# External Field Response and Applications of Metal Coated Hemispherical Janus Particles
## 4 Conclusions and Prospects  
We reported the various properties of metal coated hemispherical Janus particles from the application point of view. First of all, it was demonstrated that the particle has a property of mirror because the metal surface is flat. It was found that the orientation of the particles can be controlled by electric and magnetic fields. Controlling the orientation of the particle is very important in their applications to microoptics and microfluidics. From these characteristics, we demonstrated the function as optical shutters. They are also usable for micromirrors and micro variable color filters. Demands for micrometer-sized mirrors and their angle control is wide for the use in digital micromirror devices [37], displays, projectors, etc. They are also suited for valves in micro fluidic channels because they can be operated readily in a narrow space [38]. They can also be used as minute pumps, which cannot be done with spherical Janus particles, but takes the most advantage of the hemispherical shape. There should also be demands in medical fields. A micro-mirror device is also used as medical equipment for laser treatment [39]. To perform minimally invasive treatment, it is promising to reduce the size of each instrument while using the metal coated hemispherical Janus particles. It is expected that one can find many other applications utilizing the properties that the particles can be orientationally controlled. The fact that the particles are hemispherical and that they can be coated with a functional material on one side is of great advantage for the applicability to be unbounded.  
Figure 13: (**a**) When a magnetic field \(B\) is applied (parallel to the page) together with an electric field \(E\) (vertical to the page), the metal surface is oriented parallel to both electric and magnetic fields. (**b**) Reversing the magnetic field reverses the metal surface direction.  
**Supplementary Materials:** The following are available online at [[http://www.mdpi.com/2076-3417/8/4/653/s1](http://www.mdpi.com/2076-3417/8/4/653/s1)]([http://www.mdpi.com/2076-3417/8/4/653/s1](http://www.mdpi.com/2076-3417/8/4/653/s1)), Video S1: Response of an Ag-coated particle when an AC electric field is switched on, Video S2: Reflection change of the ITO-sandwiched suspension of Ag-coated particles with an AC electric field on, Video S3: Reflection change of the suspension of Ni-coated particles assembled in a chain-like form when the magnetic field direction is changed from perpendicular to parallel with respect to the light propagation direction.  
**Acknowledgments:** The authors would like to thank [PERSON] and [PERSON] for counting the number of the particles. Hemispherical particles are provided by [PERSON], Sekisui Plastic Co., Ltd., Nishi Tenman 2-4-4, Kita-ku, Osakashi, Osaka, Japan.  
**Author Contributions:** [PERSON] and [PERSON] conceived and designed the experiments; [PERSON] prepared samples and performed the experiments; [PERSON] contributed sample preparation; [PERSON] and [PERSON] wrote the paper.  
**Conflicts of Interest:** The authors declare no conflict of interest.