Publications by authors named "Yasutaka Iwashita"

14 Publications

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Controlled armoring of metal surfaces with metallodielectric patchy particles.

J Chem Phys 2019 May;150(17):174903

Department of Physics, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan.

A patchy colloidal particle possesses distinctive regions with different physical or chemical properties on its surface and thus exhibits anisotropic interactions with another particle or object. By utilizing the large van der Waals attraction between metal surfaces and the electric double layer repulsion originating from surface charge, we succeeded in controlling the adsorption behavior of metallodielectric particles (MDPs), which were composed of dielectric spheres each with a thin gold patch modified with dissociable groups, to gold surfaces. When MDPs were dispersed on a dielectric substrate with a thick gold pattern in aqueous solution, the particles selectively adsorbed onto the gold surface of the substrate at a moderate salt concentration. Furthermore, when MDPs were mixed with large particles coated with a thick gold film, MDPs adsorbed on the gold surface at a moderate salt concentration and formed a monolayer. In the monolayer, gold patches of MDPs bonded to the gold surface and the dielectric surface of MDPs faced outward. In other words, this monolayer was a solid dielectric layer formed on the metal surface of a large particle. Such selectivity, i.e., that a gold patch of an MDP bonded to a gold surface but the patches did not bond to each other, was realized by controlling the thickness and surface charge of gold patches.
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http://dx.doi.org/10.1063/1.5090440DOI Listing
May 2019

Emulsion Droplets Stabilized by Close-Packed Janus Regular Polygonal Particles.

Langmuir 2018 10 1;34(41):12394-12400. Epub 2018 Oct 1.

Department of Physics , Kyushu University , 744 Motooka , Nishi-ku, Fukuoka 819-0395 , Japan.

In Pickering-Ramsden emulsions, the packing structure of the colloidal particles at the liquid-liquid (or liquid-gas) interface significantly affects the structure and behavior of the emulsion. Here, using a series of platelike particles with regular polygonal shapes and Janus amphiphilicity, we created emulsion droplets stabilized by close-packed polygonal particles at the interface. The systematic variation of the particle morphology shows that the geometrical features of the regular polygons in (curved) planar packing dominate over the self-assembled structures. The structures are tessellations of triangular, square, and hexagonal particles at the surface for large droplets and regular tetrahedral, cubic, and dodecahedral particle shells of triangular, square, and pentagonal particles for small droplets, respectively. This work creates the possibility of geometrically designing the structure and functionality of emulsions.
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http://dx.doi.org/10.1021/acs.langmuir.8b02323DOI Listing
October 2018

Density dependence of orientational order in one-patch particles.

Soft Matter 2017 Jul;13(29):4997-5007

Department of Physics, Kyushu University, 819-0395 Fukuoka, Japan.

Closely packed spherical patchy particles exhibit a range of orientationally ordered equilibrium structures depending on patch size and particle arrangement due to the existence of a sticky inter-patch interaction and rotational degrees of freedom. We experimentally study the packing density dependence of such ordering in particles with a thin deposited patch which imparts a shape anisotropy of a few percent in aspect ratio. These are confined between flat substrates with a chamber thickness of up to two particle layers. When the particles are tightly packed and almost in contact with each other, the anisotropic hard-body interaction dictates the orientational order. Thus, the order depends little on patch size, with rotational motion almost frozen. A small decrease in the density allows free rotational motion while translation is restricted to vibrational motion. This drastically changes the ordering mechanism, giving rise to a patch-size dependent equilibrium orientational order. Furthermore, within this density regime, we find yet another density-dependent transition within the tetragonal bilayer. This transition is reproduced by numerical simulation assuming no shape anisotropy, indicating that shape anisotropy is unnecessary for the transition and translational entropy significantly affects the equilibrium orientational order even in such a closely packed structure. Our study demonstrates the sensitivity of the ordering mechanism and the resulting order to the packing density, where the effect of such a tiny shape anisotropy is clearly observable owing to the patch opacity. The dependence of cluster structure in particle dispersions on patch size, confinement thickness and packing density is also reported.
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http://dx.doi.org/10.1039/c7sm00565bDOI Listing
July 2017

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 2017 01 19;33(4):1030-1036. Epub 2017 Jan 19.

