Liquid
metal-based applications are limited by the wetting nature
of polymers toward surface-oxidized gallium-based liquid metals. This
work demonstrates that a 120 s CF4/O2 plasma
treatment of polymer surfacessuch as poly(dimethylsiloxane)
(PDMS), SU8, S1813, and polyimideconverts these previously
wetting surfaces to nonwetting surfaces for gallium-based liquid metals.
Static and advancing contact angles of all plasma-treated surfaces
are >150°, and receding contact angles are >140°,
with contact
angle hysteresis in the range of 8.2–10.7°, collectively
indicating lyophobic behavior. This lyophobic behavior is attributed
to the plasma simultaneously fluorinating the surface while creating
sub-micron scale roughness. X-ray photoelectron spectroscopy (XPS)
results show a large presence of fluorine at the surface, indicating
fluorination of surface methyl groups, while atomic force microscopy
(AFM) results show that plasma-treated surfaces have an order of magnitude
greater surface roughness than pristine surfaces, indicating a Cassie–Baxter
state, which suggests that surface roughness is the primary cause
of the nonwetting property, with surface chemistry making a smaller
contribution. Solid surface free energy values for all plasma-treated
surfaces were found to be generally lower than the pristine surfaces,
indicating that this process can be used to make similar classes of
polymers nonwetting to gallium-based liquid metals.