Estimating orientation of optically trapped, near vertical, microsphere dimers using central moments and off-focus imaging

• Published in: Applied optics (2004) 2022-01, Vol.61 (2), p.607
• Main Authors: Raudsepp, Allan, Jameson, Geoffrey B, Williams, Martin A K
• Format: Article
• Language: English
• Subjects: Angular velocity; Diffusion; Dimers; Flow velocity; Microspheres; Optical traps; Perturbation; Stiffness; Time measurement; Torque
• ISSN: 1559-128X; 2155-3165
• DOI: 10.1364/AO.446610

Near vertical optically trapped dimers, composed of pairs of microspheres, and constructed in situ, were imaged in bright-field in flow and at rest, and with displacement from the transverse imaging plane of an inverted microscope. Image first central moments were measured, and their dependence on the imposed flow velocity of the surrounding fluid was calculated. This dependence was related to the at-rest restricted diffusion statistics. It was assumed that, for small perturbations, the torque on the dimer was proportional to the velocity of flow and resulting angular deflection so that ∝ ∝ . Displacements at which ∝ ∝ , which are typically off focus, were examined in more detail; in this range, = . The hydrodynamics of the dimer were modeled as that of a prolate ellipsoid, and the constant of proportionality was determined by comparing the short-time mean-squared variation measured during diffusion to that predicted by the model calculation: ⟨ 012( )⟩=⟨ ( )⟩. With determined, the optical trap stiffness was determined from the long-time restricted diffusion of the dimer. The measured and can then be used compute torque: = , potentially enabling the near vertical optically trapped dimer to be used as a torque probe.