Measurements of the force fields within an acoustic standing wave using holographic optical tweezers

Bassindale, P. G., Phillips, D. B., Barnes, A. C., & Drinkwater, B. W. (2014). Measurements of the force fields within an acoustic standing wave using holographic optical tweezers. Applied Physics Letters, 104(16), 163504.


Direct measurement of the forces experienced by micro-spheres in an acoustic standing wave devicehave been obtained using calibrated optical traps generated with holographic optical tweezers. A micro-sphere, which is optically trapped in three dimensions, can be moved through the acoustic device tomeasure forces acting upon it. When the micro-sphere is subjected to acoustic forces, it’s equilibrium position is displaced to a position where the acoustic forces and optical forces are balanced. Once theoptical trapping stiffness has been calibrated, observation of this displacement enables a directmeasurement of the forces acting upon the micro-sphere. The measured forces are separated into a spatially oscillating component, attributed to the acoustic radiation force, and a constant force, attributed to fluid streaming. As the drive conditions of the acoustic device were varied, oscillating forces (>2.5 pN) and streaming forces (<0.2 pN) were measured. A 5 m silica micro-sphere was used to characterise a 6.8 MHz standing wave, λ = 220 m, to a spatial resolution limited by the uncertainty in the positioning of the micro-sphere (here to within 2 nm) and with a force resolution on the order of 10 fN. The results have application in the design and testing of acoustic manipulation devices.


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