# External Field Response and Applications of Metal Coated Hemispherical Janus Particles
## 3 Results and Discussion
### Transmission and Reflection Measurement with a Magnetic Field  
When the nickel coated hemispherical Janus particles are dispersed in water, the nickel surface faces downward due to gravity. Then, a magnetic field is applied from the side; with the result that each particle is magnetized for the particles attract each other to be self-organized. With a microscope, an ordered assembly like a chain was often found under the magnetic field, as shown in Figure 10. Similar chain formation with magnetic Janus particles has been described before [33; 34; 35]. Once magnetized, the particles tend to be self-assembled even without the magnetic field, but not in order. Before the light transmission experiment below, therefore, a stationary magnetic field was applied for nearly 10 min. Then, most of the particles were prompted to be self-assembled in an ordered structure like a chain, as shown in Figure A2. Figure 10: Result for self-organization of nickel-coated hemispherical Janus particles like a chain. When a magnetic field \(B\) is applied, particles are magnetized one by one and attract each other. Figure 9: Reflection change of the ITO-sandwiched suspension having the 15.7% cross sectional coverage of the particles observed in a dark room without (**a**) and with (**b**) the electric field when the light was incident from the equatorial side. It is clearly seen that light is reflected in (**a**) and transmitted in (**b**). The sample was irradiated from below by the light emitting diode of a smartphone. The photographic images are captured from the movie (See also Video S2) that was taken with the smartphone held by the observer below. The results of the transmitted light intensity are shown in Figure 11. The transmitted light intensity was decreased as the magnetic field was oriented perpendicular to the light propagation direction, while it was increased as the magnetic field was oriented parallel to the light direction. Suppose that the light passes through the suspension of the self-organized particles like a chain. The light passing perpendicularly to the chain is blocked and the light passing parallel to the chain is transmitted. Such a phenomenon occurs because the direction of the chain is controlled by the magnetic field. This effect is clearly seen as a reflection and transmission switch with the bulk suspension, as shown in Figure 12.  
Figure 11: Transmitted light intensity of the suspension of nickel coated hemispherical Janus particles. The transmitted light intensity was increased by 1.1 times with the magnetic field applied parallel to the light propagation direction, compared with the case of the perpendicular field. This is because the chain of self-assembled particles is aligned parallel to the light direction with the parallel magnetic field for the cross section in the light direction is minimized, while it intersects the light direction with the perpendicular field for the cross section is maximized. Figure 12: Photographs of reflections of the suspension in a dark room with a magnetic field \(B\) applied (**a**) perpendicular and (**b**) parallel to the light propagation direction. When the light direction is parallel to the magnetic field in (**b**), the light is more transmitted. The sample tube containing the suspension was irradiated from the front by the light emitting diode of a smartphone. The photographic images are captured from the movie (See also Video S3) taken with the smartphone held by the observer in the front, who rotated the magnetic field by moving the magnet from the side to the back of the tube. When an AC electric field was applied, the metal surfaces of nickel-coated particles were made parallel to the field, as well as silver-coated ones. Then, as the ferrite magnet was brought closer from the side, a particle was oriented in such a way that the equatorial plane was parallel to the stationary magnetic field. In addition, when the direction of the magnetic field was reversed, the direction of the metal surface was reversed.