Category Archives: optical trapping

Biomaterials, optical trapping

Soft particle trapping in a dual-beam optical trap

Stability and instability for low refractive-index-contrast particle trapping in a dual-beam optical trap

Alison Huff, Charles N. Melton, Linda S. Hirst, and Jay E. Sharping

Biomedical optics express, 6 (10), 3812-3819 (2015)

A dual-beam optical trap is used to trap and manipulate dielectric particles. When the refractive index of these particles is comparable to that of the surrounding medium, equilibrium trapping locations within the system shift from stable to unstable depending on fiber separation and particle size. This is due to to the relationship between gradient and scattering forces. We experimentally and computationally study the transitions between stable and unstable trapping of poly(methyl methacrylate) beads for a range of parameters relevant to experimental setups involving giant unilamellar vesicles. We present stability maps for various fiber separations and particle sizes, and find that careful attention to particle size and configuration is necessary to obtain reproducible quantitative results for soft matter stretching experiments.

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Bifurcation plots showing the axial stability locations vs. particle radius for three different traps with fiber separations of (a) 45 μm, (b) 92.9 μm, and (c) 129.1 μm. The red dots represent the simulated stable trapping location, and the symbols represent experimentally observed stably trapped particles. The trap center is located at z=0. Trapping locations are shown schematically within (c). The error bars were determined empirically through measurements of the inner and outer edges of both the fiber and beads.
Bifurcation plots showing the axial stability locations vs. particle radius for three different traps with fiber separations of (a) 45 μm, (b) 92.9 μm, and (c) 129.1 μm. The red dots represent the simulated stable trapping location, and the symbols represent experimentally observed stably trapped particles. The trap center is located at z=0. Trapping locations are shown schematically within (c). The error bars were determined empirically through measurements of the inner and outer edges of both the fiber and beads.