Near-Surface Motion and Dynamic Adhesion during Silica Microparticle Capture on a Polymer (Solvated PEG) Brush via Hydrogen Bonding

Surface modification by polymer brushes (dense end-tethered chains) is a strategy to create a steric barrier against colloidal aggregation or bioadhesion or to facilitate lubrication. The approach requires good brush solvation; however, even in a good solvent, brush chains can adhere certain objects. An important example is the hydrogen bonding of silica to PEG (poly(ethylene glycol)) chains in water. To probe how hydrogen bonding at the brush periphery facilitates adhesive capture of flowing particles, we employ a model system comprising silica microspheres on biorepellant PEG brushes sufficiently thick to screen interactions with the underlying substrate. We find that capture of silica spheres on PEG brushes is slower and less efficient than the transport-limited rate and can be hindered by the addition of salt and by flow, up to wall shears at least 500 s(-1). Individual flowing silica microparticles adhere gradually to the PEG brush (presumably by increasing the numbers of H-bonds), so that the particle motion slows prior to arrest. The deceleration length is on the order of tens of microns and is salt- and shear-dependent. By contrast, capture of the same particles on electrostatically attractive surfaces is transport-limited and occurs when particles jump tens of nanometers to the surface within a fraction of a second rather than translating near the surface. These distinctly different near-surface motion signatures may result from fundamental differences in interactions: PEG silica hydrogen bonding is short-range and requires registry of interacting groups, while electrostatic attractions are long-range. These differences in interactions likely translate to a slower forward binding constant for hydrogen bonding compared with the diffusion-limited rate of particle capture in an electrostatically attractive well. The adhesion of silica particles to a PEG brush comprises a model for other particles containing H-bond donor surface functionality (silanols, alcohols, amines) and provides a powerful example of lubrication versus adhesion at a surface presenting nanoscale deformability.