Department of Physics, Kyushu University , 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan.

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. The self-assembly shows essential commonalities with that in microemulsions of surfactant molecules, however the AJP system is stabilized only kinetically. Analysis of the dependence of the emulsion droplet size on composition shows that almost all the particles are adsorbed at the water-oil interface; i.e., the particles show ideal surface activity.
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http://dx.doi.org/10.1021/acs.langmuir.6b03723DOI Listing
January 2017

Spatial confinement governs orientational order in patchy particles.

Sci Rep 2016 06 6;6:27599. Epub 2016 Jun 6.

Department of Physics, Kyushu University, 819-0395 Fukuoka, Japan.

Orientational order in condensed matter plays a key role in determining material properties such as ferromagnetism, viscoelasticity or birefringence. We studied purely orientational ordering in closely-packed one-patch colloidal particles confined between flat substrates, where the particles can only rotate and are ordered via the sticky interaction between the patches. For the first time, we experimentally realized a rich variety of mesoscopic patterns through orientational ordering of colloids by controlling patch size and confinement thickness. The combination of experiment and numerical simulation reveals the decisive role of confinement: An ordered state(s) is selected from the (meta)stable options in bulk when it is commensurate with the system geometry and boundary conditions; otherwise, frustration induces a unique order. Our study offers a new means of systematic control over mesoscopic structures via orientational ordering in patchy particles. The system would also possess unique functionalities through the rotational response of the particles to external stimuli.
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http://dx.doi.org/10.1038/srep27599DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4893746PMC
June 2016

Orientational order of one-patch colloidal particles in two dimensions.

Soft Matter 2014 Oct;10(37):7170-81

Department of Physics, Kyushu University, 812-8581 Fukuoka, Japan.

We studied the orientational order of one-patch colloidal particles (Janus particles) in a close-packed monolayer. In an experiment on hemispherically patched particles, we realized a highly ordered zigzag stripe pattern by inducing directional growth of the pattern via a phase transition of the solvent. Upon spontaneous ordering by strengthening the inter-patch attraction, however, the particles are trapped in a poorly ordered zigzag pattern, illustrating the importance of controlling kinetics to attain a highly ordered state. The patch-size dependence of an equilibrium orientational order is experimentally observed under moderate inter-patch attraction. We also calculated the equilibrium order against the patch size and attraction in a Monte Carlo simulation. In the simulation, the rather discrete transition between a zigzag stripe, tiling of triangular trimers and tiling of dimers under strong attraction becomes continuous with weakening attraction. The experimental result not only coincides with the simulation qualitatively but also suggests that a particular cluster is selectively formed by nonuniform inter-patch attraction in the experiment. The effect of patch-substrate attraction and commonalities of the order with liquid crystals are also discussed.
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http://dx.doi.org/10.1039/c4sm00932kDOI Listing
October 2014

Hydrodynamically induced rhythmic motion of optically driven colloidal particles on a ring.

Phys Rev E Stat Nonlin Soft Matter Phys 2012 Jun 1;85(6 Pt 1):061402. Epub 2012 Jun 1.

Department of Physics, School of Sciences, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan.

We experimentally study the motion of optically driven colloidal particles on a circular path by varying their number N. Although an identical driving force is applied to each particle, their equally spaced configuration is hydrodynamically unstable, and a doublet configuration is spontaneously formed. In small-N systems, the angular difference between neighboring particles exhibits oscillatory or nonoscillatory behavior. The number of oscillatory modes that appear depends on the maximum number of doublets that the system can contain. Frequent switching between different modes was observed with increasing N. The characteristic frequencies of the oscillatory modes are discussed theoretically by linear stability analysis of the equations that govern the motion of hydrodynamically coupled particles. The evaluated frequencies of the slowest modes exhibit reasonably good agreement with those of the mainly observed modes in experiments. The relationship between the characteristic frequencies and specific configurations is confirmed experimentally by setting a specific initial configuration for the particles. An increase in N also enhances the mean angular velocity of the particles owing to the reduced effective viscosity in large-N systems.
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http://dx.doi.org/10.1103/PhysRevE.85.061402DOI Listing
June 2012

Dependence of interparticle force on temperature and cell thickness in nematic colloids.

