Optically driven oscillations of ellipsoidal particles. Part I: experimental observations

Area of Science:

• Physics
• Optical phenomena
• Soft matter physics

Background:

• Dielectric particles are influenced by light.
• Particle shape affects optical manipulation.
• Understanding particle dynamics is crucial for microfluidics and material science.

Purpose of the Study:

• To experimentally observe the mechanical effects of light on ellipsoidal dielectric particles.
• To investigate the influence of particle shape on optical manipulation.
• To model the observed particle dynamics.

Main Methods:

• Optical levitation using a moderately focused laser beam.
• Manipulation of polystyrene ellipsoids with varying aspect ratios (0.2 to 8).
• Perpendicular imaging to capture particle motion.

Main Results:

• Moderately shaped particles (moderate aspect ratio) are radially trapped with their long axis parallel to the beam.
• Highly elongated (k > 3) or flattened (k < 0.3) ellipsoids exhibit coupled translation-rotation oscillations ('dancing').
• Two bifurcations between static and oscillating states were identified at k ≈ 0.33 and k ≈ 3.

Conclusions:

• Particle shape significantly dictates its response to optical forces.
• Oscillatory motion arises from coupled translation-rotation dynamics.
• A simple model based on ray optics explains the observed oscillations.