be observing a superfluid vortex attached tothe beam and decorated by the hydrogen particles. An alternative model, consideringa solid hydrogen filament, has also been analyzed, but the observed phase betweenthe movement of the beam and the filamentary structure is better explained by thesuperfluid vortex hypothesis.

Andrew J. Dunleavy, Karoline Wiesner, Ryoichi Yamamoto, and *C. Patrick Royall,
Mutual information reveals multiple structural relaxation mechanisms in a model glassformer,
Nature Comm. 6, 6089/1-8 (2015).

[Summary] Among the key challenges to our understanding of solidification in the glass transition is thatit is accompanied by little apparent change in structure. Recently, geometric motifs have beenidentified in glassy liquids, but a causal link between these motifs and solidification remainselusive. One ‘smoking gun’ for such a link would be identical scaling of structural and dynamiclengthscales on approaching the glass transition, but this is highly controversial. Here weintroduce an information theoretic approach to determine correlations in displacement forparticle relaxation encoded in the initial configuration of a glass-forming liquid. We uncovertwo populations of particles, one inclined to relax quickly, the other slowly. Each population iscorrelated with local density and geometric motifs. Our analysis further reveals a dynamiclengthscale similar to that associated with structural properties, which may resolve thediscrepancy between structural and dynamic lengthscales.

#### 2014

Ahmed Khorshid, Philip Zimny, David Tetreault-La Roche, Geremia Massarelli, *Takahiro Sakaue, and *Walter Reisner,
Dynamic Compression of Single Nanochannel Confined DNA via a Nanodozer Assay,
Physical Review Letters 113, 268104:1-5 (2014).

[Summary] We show that a single DNA molecule confined and extended in a nanochannel can be dynamically compressed by sliding a permeable gasket at a fixed velocity relative to the　stationary polymer. The gasket is realized experimentally by optically trapping a nanosphere inside a nanochannel. The trapped bead acts like a nanodozer," directly applying compressive forces to the molecule without requirement of chemical attachment. Remarkably, these strongly non-equilibrium measurements can be quantified via a simple nonlinear convective-diffusion formalism and yield insights into the local blob statistics, allowing us to conclude that the compressed nanochannel-confined chain exhibits mean-field behaviour.

*Miho Yanagisawa, Shinpei Nigorikawa, Takahiro Sakaue, Kei Fujiwara, and Masayuki Tokita,
Multiple patterns of polymer gels in microspheres due to the interplay among phase separation, wetting, and gelation,
Proc. Natl. Acad. Sci. USA 111, 15894-15899 (2014).

[Summary] We investigate how microdroplet confinement affects pattern formation of a polymer blend in the liquid-and-gel coexisting phase, wherein interactions between the droplet surface and the polymers regulate wettability of the gelation polymer. The complete and partial wetting of the polymers produces two stable states: hollow microspheres and hemisphere microgels. In addition, gelation during phase separation produces various shapes as trapped states. The relation between capsule thickness and droplet size is changed by the dynamical coupling. Furthermore, multiple patterns with spherical asymmetric shapes are produced by the partial wetting and shape deformation along the phase boundaries between the sol/gel phases. These findings reveal a complex pattern formation arising from the interplay among the interfacial tensions, gel elasticity, and wetting in microspheres.

Shugo Yasuda and Ryoichi Yamamoto,
Synchronized molecular dynamics simulation via macroscopic heat and momentum transfer: an application to polymer lubrication,
Phys. Rev. X 4, 041011/1-10 (2014).

[Summary] A synchronized molecular-dynamics simulation via macroscopic heat and momentum transfer isproposed to model the nonisothermal flow behaviors of complex fluids. In this method, the moleculardynamicssimulations are assigned to small fluid elements to calculate the local stresses and temperaturesand are synchronized at certain time intervals to satisfy the macroscopic heat- and momentum-transportequations. This method is applied to the lubrication of a polymeric liquid composed of short chains of tenbeads between parallel plates.

Adnan Hamid, John J. Molina, and Ryoichi Yamamoto,
Direct Numerical Simulations of Sedimenting Spherical Particles at Finite Reynolds Number,

[Summary] We performed direct numerical simulations to investigate the inertial effects on the static and dynamic properties of a sedimenting suspension over a wide range of volume fractions from 0.01 to 0.4. The microstructure analysis at the high Re revealed that at Re=1 inertial forces have significant effects and these create a deficiency of particles around a given particle, which is more pronounced in the direction of gravity than in the perpendicular direction. Moreover, at Re=10, strong inertial forces generated a significant deficit of particles in both directions, which decreased velocity fluctuations and particle diffusion in both directions.

Shugo Yasuda and Ryoichi Yamamoto,
Multiscale simulation for thermo-hydrodynamic lubrication of a polymeric liquid between parallel plates,
Molecular Simulation 41, 1002 (2014).

Akira Furukawa, Davide Marenduzzo, and Michael E. Cates,
Activity-induced clustering in model dumbbell swimmers: The role of hydrodynamic interactions,
Physical Review E 90, 22303 (2014).

