The electrokinetic properties of colloidal particles in nonpolar liquids are relevant for applications such as E-ink displays and liquid toner printing. The electrokinetic behavior is determined by the particle's surface charge, size and shape, and the ionic strength of the liquid.
For non-spherical particles, there is not only an (electrical) force, but also a torque. Currently there is no commercial system that can measure the anisotropy of the electrokinetic behavior of such particles. The goal of this thesis is to employ Holographic Optical Tweezers (HOT) to control the particles orientation and measure the three-dimensional diffusion and electrokinetic mobility tensors.
An Optical Trap (or Tweezers) consists of a tightly focused laser beam. Particles with high refractive index are attracted to the focus and are trapped in this position. The source of this ‘classical’ trap is the intensity gradient of the beam; the particle is attracted towards the region with the highest optical density. HOT is an extension of Optical Trapping, where a single trapping beam can be converted into an array of multiple beams, non-Gaussian beams or even a line trap as depicted in the figure below.
With HOT, one can obtain control over a particles position and its orientation. This enables the study of the particles’ anisotropy. This project involves calculating the required SLM pattern with a (partially) selfwritten program and performing the electrokinetic measurements with the beam patterns you designed.
The staff of the LCP group will provide you with all the necessary help and know-how, but we also encourage you to take the initiative to come up with your own ideas to tackle the project. Where feasible and within the scope of the project, we'll support you to develop these ideas.
Figure: A point trap, a line trap and a superposition of both traps allows control, not only over the particles position, but also over its orientation.