Forces in the Brownian World
Abstract
On a micrometer scale, thermal noise dominates the trajectories of small particles immersed in water. These particles exhibit a diffusive, or Brownian behavior. This thesis is an experimental study of the interplay between forces and the Brownian trajectories. The particles are visualized through a microscope. Real time computer processing of the recorded images allows extensive statistical analysis of the trajectories. Different types of forces are studied. The depletion force created by a sea of small spheres (35 nm diameter) on a large sphere (1 micron diameter) next to a wall is an example of a force deriving from a potential energy. The depth of this entropic potential is measured for different volume fractions of the small spheres fluid. Hydrodynamic forces do not derive from a potential energy. We study them in the simple case of micron sized particles confined vertically between two glass plates. The observed value of the diffusion coefficient deviates from the Stokes-Einstein law. It is a boundary effect. Finally, we study the response of a Brownian particle to a localized trapping that we move in space and modulate in time. This potential is created optically by strongly focusing a laser beam: the beam focal point acts as a trap, or tweezer for the dielectric particle. Using such a device, we confine the particles diffusion in one dimension along a circle. Also, the time modulation of a one dimensional periodic asymmetric potential induces a net drift of particles. Such a motor, called a "thermal ratchet", could be used for sorting particles according to their sizes. We build in the last part of the thesis such an engine by depositing gold electrodes on a glass plate using lithography techniques. Here, the potential is created by applying an AC electric field (100 kHz) between the electrodes. The separation of particles according to their sizes is function of the modulation time.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- January 1995
- Bibcode:
- 1995PhDT........13F
- Keywords:
-
- DEPLETION FORCE;
- OPTICAL TWEEZERS;
- Physics: General