Membrane inertial impaction

The retention efficiency of membranes is dependent on particle size and concentration, pore size and length, porosity, and flow rate. Large particles that are smaller than the pore size have sufficient inertial mass to be captured by inertial impaction. In liquids the same mechanisms are at work. Increased velocity, however, diminishes the effects of inertial impaction and diffusion. With interception being the primary retention mechanism, conditions are more favorable for fractionating particles in liquid suspension. 

Particles with diameters between 1 and 5 pm are deposited in the tracheobronchial region as a result of either inertial impaction at airway bifurcations or gravitational sedimentation onto other airway surfaces. Undissolved particles may then be removed by the action of the mucociliary defense system working as an escalator particles trapped in the mucus are propelled toward the pharynx by the action of thin cilia located on the surface membrane of specialized cells. Once in the pharynx, the particles may be swallowed. The efficiency of the escalator defense system may be greatly impaired by various environmental contaminants, like sulfur dioxide, ozone, and cigarette smoke that are known to paralyze the activity of the ciliated cells and consequently the upward movement of the mucus. 

Figure 2.24 Latex particles captured on a capillary-pore membrane by inertial impaction.

For "tortuous-pore membranes" the minimum in the curves of Figures 2.22 and 2.23 is much less pronounced. This is because the tortuous path results in more and smaller particles captured by inertial impaction. Further, the longer path length through the pore results in more and larger particles captured by diffusional deposition. 

The membrane retention is determined almost solely by the size and shape of the particles (including macromolecules) in suspension. For particle sizes larger than the pore size of the membrane, capture of particles is by means of interception. For particles that are smaller than the pore size, the predominant capture mechanism is inertial impaction. As the particle size and/or inertial mass decreases, diffusion is the primary capture mechanism. In addition, increased viscosity of the fluid will greatly diminish the effect of inertial impaction and diffusion. In this case, interception would be the primary collection mechanism. 

In inertial capture, relatively large particles in the flowing liquid cannot follow the fluid flow lines through the membrane s tortuous pores. As a result, such particles are captured as they impact the pore wall. This capture mechanism is... 

A unique interaction between fluid mechanics and transport exists for filtration processes. Such processes perform better than expected based on the predicted impact of concentration boundary layers. The improvement in performance, a rare occurrence for membrane processes, arises from a combination of hydrodynamic diffusion and inertial lift [51]. Hydrodynamic interactions between particles or colloids that accumulate in the concentration boundary layer lead to shear-induced diffusion away from the membrane surface. Shear-induced diffusion can be significantly larger than molecular diffusion and thereby reduce surface concentrations. For sufficiently large particles at high shear rates, inertial lift becomes the dominant mechanism for particle movement away from the membrane. 

---