Phys Rev E Stat Nonlin Soft Matter Phys 2010 Aug 18;82(2 Pt 1):020701. Epub 2010 Aug 18.

Department of Physics, Kyushu University, Higashi-ku, Fukuoka, Japan.

We have experimentally studied the interparticle force between two particles accompanied by hyperbolic hedgehog defects in a nematic liquid crystal. The force F was measured with dual-beam optical tweezers at various temperatures and in cells with various thicknesses. In a thick cell, the dependence of F on the interparticle distance R obtained at different temperatures can be scaled to a universal curve of F∝R^(-4) for R>3a , where a is the radius of a particle. The effective elastic constant evaluated from F is found to be in good agreement with splay constant of the nematic liquid crystal. In a thin cell, the magnitude of F decreases and the dependence of F on R becomes short-ranged as the thickness of a cell, L , decreases. The reduced force curves, FL(4) against R/L , at different L are found to be scaled to a single theoretical curve which has been proposed recently.
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http://dx.doi.org/10.1103/PhysRevE.82.020701DOI Listing
August 2010

Smectic membranes in aqueous environment.

Phys Rev E Stat Nonlin Soft Matter Phys 2010 May 21;81(5 Pt 1):051709. Epub 2010 May 21.

Max Planck Institute for Dynamics and Self-Organization, 37073 Göttingen, Germany.

We present a study of thermotropic smectic liquid crystal films in aqueous environment. Macroscopic freely suspended films in water with a size up to 7.4 × 15 mm2 were prepared with the help of a surfactant, which ensures a strong homeotropic anchoring at liquid crystal/water interfaces. The films were studied by optical microscopy and ellipsometry. Attention was paid to the stability and the thinning transitions which occurred at temperatures above the bulk smectic- A -isotropic transition temperature. In addition, we investigated the formation and rupture kinetics of thin smectic membranes separating water droplets in microfluidic devices. Besides possible applications in discrete microfluidics, smectic films in aqueous environment may expand the general range of possible studies of freely suspended smectic films.
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http://dx.doi.org/10.1103/PhysRevE.81.051709DOI Listing
May 2010

Pattern evolution of an edge-dislocation array in a lyotropic lamellar phase confined in a wedge-shaped cell: defect formation, relaxation, and recombination.

Phys Rev E Stat Nonlin Soft Matter Phys 2008 Apr 16;77(4 Pt 1):041706. Epub 2008 Apr 16.

Institute of Industrial Science, University of Tokyo, Meguro-ku, Tokyo, Japan.

When a system undergoes a first-order phase transition from a disordered to an ordered state, the local energy is first minimized. This local energy minimization often prevents a system from reaching the global energy minimum state and leads to trapping in an imperfectly ordered state with many defects. In soft matter, however, a system can further relax to the global energy minimum state via slow relaxation due to its softness and fluidity. We study this relaxation process, using a lyotropic lamellar phase in a wedge-shaped cell as a model system. A lyotropic smectic liquid crystal has a large repeat unit (here, an interlayer spacing d) up to approximately 0.1 microm, and thus the motion of an individual edge dislocation in the lamellar phase can be directly observed with optical microscopy. Furthermore, a rather macroscopic spatial confinement (size h) can produce strong confinement effects, since d/h can still be large due to the largeness of d. These properties allow us to study the detailed kinetics of the relaxation process. We follow the time evolution of an edge dislocation array over 100 h from its initial stage. We reveal that the pattern evolution of an edge-dislocation array is the relaxation process of excess dislocation lines that formed initially toward the equilibrium configuration, and it is characterized by the motion of "nodes" of the topologically connected edge-dislocation network. We clarify the elementary process of this relaxation from a local to the global energy minimum state.
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http://dx.doi.org/10.1103/PhysRevE.77.041706DOI Listing
April 2008

Spontaneous onion-structure formation from planar lamellar nuclei.

