Colloidal Dispersions: Where Various Length Scales Become Correlated

On Friday September 5, Colorado School of Mines was honored to hear a lecture on “Colloidal Dispersions: Where Various Length Scales Become Correlated” by Dr. Jared Chun of Pacific Northwest National Laboratory. Dr. Chun received his Ph.D. in Chemical Engineering from Cornell University and is now performing and leading research looking at various aspects of structured fluids: multiphase flows, granular materials, and suspension rheology. He is also performing studies on millimeter wave rheometry and the prediction of magnetic permeability of ferrites.

Dr. Chun’s lecture centered on the idea of colloidal dispersions. Colloids are homogenous substances containing large molecules or one substance interspersed through a second substance, often large pieces of a solid suspended in a liquid. Examples of colloids include paints, lotions, nuclear wastes, milk, fog, and bumper foam. Dr. Chun noted that his favorite description of colloids was by D. Fennell Evans and Haken Wennerstrom who described colloids as “where physics, chemistry, biology, and technology meet.” This is, in part, because many of the various length scales are involved in colloids. Chun listed the 3 different scales: microscopic, macroscopic, and mesoscopic/particle length scale. All of these scales are involved in colloidal interactions, a unique occurrence that allows researchers to study how the various length scales correlate and interact with one another.

Many processes deal with the forces between colloids or colloidal dispersions. Often these processes are rudimentary such as flotation and detergency, or more complex such as emulsion or polymerization. Chun’s lecture focused on the idea of colloidal dispersions in industrial processes, where colloids can act as carriers in the facilitated transport of pollutants.

Dr. Chun focused mainly on the effects of colloids on cleaning up nuclear waste, or slurries. He gave the example of the nuclear waste at Hanford Site along the Columbia River in Washington state, which was generated from 1943 to 1987. By the time of its shutdown Hanford had produced around 200,000 cubic meters of radioactive waste. On the whole, this waste was converted to glass to render it stable by the use of Joule-heated melters operating at 1150˚C. However, there was a need to control the rheological properties of the slurry. Nuclear waste slurries have many unique characteristics including a very high pH, wide chemical composition, and particles with broad sizes. Nuclear waste facilities involve mixing, blending, and transporting, so controlling the rheological properties of the waste slurries was pivotal especially under challenging conditions. In order to optimize cost, time, and energy, a higher throughput, or the amount of substance put through the tank, was desirable.

But how could the rheological properties of the slurries be changed? The answer, say Chun, was introducing a small amount of “rheological modifiers” to the slurry. These modifiers can build additional steric repulsions to prevent reaction of the slurries and stabilize the slurry. Such modifiers include nonionic and polymer surfactants. Other modifiers, including weak organic acids, can strengthen electrostatic repulsions. Both of these types of modifiers work on the unstable slurry to build a stronger network formation to which stress is added, resulting in a network breaking and a stable liquid.

Another solution, Chun suggests, is to add a complex nuclear waste stimulant. These stimulants are very basic, with a pH of 11-12. They often contain oxide or hydroxide particles (Fe2O3 and Al(OH)3 are commonly used). They are abundant in electrolytes, and have a reasonably high amount of solid contents. The stimulants also have a broad size distribution. However, Chun notes that the decrease in yield is not directly from the pH effect and says that weak acids are much more effective than non-ionic polymeric surfactants. At pH 12, charges on both the particles and weak acids are negative, so the electrostatic repulsions and specific interactions are keys to understanding why these modifiers work. Particle size distribution also factors in to how the particles respond to yield stress. A higher percentage of smaller particles (less than 5 micrometers across) corresponds to higher yield stress. Modifier efficiency, or the relative change in rheological properties, also appears to correlate with the percentage of small particles. This is interesting, Chun notes, because there are many different types of slurries each of which contains particles of different sizes. All of these properties contribute to how each slurry reacts with the different rheological modifiers used to stabilize the slurry. There are still many studies being conducted on colloidal dispersions and how the different length scales are represented and interact in colloids.

Dr. Chun briefly discussed a new experiment underway as of just 7 months ago. It is called the Initial Toy Problem and involves the density fluctuation of water molecules in a mica/water/mica system. Mica is a shiny silicate mineral with a layered structure. It is generally used as a thermal or electrical insulator. This experiment is looking at how the density changes when it is near the mica versus when the mica is further away.

In summary, Dr. Chun concluded that physiochemical interactions bring valuable physical insights to colloidal dispersion correlating phenomena over a wide range of length scales. He then ended with a quick plug for his lab, Pacific Northwest National Laboratory. It is one of 10 official DOE (Department of Energy) science labs in the nation. One of their goals is to increase energy capacity and reduce dependence on imported oil through the research of hydrogen and biomass-based fuels but they offer many different research opportunities. Anyone interested in their research should contact them or investigate further at http://www.pnl.gov.