  Developing angular trapping methods, which will enable optical tweezers to
rotate a micronized bead, is of great importance for the studies of
biomacromolecules during a wide range of torque-generation processes. Here we
report a novel controlled angular trapping method based on composite Janus
particles. We used a chemically synthesized Janus particle, which consists of
two hemispheres made of polystyrene (PS) and poly(methyl methacrylate) (PMMA)
respectively, as a model system to demonstrate this method. Through
computational and experimental studies, we demonstrated the feasibility to
control the rotation of a Janus particle in a linearly polarized laser trap.
Our results showed that the Janus particle aligned its two hemisphere's
interface parallel to the laser propagation direction as well as the laser
polarization direction. In our experiments, the rotational state of the
particle can be easily and directly visualized by using a CMOS camera, and does
not require complex optical detection system. The rotation of the Janus
particle in the laser trap can be fully controlled in real time by controlling
the laser polarization direction. Our newly developed angular trapping
technique has the great advantage of easy implementation and real time
controllability. Considering the easy chemical synthesis of Janus particles and
implementation of the angular trapping, this novel method has the potential of
becoming a general angular trapping method. We anticipate that this new method
will significantly broaden the availability of angular trapping in the
biophysics community, and expand the scope of the research that can be enabled
by the angular trapping approach.
