Yasutaka Iwashita

Kyoto Sangyo University · Department of Physics

Recent developments in nanotechnology have facilitated the use of surface-active colloidal particles with tailor-made anisotropic properties. These surface-active agents have introduced unprecedented emulsion systems that exhibit qualitatively different self-assembled/organized structures and material properties from those of emulsions with convent...

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 succeed...

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 plate-like particles with regular polygonal shapes and Janus amphiphilicity, we created emulsion droplets stabilized by close-packed...

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 de...

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 structu...

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 interactio...

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 t...

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...

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 bet...

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 bet...

We experimentally studied the interparticle force between two colloidal particles with point defects in a nematic liquid crystal. The force F at various temperatures was measured using optical tweezers and with a free release method. The effective elastic constant was evaluated from F using electrostatic analogy of nematic field. At respective temp...

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...

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...

Compound refractive lenses (CRL), which are composed of many tenses aligned along the optical axis, have been developed as refractive x-ray optics. A CRL has numerous desired properties : It is robust, easy to been developed as re C align and to operate, and can be used for hard x-rays. To broaden the possibilities and functions of static CRL, we p...

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...

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 nucl...

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...

この論文は国立情報学研究所の電子図書館事業により電子化されました。 研究会報告 相分離は、ヘテロな空間構造を形成するうえで最も基本的な物理現象の一つである。しかし多くのソフトマターのようにメゾスケールの内部秩序を持つ系の相分離に関しては、幾つかの興味深いパターン形成が報告されているものの、その秩序が相分離キネティクスに与える影響については未だ十分に解明されていない。本研究ではリオトロピック液晶のラメラ(スメクチック)-スポンジ(等方)相分離を用いた実験を行い、基本的な(低次元の)秩序であるスメクチック秩序が相分離パターン形成においてどのような役割を果たすか調べた。その結果スメクチック秩序と相分離のキネティックな競合により、ネットワーク、フォーム、ドロップレットといった全く異なる空間構造が形成...

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 r...

Phase separation is the most fundamental physical phenomenon that produces various heterogeneous structures. Here we study a phase separation of lyotropic liquid crystal into ordered (lamellar) and disordered (sponge) phases. We found two types of novel pattern formation caused by the interplay between a phase separation and smectic ordering in add...

We study the formation of lamellar structure of lyotropic liquid crystal composed of C_12E_5/H_2O in wedge-shaped cell. The equilibrium lamellar structure in this cell is known to be an edge dislocation array, which is formed if lamellar layers well align homeotropically to cell surface. When we formed the lamellar phase in the cell, however, some...

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 opti...

この論文は国立情報学研究所の電子図書館事業により電子化されました。 我々はリオトロピック液晶であるC_<10>E_3/H_2Oを用い、そのスポンジ相(無秩序相)からのラメラ相(秩序相)の核形成キネティクスについて研究した。その結果特に低濃度の試料において、円盤状のラメラ核が変形して球状のオニオン構造を形成する、新たなオニオン相の自発的形成機構を発見した。これは界面エネルギーと変形の自由度が結合した、ソフトマターに特有の機構と考えられる。更に高濃度の試料では、強い非平衡状態下において、異方的で速い特有の枝形成・成長機構が見られた。発表では、セルに閉じ込められた状態における、セル表面での不均一核形成と表面から離れたパルク中での均一枝形成の競合も含めて報告する予定である。