Particle Capture Rate Research Articles

The influence of interparticle forces in the magnetic separation of submicron particles

The magnetic separation of ultrafine particles is described in terms of a number of particle - particle - fibre interactions some of which are important only in this size regime. Conditions for solid and diffuse buildup are established in terms of the total potential energy of a particle. The detailed form of the buildup is shown to be strongly dependent on the surface potential of particles and, consequently, the pH of the colloid. The calculated results are found to compare favourably with diffusion theory. The initial rate of particle capture is shown to be a quadratic function of time.

A non-motorized dye ejector for visualization of flow in situ and its use with coral reef crinoids

We describe a portable, non-motorized device for delivering a tracer dye into seawater under field conditions. Dye is ejected at a constant flow rate over a period of tens of minutes. The ejector works in a wide range of ambient pressures without external energy requirements. The flow rate is adjusted simply by varying the length of the delivery tube. The dye streams permitted observations of the upcurrent and downcurrent flow regimes for a filter-feeding crinoid (Comanthus bennetti) living at a depth of 8 m on a coral reef. The results indicate that the crinoid may enhance the rate of particle capture by changing the scale of turbulence in the water passing through the mesh of the filtration fan.

Effects of inertia on the diffusional deposition of small particles to spheres and cylinders at low Reynolds numbers

A formalism that accounts for inertial and diffusive effects in the dynamics of a dilute gas-particle suspension is introduced. The treatment is purely deterministic away from a very thin Brownian diffusion sublayer, while, within the sublayer, inertial effects are small, permitting a near-equilibrium expansion in powers of the Stokes number (particle relaxation time divided by flow characteristic residence time). This expansion provides phenomenological expressions for the particle velocity including two terms : the standard Brownian diffusion, and an additional inertial drift velocity which is closely related to the pressure diffusion term of the Chapman-Enskog expansion. As an example, the general formalism is applied in detail to the case of Stokes flow about a sphere, and sketched for the similar case of a cylinder. Two competing mechanisms are seen to affect the total rate of particle capture by the sphere : (if the stagnation-point region is considerably enriched in particles owing to the high compressibility of the particle phase, which leads to locally enhanced deposition; (ii) centrifugal forces tend to deplete the Brownian diffusion sublayer of particles, reducing diffusion rates away from the stagnation point to the surface. The first effect is seen to dominate over the second except in a very narrow zone of small Stokes numbers. Our method bridges the gap between Levich's solution for the ‘pure-diffusion’ limit and Michael's treatment in the ‘pure-inertia’ limit.

The influence of colloid stability on the formation and properties of dynamic hydrous Zr(IV) oxide membranes

A study of dynamic Zr(IV) hydrous oxide membrane formation was carried out on microporous supports in unstirred batch cells. Analysis of the flux decline behavior during the filtration shows two processes marked by a transition time τ: (a) a precake formation period during which the pores of the support are closed by particle capture and (b) the buildup of a surface cake. The stability of the Zr(IV) hydrous oxide colloidal solution and the chemical nature of the pore wall determine the rate of particle capture and provide the major influence on τ. The variation of the cake characteristics, its flux, and salt rejection with the pH, indicates an optimal value of formation pH between 3.2 and 3.8. At maximum performance, the equivalent diameter of capillaries in the cake is ∼50 Å.