LAUSR

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 Δz from the transverse xy imaging plane of an inverted microscope. Image first central moments μ01 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 T on the dimer was proportional to the velocity of flow v and resulting angular deflection Δθ so that T∝v∝Δθ. Displacements Δz at which v∝Δμ01∝Δθ, which are typically off focus, were examined in more detail; in this range, Δθ=hΔμ01. The hydrodynamics of the dimer were modeled as that of a prolate ellipsoid, and the constant of proportionality h was determined by comparing the short-time mean-squared variation measured during diffusion to that predicted by the model calculation: h2⟨Δμ012(t)⟩=⟨Δθ2(t)⟩. With h determined, the optical trap stiffness kθ was determined from the long-time restricted diffusion of the dimer. The measured kθ and Δθ can then be used compute torque: T=kθΔθ, potentially enabling the near vertical optically trapped dimer to be used as a torque probe.