Adhesion and Particle Removal from Surface-Tethered Poly(N-Isopropylacrylamide) Coatings Using Hydrodynamic Shear...

Thermally responsive coatings of poly(N-isopropylacrylamide), or poly(NIPAAm), have a volume phase transition temperature (VPTT) near 32 °C. Below this temperature, the coating imbibes water and swells. Above this temperature, the coating rejects water and collapses. Herein, a spinning disk method is used to determine the hydrodynamic shear stress necessary to remove 10 μm polystyrene (PS) microspheres capped with either carboxylic acid (COOH) functionality or immunoglobulin (IgG) proteins from the coatings as a function of coating thickness and temperature. In the case of the PS-COOH, the hydrodynamic shear stress necessary to remove the microspheres was consistently larger below the VPTT than above the VPTT of the poly(NIPAAm) coating. In the case of PS-IgG, the trend was reversed, in which the hydrodynamic shear stress necessary to remove the microspheres was consistently smaller below the VPTT than above the VPTT. Simple scaling relationships were developed to explain the findings within the Johnson-Kendall-Roberts (JKR) model of contact mechanics, which illustrates the delicate interplay between the pull-off force and contact radius (as determined by the coating shear modulus) in governing particle removal from soft surfaces with hydrodynamic forces.