Civil and Environmental Engineering at its Finest ... Scales

Nanoscale research focuses on processes that occur at the finest length scales from individual atoms to tens of nanometers. The inset image on the cover for this issue illustrates the nucleation of atomic scale defects that are ultimately responsible for macroscopic effects like ductility and fatigue fracture of metals. Often, the high surface area to volume ratio of nanoscale particles and evices makes them behave differently than their microscopic or macroscopic counterparts. These behaviors can be both beneficial for the environment or pose potentially negative consequences. Other times, nanoscale devices are useful because their compact size and low costs make them easy to deploy in locations previously inaccessible or uneconomical. The following examples highlight some of the ways in which CEE faculty are enhancing our nderstanding of nanoscale systems.

 

Figure 1: Maximum stresses compared to actual defect location found using atomistic simulation

Dislocations, which are defects in a crystal lattice, are responsible for the strength and ductility of crystalline solids such as Nickel, and predicting their emergence under load is important for understanding material behavior and failure. Professor Amit Acharya is developing theories and computational models of dislocations at the atomic level and higher in crystalline materials. The image shown in figure 1 shows dislocation nucleation predictions for a nanoindentation of a perfect FCC Ni crystal, based on research performed by Professor Acharya in collaboration with Ronald Miller at Carleton University, Canada. The conditions under which dislocations emerge in a perfect crystal lattice are not well-understood, in contrast to the understanding of the conditions under which preexisting dislocations move. Dislocation motion is governed by the local stress level. In the absence of better theory, dislocation nucleation has also been assumed to be dependent on local stress . However, Professor Acharya has shown the feasibility of a nucleation criterion related to the local gradient of the stress exceeding a material-specific value. The image in Figure 1 also shows a comparison of the maximum (resolved shear) stress (colored contours, where red indicates the highest magnitudes) with the actual locations of defect nucleation as predicted by an atomistic simulation and using the stress gradient-based criterion being developed by Professors Acharya & Miller. The comparison demonstrates that the stress-based criterion is not an accurate predictor of nucleation. Prof. Acharya’s research has shown that the stress-gradient based criterion he has developed, on the other hand, accurately predicts the location and the type of nucleated defect.

A significant amount of roof degradation and damage goes undetected until a major amount of damage has already occurred and the damage is visible. While moisture penetration is a prevalent problem for all building envelope systems, including foundation and nanoscale research focuses on processes that occur at the finest length scales from individual atoms to tens of nanometers. Figure 1 illustrates the nucleation of atomic scale defects that are ultimately responsible for macroscopic effects like ductility and fatigue fracture of metals. Often, the high surface area to volume ratio of nanoscale particles and devices makes them behave differently than their microscopic or macroscopic counterparts. These behaviors can be both beneficial for the environment or pose potentially negative consequences. Other times, nanoscale devices are useful because their compact size and low costs make them easy to deploy in locations previously inaccessible or uneconomical. The following examples highlight some of the ways in which CEE faculty are enhancing our understanding of nanoscale systems. basement floor slabs, its impact is the most fatal to roof systems. Professor Jim Garrett, CEE department head, is working with CMU collaborators Metin Sitti (Mechanical Engineering) and Omer Akin (Architecture) to explore a nanotechnology-based high sensitivity, low cost and compact sensor to detect the presence of moisture between the outer layers of a roofing system and its underlying support structure. A version of this sensor has been designed and manufactured by Professor Metin Sitti.

The team explored several different membranes with nanometer sized pores into which water could be collected: polycarbonate, cellulose acetate and polyester (nylon). These materials are flexible and inexpensive, and show a highly sensitive change in resistance in response to water adsorption inside their nano-pores. To be able to detect the change in resistance of the membrane in response to uptake of moisture, an electrode is deposited over this material and used to detect resistance changes in this material as it takes on different levels of moisture. The team characterized the sensors by comparing their output values with those from a commercial humidity sensor. For example, for the polycarbonate membrane, resistance changes from 4.7 megaohms to 3.3 megaohms when the relative humidity changes from 35% to 92% at room temperature. It is anticipated that these membranes can be manufactured in large sheets with arrays of electrodes deposited onto them for deployment on roofs and in other parts of the building envelope. Further testing of this concept using an array of these sensors is still in progress.

 

Figure 2: Optical microscope image showing the ability of polymer-modified nanoiron to adsorb to the interface between water and tri-chloroethylene (TCE).

