Colloidal interactions are involved in numerous natural and engineered processes, such as the filtration of pollutants from water, the adhesion of microorganisms to surfaces and the site-specific delivery of drugs to mitigate side effects. For these systems, there is a great deal of interest in how physical and chemical heterogeneities may influence the trajectories and subsequent deposition of colloidal bodies near interfaces. Considerable effort by many researchers has provided a fairly good understanding of the mechanisms of particle deposition to a leading order of approximation. Some research has even attempted to determine how the physical and chemical properties of a surface can be altered to encourage the deposition of colloidal particles at particular locations. However, various aspects of particle deposition phenomena are still persistently debated, and the level of sophistication of the models of the systems is often the source of debate. Most studies of colloidal interactions employ the DLVO (Derjaguin-Landau-Verwey-Overbeek) theory to determine interaction force magnitudes. These approaches typically focus on the forces acting normal to the surfaces of the interacting media only, and treat the colloids as homogeneous bodies. However, at micro- and nano- scales, most natural substrate surfaces are inherently heterogeneous, possessing roughness and variations in chemical composition throughout. Discrepancies between theory and experiment for these colloidal interactions can, at least in part, be attributed to theoretical models not accounting for the inherent heterogeneity of the system. Any lateral (i.e. along the surface of the substrate) heterogeneity in a colloidal system clearly requires a look at interaction forces in three dimensions. Although lateral forces have been mentioned in studies investigating surface roughness, there has been little substantive effort to quantify these forces on physically and chemically heterogeneous substrates within the framework of DLVO interactions. In considering the three-dimensional nature of the DLVO interaction forces, the influence of lateral forces caused by heterogeneities on colloidal phenomena, such as the attraction of particles to particular regions of a surface, can sometimes be significant [1]. The main challenge is to impart heterogeneity to a theoretical model that is as realistic as possible while avoiding the computational costs and limitations involved with simulations of these mesoscale systems at an atomic level resolution. The prime objective of this research is to observe how the presence of physical and chemical heterogeneities on a macroscopically planar substrate can influence deposition of particles on the substrate. The novelty of the approach lies in the use of a mathematical model that treats the substrate as an array of nanoscale subunits (which are typically spherical). Each subunit of this model substrate can be assigned individual chemical properties such as the effective Hamaker constant and surface charge or potential. The ability to assign a chemical identity to individual subunits, as well as to arrange them to emulate topological features of textured surfaces, enables one to simultaneously impart physical and chemical heterogeneity to the substrate. The DLVO interactions between the substrate and the approaching spherical particle are computed using a pairwise summation technique [1]. We incorporate the three-dimensional force field in a modified trajectory analysis, henceforth referred to as the particle-tracking model, to study the deposition of nanoparticles onto chemically and topologically heterogeneous substrates. The particle-tracking model incorporates the Brownian forces as well as the hydrodynamic interactions in conjunction with the three-dimensional colloidal force field. This results in a considerably realistic rendering of the particle motion near chemically and topologically textured interfaces. We use this particle-tracking model to study the deposition of colloidal particles from a dilute suspension onto a heterogeneous surface in an impinging jet flow field. The simulated system considers a macroscopically planar substrate with chemically heterogeneous patches and a single cylindrical protrusion into the fluid volume. The complete flow field in this simulation box is calculated by numerically solving the Stokes equations. For this system, the trajectories of consecutive particles released from a given location are followed, emulating a sequential adsorption process. With each particle that is irreversibly adsorbed on a surface, the surface becomes more topologically and chemically heterogeneous. These simulations depict the time evolution of the particle deposit structure evolving from the combined influence of hydrodynamic, Brownian, colloidal, and heterogeneity effects. Some key observations from this study will be presented, particularly focusing on how the particle deposition behavior is influenced by the presence of chemical heterogeneity located at different positions of the cylindrical protrusion from the substrate. The studies provide some quantitative markers for distinguishing between a hydrodynamic- and diffusion-controlled deposition regime, and a colloidal interaction (heterogeneity) governed deposition regime. We also demonstrate that by altering the physical and chemical heterogeneity of the substrate, one can achieve significant control on the resulting deposited particle morphology on the substrate.

References [1] Kemps, J. A. L. and Bhattacharjee, S. Langmuir., 2005, 21, 11710-11721.