[Summary] Using a fluid-particle dynamics approach, we numerically study the effects of hydrodynamic interactions on the collective dynamics of active suspensions within a simple model for bacterial motility: each microorganism is modeled as a stroke-averaged dumbbell swimmer with prescribed dipolar force pairs. Using both simulations and qualitative arguments, we show that, when the separation between swimmers is comparable to their size, the swimmers' motions are strongly affected by activity-induced hydrodynamic forces. To further understand these effects, we investigate semidilute suspensions of swimmers in the presence of thermal fluctuations. A direct comparison between simulations with and without hydrodynamic interactions shows these to enhance the dynamic clustering at a relatively small volume fraction; with our chosen model the key ingredient for this clustering behavior is hydrodynamic trapping of one swimmer by another, induced by the active forces. Furthermore, the density dependence of the motility (of both the translational and rotational motions) exhibits distinctly different behaviors with and without hydrodynamic interactions; we argue that this is linked to the clustering tendency. Our study illustrates the fact that hydrodynamic interactions not only affect kinetic pathways in active suspensions, but also cause major changes in their steady state properties.

Ken-ichi Mizuochi, Hiizu Nakanishi, and *Takahiro Sakaue,
Dynamical scaling of polymerized membranes,
Europhysics Letters 107, 38003:p1 -p6 (2014).

[Summary] Monte Carlo simulations have been performed to analyze the sub-diffusion dynamics of a tagged monomer in self-avoiding polymerized membranes in the flat phase. By decomposing the mean square displacement into the out-of-plane (//) and the in-plane (⊥) components, weobtain good data collapse with two distinctive diffusion exponents 2 \alpha_{//} = 0.36 ± 0.01 and 2 \alpha_{⊥} =0.21 ± 0.01, and the roughness exponents \zeta_{//} = 0.6 ± 0.05 and \zeta_{⊥ }= 0.25 ± 0.05, respectively foreach component. Their values are consistent with the relation from the rotational symmetry. We derive the generalized Langevin equations to describe the sub-diffusional behaviors of a tagged monomer in the intermediate time regime where the collective effect of internal modes in the membrane dominate the dynamics to produce negative memory kernels with a power law. Wealso briefly discuss how the long-range hydrodynamic interactions alter the exponents.

*Kazuya Fujimoto, Makoto Tsubota,
Spin-superflow turbulence in spin-1 ferromagnetic spinor Bose-Einstien condensates,
Physical Review A 90, 013629/1-7 (2014).

[Summary] Spin-superflow turbulence (SST) in spin-1 ferromagnetic spinor Bose-Einstein condensates is theoretically and numerically studied. We found that the -5/3 and -7/3 power laws appear in spectra of the superflow kinetic and the spin-dependent interaction energy, respectively.

Adnan Hamid, John J. Molina, and Ryoichi Yamamoto,
Simulation studies of microstructure of colloids in sedimentation,
Molecular Simulation 41, 968 (2014).

Chunyu Shih and Ryoichi Yamamoto,
Dynamic electrophoresis of charged colloids in an oscillating electric field,
Physical Review E 89, 062317-1-11 (2014).

[Summary] The dynamics of charged colloids in an electrolyte solution is studied using direct numerical simulations via the smoothed profile method. We calculated the complex electrophoretic mobility μ∗(ω) of the charged colloidsunder an oscillating electric field of frequency ω. We show the existence of three dynamically distinct regimes, determined by the momentum diffusion and ionic diffusion time scales. The present results agree well with approximate theories based on the cell model in dilute suspensions; however, systematic deviations between the simulation results and theoretical predictions are observed as the volume fraction of colloids is increased, similarto the case of constant electric fields.

Ai Nakatsuji, *Makoto Tsubota, and Hideo Yano,
Statistics of vortex loops emitted from quantum turbulence driven by an oscillating sphere,
Physical Review B 89, 174520/1-7 (2014).

[Summary] We perform numerical simulation of quantum turbulence at zero temperature generated by an oscillating sphere. In this simulation, we injected vortices on the sphere to generate turbulence. Although we prepare injected vortex loops of identical length, they are extended by the oscillating sphere to form a tangle through numerous reconnections. The resulting tangle around the sphere is anisotropic and affected by the oscillation. The vortex tangle continues to emit vortex loops, which leave the sphere. The statistics of emitted loops differ significantly from those of the original injected vortices. First, the sizes of the emitted loops are widely distributed, ranging from smaller to much larger than the size of the initial injected loop. Second, the propagation direction of the emitted loops exhibits anisotropy: Small loops move away almost isotropically but large ones do so anisotropically along the oscillation direction of the sphere. Thus, the oscillating object stirs the initial injected vortices to reproduce a group of vortices with different statistics. Such physics is compared with the experiments of vibrating objects.

Xiaowei He, Naoki Fujimura, J. Meagan Lloyd, Kristopher J. Erickson, A. Alec Talin, Qi Zhang, Weilu Gao, Qijia Jiang, Yukio Kawano, Robert H. Hauge, *François Léonard and *Junichiro Kono,
Carbon Nanotube Terahertz Detector,
Nano Letters 14, 3953–3958 (2014).

[Summary] Terahertz (THz) technologies are promising for diverse areas such as medicine, bioengineering, astronomy, environmental monitoring, and communications. However, despite decades of worldwide efforts, the THz region of the electromagnetic spectrum still continues to be elusive for solid state technology. Here, we report on the development of a powerless, compact, broadband, flexible, large-area, and polarization-sensitive carbon nanotube THz detector that works at room temperature. The detector is sensitive throughout the entire range of the THz technology gap, with responsivities as high as ∼2.5 V/W and polarization ratios as high as ∼5:1. Complete thermoelectric and opto-thermal characterization together unambiguously reveal the photothermoelectric origin of the THz photosignal, triggered by plasmonic absorption and collective antenna effects, and suggest that judicious design of thermal management and quantum engineering of Seebeck coefficients will lead to further enhancement of device performance.