Part II Undergraduate Student, University of Oxford, 2000–2001
D.Phil. Student, University of Oxford, 2001–2005
Optical Trapping of Colloidal Particles
The forces between two oscillating hydrodynamically coupled colloidal particles have been investigated to surface separations of ~ 50 nm with OPTICAL TRAPPING. The colloidal systems chosen for study were a model hard sphere polystyrene suspension in deionised water and a 2-ethlynaphthalene oil-in-water emulsion stabilised with the anionic surfactant AOT. The hydrodynamic and electrostatic forces between two particles held in harmonic traps were modelled. The model successfully predicted the gentle changes in the amplitude of oscillation for synchronously oscillating particles and the phase lag between the particles and the laser beams creating the optical traps. It also predicted the steep drop off of the particle amplitude for asynchronously oscillating particles, and the change in the phase behaviour when electrostatic interactions are present. Alterations to the normal optical tweezers set up were identified and the solutions to them presented.
Polystyrene particles have been successfully trapped in an evanescent wave interference pattern at the water/silica interface. The structures formed in the evanescent interference pattern was found to depend on the particle size and density of particles in the pattern. For large particles (700 nm – 945 nm) the particles form lines across the interference fringes. For smaller particles (390 nm – 520 nm) the particles form 2 and 3 D arrays that have their long axis across the interference fringes. The separation between particles in this direction was found to be twice the interference fringe spacing. The spacing along the fringes was found only to depend on the particle size and salt concentration. A second phase was observed to spontaneously form with hexagonal packing.
The structures in the evanescent wave were not restricted to interference fringes. A hexagonal phase (rotated by 30º to that observed with interference fringes present) was observed when the interference fringes were turned off. Therefore the interference fringes were only pinning the structure formed, not causing the 2 D structure to form.
C. D. Mellor and C. D. Bain "Array Formation in Evanescent Waves" ChemPhysChem, in press (DOI).
C. D. Mellor, C. D. Bain and J. Lekner "Pattern Formation in Evanescent Wave Optical Traps" Proceedings of SPIE – The International Society for Optical Engineering 2005, 5930, 352–361 (DOI).
C. D. Mellor, M. A. Sharp, C. D. Bain and A. D. Ward "Probing Interactions between Colloidal Particles with Oscillating Optical Tweezers" Journal of Applied Physics 2005, 97, 103114/1–11 (DOI).