This video shows direct observation of colloid microspheres of 2 
m diameter being transported in a microscopic granular media domain of glass beads that are approximately 1 mm diameter. The main flow direction is from bottom left to top right. Colloids show as bright white dots. For favorable conditions, attachment occurs immediately once the colloid enters the near‑surface fluid domain; there is no action at the grain surface other than the sudden appearance of the attached colloid. In contrast, under unfavorable conditions colloids spend a significant time in the near‑surface fluid domain prior to attachment or re‑entrainment back to bulk fluid; there is frequent slow colloid motion along the grain surface, with a subset of the colloids becoming attached as shown here.
A discussion of the material in this video is provided in Section 5.1.1 ‘Colloid Motion in Near Surface Fluid Domain in Pore Scale Experiments’ of the Groundwater Project book ‘Colloid (Nano- and Micro-Particle) Transport and Surface Interaction in Groundwater’, the PDF can be downloaded at that link.
Citation: Johnson, W.P. & Pazmiño, E.F. (2024). Colloid Attachment to Porous Medium Under Favorable Conditions. Video of The Groundwater Project, Guelph, Ontario, Canada.