'Lissajous-like' trajectories in optical tweezers

Area of Science:

• Fluid dynamics
• Soft matter physics
• Optical manipulation

Background:

• Microscopic particles in low Reynolds number fluids generate hydrodynamic forces on their surroundings.
• Optical traps are used to manipulate and study micro-scale systems.

Purpose of the Study:

• To investigate the trajectories of an optically trapped microsphere subjected to time-varying hydrodynamic flow fields.
• To demonstrate and analyze the breaking of time-reversal symmetry in microparticle motion.
• To explore the resulting fluid-pumping effect.

Main Methods:

• Experimental observation of 'Lissajous-like' trajectories of a probe microsphere.
• Utilizing a nearby moving 'actuator' particle to generate time-varying hydrodynamic flow fields.
• Comparison of experimental results with Stokesian dynamics simulations.

Main Results:

• Observed 'Lissajous-like' trajectories for the optically trapped probe particle.
• Demonstrated breaking of time-reversal symmetry in the probe's motion under specific actuator movements.
• Identified a fluid-pumping effect resulting from the interplay of hydrodynamic flow and optical restoring force.
• Experimental trajectories showed good agreement with Stokesian dynamics simulations.

Conclusions:

• The interaction between optically trapped particles and externally generated hydrodynamic fields can lead to non-reciprocal motion and fluid pumping.
• The presence of an optical trap can inadvertently perturb the micro-environment being studied.
• Findings have implications for active micro-rheology and micro-scale flow measurements.