Phys Rev Lett 2007 Apr 6;98(14):145703. Epub 2007 Apr 6.

Institute of Industrial Science, University of Tokyo, Meguro-ku, Tokyo 153-8505, Japan.

Nucleation and growth are a basic elementary process of ordering. The nucleation process is controlled by a competition between interfacial and bulk energy. Thus an ordered structure of a nucleus at its birth is not necessarily the most stable thermodynamically: Ostwald step rule. In addition to this, we found the topological transformation of nuclei from the most stable bulk structure (planar lamella) to a metastable one (onion) in a lyotropic liquid crystal. This indicates that the fate of nuclei of low-dimensional internal order can also be seriously affected by an additional competition between interfacial and elastic deformation energy.
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http://dx.doi.org/10.1103/PhysRevLett.98.145703DOI Listing
April 2007

Self-organization in phase separation of a lyotropic liquid crystal into cellular, network and droplet morphologies.

Nat Mater 2006 Feb 22;5(2):147-52. Epub 2006 Jan 22.

Institute of Industrial Science, University of Tokyo, Japan.

Phase separation is one of the most fundamental physical phenomena that controls the morphology of heterogeneous structures. Phase separation of a binary mixture of simple liquids produces only two morphologies: a bicontinuous or a droplet structure in the case of a symmetric or an asymmetric composition, respectively. For complex fluids, there is a possibility to produce other interesting morphologies. We found that a network structure of the minority phase can also be induced transiently on phase separation if the dynamics of the minority phase are much slower than those of the majority phase. Here we induce a cellular structure of the minority phase intentionally with the help of its smectic ordering, using phase separation of a lyotropic liquid crystal into the isotropic and smectic phase. We can control the three morphologies, cellular, network and droplet structures, solely by changing the heating rate. We demonstrate that the kinetic interplay between phase separation and smectic ordering is a key to the morphological selection. This may provide a new route to the formation of network and cellular morphologies in soft materials.
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http://dx.doi.org/10.1038/nmat1580DOI Listing
February 2006

Surface-assisted monodomain formation of an ordered phase of soft matter via the first-order phase transition.

Phys Rev Lett 2005 Jul 22;95(4):047801. Epub 2005 Jul 22.

Institute of Industrial Science, University of Tokyo, Meguro-ku, Japan.

We produced a monodomain ordered structure with the help of a surface-mediated first-order transition, using a sponge-to-lamella transition in a membrane suspension as a model system. The long characteristic length and time scales of the system allow us to observe the process of the phase ordering in a confined geometry with optical microscopy in real time. We demonstrate that a homogeneously ordered domain can be attained if we prevent nucleation in bulk by using its kinetic coupling to nucleation on the surface.
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http://dx.doi.org/10.1103/PhysRevLett.95.047801DOI Listing
July 2005

Optical manipulation of defects in a lyotropic lamellar phase.

Phys Rev Lett 2003 Jan 28;90(4):045501. Epub 2003 Jan 28.

Institute of Industrial Science, University of Tokyo, Meguro-ku, Tokyo 153-8505, Japan.

Here we study the line defect in a hyperswollen lamellar phase of lyotropic liquid crystal by applying a laser trapping method. We have succeeded in directly measuring the tension of a single isolated line defect and the adhesion energy between two defects. We demonstrate a new possibility of intentional patterning of various defects by direct optical manipulation. Furthermore, local rheological measurements provide information on the membrane organization around a particle and also evidence suggesting that flow in a lamellar phase has a two-dimensional nature.
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http://dx.doi.org/10.1103/PhysRevLett.90.045501DOI Listing
January 2003