Green Engineering of Dispersed Nanoparticles: Measuring and Modeling Nanoparticle ForcesEPA Grant Number: R829605
Title: Green Engineering of Dispersed Nanoparticles: Measuring and Modeling Nanoparticle Forces
Investigators: Velegol, Darrell , Fichthorn, Kristen
Institution: Pennsylvania State University
EPA Project Officer: Lasat, Mitch
Project Period: February 1, 2002 through January 31, 2004 (Extended to January 31, 2005)
Project Amount: $370,000
RFA: Exploratory Research: Nanotechnology (2001) RFA Text | Recipients Lists
Research Category: Safer Chemicals , Nanotechnology
Nanotechnology will be critical to advances in electronics, materials, medicine, and the environment. This is in large part due to their remarkable electronic, optical, magnetic, and mechanical properties. However, a significant limitation of nanotechnology is the ability to produce bulk quantities of dispersed particles. One possibility for dispersing nanoparticles (which have a high area/mass ratio) is to use adsorbed polymer, oligomer, or surfactant molecules; however, disposal of the enormous quantity of additives would involve huge environmental and financial stresses. The expected engineering breakthrough of the proposed research is to identify whether solvation or depletion forces can be manipulated to produce dispersed suspensions of "bare" nanoparticles (i.e., without adsorbed additives).
The central scientific questions to be answered are these: What are the magnitudes of the van der Waals, solvation, and depletion forces for nanoparticle systems, and what variables can we control to alter these forces? The research will involve two primary components: 1) development and use of "particle force light scattering" (PFLS), an experimental method for measuring sub-piconewton nanoparticle forces, 2) use of molecular dynamics (MD) simulations to predict the individual forces. Synergy is essential to this research: PFLS is the first technique capable of measuring the nanoparticle forces, and MD enables the interpretation of exactly how the forces are acting. Measured forces will be compared with bulk stability and rheology measurements.
Nanoparticle forces are sub-piconewton in magnitude. This is more than two orders of magnitude below the resolution of AFM. Of the methods available for measuring such small forces between Brownian particles with small interparticle gaps, the best is perhaps "differential electrophoresis". For two Brownian particles held together by colloidal forces, the technique can measure attractive interparticle forces with sub-piconewton resolution, by measuring doublet breakup in an electric field. The technique previously used to measure doublet breakup (video microscopy) is inadequate for visualizing nanoparticle systems, and the fluid mechanics results required to interpret the experiments will not extend from doublets to triplets. Thus, the experimental work has two components: 1) develop a Rayleigh light scattering device to measure nanoparticle triplet breakup (similar to the system used to measure colloidal stability), 2) derive the force expression from the electrokinetic equations for triplets of nanoparticles.
In the simulation component of the research, we will use MD simulations to characterize the interaction between two, model nanoparticles (Lennard-Jones Au solids) immersed in solvent (Lennard-Jones spheres, n-alkanes, and water) in the presence and absence of polymer. We will quantify the interaction as a function of particle size, particle separation, fluid type, and polymer model. One anticipated outcome of the proposed research is increased fundamental insight into the interaction between two, nonpolar nanoparticles in various fluids. A second anticipated outcome is that we will develop a new simulation method to quantify interparticle forces. Both depletion and solvation forces will be modeled, and the results used to develop the experiments.
The expected engineering outcome of this work is knowing whether bare nanoparticles can be stabilized by appropriately-engineered solvation or depletion force systems. A positive result will avert a huge waste stream of additives that would otherwise be necessary to stabilize nanoparticle systems. In order to achieve our outcome, the following scientific objectives must be met: 1) MD modeling will be done to delineate the magnitude of van der Waals, solvation, and depletion forces for nanoparticles systems; the modeling will demonstrate the pertinent variables that control these forces. 2) PFLS will be developed, and measurements of nanoparticle forces between silica, titania, and barium titanate particles in water will be performed. Results from the MD modeling will be tested.