Mechanical Motion Of Janus Particles In Optical Tweezers

Since optical tweezers were invented by A.Ashkin,they have become powerful tools to achieve three-dimensional trapping and manipulation of micro-/nano-sized dielectric particles and nano-sized metal particles.Moreover,many researchers utilize the trapped particles as handles to explore the unsolved mysteries of biological single cells and single molecules.Due to the isotropic optical properties of pure dielectric or metal micro-spheres,they will be firmly captured in the optical tweezers.Thus,it is difficult to observe various types of mechanical motions in microscopic world.But particles called Janus particles involve both dielectric and metal material and naturally provide a route to break the structure symmetry.They combine different optical properties,which makes the force process become too complicated to estimate the motion in the optical trap.So far,researches on the optical trapping of these special particles are basically blank.In this paper,we investigate the mechanical motion of Janus particles with dielectric-metal structure in various optical traps.This paper mainly includes five aspects.1.We developed the ray optics method to calculate the optical force and torque of gold film half-coated Janus particles in an optical trap.Combined with the theory of metal film and law of conservation of momentum,the optical force and torque can be calculated through the refraction,reflection and absorption of rays interactions on the Janus particles surfaces.2.We reported the systematic high-resolution optical observation of the spontaneous crystallization of monodisperse polystyrene(PS)micro-spheres in aqueous solution.The PS micro-spheres were drived into close-packed arrays in a static line optical tweezers(LOT).This experiment demonstrated two major crystal growth modes,the epitaxy growth pattern and the inserted growth pattern.These two growth modes played a key role in shaping the dynamics of both the one-dimensional(1D)and one-dimensional(2D)crystallization process.In addition,the complicated dynamic transformation process from 1D to 2D crystallization was clearly illustrated.These observations offer invaluable insights for in-depth research about colloidal crystal crystallization.3.We fabricated Janus particles by magnetron sputtering of a thin gold layer onto PS micro-spheres.We controlled the gap of particles through altering the density of PS solution.In this way,we can get the patterned and ideal Janus particles,respectively.In the experiments,it was found that a patterned Janus particle could stably rotate around the optical axis while an ideal Janus particle was hard to rotate.Both the rate and the direction of the rotor could be controlled by adjusting the position and intensity of the focused laser beam,respectively.Numerical analyses show that the observed directional rotation is the result of spontaneous symmetry breaking induced by the uneven patterns of metal coatings on the particles.4.We observed the self-propelled cyclic round-trip motion of a metallo-dielectric Janus particle in a LOT.Considering the thermal motion of metal film in a highly focused single optical trap,we used a cylindrical lens to generate the LOT,which decreases the confinement in one direction and reduces the intensity of optical field in each unit area.The intensities are lower at the two ends of the line and higher at the center,creating a small yet non-zero gradient force in the longitudinal direction.This gradient force always points towards the focal line center and varies with particle positions.There are two main reasons for this cyclic round-trip motion.One reason is the competing action of the propulsion force and gradient force.The other reason is that the spontaneous symmetry breaking provides the torque to change the orientation of particles.These two factors provide the necessary conditions for the round-trip motion.5.We observed that the Janus particles rotated synchronously around the annular optical trap.Taking the strong coupling ability of the translational and rotational motion of the Janus particles into account,we utilized an annular optical trap generated by the axicon lens to create a uniform intensity of the light railway.It reduced the thermal effect and provide a circular path with consistent curvature.So we can focus on the coupling motion of the translational and rotational motion of a Janus particle.In the experiments,we observed that the Janus particles rotated synchronously around the annular optical trap,which is similar to the moon rotating around the earth.The Janus particle revolves around the annular optical trap,this motion is called translation.At the same time,it rotates around its own axis,and this motion is called rotation.The frequencies of translation and rotation are the same.In a summary,Janus particles show a variety of motions in the point,linear,and circular optical fields,which exhibit its strong coupling ability of translation and rotation.In addition,the orientation of a Janus particle is automatically adjusted with the optical field.Janus particles have a high optical adaptability,and a strong potential application for the intelligent motion control of micro-/nano-sized particles.