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Biflagellate algal cells of the genus Volvox form spherical colonies that propel themselves, vertically upwards in still fluid, by the coordinated beating of thousands of flagella, that also cause the colonies to rotate about their vertical axes. When they are swimming in a chamber of finite depth, pairs (or more) of Volvox carteri colonies were observed by Drescher et al. (Phys. Rev. Lett., vol. 102, 2009, 168101) to exhibit hydrodynamic bound states when they are close to a rigid horizontal boundary. When the boundary is above, the colonies are attracted to each other and orbit around each other in a ‘waltz’; when the boundary is below they perform more complex ‘minuet’ motions. These dances are simulated in the present paper, using a novel ‘spherical squirmer’ model of a colony in which, instead of a time-independent but
-dependent tangential velocity being imposed on the spherical surface (radius
is the polar angle), a time-independent and uniform tangential shear stress is applied to the fluid on a sphere of radius
$(1+\epsilon )a, \epsilon \ll 1$
represents the length of the flagella. The fluid must satisfy the no-slip condition on the sphere at radius
. In addition to the shear stress, the motions depend on two dimensionless parameters that describe the effect of gravity on a colony:
, proportional to the ratio of the sedimentation speed of a non-swimming colony to its swimming speed, and
, that represents the fact that colonies are bottom heavy;
is the ratio of the time scale to swim a distance equal to the radius, to the time scale for gravitational reorientation of the colony's axis to the vertical when it is disturbed. In addition to reproducing both of the dancing modes, the simulations are able to determine values of
for which they are stable (or not); there is reasonable agreement with the experiments. A far-field model for the minuet motions is also shown to have qualitative agreement, but does not describe some features that are reproduced in the full simulations.
We report the investigation on the properties of a novel Te precursor (i-C3H7)2Te and its effectiveness in fabricating MoTe2. The vapor pressure of the precursor was obtained by measuring the pressure as a function of its temperature in a sealed chamber. As a result it showed a high vapor pressure of 552.1 Pa at room temperature. The decomposition of the precursor was also investigated using DFT calculation. It was shown that the most likely reaction during the course of the decomposition of (i-C3H7)2Te is (i-C3H7)2Te → H2Te + 2 C3H7. The effectiveness of the precursor on the fabrication of MoTe2 was also investigated. Sputter-deposited MoO3 was tellurized in a quartz-tube furnace at the temperature up to 440°C. The resulting film showed that the 80% of the original MoO3 was tellurized to form MoTe2. It was also shown that further optimization of tellurization is required in order to prevent formation of metal Mo and elemental Te.
We report the synthesis of MoS2(1-x)Te2x by co-sputtering deposition and effect of mixture on its bandgap. The deposition was carried out at room temperature, and the sputtering power on individual MoS2 and MoTe2 targets were varied to obtain films with different compositions. Investigation with X-ray photoelectron spectroscopy confirmed the formation of Mo-Te and Mo-S bonds after post-deposition annealing (PDA), and one of the samples exhibited composition ratio of Mo:S:Te = 1:1.2:0.8 and 1:1.9:0.1 achieving 1:2 ratio of metal to chalcogen. Bandgap of MoS1.2Te0.8 and MoS1.9Te0.1 was evaluated with Tauc plot analysis from the extinction coefficient obtained by spectroscopic ellipsometry measurements. The obtained bandgaps were 1.0 eV and 1.3 eV. The resulting bandgap was lower than that of bulk MoS2 and higher than that of bulk MoTe2 suggesting mixture of both materials was achieved by co-sputtering.
Molybdenum disulfide (MoS2) thin films were fabricated by two-step chemical vapor deposition (CVD) using (t-C4H9)2S2 and the effects of temperature, gas flow rate, and atmosphere on the formation were investigated in order to achieve high-speed low-temperature MoS2 film formation. From the results of X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) investigations, it was confirmed that c-axis orientation of the pre-deposited Mo film has a significant involvement in the crystal orientation after the reaction low temperature sulfurization annealing and we successfully obtained 3 nm c-axis oriented MoS2 thin film. From the S/Mo ratios in the films, it was revealed that the sulfurization reaction proceeds faster with increase in the sulfurization temperature and the gas flow rate. Moreover, the sulfurization under the H2 atmosphere promotes decomposition reaction of (t-C4H9)2S2, which were confirmed by XPS and density functional theory (DFT) simulation.
