Influence of viscous force on the dynamic process of micro-sphere in optical tweezers

With the advantages of noncontact, high accuracy, and high flexibility, optical tweezers hold huge potential for micro-manipulation and force measurement. However, the majority of previous research focused on the state of the motion of particles in the optical trap, but paid little attention to the early dynamic process between the initial state of the particles and the optical trap. Note that the viscous forces can greatly affect the motion of micro-spheres. In this paper, based on the equations of Newtonian mechanics, we investigate the dynamics of laser-trapped micro-spheres in the surrounding environment with different viscosity coefficients. Through the calculations, over time the particle trajectory clearly reveals the subtle details of the optical capture process, including acceleration, deceleration, turning, and reciprocating oscillation. The time to equilibrium mainly depends on the corresponding damping coefficient of the surrounding environment and the oscillation frequency of the optical tweezers. These studies are essential for understanding various mechanisms to engineer the mechanical motion behavior of molecules or microparticles in liquid or air.