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WFU Physics Colloquium

TITLE: Confinement and Tracking of Brownian Particles in a Bessel Beam Standing Wave

SPEAKER: Chad McKell

M. S. Presentation
Mentor: Keith Bonin

TIME: Friday December 4, 2015 at 2:00 PM

PLACE: Room 101 Olin Physical Laboratory

All interested persons are cordially invited to attend.


Optical trapping is a useful tool for manipulating microscopic particles and probing the physical interactions of matter. However, previous optical trapping techniques introduced complications for analyzing Brownian particle diffusion in viscous media because they either restricted the particles’ motion to one dimension or trapped the particles too close to a surface. To our knowledge, this thesis presents the first realization of two-dimensional transverse tracking of Brownian microparticles in multiple, surface-isolated traps. To accomplish this, we used an axicon-generated, zeroth-order Bessel beam standing wave whose parameters were adjusted to allow tight axial confinement and loose transverse confinement of microscopic size particles in the central maximum of the Bessel beam. For our diffusion analysis, we collected tracking data of individually trapped fluorescent microspheres submerged in water and then quantified their motion using mean square displacement algorithms. To calculate the diffusion coefficients of each particle, we fit the mean-square displacement points to the generic expression for confined diffusion. As expected, our results using the expression for confined diffusion to model the particles' motion proved to be more accurate than the values obtained using Einstein's unconfined diffusion equation. In the future, we propose to analyze the data by adopting a more rigorous Bayesian statistical technique so as to better account for the optical potential of the Bessel beam standing wave. Our research has potential applications in microrheology and microfluidics; specifically, the diffusion coefficients extracted from our tracking data may be used to approximate the viscosity of semi-dilute polymers according to de Gennes’ viscosity theory, and the Bessel beam standing wave trap may be further utilized to measure fluid drag forces near surfaces.

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