Effect of particle morphology on thermophoretic velocity of aggregated soot particles

A particle under a temperature gradient experiences a force toward the colder region if a particle is very small. This phenomenon is called thermophoresis, a mass transfer phenomenon induced by the temperature gradient. It is well known that the magnitude of the thermophoretic force depends on the size of particle and the temperature gradient. In addition, the magnitude of the thermophoretic force depends on various factors such as the ambient gas's kinematic viscosity and thermal conductivity and the morphology of the particle. To understand thermophoresis in detail, the effects of these factors need to be evaluated. In this study, we accurately measured the thermophoretic velocity of aggregated particles in order to understand the effect of particle morphology. We used carbon black particles of well-defined aggregation parameters to systematically understand the effect of morphology. In addition, we introduced a new optical system to measure the velocity and the size of each particle simultaneously. Five different samples of carbon black particles with different aggregation parameters were used to systematically understand the effect of morphology. The measured thermophoretic velocities were almost proportional to ν∇T/T. The measured dimensionless thermophoretic velocities, UT/(ν∇T/T), were much larger than those expected based on the size of aggregates and rather close to those expected based on the primary particle size even when the size of aggregates are larger than 100 mm. This result infers that thermophoretic velocity of an aggregated particle is governed by the primary particle size. The dimensionless density, the ratio of the bulk density to the true density, which represents the overall packing degree of aggregate, is also found to have a significant effect on the thermophoretic behavior.