The Effects of Short-Range Intermolecular Repulsive Forces on Hamaker Constant Estimation Using Atomic Force Microscopy.

The atomic force microscope (AFM), as it is able to characterize surface topography as well determine the AFM cantilever tip-surface force, proves effective at estimating the value of the Hamaker constant, , for a given solid material. Two main AFM-based methods have been proposed for estimating values of . In the approach-to-contact (AtC) method, Hamaker constants are inferred from the deflections at which the AFM tip first jumps into contact with the substrate.

Amorphous solid dispersions containing residual crystallinity: Influence of seed properties and polymer adsorption on dissolution performance.

The solubility advantage of amorphous solid dispersions (ASDs) is contingent upon supersaturation being generated and maintained. If crystals are present within an ASD, these crystals directly result in lost solubility advantage, and may also seed crystal growth leading to desupersaturation. The goal of this study was to evaluate the impact of residual crystals on ASD supersaturation profiles.

London-van der Waals Force Field of a Chemically Patterned Surface To Enable Selective Adhesion.

The London-van der Waals (L-vdW) force between a particle and a surface strongly depends on the topography and the chemical properties of the interacting surfaces. Although a great deal of work has been done to understand the effect of topographical heterogeneity on the L-vdW adhesion, the role of chemical heterogeneity has been discussed only rarely. This study makes an attempt to quantify the magnitude and range of the L-vdW force acting on a spherical particle in the vicinity of a chemically patterned surface.

Crystallization Inhibition Properties of Cellulose Esters and Ethers for a Group of Chemically Diverse Drugs: Experimental and Computational Insight.

Amorphous solid dispersions are widely used to enhance the oral bioavailability of poorly water-soluble drugs. Polymeric additives are commonly used to delay crystallization of the drug from the supersaturated solutions formed upon ASD dissolution by influencing the nucleation and growth of crystals. However, there is limited evidence regarding the mechanisms by which polymers stabilize supersaturated drug solutions.

Non-contact AFM measurement of the Hamaker constants of solids: Calibrating cantilever geometries.

Hypothesis: Surface effects arising from roughness and deformation can negatively affect the results of AFM contact experiments. Using the non-contact portion of an AFM deflection curve is therefore desirable for estimating the Hamaker constant, A, of a solid material. A previously validated non-contact quasi-dynamic method for estimating A is revisited, in which the cantilever tip is now always represented by an "effective sphere".

A New "Quasi-Dynamic" Method for Determining the Hamaker Constant of Solids Using an Atomic Force Microscope.

In order to minimize the effects of surface roughness and deformation, a new method for estimating the Hamaker constant, A, of solids using the approach-to-contact regime of an atomic force microscope (AFM) is presented. First, a previous "jump-into-contact" quasi-static method for determining A from AFM measurements is analyzed and then extended to include various AFM tip-surface force models of interest. Then, to test the efficacy of the "jump-into-contact" method, a dynamic model of the AFM tip motion is developed.

Contact between traps and surfaces during contact sampling of explosives in security settings.

Realistic descriptions of interfacial contact between rough, deformable surfaces under load are difficult to obtain; however, this contact is of great import in a wide range of applications. Here, we detail, through experiment and computational simulation, the interfacial contact between four common traps and five commonly investigated surfaces encountered in explosives detection applications associated with airport security. The Young's modulus and hardness of four traps and seven substrates were measured using nanoindentation.

Influence of Polymers on the Crystal Growth Rate of Felodipine: Correlating Adsorbed Polymer Surface Coverage to Solution Crystal Growth Inhibition.

The bioavailability of orally administered drugs that exhibit poor aqueous solubility can be enhanced with the use of supersaturating dosage forms. Stabilization of these forms by preventing or inhibiting crystallization in solution is an important area of study. Polymers can be used to stabilize supersaturated systems; however, the properties that impact their effectiveness as crystal growth rate inhibitors are not yet fully understood.

Impact of polymer conformation on the crystal growth inhibition of a poorly water-soluble drug in aqueous solution.

Poor aqueous solubility is a major hindrance to oral delivery of many emerging drugs. Supersaturated drug solutions can improve passive absorption across the gastrointestinal tract membrane as long as crystallization can be inhibited, enhancing the delivery of such poorly soluble therapeutics. Polymers can inhibit crystallization and prolong supersaturation; therefore, it is desirable to understand the attributes which render a polymer effective.

