Study of a colloidal sphere near flat walls using oscillating optical tweezers

Abstract

ABSTRACT We study the dynamics of a micron-sized colloidal sphere in two cases; one is for a particle near a single flat wall and the other is for a particle confined between two parallel flat walls. In this geometry, the force felt by a moving particle is quite different from that of a moving particle in unbounded space. Even though the presence of wall(s) complicates the flow field surrounding the colloidal sphere, the dynamics of a colloidal sphere near flat walls provides a model system with which to understand the phenomenon of more complex systems whose boundaries can be modeled as effective walls.[1] In this work, hydrodynamic interactions of colloidal sphere with nearby plat wall(s) are studied by using oscillating optical tweezers and compared with known theories and other experimental results using different techniques. Keywords: Oscillating optical tweezers, Fa xen’s law, colloidal sphere 1. INTRODUCTION In many experiments using optical tweezers,[2] colloidal particles are used as handles to study the mechanical properties of biological systems. If these particles are positioned near flat surfaces, the static and dynamic forces between the colloidal particles and the surfaces influence the mechanical properties of the system. A well-known “wall effect”[3] is a good example in which nearby flat walls have hydrodynamic effects on the colloidal particles. Similarly when a colloidal particle near flat wall(s) is trapped and oscillated by oscillating optical tweezers,[4] the motion of particle is also influenced by the presence of the walls. Therefore, the hydrodynamic force on the particle positioned near walls is quite different from the hydrodynamics drag force (known as Stokes’ drag) on the particle in unbounded space. The hydrodynamic dr ag force on an isolated partic le at very low Reynolds number near a flat wall(or walls) had been theoretically studied by several researchers. Most works were based on the solution of the linear hydrodynamic equations obtained under the creeping approximation, known as Stokes’ equation. The most known theoretical work is Faxen’s early study for a single sphere’s hydrodynamics near a rigid planar surface.[3] In biological processes, this situation is ubiquitous and governs the biological particle’s hydrodynamics. Therefore, in order to estimate the mechanical properties of biological systems, detailed study of colloidal particle-wall interaction is necessary. Most previous experimental works were measuring the diffusion coefficient of the Brownian motion of an isolated particle near wall(s) using the video imaging microscopy technique.[1] In this work, we measure the hydrodynamics drag force on a single colloidal particle in two cases: one is for a particle n ear a single flat wall and the other is for a particle confined between the two parallel flat glass walls. The particle is trapped at selected distance far from the wall and oscillated by using an oscillating optical tweezers. By simultane ously detecting both the position and phase information of the particle displacement, we measure the hydrodynamic drag coefficient and compare with the theories and known experimental results.