Fabrication and holonomic control of ‘dot’ Janus particle
Through the use of optical trapping, the three translational degrees of freedom of particles that range from tens of nanometers to micrometers in size can be controlled. This technique alone, however, cannot control both a particle’s three translational and three orientation degrees of freedom. Adding a magnetic force has been found to give control over four degrees of freedom of asymmetric particle or particle aggregate, but these magnetic particles have made the optical traps unstable. In order to achieve five or more degrees of freedom, Janus particles can be used.
Janus particles generally refer to a class of colloids with two dissimilar faces having unique material properties. The spherical asymmetry associated with Janus particles is the key to realizing many commercial applications, including electrophoretic displays, nanosviscometers, and self-propelling micromachines. A Janus particle with a magnetic metal coating on one of its faces would be ideal for manipulating the particle’s six degrees of freedom. These types of Janus particles, however, have been found to be unstable in optical traps. The metal coating inhibits light transmission through half of the particle, leading to unbalanced gradient and scattering forces, which consequently expel a Janus particle from the center of the trap.