An oil repellent layer is applied to {HEAD ~ either the surface portion} having grooves thereon or {TAIL ~ its opposing surface} in the vicinity of the narrower gap 
An oil repellent layer is applied to { HEAD ~ either the surface portion } having grooves thereon or { TAIL ~ its opposing surface } in the vicinity of the narrower gap
OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH