Humidity-dependent wetting properties of high hysteresis surfaces.
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 When water is withdrawn from a drop on 6KC10, the contact line does not recede.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.
Mackel
Michael J
MJ
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, USA.
Sanchez
Sergio
S
Kornfield
Julia A
JA