Analyzing adhesion in microstructured systems through a robust computational approach

Interactions between nanoparticles and fractal surfaces

This study evaluated attachment of a 30-nm nanoparticle to and detachment from fractal surfaces by calculating Derjaguin-Landau-Verwey-Overbeek (DLVO) interaction energies in three-dimensional space using the surface element integration technique. The fractal surfaces were generated using the Weierstass-Mandelbrot function with varying values of fractal dimension D (2.3 ≤ D ≤ 2.7) and fractal roughness G (0.000136 ≤ G ≤ 0.136). Results show that maximum energy barrier is reduced at peak areas of a fractal surface, and hence attachment in primary minima is favored. Some nanoparticles attached in primary minima at the peak areas can be detached by decreasing ionic strength (IS) due to monotonic decrease of interaction energy with increasing separation distance at low ISs. While the attachment in primary minima at valley areas is irreversible to IS reduction, the attachment is inhibited due to enhanced maximum energy barrier at these areas. A nonmonotonic variation of attachment efficiency in primary minimum (AEPM) with IS is present at high fractal dimension (D ≥ 2.4) or low fractal roughness (G < 0.00136), whereas the AEPM decreases monotonically with decreasing IS at low fractal dimension (D < 2.4) or high fractal roughness (G ≥ 0.00136). The AEPM decreases monotonically with increasing D or decreasing G at ISs from 1 mM to 200 mM. The decrease of AEPM with D or G is much slower at 10 mM compared to other ISs. These theoretical findings can explain various experimental observations in the literature, and can have important utility to development of water filtration techniques in engineered systems and to assessment of environmental risks of nanoparticles.

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. Specifically, an ideal system of a smooth spherical particle approaching a surface composed of parallel stripes of chemically distinct materials with different Hamaker constants is considered. The L-vdW forces for such systems are determined by solving the London dispersion potential for the entire volumes of the adhering bodies from first principles, using Hamaker's microscopic approach. The computational results elucidate that a chemical interface can apply a tangential L-vdW force, in addition to the normal L-vdW force, on nearby particles. This can cause lateral motion of particles neighboring a chemically inhomogeneous surface. The magnitude of the tangential L-vdW force is found to be maximum when the particle is centered at the interface and shows a gradual drop as it moves away from this location. The magnitude and range of the tangential L-vdW force can be large for large colloidal particles in close contact with a chemically patterned surface whose materials have distinct Hamaker constants. This study suggests that the tangential L-vdW force field generated by a chemical interface can be utilized as a tool to manipulate the path of an approaching particle to facilitate selective adhesion.