In this study microscopic gas bubbles (7-12 μm diameter) suspended in water were optically trapped in a custom-built
microfluidic slide using holographically generated Laguerre-Gaussian (‘doughnut’) beam optical tweezers. The optical
potential was then characterized as a function of bubble size, trapping laser power and trapping beam diameter
(Laguerre-Gaussian beam mode) using the trap spring constant in the plane transverse to the beam propagation direction,
obtained from the position fluctuations of the bubble in the trap measured by video microscopy and particle tracking. It
was found that microbubbles were held at the equilibrium position of buoyant and optical forces at a distance from the
focus of the beam that increased with laser power, and that optical trapping in this configuration was only possible within
a specific range of trap and bubble parameters. Furthermore an optimum size of the doughnut beam to microbubble
diameter which maximized the transverse spring constant was found . A ray optics model of the optical forces acting on
microbubbles in a focused Laguerre-Gaussian beam was used in order to calculate the trap spring constants and
equilibrium trapping position as a function of the different parameters, and highlight key physical behaviours.
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Chris Fury ; Caroline Harfield ; Philip H. Jones ; Eleanor Stride and Gianluca Memoli
Experimental characterisation of holographic optical traps for microbubbles
", Proc. SPIE 9126, Nanophotonics V, 91263L (May 2, 2014); doi:10.1117/12.2055889; http://dx.doi.org/10.1117/12.2055889