Just a question. Does Stokes Law apply to colloidal suspensions?? Does the + charge on the particles modify the gravitational action on those particles??
Dean -----Original Message----- From: Jeffrey La Favre [mailto:[email protected]] Sent: Sunday, September 06, 1998 10:23 PM To: Silver List Subject: Measuring the particle size of CS I would like to submit a possible approach to measuring the size of silver particles in CS that does not require a microscope. The method is drawn from soil science, specifically, methods used to analyze particle sizes in soil. Particles of soil are routinely sized by passing a sample through a series of sieves. However, it is only practical to sieve particles down to a diameter of about 50 microns. Nearly all soils have particles smaller than this. In fact, clay particles are defined as particles with a diameter of 2 microns or less. Beyond the sieving limit of 50 microns, a sedimentation method is usually employed to determine silt and clay particle sizes. These methods rely on Stokes' Law, which can be used to calculate the settling rate of various sizes of particles of known density in a fluid medium of known viscosity. I will give a version of Stokes' Law below in equation form: t =18hn/d^2g(ps-pf) t = time in seconds h = settling distance in centimeters n = viscosity of the fluid in poise d = the diameter of the particle in centimeters ps = the density of the particle pf = the density of the fluid g = gravitational constant = 980.7 centimeters per second squared now some values, all for a temperature of 20 degrees centigrade viscosity of water = 0.01002 poise density of silver = 10.5 grams per cubic centimeter density of water = 0.998 grams per cubic centimeter Suppose we do our measurements of settling in water at 20 degrees centigrade. Furthermore, we want to know how much time it will take a silver particle of specific diameter to settle 10 centimeters in the fluid. Then substituting the appropriate values into Stokes' equation ( I have excluded the units for simplicity, they reduce to centimeters): t = 1.8/(d^2)(9320) Now suppose we want to know how long it will take silver particles of various sizes to settle a distance of 10 cm in water. All that is needed is to enter the particle diameter (in centimeters) into the above equation and solve for t. (note that d^2 is my notation for d squared). Now I will solve the equation for particles of the following sizes: 10 microns (=0.001 cm), 1 micron (=0.0001 cm), 0.1 micron (=0.00001 cm), 0.01 micron (=0.000001 cm). Then here are the settling times: 10 micron particles will settle 10 cm in 3.2 minutes 1 micron particles will settle 10 cm in 5.4 hours 0.1 micron particles will settle 10 cm in 22 days 0.01 micron particles will settle 10 cm in 6.1 years There is one key assumption in these figures and that is that there is no significant effect of thermal motion in the atoms of the fluid to keep particles suspended, i.e. Brownian Motion. Since I am not a physicist, I am not able to comment specifically on this point. I know from soil science that the 1 micron particles will not be kept in suspension to any significant degree by Brownian Motion. However, Brownian Motion may have some effect on the 0.1 and 0.01 micron particles. If so, these particles would stay in suspension longer than the figures given above, possibly an indefinite amount of time. So how can we use this information to measure the particle sizes of silver in CS? First you make up a fresh batch of CS and analyze it immediately for PPM of silver. You may need to shake the CS to suspend larger particles (if you have particles in the 10 micron or higher size range). Then suppose you set the CS aside for 5.4 hours with the temperature at 20 degrees centigrade. After this time you carefully withdraw a sample with a pipet at a depth of 10 cm from the liquid surface. The amount of the sample must be small compared to the total amount of CS. Then you analyze this sample for PPM. The second sample should contain silver particles of 1 micron diameter or less. If the PPM of both samples are about the same, then you can assume that your CS contains particles of 1 micron or less. Now if you let your CS sit for 22 days and take a third sample at 10 cm below the liquid surface, you can determine how much of your silver is 0.1 micron or less in size (this assumes that a 0.1 micron particle will indeed settle 10 cm in 22 days - a fact that I can't guarantee without further study). There may be several pitfalls to this approach. Nevertheless, I submit it to you as food for thought. 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]> -- 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]>
