Hiroshi Frusawa

Precise switching control of liquid crystalline microgears driven by circularly polarized light

Liquid crystalline molecules carrying photopolymerizable end groups absorb photon energy via a two-photon process, enabling the photofabrication of 3D structures. In this work, we prepared microgears with different heights and tooth lengths. These birefringent microgears can be induced to rotate by circularly polarized light. Here, we demonstrate that the use of phase plate for switching between left- and right-handed polarization reverses the optically induced rotation while maintaining the same rotational frequency. Due to the precise switching control, these birefringent microgears have advantages over previous microrotors that are fabricated from non-birefringent light-curing resins.

Anisotropic micro-cloths fabricated from DNA-stabilized carbon nanotubes: one-stop manufacturing with electrode needles

Among a variety of solution-based approaches to fabricate anisotropic films of aligned carbon nanotubes (CNTs), we focus on the dielectrophoretic assembly method using AC electric fields in DNA-stabilized CNT suspensions. We demonstrate that a one-stop manufacturing system using electrode needles can draw anisotropic DNA-CNT hybrid films of 10 to 100 µm in size (i.e., free-standing DNA-CNT micro-cloths) from the remaining suspension into the atmosphere while maintaining structural order. It has been found that a maximal degree of polarization (ca. 40%) can be achieved by micro-cloths fabricated from a variety of DNA-CNT mixtures. Our results suggest that the one-stop method can impart biocompatibility to the downsized CNT films and that the DNA-stabilized CNT micro-cloths directly connected to an electrode could be useful for biofuel cells in terms of electron transfer and/or enzymatic activity.

Thin-Film Transistors Using Uniform and Well-Aligned Single-Walled Carbon Nanotubes Channels by Dielectrophoretic Assembly

A single-walled carbon nanotube thin-film transistor (SWCNT TFT) was formed by an aligned SWCNTs channel assembled by the dielectrophoretic (DEP) process. In this work, we investigated the effects of the DEP factors (frequency, solution concentration) on structural (orientation and density in the SWCNT channels) and electrical properties of SWCNT TFTs. A uniform, well-aligned and density controlled SWCNT channel was achieved by optimizing the DEP assembly process, and as a result, electrical properties (mobility and on/off current ratio) of SWCNT TFTs were improved. In addition, we also discussed the effect of uniformity of assembled SWNTs in a channel on performance variation of the SWCNT TFTs. We found that the tube density and uniformity are key parameters which determine electrical properties and performance variation of SWCNT TFTs.

Scanning Dielectrophoresis for the Directed-Assembly and Non-Contact Manipulation of Colloidal Particles

Abstract The scanning dielectrophoresis system consists of a pair of microneedles connected to a waveform generator of arbitrary ac electric fields. Consequently, our scanning method is able to place and move the strong electric field spot between the electrode tips anywhere, just as optical tweezers have done the focus of light beam. In this paper, we demonstrate non-contact manipulations unique to our scanning method. We also propose that the use of the frequency modulated electric field provides a novel way for one-step assay of crossover frequency in dielectrophoresis of a single colloid, which has been validated by the scanning method. Furthermore, the dielectrophoretic assembly forms amicron-film with stripe pattern of single-walled carbon nanotube bundles surrounded by polymer beads, similarly to depletion induced clusters in binary colloids.

Emerging quasi-0D states at vanishing total entropy of the 1D hard sphere system: A coarse-grained similarity to the car parking problem

Abstract A coarse-grained system of one-dimensional (1D) hard spheres (HSs) is created using the Delaunay tessellation, which enables one to define the quasi-0D state. It is found from comparing the quasi-0D and 1D free energy densities that a frozen state due to the emergence of quasi-0D HSs is thermodynamically more favorable than fluidity with a large-scale heterogeneity above crossover volume fraction of ϕ c = e / ( 1 + e ) = 0.731 ⋯  , at which the total entropy of the 1D state vanishes. The Delaunay-based lattice mapping further provides a similarity between the dense HS system above ϕ c and the jamming limit in the car parking problem.

Frequency-Modulated Wave Dielectrophoresis of Vesicles And Cells: Periodic U-Turns at the Crossover Frequency

We have formulated the dielectrophoretic force exerted on micro/nanoparticles upon the application of frequency-modulated (FM) electric fields. By adjusting the frequency range of an FM wave to cover the crossover frequency fX in the real part of the Clausius-Mossotti factor, our theory predicts the reversal of the dielectrophoretic force each time the instantaneous frequency periodically traverses fX. In fact, we observed periodic U-turns of vesicles, leukemia cells, and red blood cells that undergo FM wave dielectrophoresis (FM-DEP). It is also suggested by our theory that the video tracking of the U-turns due to FM-DEP is available for the agile and accurate measurement of fX. The FM-DEP method requires a short duration, less than 30 s, while applying the FM wave to observe several U-turns, and the agility in measuring fX is of much use for not only salty cell suspensions but also nanoparticles because the electric-field-induced solvent flow is suppressed as much as possible. The accuracy of fX has been verified using two types of experiment. First, we measured the attractive force exerted on a single vesicle experiencing alternating-current dielectrophoresis (AC-DEP) at various frequencies of sinusoidal electric fields. The frequency dependence of the dielectrophoretic force yields fX as a characteristic frequency at which the force vanishes. Comparing the AC-DEP result of fX with that obtained from the FM-DEP method, both results of fX were found to coincide with each other. Second, we investigated the conductivity dependencies of fX for three kinds of cell by changing the surrounding electrolytes. From the experimental results, we evaluated simultaneously both of the cytoplasmic conductivities and the membrane capacitances using an elaborate theory on the single-shell model of biological cells. While the cytoplasmic conductivities, similar for these cells, were slightly lower than the range of previous reports, the membrane capacitances obtained were in good agreement with those previously reported in the literature.

Histone-based self-assembly into DNA-wrapped meso-clusters

The recent discovery of meso-cluster phase includes not only colloidal molecules of synthetic polymer particles but also equilibrium protein clusters. Here we report self-assembly of histone protein into stable submicron clusters that can be generated even in centrifuged supernatants containing no initial aggregates. Furthermore, dark-field microscopy of the electrophoresis has verified charge reversal of individual histone clusters by adding DNA. We have determined the critical nucleotide concentration at which the electrophoretic mobility vanishes in three types of DNA, revealing the coexistence of nucleosomes with DNA-wrapped meso-clusters.

Micelle Formation in a Highly Charged Polyelectrolyte Solution

A highly charged polyelectrolyte, sodium polystyrenesulfonate, was investigated in various quality of solvent by using dynamic light scattering and viscosity measurements. As the solvent quality was lowered, the relaxation dynamics by the scattering measurements exhibited a drastic change from two relaxations to single one. Furthermore, an obvious reduction of zero‐shear viscosity coincided with the transition. These results indicate that the polyelectrolyte solution system in the solvent region has formed mono‐disperse micelles by strong segregation. The micelle size was estimated to be about 20 nm by Einstein‐Stokes relation. This micelle formation of polyelectrolyte solution system has been achieved experimentally for the first time despite theoretical predicts so far.