Chapter 18 - Diffusiophoresis of Porous Particles

Abstract

Abstract Diffusiophoretic motion of a single charged porous particle suspended in an infinite medium is analyzed theoretically here in this chapter. Corresponding behavior in suspensions, either dilute or concentrated, is investigated as well. The fundamental mechanisms are essentially the same as those discussed earlier in Chapters 16 and 17 for rigid particles and liquid droplets or gas bubbles, respectively. The only difference here is the permeability of the porous particle as well as the fixed charges distributed within the entire particle interior. As a result, the hydrodynamic drag force is drastically decreased when the particle is highly permeable and the electric driving force is significantly larger than the rigid particle situation for a highly charged porous particle due to the much larger total amount of charges. The diffusiophoretic mobility for a porous particle, thus, is much larger than the corresponding rigid particle situation, similar to the electrophoresis phenomenon discussed in Chapters 7 , especially for particles both highly charged and highly permeable. Theoretical investigation conducted here on the diffusiophoresis of porous particles can be applied in biochemical and biomedical fields involving proteins and DNAs, for instance, by far the two most important polyelectrolytes that can be modeled as charged porous particles successfully. Both chemiphoresis in KCl solution and electrophoresis in NaCl solution are investigated as two classic sample systems. The results are presented in the form of general Henry's charts, with further examination of the impact by various electrokinetic parameters, such as the fixed charge level, the double layer thickness, the particle mobility, and the β factor. The diffusiophoretic motion of a porous particle is slower than the corresponding electrophoresis in general, although still much faster than the diffusiophoretic motion of a rigid particle. The particle mobility increases with increasing volume fraction in a suspension sometimes. And a lesser charged particle may move faster than a highly charged one.