Detailed investigations of capillary and van der Waals forces in the adhesion between solids
The primary focus of this dissertation is on characterizing two fundamental forces common to adhesion between solids: capillary and van der Waals forces. These two forces have a significant impact on how solids flow, stick to surfaces, agglomerate and break; therefore, understanding their behavior can lead to better processing techniques. Capillary forces are responsible for caking in the food, cosmetic, and pharmaceutical industries and are caused by the spontaneous formation of liquid bridges between two surfaces in close contact. To better understand how capillary forces depend on the system relative humidity (RH), solid surface separation, and surface hydrophilicity, the Wang-Landau Monte Carlo technique on a lattice-gas framework has been used. For a smooth, hydrophilic AFM tip against a flat, hydrophilic plate on a 45×45 lattice, a maximum capillary force occurs when the separation distance is one molecular diameter of the adsorbate and the capillary force decreases as the separation distance increases, except when the tip and the surface are in contact. The capillary forces associated with completely wetting AFM tips are strongly dependent on the system RH, while partially wetting and partially drying tips are relatively independent of RH. Interestingly, capillary forces can be significant in low RH environments and thus cannot be ignored in AFM studies involving hydrophilic surfaces. Another aspect in predicting capillary forces involves the use of the Kelvin equation, which describes the relationship between the system saturation and the curvature of the liquid-vapor interface. Using a two-dimensional lattice-gas model with mean-field density functional theory, the effect of meniscus curvature on the prediction of the Kelvin equation has been studied. First, the dependence of the surface tension on the curvature of the liquid-vapor interface is established for critical bubbles forming within a bulk liquid. It is demonstrated that for a pure-component, bulk system the Kelvin equation properly describes the curvature of the interface at the Gibbs surface of tension, even for very small bubbles. Next, the system is modified to include parallel, hydrophilic surfaces between which capillary bridges can form. The curvature of these capillary bridges is quantified at differing saturation levels, separation distances and contact angles and then compared to the Kelvin equation. For these capillary bridges, it is found that the radius of curvature is not constant (i.e., the meniscus is not circular) and that the Kelvin equation is a non-physical extrapolation as the system approaches zero saturation or as the separation distance decreases. Therefore, the Kelvin equation best describes curvatures for pure-component systems or for capillary bridges that are near or at saturation and with large plate separations. Dry adhesion caused by dipole-dipole interactions (i.e., van der Waals forces) have also been considered. The Hamaker constants (which are a measure of the van der Waals forces) and dispersive surface energies have been characterized for ten energetic powders using inverse gas chromatography. It has been determined that the effect of the amount of fuel additives in the energetic powders on Hamaker constants is not statistically significant. In addition, the Hamaker constants agree with Lifshitz theory indicating that inverse gas chromatography is a possible alternative for characterizing the dry adhesion of powders. Lastly, when combined with a better prediction of capillary forces, these experimentally determined Hamaker constants can lead to better models describing the removal of these energetic materials. Finally, an ancillary project investigating the nanoscopic, chemical properties of crystalline griseofulvin embedded in polymer is also included in this dissertation. Obtaining small-scale chemical information from traditional infrared (IR) spectroscopy has recently been improved from 3–30 µm to 10–100 nm by combining atomic force microscopy (AFM) with IR and is known as AFM-IR. It is shown that AFM-IR can chemically distinguish drug nanoparticles embedded in polymer with a sub-100 nm resolution, which is a considerable improvement over traditional IR techniques.
Beaudoin, Purdue University.
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