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UCL Optical Tweezers Group:  3C00 Project on Optical Tweezing of Microbubbles

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The aim of John and Steffen's project was to study the trapping of microscopic bubbles in an optical tweezers, and the factors that affect the magnitude of the trapping force, such as laser power, bubble size, and trap size and geometry.

The microbubbles are a commercially available ultrasound contrast agent, and have diameters of up to 10 mm.  They are interesting to trap because their refractive index is lower than that of the suspending fluid, so they are repelled from the part of the laser beam with highest intensity.

 

Contact Phil Jones if you are interested in a BSc, MSci or MSc project for the coming year

Steffen Hilgemann operating the optical tweezers

We overcome this by rapidly scanning the laser beam in a circle such that the bubbles experience a time-averaged trap that is a dark volume surrounded by higher intensity in all directions.  

The picture below shows two optically trapped microbubbles in the tweezers, demonstrating that they may be manipulated in all three dimensions.

 

John Wright preparing microbubbles on a microscope slide

John and Steffen's results helped us to determine how the optical trapping force depends on the microbubble and trap size, finding that the efficiency of the trap (in the axial direction) is

Qax (rb/RT)2

Some of the data from this project appeared in a paper in a special issue of the Journal of Optics A (2007).

 

Two microscopic bubbles being manipulated in the optical tweezers


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