Professor of Chemical Engineering
School of Chemical Engineering
Forney Hall of Chemical Engineering
480 Stadium Mall Drive
West Lafayette, IN 47907-2100
Mesoscopic Modeling of Capillary Forces between Micro- or Nano-Scale Probes and Substrates
- Develop experimentally-validated models for the effects of water vapor on the adhesion between particles and surfaces
- Develop a stand-alone simulator to predict these effects for a broad range of materials
Water vapor can significantly change the adhesion between a particle and a substrate by condensing between the two solid surfaces and forming a meniscus (Figure 1). Macroscopically, the capillary force is a direct result of the pressure difference caused by the formation of the meniscus and can be described by the Young-Laplace and Kelvin equations. These models fail at low to moderate relative humidity levels where ‘condensed’ moisture cannot be considered continuous. We are developing a mesoscopic simulator based on the lattice-gas model and the Wang-Landau Monte Carlo method to calculate the capillary force between a nanoscale pyramidal solid (an AFM cantilever) and a smooth surface. In this procedure the energetic interactions among water molecules, the probe and the surface are used to calculate the system’s Hamiltonian. The Hamiltonian is then used to calculate the density of states of a given configuration, the partition function and, ultimately, the capillary force between the probe and the substrate.
Figure 1: (Top) Rupture of a capillary bridge between a particle and a substrate. (Bottom) Magnified Area: water molecules placed onto a lattice between the particle and the surface.
Adhesion/Cohesion in Systems involving Energetic Materials Composites and Solid Surfaces
- Measure and model the adhesion between residues of explosives compounds and surfaces of interest in Homeland Security environments
- Use understanding to develop improved methods for detecting explosives residues
By improving the performance of swabs used in the detection of explosives residues in airport security settings, it is possible to improve the efficiency and accuracy of interdiction efforts. Explosives residues include a matrix of binder material which encapsulates particles of energetic material , as shown in Figure 2. The adhesion of these composites, and the mode of failure when they are removed from the surface, must be understood in order to optimize the design and method of implementation of the swabs used during residue detection. Our preliminary work is based on an approach taken to evaluate the breakup of granules during granulation processes. This approach categorizes cohesive (within granule) failure as either brittle or plastic, based on the measured values of the dimensionless strength and capillary numbers.
Figure 2. Schematic of explosive residue. A) Swab/Explosive interaction; B) Energetic material/Matrix interaction; C) Explosive/Substrate interaction.
Polymer Adsorption onto Crystals
- Measure the conformation of polymers adsorbed to crystalline drug surfaces using atomic force microscopy (AFM)
- Relate the polymer conformation to changes in crystal growth and dissolution rates
About 75% of active pharmaceutical ingredients (APIs) in the drug development pipeline demonstrate poor aqueous solubility in crystalline solid form. This poses a problem in oral dosage forms due to low bioavailability. Amorphous solid forms, however, possess much greater free energy and are correspondingly much more soluble. One approach to stabilize the inherently unstable amorphous form and prevent crystallization utilizes adsorbed polymers to occupy growth sites and serve as mechanical barriers to growth. The effectiveness of this method depends critically on the conformation of the polymer once it is adsorbed onto the solid drug. For example, if the polymer chain is extended, a single adsorbed molecule can block multiple sites in addition to its adsorption site (Figure 3); however, a coiled polymer has limited ability to block multiple sites (Figure 4). Using AFM, we are studying the conformation of adsorbed polymers and relate this parameter to crystal growth kinetics.
Figure 3: Extended polymer confirmation blocking multiple growth sites.
Figure 4: Coiled polymer confirmation occupying a limited number of growth sites.
Enhanced Centrifuge-Based Approach to Powder Characterization
- Develop improved centrifuge-based technique for advanced powder characterization.
Powder behavior during solids handling, such as in the pharmaceutical, food, and personal care industries, is controlled to a large degree by the adhesion characteristics of the powder’s particulate ensemble. Key physical properties of a particulate system, chiefly size, geometry, composition, and surface roughness, generally control the particle adhesion. A classic method for measuring powder adhesion involves coating metal plates with powder, mounting these plates in centrifuge tubes so that their coated surfaces face outward, and then tracking the removal of particles from the plates when the centrifuge is rotated. While the technique is simple to use, it gives low-quality information about the powder adhesion characteristics. Our work pursues the use of rationally-designed plates in the centrifuge method. By tracking the removal of small quantities of powder from these plates, we are able to extract a great deal of high-value added information about the particle adhesion. Specifically, substrates designed with hemispherical indentations of different sizes and roughness, as shown in Figure 5 (roughness not shown) allow us to characterize the size, shape, roughness, and composition of the particles in the powder.
Figure 5. Schematic of a substrate with hemispherical indentations: top-down view (left), side view with different particle-indentation orientations (right).
This work is supported by the Department of Homeland Security Science and Technology Directorate.
- Aaron Harrison
- Darby Hoss (co-advised with Professor Bryan Boudouris)
- Jennifer Laster (co-advised with Professor Bryan Boudouris)
- Caitlin Schram
- Kathryn Smith
- Melissa Sweat
- Myles Thomas
Awards and Honors
“Incorporation of a Decorin Biomimetic Enhances the Mechanical Properties of Electrochemically Aligned Collagen Threads," V. Kishore, J. E. Paderi, A. Akkus, K. M. Smith, D. Balachandran, S. Beaudoin, A. Panitch, and O. Akkus, Acta Biomaterialia, 7, 2428–2436 (2011)
“Nanoparticle Adhesion Models: Applications in Particulate Contaminant Removal from Extreme Ultraviolet Lithography Photomasks,” R. Jaiswal, and S. Beaudoin, Journal of Adhesion Science and Technology, 25, 781-797 (2011)
“Adhesion of Contaminant Particles to Advanced Photomask Materials,” C. Kilroy, R. Jaiswal, and S. Beaudoin, IEEE Transactions on Semiconductor Manufacturing, 25 (1), 37 - 44 (2012)
“An Approximate Scheme for Calculating van Der Waals Forces between Finite Cylindrical Volume Elements,” R. Jaiswal, and S. Beaudoin, Langmuir, in press (2012)
“Adhesion of Dry Nano-coated Microparticles to Stainless Steel: A Physical Interpretation,” D. Balachandran, L. Jallo, R. Dave, and S. Beaudoin, Powder Technology, in press (2012)