Atomic force microscope infrared spectroscopy of griseofulvin nanocrystals.

The goal of this work was to evaluate the ability of photothermal-induced resonance (PTIR) to measure the local infrared absorption spectra of crystalline organic drug nanoparticles embedded within solid matrices. Herein, the first reports of the chemical characterization of sub-100 nm organic crystals are described; infrared spectra of 90 nm griseofulvin particles were obtained, confirming the chemical resolution of PTIR beyond the diffraction limit. Additionally, particle size distributions via dynamic light scattering and PTIR image analysis were found to be similar, suggesting that the PTIR measurements are not significantly affected by inhomogeneous infrared absorptivity of this system.

Effect of relative humidity on onset of capillary forces for rough surfaces.

Atomic force microscopy (AFM) was used to investigate the effect of relative humidity (RH) on the adhesion forces between silicon nitride AFM probes, hydrophilic stainless steel, and hydrophobic Perspex® (polymethylmethacrylate, PMMA). In addition, AFM-based phase contrast imaging was used to quantify the amount and location of adsorbed water present on these substrates at RH levels ranging from 15% to 65% at 22°C. Both the adhesion forces and the quantities of adsorbed moisture were seen to vary with RH, and the nature of this variation depended on the hydrophobicity of the substrate.

Adhesion of explosives.

It is of increasing importance to understand how explosive particles adhere to surfaces in order to understand how to remove them for detection in airport or other security settings. In this study, adhesion forces between royal demolition explosive (cyclotrimethylenetrinitramine) (RDX), pentaerythritol tetranitrate (PETN), and trinitrotoluene (TNT) in their crystalline forms and aluminum coupons with three finishes, acrylic melamine (clear coating), polyester acrylic melamine (white coating) automotive finishes, and a green military-grade finish, were measured and modeled. The force measurements were performed using the atomic force microscopy (AFM)-based colloidal probe microscopy (CPM) method.

Approximate scheme for calculating van der Waals interactions between finite cylindrical volume elements.

A successful approach to calculating van der Waals (vdW) forces between irregular bodies is to divide the bodies into small cylindrical volume elements and integrate the vdW interactions between opposing elements. In this context it has been common to use Hamaker's expression for parallel plates to approximate the vdW interactions between the opposing elements. This present study shows that Hamaker's vdW expression for parallel plates does not accurately describe the vdW interactions for co-axial cylinders having a ratio of cylinder radius to separation distance (R/D) of 10 or less.

Modeling and validation of the van der Waals force during the adhesion of nanoscale objects to rough surfaces: a detailed description.

The interactions between nanoparticles and rough surfaces are of great scientific and engineering importance and have numerous applications in surface science and biotechnology. Surface geometry and roughness play crucial roles in observed particle adhesion forces. We previously developed a model and simulation approach to describe adhesion between microscale bodies.

Hamaker constants in integrated circuit metalization.

A new method for determining Hamaker constants was examined for materials of interest in integrated circuit manufacture. An ultra-high vacuum atomic force microscope and an atomic force microscope operated in a nitrogen environment were used to measure the interaction forces between metals, dielectrics, and barriers used during the metalization portion of integrated circuit manufacturing. The materials studied included copper, silver, titanium nitride, silicon dioxide, poly(tetrafluoroethylene), and parylene-N.

Quantification of cytokines involved in wound healing using surface plasmon resonance.

Sensing of three cytokines related to chronic wound healing, interleukin-1 (IL-1), interleukin-6 (IL-6), and tumor necrosis factor-alpha (TNF-alpha), with detection limits at or below 1 ng/mL in buffered saline solution and spiked cell culture medium (CCM) has been achieved. Fiber-optic surface plasmon resonance (SPR) sensors are coated with an antibody binding layer and antibodies specific to the cytokine of interest are covalently attached to this layer. To achieve such detection limits in a complex medium such as CCM, total protein content of 4 mg/mL, the use of a novel N-hydroxysuccinimide ester of 16-mercaptohexadecanoic acid (NHS-MHA) is necessary.