Theoretical and experimental studies of electric field enhanced deposition of flame-synthesized silica onto a disk target were carried out in relation to applications in optical waveguide preform manufacturing. The deposition method utilized an imposed electric field to induce a charge on the particle and cause electrophoretic drift in addition to thermophoretic drift towards the deposition target. An analytical model utilizing an axisymmetric, viscous stagnation-point flow analysis was developed. From the model calculations, the overall thermophoretic deposition rate (mass/time) onto the target was found to be proportional to the target size to the power of 3 2. In the presence of an applied electric field the model calculation results showed that for a constant particle density and a constant particle charge the overall deposition rate increased as the strength of the applied electric field increased. The results also showed that for a constant particle density and a constant applied potential the deposition rate increased as the degree of particle charging increased. To confirm the analytical results, experiments were carried out. After preliminary experiments to establish optimal conditions for deposition measurements, silica deposition rates onto targets were measured both in the absence and in the presence of applied electric fields. The experimental results for thermophoretic deposition were found to be within 12% with respect to the deposition rates predicted by the model. When a potential of -1.6 kV was applied to the 9 cm target, for particle average charge of 2.78e (e: electronic charge) the deposition rate (0.28 g min-1) was increased by approximately 35% compared to the thermophoretic deposition rate (0.2 g min-1). Related to the deposition measurements, interesting aspects of silica charging mechanisms were discussed.
Bibliographical noteFunding Information:
Acknowledyements--The work has been supported by the National Science Foundatton under Grant CTS9005765 and by the Center for Combustion Research at the University of Colorado at Boulder. The authors acknowledge many fruitful conversations with Dr. Jerry R. Bautista at the AT&T Bell Laboratories, Professor Ralph Greif at the University of California at Berkeley, and Professor F. J. Weinberg and Professor J. Lawton at the Imperial College of Science, Technology and Medicine at London.
All Science Journal Classification (ASJC) codes
- Environmental Engineering
- Mechanical Engineering
- Fluid Flow and Transfer Processes
- Atmospheric Science