The variation of s of oil droplets is more complicated,
as evidenced by the results of Fig. 3b. Treated silk reaches superoleophobicity (s of oil droplets > 150◦) at the nanoparticle
concentration of 2% w/w. The angle is high and roughly stable
for a short nanoparticle concentration range (2–3% w/w). Further
increase in SiO2 concentration (>3% w/w) results in a rapid decrease
of s according to the results of Fig. 3b, whichshows that s becomes
around 97 ◦ for 4 and 5%w/w nanoparticle concentration.