ACS PRF | ACS
All e-Annual Reports
44936-GB5
Nanotribology Studies of Petroleum Derivatives for Nanotechnology
One of the projects outlined in my original proposal was to study the potential of alcohols as possible vapor phase lubricants for micromachines. Recent research has suggested that alcohols such as propanol, ethanol, butanol, or pentanol might make excellent vapor phase lubricants for micromachines. Kim and colleagues believe that by enclosing a small amount of the liquid alcohol in the packaging of the micromachine, thin layers of the vapor phase lubricants should coat all the surfaces of the micromachine due to the relatively high vapor pressures of these alcohols. This would be a great improvement over the solid lubricant coatings, which don't always cover buried contacts. AFM studies of these molecules have shown that at their vapor pressures, they can reduce the friction as compared to bare SiO2 surfaces by a factor of 3 or more.
I have entered into a collaboration with Dr. Kim of Pennsylvania State University on this topic involving research into the adsorption of alcohols atop self assembled monolayers (SAMs). SAMs are also touted as a possible lubricant coating for micromachines. Dr. Kim's laboratory is focusing on acquiring vibrational spectroscopy of the aforementioned alcohols atop self assembled monolayers (SAMs) on Au(111). My laboratory is focusing on acquiring quartz crystal microbalance (QCM) isotherms of those same alcohols using QCM's coated with SAM's atop Au(111) electrodes.
QCM's consist of single crystal quartz that oscillates in transverse shear motion (resonance frequencies are typically at 5-10 MHz) with very little internal dissipation (quality factors (Q) near 100000). The oscillations are driven by applying an alternating voltage to thin metal electrodes deposited on the surface of the quartz. Atomically thin films later adsorbed onto the QCM electrodes produce shifts in both the frequency (df) and Q. For our QCM measurements of alcohols on SAMs, we will be monitoring both df and Q. The frequency shift gives us the mass uptake, via the equation -df = c(dm), where c is a constant that is dependent on the QCM used and dm is the change in mass atop the crystal. We are in the process of acquiring QCM isotherms at several temperatures, and using this data to find the heat of adsorption for the alcohols on the SAM's. We are also monitoring the shift in Q, which gives the changing dissipation. We plan to compare the dissipation data from the QCM with the vibrational spectroscopy data acquired by Dr. Kim's laboratory, to see what can be learned about the damping and vibrational modes of the alcohols' interactions with the SAM's.
An interesting side project has been a study of the Diet Coke and Mentos reaction. My summer undergraduate research students often had spare time while waiting for shipment of new parts or waiting to use our vacuum chamber. While they waited, they often worked on this project. The abrupt ‘fountain' response of diet soda mixed with Mint Mentos is well documented and much debated. It has been speculated that this phenomenon is most likely a combination of two physical reactions. First, the rough surface of Mint Mentos provides many nucleation sites for CO2 bubbles. Second, ingredients in the Mentos, such as gum arabic, and ingredients in Diet Coke, such as caffeine, potassium benzoate and aspartame, significantly reduce the surface tension of Diet Coke, allowing rapid escape of CO2. We selected sample-soda pop combinations for testing based upon the surface roughness of the samples and the presence or absence of ingredients that vary surface tension of soda pop. The samples include: Mint Mentos, Fruit Mentos, Cake Mates, Wint-o-green Lifesavers, liquid Gum Arabic, an emulsion of Dawn detergent and water, sand, rock salt, and table salt which we tested in Diet Coke, Caffeine Free Diet Coke, Coke Classic, Caffeine Free Coke Classic, and seltzer water. An SEM was used to determine surface morphology of the testing materials microscopically. An AFM was used to obtain quantitative surface roughness measurements. We systematically varied the ingredients in the liquid, and contact angle measurements were conducted to determine the minimum work required for bubble formation in the liquids. This analysis of our materials was then coupled with observations of the explosive reaction. Intensity of reaction was determined by measuring the amount of liquid lost during the reaction and distance traveled by the spray.
We found that the caffeine in the Diet Coke is not abundant enough to contribute heavily to the explosive reaction, contradicting previous results from the Mythbusters team. We found that hotter beverages led to more explosive reactions, due to Henry's Law and Le Chatelier's principle. We found that although rougher surfaces lead to more explosive reactions, the presence of surfactants in the candy coating is also a large part of the explanation for the explosiveness of the Diet Coke and Mentos reaction. This work has been accepted for publication in the American Journal of Physics.
