Chungil Ha

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

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.

Optical rotation of the shrunken multi-lamellar vesicle using optical tweezers

We study a geometrically anisotropic internal nano-layered structure of the shrunken multi-lamellar vesicle(SMLV) by using optical tweezers with a polarized beam. The SMLVs are synthesized from soybean asolectin by using gentle hydration method and has an optically birefringence property, known as a form birefringence. When a laser beam of optical tweezers with an elliptical polarization passes through the material with an optical birefringence, the ordinary and extraordinary components of the laser light experience different phase shift, respectively. Therefore, an optical torque due to the angular momentum conservation can be exerted on the optically birefringence material generating a rotational motion of the system. In this work we analyze the distance between the next bilayers of the membrane structure in SMLV from the experimentally measured data of the rotational motion and propose a simple model in which the SMLV is idealized as a multi-regularly-thin parallel plate structure.