The reorientation phenomenon of a single red blood cell during sedimentation is simulated using the boundary element method. The cell settles downwards due to a density difference between the internal and external fluids, and it changes orientation toward a vertical orientation regardless of Bond number or viscosity ratio. The reorientation phenomenon is explained by a shape asymmetry caused by the gravitational driving force, and the shape asymmetry increases almost linearly with the Bond number. When velocities are normalised by the driving force, settling/drifting velocities are weak functions of the Bond number and the viscosity ratio, while the angular velocity of the reorientation drastically changes with these parameters: the angular velocity is smaller for lower Bond number or higher viscosity ratio. As a consequence, trajectories of the sedimentation are also affected by the angular velocity, and blood cells with slower reorientation travel longer distances in the drifting direction. We also explain the mechanism of the reorientation using an asymmetric dumbbell. From the analysis, we show that the magnitude of the angular velocity is explained by two main factors: the shape asymmetry and the instantaneous orientation angle.
We present a numerical analysis of the rheology of a dense suspension of spherical capsules in simple shear flow in the Stokes flow regime. The behaviour of neo-Hookean capsules is simulated for a volume fraction up to
by graphics processing unit computing based on the boundary element method with a multipole expansion. To describe the specific viscosity using a polynomial equation of the volume fraction, the coefficients of the equation are calculated by least-squares fitting. The results suggest that the effect of higher-order terms is much smaller for capsule suspensions than rigid sphere suspensions; for example,
terms account for only 8 % of the specific viscosity even at
for capillary numbers
. We also investigate the relationship between the deformation and orientation of the capsules and the suspension rheology. When the volume fraction increases, the deformation of the capsules increases while the orientation angle of the capsules with respect to the flow direction decreases. Therefore, both the specific viscosity and the normal stress difference increase with volume fraction due to the increased deformation, whereas the decreased orientation angle suppresses the specific viscosity, but amplifies the normal stress difference.
Molybdenum disulfide (MoS2), one of the transition-metal dichalcogenides, is a 2-dimensional semiconducting material that has a layered structure. Owing to excellent optical and electronic properties, the ultra-thin MoS2 film is expected to be used for various devices, such as transistors and flexible displays. In this study, we investigated the physical and chemical properties of sputtered-MoS2 film in the sub-10-nm region by Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). As the results of Raman spectroscopy investigations, we observed two Raman modes, E12g and A1g, in the 2-dimensional MoS2 films. As the thickness of the MoS2 film decreased, the peak frequency difference between E12g and A1g modes increased. From the XPS investigations, we confirmed sulfur reductions from the 2-dimensional MoS2 films. Therefore, we considered that the sulfur vacancies in the MoS2 film affected the Raman peak positions. Moreover, we performed the additional sulfurization of sputtered-MoS2 films. From the XPS and Raman investigations, the quality of the sputtered-MoS2 films was improved by the additional sulfurization.
A fully coherent free electron laser (FEL) seeded with a higher-order harmonic (HH) pulse from high-order harmonic generation (HHG) is successfully operated for a sufficiently prolonged time in pilot user experiments by using a timing drift feedback. For HHG-seeded FELs, the seeding laser pulses have to be synchronized with electron bunches. Despite seeded FELs being non-chaotic light sources in principle, external laser-seeded FELs are often unstable in practice because of a timing jitter and a drift between the seeding laser pulses and the accelerated electron bunches. Accordingly, we constructed a relative arrival-timing monitor based on non-invasive electro-optic sampling (EOS). The EOS monitor made uninterrupted shot-to-shot monitoring possible even during the seeded FEL operation. The EOS system was then used for arrival-timing feedback with an adjustability of 100 fs for continual operation of the HHG-seeded FEL. Using the EOS-based beam drift controlling system, the HHG-seeded FEL was operated over half a day with an effective hit rate of 20%–30%. The output pulse energy was
at the 61.2 nm wavelength. Towards seeded FELs in the water window region, we investigated our upgrade plan to seed high-power FELs with HH photon energy of 30–100 eV and lase at shorter wavelengths of up to 2 nm through high-gain harmonic generation (HGHG) at the energy-upgraded SPring-8 Compact SASE Source (SCSS) accelerator. We studied a benefit as well as the feasibility of the next HHG-seeded FEL machine with single-stage HGHG with tunability of a lasing wavelength.
