Shape and orientation of bare silica particles influence their deposition under intermediate ionic strength: A study with QCM–D and DLVO theory

Theory developed by Derjaguin, Landau, Verwey, and Overbeek (DLVO) is commonly used to interpret and predict the deposition of particles, but it was originally simplified for spherical particles. Many types of bacteria and particles in nature are not spherical. Previous literature has experimentally shown that particle shape has an effect on drug delivery, retention in porous media, self-assembly, and flotation, but the quantitative interpretation of these results has been hindered by experimental (e.g., uniform rod shaped particles and flow fields at controlled chemistries) and theoretical (e.g., consideration of rod shape particles with different surface orientations) challenges. For example, the deposition of ellipsoidal polystyrene particles has been investigated in the presence of non-DLVO interactions, due to residual poly(vinyl) alcohol, hindering the effect of shape. In our study, bare spherical and bullet-like silica particles of well-defined surface chemistry were used for deposition tests on bare silica surfaces using a Quartz Crystal Microbalance with Dissipation (QCM–D) over six ionic strengths (IS). At the same time, the DLVO theory was modified to consider particle shape and orientation of deposition. We found that particle shape had an effect on deposition at intermediate IS. A modified DLVO approach was able to interpret the deposition of bullet-like silica particles. Specifically, bullet-like silica particles of certain aspect ratio may find angles that minimize repulsive energies and overcome energy barriers so that deposition on the silica surfaces is energetically favorable. Therefore, there are conditions of water chemistry where particle shape and orientation cannot be ignored and their deposition must be systematically investigated.