Demonstrating the environmental benefits of nanotechnology, Professor Greg Lowry and his co-workers have recently received several research grants to synthesize, characterize, and evaluate the fate and mobility of reactive polymer-coated iron-based nanoparticles, which can be used for remediating groundwater contaminated with chlorinated organic solvents (see Figure 2). Sites containing these carcinogenic contaminants, present as Dense Non-aqueous Phase Liquids (DNAPLs), are ubiquitous in the US and the world. Prof. Lowry’s research is showing that novel nanotechnologies can be used to effectively remediate these sites at a lower cost than conventional remediation methods. He is part of a multi-disciplinary team, including Professors Robert Tilton (Chemical Engineering), Krzysztof Matyjaszewski (Chemistry), and Edwin Minkley (Biology). Graduate students from CEE Navid Saleh, Tanapon Phenrat, Hye-Jin Kim and these departments are working together to develop reactive Fe0/Fe-oxide nanoparticles coated with triblock copolymers that allow the particles to seek out subsurface contaminants so that they are used efficiently. This process is inspired by drug delivery schemes that directly target drugs to the diseased tissues. The effects of nanoparticle addition on microbial health and diversity are being determined in an EPA target sponsored project. Polymer architectures to improve targetability are being evaluated in an NSF study. In a DOD sponsored project directed by Prof. Lowry, in collaboration with Tissa Illangasakare at the Colorado School of Mines, studies the fundamental hydrogeochemical processes affecting the ability to contact these reactive particles with residual DNAPL, and measures the effects of treatment with these particles on mass emission from the DNAPL source zone. Results from these studies will improve the eventual field application of this technology. Results from this study will be used to guide policy decisions related to the potential ecological and human health risks associated with the burgeoning use of nanotechnologies.

 

Figure 3: Measurement of a nucleation event in Pittsburgh on July 27, 2001. Time of day is on the horizontal axis and particle size on the vertical axis. Warmer colors indicate higher numbers of particles observed of that size and at that time. The figure shows that high numbers of freshly nucleated clusters were observed just after sunrise (about 6 AM) and continued to form through midafternoon.

In the field of air pollution, scientists and regulators have become concerned about progressively smaller and smaller particles until “ultrafine” nanoparticles, defined as particles whose diameter is less than 100 nm, are a major area of research interest today. Their small size has several implications for human health and the Earth’s climate. Toxicological studies have shown that, once inhaled, nanoparticles can spread rapidly throughout the body leading some to suspect that they may be more harmful than larger particles.

Airborne particles have always been important to the Earth’s climate as they are the “seeds”, known as cloud condensation nuclei, onto which water condenses to form cloud droplets. Human emissions have increased the number of cloud condensation nuclei since preindustrial times, making clouds brighter and potentially less likely to rain. Not all atmospheric particles are good cloud condensation nuclei: nanoparticles are small enough that their surface tension hinders their growth. Only the largest and most soluble nanoparticles in the atmosphere have a chance to become a cloud droplet. Professor Peter Adams and his research group have been developing models of airborne particles that predict the number, composition, and sizes of particles present around the globe. These models are then embedded into global climate models to see how much cloud condensation nuclei concentrations have increased, how much brighter that makes clouds, and how much the brighter clouds have offset global warming from greenhouse gases. Combustion sources are known to emit a large number of nanoparticles to the atmosphere, but nanoparticles can also form spontaneously in the atmosphere during “nucleation” events. In a nucleation event, a non-volatile gas such as sulfuric acid reaches a supersaturated concentration. Molecules of the gas then cluster together until a new particle is formed. Again, the small size of the nucleating cluster means that surface tension effects tend to destabilize it. Exactly how the clusters overcome this barrier to growth is still the subject of debate since nucleation events are observed to occur in the atmosphere at sulfuric acid concentrations lower than simple theories would predict. During the multi-year Pittsburgh Air Quality Study, CEE Professor Cliff Davidson and co-workers observed that nucleation events occurred in Pittsburgh on one out of three days. [Figure 3 shows an example of one such observation for July 27, 2001.] This was something of a surprise as conventional wisdom was that nucleation events tended to occur only in more pristine parts of the atmosphere. Subsequent modeling work by Prof. Adams and others is suggesting that a mixture of sulfuric acid, mostly from coal power plants, and ammonia, mostly from agricultural sources, is responsible for the nucleation events. Ironically, current air pollution regulations will likely increase the number of atmospheric nanoparticles from nucleation events, although it is not yet clear whether this is a health concern.

In summary, while Civil Engineering is often depicted as the engineering profession concerned with the mega-scale, we are also a profession with many concerns that require attention to the nano-scale. This short article highlights some of those concerns being researched here in Civil and Environmental Engineering.