# External Field Response and Applications of Metal Coated Hemispherical Janus Particles
## 3 Results and Discussion
### Transmission and Reflection Measurement with an Electric Field  
The results of application of an AC electric field to the silver coated particles are shown in Figure 7. It turned out that the metal surface was oriented parallel to the field, which was kept while the field was on. This is because the electric-field induced electric dipole moment is maximally aligned in the direction of the electric field.  
Figure 6: Relative reflectance (\(R\)) when the light was incident on a metal coated hemispherical particle from the spherical surface and from the equatorial plane. \(R\) reaches 80% when the light is incident on the equatorial plane.  
The transmittance of the suspension before and after the electric field was applied is shown in Figure 8. When the electric field was applied, the transmitted light intensity increased by 10 to 15 percent. This is explained as follows. Without an electric field, metal coated hemispherical Janus particles in water are stabilized in such a way that the metal surface is perpendicular to the direction of gravity. Therefore, the light that is passing perpendicularly to the ITO electrode is blocked by the metal surface. With the electric field that is applied, the metal surface becomes parallel to the field. Then, light passing perpendicularly to the ITO electrode does not hit the metal surface, but is transmitted. In other words, it functions as an electric-field controlled shutter, in such a way that it blocks and passes light without and with the electric field, respectively.  
From the number density of the particles that were estimated from the microscope image, the coverage of the particles was 15.7% of the transmission area if the cross section of all the particles is assumed to be a circle with a diameter of 2.5 \(\upmu\)m. When the electric field is applied, the cross-section turns to a semicircle for the effective coverage to be 0% (if the light is perfectly transmitted through the transparent semicircle) to 8% (the light is completely deflected by the transparent semicircle, which is  
Figure 8: Change in the transmitted light intensity of the ITO-sandwiched suspension of silver coated hemispherical Janus particles with the 15.7% cross sectional coverage observed in the experimental setup in Figure 4. (**a**) When the electric field was applied across the ITO electrodes, the transmitted light intensity was increased. (**b**) The ratio of the transmitted intensity with the electric field versus that without the field, showing the transmittance is increased by 10% to 16% with the application of the electric field.  
Figure 7: AC electric field response of a silver coated hemispherical Janus particle. (**a**) Without an electric field, the metal surface was horizontally oriented (perpendicularly to the direction of gravity). (**b**) With an electric field \(E\), the metal surface was oriented parallel to the field. (See also Video S1).  
actually the transparent hemisphere, due to refraction etc.), which is in reasonable agreement with the observed increase in the transmittance by 10-15%. Therefore, if the number density of particles is increased, the function as a shutter will be more enhanced. This effect is clearly seen in the photograph in Figure 9, where a reflection and transmission switch is demonstrated with the bulk suspension.