Shear-Induced Desorption of Isolated Polymer Molecules from a Planar Wall

Shear-induced desorption of isolated polymer molecules is studied using Brownian
Dynamics simulation. The polymer molecules are modeled as freely-jointed
bead-spring chains interacting with an adsorbing wall via a short-range
potential. The simulations include both intrachain and chain-wall hydrodynamic
interactions. Shear flow is found to cause chain flattening, resulting in an
increased bound fraction. However, above a certain critical shear rate chains
desorb completely from the wall. The desorption process is nucleated by random
protrusions in the shear gradient direction, which evolve under the combined
effect of drag, hydrodynamic lift force, and vorticity induced rotation toward
the wall leading to subsequent recapture. Above the critical shear rate, these
protrusions grow in length until the entire chain is peeled off the wall. For
free-draining chains, the protrusions do not sustain and no desorption is
observed even at shear rates much higher than the critical value. These
simulations can help in understanding several features of experimentally
observed shear-induced desorption in polymer films as well as brushes.