Underwater superoleophobic composite films containing hydrophilic SiO2 nanoparticles and chitosan were prepared on glass substrates by a simple dip-coating method.By observing the surface morphologies with scanning electron microscope (SEM) and atomic force microscope (AFM) and measuring oil contact angles

factors affecting the morphology

roughness

contact angle and sliding angle of the film

such as the immersion time in the water and the volume ratio of hydrophilic SiO2-ethanol suspension to chitosan solution

were investigated in detail.With the increase of the volume ratio of hydrophilic SiO2-ethanol suspension to chitosan solution

more hydrophilic SiO2 powders accumulated on the surfaces of composite films

leading to a significant increase of underwater oleophobicity.With the increase of immersion time in the water

underwater oil contact angle (OCA) of the composite films first increased gradually and then slightly decreased after the films were immersed for 168 h.Meanwhile

the sliding angle (SA) of the films was also affected by the immersion time.When the volume ratio of hydrophilic SiO2-ethanol suspension to chitosan solution was 1:10 and 1:5

the chitosan-SiO2 composite films were oleophobic; and when the volume ratio of hydrophilic SiO2-ethanol suspension to chitosan solution was equal to or higher than 1:2

the chitosan-SiO2 composite films exhibited underwater superoleophobicity with underwater oil contact angles (OCA) higher than 150.When the chitosan-SiO2 composite film with the volume ratio of hydrophilic SiO2-ethanol suspension to chitosan solution 1:1 immersed in the water for 24 h

it showed an underwater oil contact angle (OCA) 157.8 and sliding angle (SA) 1.9

exhibiting outstanding underwater superoleophobicity.The chitosan-SiO2 composite films was applied to the manipulation of oil droplets underwater and realized the addition reaction of Br2 and styrene in oil droplet microreactors underwater.