Page 286 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

Nanoparticle Transport, Aggregation, and Deposition 271

Greater surface roughness has been observed to increase deposition rate
coefficients observed for some nanoparticles [65]. Similar observations
have also been made for colloid deposition onto membrane surfaces [78,
79]. Enhanced particle deposition rates on rough surfaces are attributed
to a combination of a reduced energy barrier on approach and physical
trapping of the nanoparticles by surface features. For ideally smooth sur-
faces, interactions occur normal to the interacting surfaces; however, for
a particle approaching a rough surface, it experiences both normal and
tangential forces that can capture it in surface depressions. Furthermore,
the variable height of the surface topography means that the approach-
ing particle is experiencing a range of interactions as some are more
prominent than others at different separation distances. For this reason
it is difficult to fully describe a true surface interaction between hetero-
geneous surfaces in terms of a single energy curve. Surface roughness can
also alter the magnitude and type (repulsive or attractive) of interaction
between two surfaces [78]. This results from a reduction in the relative
interaction area between the particle and the collector surface. One
approach is to model surface asperities as a series of small hemispheres
instead of a flat surface. The resulting calculations yield smaller inter-
action energies as the interaction energy is determined by the radii of
curvature of the protrusions rather than that of the interacting bodies.
Consequently, roughness reduces the height of the energy barrier between
two surfaces in aqueous media, thus making particle deposition more
favorable [74, 75]. For instance, Suresh and Walz [74] found that the van
der Waals interaction energy significantly increased when the separation
distance between two surfaces approached the height of the surface asper-
ities. The number of asperities per unit area on the surface had a smaller
influence on the magnitude of the interaction energy than did asperity
size. Asimilar conclusion was reached by these authors regarding the elec-
trostatic interactions—electrostatic repulsion occurs at a larger separa-
tion distance than would be expected for a smooth surface. At small
separation distances, surface roughness has a more profound impact on
van der Waals interactions (i.e., makes the attraction stronger). Hence,
at shorter separations the height of the energy barrier is substantially
reduced. In summary, surface roughness tends to reduce the depth of a
secondary minimum and shifts it to longer separation distances while the
repulsive energy barrier is decreased and the primary minimum is moved
to larger separation distances.
Roughness features may also physically trap nanoparticles on the sur-
face once contact has been made. This occurs through a combination of
enhanced surface adhesion and modification of the surface fluid velocity
pattern. Upon deposition, particles are principally subject to shearing
forces that would cause them to “roll” across a surface. This rolling motion
is resisted by the adhesion force acting between the surfaces, as discussed