The deformation of a spherical capsule in oscillating shear flow is presented. The boundary element method is used to simulate the capsule motion under Stokes flow. We show that a capsule at high frequencies follows the deformation given by a leading-order prediction, which is derived from an assumption of small deformation limit. At low frequencies, on the other hand, a capsule shows an overshoot phenomenon where the maximum deformation is larger than that in steady shear flow. A larger overshoot is observed for larger capillary number or viscosity ratio. Using the maximum deformation in start-up shear flow, we evaluate the upper limit of deformation in oscillating shear flow. We also show that the overshoot phenomenon may appear when the quasi-steady orientation angle under steady shear flow is less than
. We propose an equation to estimate the threshold frequency between the low-frequency range, where the capsule may have an overshoot, and the high-frequency range, where the deformation is given by the leading-order prediction. The equation only includes the viscosity ratio and the Taylor parameter under simple shear flow, so it can be extended to other deformable particles, such as bubbles and drops.
Polysilsesquioxane passivation layers were used to passivate bottom gate a-InGaZnO (a-IGZO) thin film transistors (TFT). The a-IGZO TFTs passivated with polysilsesquioxane showed highly stable behavior during positive bias stress, negative bias stress, and negative bias illumination stress. A voltage threshold shift of up to 0.1 V, less than -0.1 V and -2.3 V for positive bias stress, negative bias stress, and negative bias illumination stress, respectively. We also report the effect of reactive ion etching (RIE) on the electrical characteristics of a-InGaZnO (a-IGZO) thin-film transistors (TFT) passivated with the polysilsesquioxane-based passivation layers. We show how post-annealing treatment using two different atmosphere conditions: under O2 ambient and combination of N2 and O2 ambient (20% O2), can be performed to recover the initial characteristics. Furthermore, we present a highly stable novel polysilsesquioxane photosensitive passivation material that can be used to completely circumvent the reactive ion etching effects.
The diffusion of red blood cells (RBCs) in blood is important to the physiology and pathology of the cardiovascular system. In this study, we investigate flow-induced diffusion of RBCs in a semi-dilute system by calculating the pairwise interactions between RBCs in simple shear flow. A capsule with a hyperelastic membrane was used to model an RBC. Its deformation was resolved using the finite element method, whereas fluid motion inside and outside the RBC was solved using the boundary element method. The results show that shear-induced RBC diffusion is significantly anisotropic, i.e. the velocity gradient direction component is larger than the vorticity direction. We also found that the motion of RBCs during the interaction is strongly dependent on the viscosity ratio of the internal to external fluid, and the diffusivity decreases monotonically as the viscosity ratio increases. The scaling argument also suggests that the diffusivity is proportional to the shear rate and haematocrit, if the suspension is in a semi-dilute environment and the capillary number is invariant. These fundamental findings are useful to understand transport phenomena in blood flow.
KNbO3 thick films were deposited on (100)c SrRuO3//(100)SrTiO3 substrates at 240 °C for 3 h by hydrothermal method. Film thickness increased linearly with increasing the deposition number of times and 130 μm thickness was achieved by the 6 time deposition. XRD analysis showed the growth of epitaxial orthorhombic films with the mixture orientation of (100), (010) and (001). Cross-sectional SEM observation showed that the 130 μm-thick film was dense and no obvious voids inside the film. In addition, the crystal structure change along film thickness direction was not detected from the cross-sectional Raman spectral observation.
Low sheet resistance (high mobility) with high transmittance in all wavelength is required for front TCO. High haze value is also required for effective light trapping. For this purpose, we have combined F-doped SnO2 (FTO) with high mobility deposited by LPCVD and reactive ion etching (RIE) processed glass substrate. However, two problems have been found. (1) The mobility of FTO on RIE substrate dropped from that on flat glass (75 to 36 cm2/Vs). To avoid this drop, thicker film is needed. (2) To keep high transmittance with thicker film, lower carrier concentration is needed. But the mobility dropped with lower carrier concentration. In order to solve these constrains, we have adopted a stacked structure using thick non-doped layer of 2700 nm and thin F-doped layer of 500 nm. With this novel approach, we have successfully achieved the high mobility (80 cm2/Vs), low carrier concentration (2.2x1019 /cm3) and high haze value (77% at wavelength of 1000 nm) at the same time. This new developed high-haze SnO2 is a new promising TCO for thin-film Si solar cells.