title: Humidity-dependent wetting properties of high hysteresis surfaces.
abstract: The advancing contact angle (thetaadv) of water on thin films ( approximately 1 microm) of poly(ethylene glycol) (PEG) with fluoroalkyl endgroups (6 kg/mol PEG with 10-carbon fluoroalkyl, denoted 6KC10) changes strongly with relative humidity (RH). Films of 6KC10 on silicon wafers pretreated with a fluorinated alkylsilane (TFOS) display thetaadv increasing from 75 degrees at 12% RH to 95 degrees at 94% RH. The surprising transition to nonwetting character at high humidity is attributed to fluoroalkyl groups ordering at the air-hydrogel interface when they are liberated by dissolution of PEG crystallites above 85% RH. When water is withdrawn from a drop on 6KC10, the contact line does not recede. This extreme hysteresis is attributed to restructuring of the gel to bury the fluoroalkyl groups when in contact with water.
authors.affiliation: Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, USA.
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Mackel
authors.forename: Sergio
authors.initials: S
authors.lastname: Sanchez
authors.forename: Julia A
authors.initials: JA
authors.lastname: Kornfield