Dual scale roughness driven perfectly hydrophobic surfaces prepared by electrospraying a polymer in good solvent-poor solvent systems

Official URL: http://192.168.1.20/record=b1489828 (Table of Contents)

A facile method to produce perfectly hydrophobic surfaces (advancing and receding water contact angles both 180°) via electrospraying is demonstrated. When a copolymer of styrene and a perfluoroalkyl acrylate monomer was electrosprayed in good solvents, surfaces composed of micron size beads were formed and fairly low threshold sliding angles could be achieved. Addition of high boiling point poor solvents to the solutions resulted nanoscale roughness on the beads. However, even the nanoscale roughness dominated topographies achieved by this method exhibited contact angle hysteresis although deducted to be relatively small. On the other hand, when the electrospraying process parameters were set such that micron size hills of nanoscopically rough beads were formed, 0° sliding angles, implying zero contact angle hysteresis, were measured. Videos of droplets recorded and the adhesive forces measured during a contact and release experiment revealed that these dual scale rough surfaces were indeed perfectly hydrophobic. Application of the method with other binary good solvent-poor solvent systems also resulted in perfect hydrophobicity. Overall results showed how the differences in surface topology affected the wettability of surfaces within a very narrow range between perfect and extreme hydrophobicity (advancing and receding water contact angles both close to 180°). In order to interpret the formation of different surface topographies achieved by electrospraying the corresponding copolymer in good, poor and binary solvent systems, dissipative particle dynamics simulations and dynamic light scattering analysis were performed. Simulations of the polymer in good solvent revealed relatively homogenous solutions at all concentrations, whereas phase separation was observed in the poor solvent even at low concentrations. Light scattering experiments yielded useful information about the hydrodynamics of the real chains in the corresponding solvent systems in the dilute regime. It was found that the polymer forms stable aggregates in the poor solvent due to weak interaction with the solvent. Overall results indicated that formation of smooth bead morphologies is due to homogenous drying of the polymer from the good solvent. On the other hand, polymer aggregates lead to nanoscopic features in the regions where the solidi cation occurs mainly in the poor solvent environment.