"Towards a dynamic control of spurious forces with optical tweezers"

Towards a dynamic control of spurious forces
with optical tweezers

Ignacio Bordeu (a), Giovanni Volpe (b), Juan Pablo Staforelli (a)
(a) Center for Optics and Photonics, Universidad de Concepción, Casilla 160-C, Concepción, Chile.
(b) Department of Physics, Bilkent University, Çankaya, Ankara 06800, Turkey.

A spatial diffusion gradient appears in situations where the diffusion coefficient of a colloidal object
becomes position dependent, which causes a spurious drift- a shift of the position distribution mean.
In this work, the stochastic nature that account in this phenomena is studied preliminarily in a system
of two polystyrene micro-spheres in an optical tweezers set-up. Here, the spurious drift, and the
related, spurious forces account for an asymmetry in the particle position distribution which is
modulated via spatial control of the initial particle distribution. Experimental data gives the
diffusion term, the drift of the mean position, the propagator shape and some fundamental stochastic
parameters of the system. Additionally, it is a challenge to perform a real-time spatial control by
setting diffusion gradients through dynamic wall geometries. This would allow one to observe the
brownian particle transport through a channel whose walls - multiple trapped particles- are modulated
in real-time. To perform this, a real-time feedback control mechanism with holographic optical traps
is engineered to adjust the position of the channel in response to information of the particle drift
from its initial position. This work aims to study the behavior of the drift term in the interaction
between few particles, particle-wall and particle-potential, as well to get new insight in natural
scenarios concerning diffusive process as those found in cell membrane biology.

Keywords: Brownian motion, thermodynamics, diffusion gradient, holographic optical tweezers,
feedback control, digital video microscopy.

Place: RIAO/Optilas 2013
22-26 July 2013
University of Porto, Porto, Portugal