Dean, I will try to answer your questions, pasted below:
Just a question. Does Stokes Law apply to colloidal suspensions?? Does the + charge on the particles modify the gravitational action on those particles?? According to Stokes' Law, the terminal velocity of a spherical particle settling under the influence of gravity in a fluid of a given density and viscosity is proportional to the square of the particle's radius. With the particles we are dealing with, the terminal velocity is attained almost instantaneously. Therefore, we can assume that a particle starts to settle at a constant terminal velocity immediately after the container of fluid is set to rest. However, Stokes' Law rests upon several simplifying assumptions which are listed below: 1. The particles are sufficiently large to be unaffected by the thermal (Brownian) motion of the fluid molecules 2. The particles are rigid, spherical, and smooth. 3. All particles have the same density 4. The suspension is sufficiently dilute that particles do not interfere with one another and each settles independently. 5. The flow of the fluid around the particles is laminar, i.e., no particle exceeds the critical velocity for the onset of turbulence. In the case of CS preparations at concentrations typically produced, I believe that points 2 through 5 do not have any significant effect on the settling rate calculated by Stokes' Law. It is point 1 that is problematic. Very small particles, those less than about 1 micron diameter, can be retained in suspension against the force of gravity due to the random bombardment of the particle by molecules in the fluid suspending agent. This is the reason why small particles can form a stable colloid in a fluid even though they are not in solution. In my chemistry books, a colloid is defined as a fluid containing particles in a size range from 10 angstroms to 1 micron. So apparently, a particle smaller in size than about one micron does not settle at a rate that can be described purely by Stokes' Law. We must take into account the effects of Brownian motion, which tend to keep the particles suspended. If there is someone on this list who is familiar with colloid chemistry, perhaps they could add information about what is known about settling rates of particles less than one micron in diameter. As I have time, I will try to do some library research on this point. As regards the charge on the silver particles, I will try to give my best answer. The silver particles in the CS all have a positive charge as I understand it. Like charges repel each other, which means that the silver particles will want to stay apart. These repelling forces will be active in all directions so the net effect is that they will not interfere with the settling rate imposed by gravity. Now if we have particles of opposite charge in the fluid, they will be attracted to each other and form larger aggregate particles. The aggregates would settle at a faster rate then the individual particles. This is the principle used with flocculating agents, which have opposite charges from material in suspension. They are often used in treatment of liquid wastes to settle out suspended solids. In the case of CS preparations, at least those produced with pure water, the only particles in suspension should be the silver particles. Therefore, there should not be any flocculation (aggregation) of particles which would increase the settling rate. I would invite any others on the list to contribute their knowledge on this point. I hope this answers your questions. Jeff La Favre -- The silver-list is a moderated forum for discussion of colloidal silver. To join or quit silver-list or silver-digest send an e-mail message to: [email protected] -or- [email protected] with the word subscribe or unsubscribe in the subject: line. To post, address your message to: [email protected] List maintainer: Mike Devour <[email protected]>
