Abstract
We have studied the transverse and axial equilibrium positions of dielectric micro-spheres trapped in a single-beam gradient optical trap and exposed to an increasing fluid flow transverse to the trapping beam axis. It is demonstrated that the axial equilibrium position of a trapped micro-sphere is a function of its transverse position in the trapping beam. Moreover, although the applied drag-force acts perpendicularly to the beam axis, reaching a certain distance r(0) from the beam axis (r(0)/a approximately 0.6, a being the sphere radius) the particle escapes the trap due to a breaking axial equilibrium before the actual maximum transverse trapping force is reached. The comparison between a theoretical model and the measurements shows that neglecting these axial equilibrium considerations leads to a theoretical overestimation in the maximal optical transverse trapping forces of up to 50%.
Highlights
The use of optical forces for trapping, manipulating and sorting particles or living cells in microfluidic devices is a growing field of research, with potential applications in biotechnology and gene technology [1, 2, 3, 4, 5, 6, 7]
(13), predicting the axial equilibrium position z eq 0 of a trapped microsphere as a function of its transverse position r0, is presented in Fig. 6(a) for the parameters given in Tab. 1 and in the pure ray-optics (RO) approximation
The model predicts that, as the particle is displaced from the optical axis, its axial equilibrium position shifts in the positive direction
Summary
The use of optical forces for trapping, manipulating and sorting particles or living cells in microfluidic devices is a growing field of research, with potential applications in biotechnology and gene technology [1, 2, 3, 4, 5, 6, 7]. Provided that the numerical-aperture is sufficiently high, the force due to the field-gradient can overcome the forces due to backscattered light, a three-dimensional optical trap is created This optical trapping technique, first reported by Ashkin et al [8], is commonly referred as optical tweezers or single-beam gradient optical trap. It is implicitly assumed that the particle only displaces radially (orthogonally to the beam axis) and stays in the focal plane of the focused laser beam. As this was already observed by Sato et al [17] and Ashkin [10], and theoretically proposed by Mazolli et al [18], this is not correct. When considering an optically trapped particle submitted to an increasing transverse liquid flow, as the drag-force increases the microsphere displaces from its resting equilibrium position - located somewhat above the focus - both in the radial and the axial directions, before it escapes the trap
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