Effect of shear stress on adhering polyelectrolyte capsules

A parallel plate flow chamber was implemented to study the deformation and adhesion of individual spherical hollow polyelectrolyte multilayered shells adhering to a coated surface. The device provides a well-defined laminar flow allowing the determination of the shear stress to which the capsules are being exposed up to 15 N/m(2) The results of the investigations indicate a strong dependence of the adhesion and mechanical resistance on the capsule size and wall thickness. Thin walled capsules, constituted of 8 polyelectrolyte layers (thickness congruent to 12 nm), are immediately deformed when exposed to flow while thick capsules, constituted of 16 layers (thickness congruent to 24 nm), of equal dimensions are detached from the surface for drag forces below 50 nN. It was observed that adhering capsules exposed to flow undergo an increase in their adhesion area in the direction of flow, resulting in rolling of the capsules. It was also found that the resistance of the capsules decreases after acetone treatment, indicating a weakening of the polyelectrolyte multilayer structure in the presence of this solvent. (C) 2004 Elsevier Inc. All rights reserved.